Chapter 3 Dimethacrylate-POSS networks: processing,...

Chapter III Dimethacrylate-POSS networks

Marco Amici– INSA 2006 72

Chapter 3 Dimethacrylate-POSS networks: processing,

characterization, structure-morphology and properties

BisEMA-POSS networks have been realized with three different types of POSS, two

monofunctional-POSS and one octafunctional-POSS. A reactive solvent, namely CHMA, has

been used in conjunction with the BisEMA dimethacrylate resin. Free radical polymerization

has been used, either via UV curing or via thermal curing. For most of the systems, thermal

post-curing has been done in order to maximize the extent of the polymerization. The degree

of polymerization of these hybrid networks has been measured with NIR spectroscopy. The

structure-morphology has been analysed using WAXS, SEM, TEM and AFM. DMTA

analysis has been used to determine the glass transition temperature, the degree of

heterogeneity and the average crosslinking density of the networks. Tensile testing has been

used to asses the influence of POSS structure and dispersion in the final hybrid networks on

the values of the Young moduli and the tensile strength. Surface properties as hardness and

surface energy have been studied as well. From this body of work, structure-properties

relationships of BisEMA-POSS networks will be drawn in the conclusions.

3.1 General characteristics of methacrylate-based networks and

thermoplastics

In this section the methacrylate-based resins and thermoplastic polymers, with a particular

attention to the materials used in this research, are introduced from a general point of view.

The family of methacrylate based polymers represents a common subject of interest both for

researchers and industries.

The common feature of these polymers is the presence of the methacrylate group in figure 3-

1:

Figure 3-1 Schematic structure of the methacrylate group. The C=C bond is the polymerizable group.



Organic chemistry allows the tailoring of the chemical units attached to the methacrylate

group. Due also to the early synthesis of the first methacrylate polymers [Van97], a wide

variety of methacrylate monomer and polymers is nowadays commercially available [Bra03].

Some of the more general features of methacrylate-based polymers are listed below:

- Good mechanical properties like moderately high Young modulus, strength and

hardness [Bra03, Rey02, Sid96]. These properties are for example the reason of the

use of the methacrylate-based polymers for bio-medical implants [Bel00, Tan03],

dental resins [Mos04] and coatings [Dus00, Har03, Mam05a, Mam05b, Solo02].

- Good solvent resistance, transparency and wettability with inorganic substrates,

making these systems suitable for coating applications [Sta91].

- Ability to steadily polymerize in free-radicals conditions, both under UV radiation and

under heating. This feature, probably the most important one for industrial

applications, directly results in the use of dimethacrylates for dental resins ([Ken03,

Kla05, Tan03]), coating ([Bur00]) and materials for electronic and opto-electronic

applications [Kan97, Kan01, Kan98, Sid04, Sid96, Sim01].

- Presence of both polar and non-polar groups in the molecules, which makes these

polymer a good compromise for some applications where the water adsorption and the

hydrophylicity are a key issue (for instance humidity detectors [Bur00]).

- Capacity to undergo rapid chemical decomposition-depolymerization in presence of

radiations, which makes this materials very suitable for lithography applications,

where the methacrylate polymers are etched and then removed via dissolution in

specific solvents [Che05].

All the above considerations can be applied to the three different organic monomers used in

this research, presented in the figure 3-2:



CH3

CH3

OOO

CH3

O

O

CH3

O2

2

( A )

C

H3C

H2C

O

O

( B )

C

H3C

H2C

O

O CH3

( C )

Figure 3-2 Schematic chemical structure of the methacrylate monomers used in this research:

(A) BisEMA, tetraethoxylated bisphenol A dimethacrylate, (B) CHMA, cyclohexyl-methacrylate,

(C) MMA, methylmethacrylate.

BisEMA (figure 3-2 (A)) is the acronym for tetraethoxylated bisphenol A dimethacrylate.

BisEMA is a commercial dimethacrylate resin from Sartomer. The molar mass is 540 g.mol-1

and the density is 1.12 g/cm3.

CHMA, shown in figure 3-2 (B), stays for cyclohexyl-methacrylate. CHMA has a molar mass

of 168 g.mol-1, a purity higher than 97 % and a density of 0.96 g/cm3.

MMA indicates the methyl-methacrylate (figure 3-2 (C)). MMA has a molar mass of 100

g.mol-1, a purity higher than 99 % and a density of 0.94 g/cm3.



In this chapter the attention will be focused on the dimethacrylate-POSS networks, leaving all

the analysis of the thermoplastic POSS-MMA copolymers for the Chapter IV.

3.2 Synthesis of the dimethacrylate-based networks

The synthesis of the dimethacrylate-based networks has been performed via free radical

polymerization. The synthesis protocol is the same of Bizet [Biz04]. The most important

aspects will be summarized here in order to give the reader a glimpse view on the subject and

to explain properly the properties as a function of the structure-morphologies originated

during the processing.

3.2.1 Polymerization mechanism and network build-up

The free radical polymerization is a chain-polymerization, which means that each

macromolecule propagates from an active centre, in this case a radical. This reaction is based

essentially on three steps: initiation, propagation and termination, as shown in figure 3-3:

Figure 3-3 Reactions involved in the free radical polymerization.

For each of the three steps the kinetic constants are reported.

As can be seen, the chemical species I, called “initiator”, dissociates giving rise to two

radicals. These radicals “attack” the monomer, originating the first oligomers, then the chain

Initiation

Propagation

Termination

Mn + M Mn+1kp

I 2 Rkd

R + M RMki

Mn Mm+ Mn+mktc

Mn Mm+ Mn + Mmktd

Combination

Disproportionation



Mn + T Mn + Tktr

M+T TMk'i

propagates via addition of new monomers. In this way, high molar masses are reached very

early in the polymerization. The end of the polymerization is usually due either to

combination or disproportionation of the growing chains. Another reaction which could be

originated by a radical is the transfer reaction, as shown in figure 3-4:

Figure 3-4 Transfer reaction in free radical polymerization.

For each step the kinetic constant is reported.

This reaction consists in the transfer of a macro-radical to a monomer, to another polymer, to

a solvent or to a transfer agent. The result of this reaction is usually the decrease of the molar

mass of the final polymer.

The dissociation of the initiator molecules can be originated by heating or radiation. Among

the latter, the most used radiation are UV-light and accellerated electrons.

In this case, the main technique chosen for the processing of dimethacrylate-POSS systems is

the UV polymerization, usually followed by a thermal process to ensure the maximum extent

of the reaction. The reason of this choice is that UV-processing is a very cost-effective

technique, therefore used widely in the industrial field, while at the same time ensuring a high

extent and reproducibility of the polymerization.

The free radical polymerization can be done in the bulk, in solution, suspension or emulsion.

In this research, the bulk polymerization has been used in the case of the dimethacrylate-

POSS networks, while the polymerization in solution has been chosen for the MMA-POSS

copolymers. However, for the dimethacrylate-networks, a reactive solvent, the CHMA, has

been used in order to improve the solubility of the POSS molecules, as shown in the next

section.



- Network build-up

The networks polymerized via free radical reaction are well known to be heterogeneous

[Coo91, Dus80, Kar97, Pas02, Rey02, Rey00].

This feature is inherent to the radical polymerization process taking place whenever a network

is formed by the presence of a multifunctional vinyl monomer in the reactive system. The

development of the heterogeneities happens in different steps during the polymerization, as

shown in figure 3-5:

Figure 3-5 Different steps of the mechanism of formation of networks via radical polymerization [Pas02].

It should be noted that the presence of heterogeneities is not due to an eventual heterogeneity

of the reactive mixture, i.e. the development of heterogeneities happens from completely

homogenous reactive mixtures.

In figure 3-5 are visible four steps of the formation of the heterogeneous morphology:

Step I: Induction period, in which there is the consummation of the inhibitor

Step II: Formation of the microgels (zones at high crosslinking density). A little insight in the

chemical processes happening during this stage shows the presence of unreacted double bonds

pending from the macromolecular growing chains. These double bonds can react with a

monomer (chain propagation), with another double bond on the same macromolecule

(cyclization) or on another macromolecule (intermolecular crosslinking reaction).

Step III: The microgels present unreacted double bonds on their shell. Once two microgels

become in contact, these double bond can react.



Step IV: The reaction of these double bonds among different microgels leads to the formation

of macrogels via clustering, as depicted in figure 3-5.

It is worthy to go a bit deeper in the analysis of the role played by the microgels in forming

the final nano-morphologies.

First of all, the formation of the microgels, due to cyclization of macromolecules, is more

probable than the intermolecular crosslinking reaction [Gan02]. This cyclization happens

since the beginning of the polymerization, hence leading to the presence of microgels from a

very low conversion rate.

As shown by Rey et al. [Rey02, Rey00], the size of a microgel in a dimethacrylate-based

systems ranges around 50 nm. The microgels, which are very compact zones at high

crosslinking density, can be idealized as in figure 3-6:

Figure 3-6 Schematic representation of a microgel. It is well represented the high crosslinking density and the

presence of unreacted double bonds on its surface [Dus80].

Symplifying, the microgels can be imagined as composed of a highly crosslinked core and a

less crosslinked shell. On the other hand, it would be more appropriate to imagine the

crosslinking density varying continuously from the core to the shell of the microgels, rather

than the existence of two distinct phases with massively different density of crosslinking.

The main feature of the microgels is the very high density of crosslinking in the core, which

makes it effectively unaccessible to solvents or monomers molecules. Therefore it is very

probable that unreacted or partially reacted monomers could be trapped inside the microgels

without further possibility of polymerising. This effect becomes even more important after the



macrogels are formed, that is after the so-called “gelation time”. In fact, in the light that the

free radical polymerization of dimethacrylates is essentially a chemical reaction controlled by

diffusion [Rey00], the presence of macrogels largely reduces the extent of this diffusion,

hence the radicals trapped in the microgels are even less likely to react. The result is that

unreacted or partially-reacted monomers are blocked since the beginning of the

polymerization in the forming microgels, without further possibility to react. This trapping of

unreacted or partially-reacted monomers becomes more and more effective with the

polymerization extent.

3.2.2 Processing conditions

For the UV processing, a photo-curing set of three compounds has been chosen. This set is

constituted by a photocuring agent, a so-called photosensibiliser and an amine as co-curing

agent. The photocuring agent is an alkylphenone, sold by Ciba-Geigy with the commercial

name DAROCUR 1173. The photosensibiliser is the benzophenone, sold by Ciba-geigy as

DAROCUR BP. The amine reducing agent is the n-methyl diethanol amine (n-MDEA from

Aldrich Chemicals). The percentage of each of these three compound has always been 2 % of

the weight of the total system.

Alternatively, some systems have been processed using thermal curing. In this case, azo-bis-

iso-butyronitrile (AIBN) has been used at 0.5 wt % instead of the photocuring set, and the

polymerization has been performed for 8 hours at 70°C under inert atmosphere, followed in

most cases by postcuring for 2 hrs at 130°C.

The composition (intended as weight percentage) of the reactive mixture of a generic system

BisEMA-x % weight fraction of POSS is:

BisEMA: 100% – x% – 6%

POSS: x %

Photocuring set: 6 % (2 % each of the three compounds)



A similar situation happens in the case of networks based on 50/50 BisEMA-CHMA:

BisEMA: (100 % - x % - 6 %) / 2

CHMA: (100 % - x % - 6 %) / 2

POSS: x %

Photocuring set: 6 % (2 % each of the three compounds)

The POSS concentrations used were 1, 2.5, 5 and 10 wt % with respect to the total system.

Some samples with 20 and 30 wt % of POSS have also been processed to study particular

properties or morphologies.

All the samples have been prepared according to the following protocol (see for all the details

[Biz04]). Reactive mixtures have been prepared in a 10 ml glass vial and then stirred at room

temperature under air for a range of time between 1 and 2 hours. After the mixing, the liquid

systems were poured either into “dog-bone” (thickness of ca. 1 mm, width ca. 2mm and gauge

length of 10 mm) or rectangular shaped silicon-rubber moulds (to prepare bulk samples 1 mm

thick) or in glass plates with teflon walls (to prepare 100µm thick films). The samples were

thus irradiated for ~70s with an UV light lamp (Fusion F300 Fusion UV Systems Inc.).

During the irradiation, the samples were subsequently carried back and forth under the lamp

beam by an automatic belt to have the samples homogeneously irradiated. Post-curing has

been performed to maximise the conversion of the methacryl double bonds along with the

crosslinking of the materials. The post-curing was carried out in an oven under inert

atmosphere for 2 hrs at 130 °C (for the MonoPOSS-based materials) and 150 °C (for the

OctaPOSS-based materials).

3.2.3 Miscibility – Solubility of the POSS in the organic monomers

A key issue of this research has been the dispersion of the POSS in the final systems. As

shown in Chapter 1, a better dispersion is usually accompained by an improvement of the

properties due to the POSS presence, like in Fu et al. [Fu04], Li et al. [Li02] and Strachota et

al. [Str04]. This dispersion in turn could be ensured only in very specific conditions. In fact,

despite the POSS molecules have a twofold nature (inorganic at the core, organic on the



shell), their miscibility-solubility into organic monomers is somehow limited. This aspect has

been already analysed in the Chapter 1, where it has been shown that most of the literature on

POSS-polymer systems is focused on the POSS dispersion in the final systems.

Rigorously, it should be used miscibility in case of mixtures BisEMA-OctaPOSS (two liquid

systems), while the term solubility should be adopted when speaking of systems BisEMA-

MonoPOSS (powder into a liquid).

The OctaPOSS is very miscible in BisEMA resin, up to 30 % in weight, probably because of

the presence of the methacrylate groups. On the contrary, the solubility of the two MonoPOSS

in the BisEMA resin is much lower, as can be seen in Bizet [Biz04] (figure A-8, pag. 196).

The iBuPOSS is soluble at 80°C only up to 4 % in weight, while the CyPOSS is much less

soluble, with only 1 % in weight of CyPOSS soluble in the BisEMA at more than 130°C. The

explanation of this difference has to be found in the higher compatibility between isobutyl

than cyclohexyl groups with methacrylate monomers.

A way to improve the POSS miscibility-solubility in the reactive systems is the use of

solution polymerization, and this technique will be used in Chapter 4 to synthesize the POSS-

MMA copolymers. Another way to improve the miscibility-solubility of the POSS in the

reactive mixture is the use of a reactive solvent. This technique has been used for the

thermoset POSS-dimethacrylate systems because of the problems connected with the solution

polymerization of networks, in particular in the step of solvent evaporation. Among different

mono-methacrylate monomers, the CHMA has been chosen because the shrinkage observed

in the networks reached a minimum with this compound rather than with other methacrylate

monomers, like isobutyl-methacrylate or MMA.



- Systems considered in this study

The table 3-1 sum up the UV-polymerized systems studied in this research.

Table 3-1 Resume of the UV-polymerized systems studied in this research.

Matrix POSS type POSS weight fraction (%)

iBuPOSS 0, 1, 2.5, 5, 10 BisEMA

OctaPOSS 0, 1, 2.5, 5, 10

iBuPOSS 0, 1, 2.5, 5, 10 BisEMA-CHMA (50/50)

OctaPOSS 0, 1, 2.5, 5, 10

The other systems studied, either with different POSS contents or thermally cured, will be

indicated when presented.

Remarkably, the reactive mixtures of all the BisEMA-iBuPOSS systems were opaque (1, 2.5

and 5 %) or milky (10 %). The reactive mixtures of BisEMA-CHMA-iBuPOSS were

transparent, as well as all the reactive mixtures with OctaPOSS. The lack of transparency of

the systems BisEMA-iBuPOSS (indication of lack of solubility of the POSS) is due to the

incapacity of BisEMA to act as a solvent for the iBuPOSS crystals. In this system, the

iBuPOSS crystals are present since the very beginning of the reaction, even if it could be logic

that a part of the iBuPOSS are however dissolved in the BisEMA matrix.

3.2.4 Conversion of the methacrylate double bonds

For conversion (or alternatively degree of conversion) it is meant the degree of reaction of the

methacryl double bonds of the final systems (after curing and post-curing) with respect to the

reactive mixtures.

As stated in the previous sections, the conversion of the methacryl double bonds at the end of

the polymerization process is an important parameter to keep into account in the

dimethacrylate matrices. The conversion in fact quantifies directly the number of double

bonds unreacted. Therefore, only the conversion at the end of the process of the

polymerization will be studied in this research. The reader is referred to the work of Bizet for

the POSS influence on the polymerization kinetics of dimethacrylate networks [Biz04].



Hereafter, the term conversion will identify throughout all this thesis the final conversion after

the polymerization process.

The conversion has been measured using Near Infrared Spectroscopy on all the samples

reported in the table 3-1 plus the networks containing CyPOSS to better understand the effect

of the presence of the POSS crystalline phase on the final conversion. Furthermore, four of

these six different systems (that is BisEMA-iBuPOSS, BisEMA-OctaPOSS, BisEMA-

CHMA-iBuPOSS and BisEMA-CHMA-OctaPOSS) have been prepared with higher contents

of POSS (generally 20 and 30 wt %) to check for possible trends both in conversion and in

properties with the increasing of the POSS content. As can be seen, the networks containing

CyPOSS have not been prepared with POSS content higher than 5 wt % because of the low

solubility of this compound in both matrices.

The technique used, as well as the theoretical basis, is reported in the annexes.

The results are visible in the figures 3-7, 3-8 and 3-9:

0 5 10 15 20 25 3080

85

90

95

100

105

Con

vers

ion

(%)

Weigth fraction of iBuPOSS (%)

Figure 3-7 Conversion (%) of nanocomposites with iBuPOSS and BisEMA (full squares ■) or BisEMA-CHMA

(empty circles ○) as matrices.



0 5 10 15 20 25 3080

85

90

95

100

105

Weigth fraction of CyPOSS (%)

Con

vers

ion

(%)

Figure 3-8 Conversion (%) of nanocomposites with CyPOSS and BisEMA (full squares ■) or BisEMA-CHMA

(empty circles ○) as matrices.

0 5 10 15 20 25 3080

85

90

95

100

105

Weigth fraction of OctaPOSS (%)

C

onve

rsio

n (%

)

Figure 3-9 Conversion (%) of nanocomposites with OctaPOSS and BisEMA (full squares ■) or BisEMA-

CHMA (empty circles ○) as matrices.

Complementary information can be found in the table 3-2, where are reported the theoretical

percentages of double bonds due to the POSS with respect to the total number of double

bonds of reactive mixtures, as a function of the POSS type and concentration:



Table 3-2 Theoretical percentages of double bonds due to the POSS with respect to the total number of double

bonds of the reactive mixtures, as a function of POSS type and concentration.

*: OctaPOSS have been assumed as composed only of (SiO1.5)8 cage elements.

POSS

concentration

(wt %)

% C=C POSS

BisEMA matrix

% C=C POSS

BisEMA-CHMA

matrix

0 0.0 0.0

1 0.3 0.2

2.5 0.8 0.6

5 1.6 1.2

10 3.3 2.5

20 7.2 5.6

iBuPOSS

30 11.8 9.3

0 0.0 0.0

1 0.3 0.2

2.5 0.7 0.5 CyPOSS

5 1.3 1.0

0 0.0 0.0

1 1.6 1.2

2.5 4.0 3.1

5 7.8 6.1

10 15.2 12.1

20 28.9 23.8

OctaPOSS *

30 41.4 35.1

Remarkable differences are present in the trends of the conversion as a function of POSS

weight fraction in systems containing differents of POSS.



The conversion tends to decrease with the increasing of POSS content for a weight fraction

above 10%. The cause of this phenomenon is twofold: the lower reactivity of all the three

POSS molecules with respect to the methacrylate-based organic monomers [Hyb05] and, in

the MonoPOSS based systems, the fact that the POSS remain associated into crystals.

For the BisEMA-MonoPOSS systems, POSS crystals have been detected using WAXS (see

figure 3-11). Moreover micro-aggregates have been observed with SEM for all the iBuPOSS

concentrations while the CyPOSS were aggregated at a mm scale and visible even with

unaided eye. It should be recalled here, that the MonoPOSS are also poorly soluble in

BisEMA, hence aggregates are present even before the polymerization starts. Consequently,

the presence of the MonoPOSS aggregates ‘traps’ the double bonds not allowing them to

react.

In the systems BisEMA-CHMA-MonoPOSS such an effect does not occur, because the

solvent presence is able to destroy the aggregates and POSS cubes are individually dispersed.

In the light of what is stated here it is logic that, both for iBuPOSS- and CyPOSS-based

BisEMA-CHMA networks, the conversion does not decrease so markedly with the POSS

content as in the network based on BisEMA (see figure 3-7 and 3-8), even if the number of

(less reactive) C=C bonds from the POSS in the reactive mixtures is similar in percentage

between the MonoPOSS-systems based on BisEMA and BisEMA-CHMA matrices (see table

3-2).

For both the BisEMA-OctaPOSS and BisEMA-CHMA-OctaPOSS networks the decrease of

the conversion is likely to be related only to the low reactivity of the methacrylate groups of

the OctaPOSS molecules (a very important parameter in the OctaPOSS-systems, also

according to the C=C bonds percentage in the reactive mixtures due to the OctaPOSS, which

is rather high, as shown in table 3-2). Also the reduction of the diffusion of reactive species

during the polymerization, which results in another type of “trapped radicals” in the network

[Rey02], can play a role in decreasing the conversion. It will be shown later that the

experimental results about the structure-morphology of these systems do not reveal any

aggregation.

Furthermore, the very similar conversions of BisEMA and BisEMA-CHMA networks (that is,

without POSS) indicates that the solvent is effectively polymerized, albeit it is not clear at this

point if it has copolymerized with the matrix or not.



The decrease of the conversion in most systems with the POSS weight fraction will supply

one of the explanations of the trend of the tensile strength, as will be shown in the following

of this chapter.

3.3 Structural characterization via WAXS analysis

The samples analysed were dog-bone shaped (see §3.2.3), UV cured and thermally post-

cured. The very same samples have been afterwards used for mechanical testing.

Due to the different structures of the neat iBuPOSS and the OctaPOSS, as shown in Chapter

2, this section will deal separately with the systems based on each of the two types of POSS.

3.3.1 Influence of the amount of the reactive solvent CHMA on the POSS

organization

WAXS has been used to determine the influence of the reactive solvent on the dispersion of

the iBuPOSS in the final networks. The weight percent of the reactive solvent CHMA has

been varied from 10 to 60 % while the amount of iBuPOSS has been kept costant (7.5 wt %).

In figure 3-10 are reported the WAXS patterns for the different networks.

5 10 15 20 25 30

Neat iBuPOSS

Rat

io B

isE

MA

/CH

MA

Inte

nsity

(a.u

.)

2θ (°)

100/0

90/10

80/2070/30

60/4050/5040/60

Neat iBuPOSS BisEMA-CHMA 40:60 - 7.5 wt % iBuPOSS BisEMA-CHMA 50:50 - 7.5 wt % iBuPOSS BisEMA-CHMA 60:40 - 7.5 wt % iBuPOSS BisEMA-CHMA 70:30 - 7.5 wt % iBuPOSS BisEMA-CHMA 80:20 - 7.5 wt % iBuPOSS BisEMA-CHMA 90:10 - 7.5 wt % iBuPOSS BisEMA-7.5 wt % iBuPOSS

Figure 3-10 WAXS patterns of networks BisEMA-CHMA-7.5 wt % iBuPOSS with different ratio BisEMA-

CHMA. The samples analysed were thick films UV cured. The data are offset for clarity.



The iBuPOSS were present in the crystalline state for all the networks with less than or equal

to 20 wt % CHMA. In the case BisEMA-CHMA with a ratio 70/30, the WAXS pattern

presents a halo centred at 2θ~8°, which broadens when increasing the amount of CHMA. For

the network with 7.5 wt % iBuPOSS and BisEMA-CHMA in a ratio 60/40, the reactive

solvent was therefore able to solubilise the iBuPOSS in the reactive mixture while at the same

time the iBuPOSS do not recrystallise during the polymerization. Therefore, to be on the safe

side up to 10 wt % iBuPOSS, it has been chosen for this research the ratio BisEMA-CHMA

50/50 in weight.

3.3.2 BisEMA-iBuPOSS and BisEMA-CHMA-iBuPOSS networks

The WAXS patterns of BisEMA-iBuPOSS networks are reported in figure 3-11:

5 10 15 20 25 30

100% iBuPOSS

30% iBuPOSS

20% iBuPOSS

10% iBuPOSS5% iBuPOSS2.5% iBuPOSS0% iBuPOSS

neat iBuPOSS BisEMA-30 wt % IBuPOSS BisEMA-20 wt % IBuPOSS BisEMA-10 wt % IBuPOSS BisEMA-5 wt % IBuPOSS BisEMA-2.5 wt % IBuPOSS BisEMA

2θ = 18.9°d-spacing = 4.5 Å

2θ = 8.05°d-spacing = 11.0 Å

Inte

nsity

(a.u

.)

2θ (°)

Figure 3-11 WAXS pattern of BisEMA-iBuPOSS networks (UV cured and thermally postcured). All the

patterns have been normalized with respect to the absorption coefficient of the sample, except the one of the neat

iBuPOSS (for reasons of graphic scaling). The curves are offset for clarity.

The BisEMA network shows an amorphous pattern, with a broad halo centered at 2θ~18°.

The network containing 2.5 wt % iBuPOSS shows a X-ray pattern very similar to the neat

matrix, with just a small halo centred at 2θ~8°. The networks containing at least of 5 wt %

POSS start to exhibit crystalline features in the WAXS patterns for a iBuPOSS content. This

is clearly visible from the appearance of the crystalline peak centered at 2θ = 8.05°. This peak



at 2Ө~8° will be used through this chapter as an indicator of the existence of POSS

crystallization as it is the most prominent one of the neat iBuPOSS and CyPOSS (see for

reference Chapter II).

The position of this peak is the same both for the neat iBuPOSS and for the networks, which

means that the crystalline structure of the iBuPOSS does not vary after the POSS

incorporation and reaction in the dimethacrylate matrix. In other words, it seems that the

dimethacrylate matrix does not alter the iBuPOSS crystalline structure when the content of

POSS is high enough to have immiscibility of POSS and BisEMA (i.e. ≥ 10 wt %).

The analysis of the crystalline contribution to the WAXS patterns of the networks has

supplied information on the mean crystallite size via the Scherrer equation. For this purpose

the peak at 2θ = 8.05° has been analysed, as displayed in figure 3-12:

7,0 7,5 8,0 8,5 9,0 9,5

Inte

nsity

(a.u

.)

2θ (°)

neat iBuPOSS BisEMA-30% iBuPOSS BisEMA-20% iBuPOSS BisEMA-10% iBuPOSS

Figure 3-12 WAXS pattern of BisEMA-iBuPOSS networks (UV cured and thermally postcured): crystalline

peak at 2θ=8.05° after removal of the amorphous background. The colours are the same of figure 3-11. The data

are offset for clarity.

As can be seen, this peak seems to narrow with the increasing of the POSS content. Analysing

the FWHM of the peaks with the Scherrer equation allows one to estimate the size of the

POSS crystallites once in the BisEMA networks and to compare them with the neat iBuPOSS.



In the light that this peak corresponds to the crystallographic planes (010), the crystallite size

calculated via the Scherrer equation represents the mean size of the crystallite in the direction

perpendicular to this set of planes. The results of this analysis are shown in figure 3-13:

0 20 40 60 80 100 1200

200

400

600

800

1000

1200

1400

0

200

400

600

800

1000

1200

1400

Mean Crystallite Size (Å) Linear Fit

M

ean

Cry

stal

lite

Size

(Å)

iBuPOSS Wt (%)

Figure 3-13 Mean Crystallite size (Å) as a function of the POSS content in BisEMA-iBuPOSS systems.

The average size of the crystallites in the direction perpendicular to the planes (010) is of

1150 Å for the case of the neat iBuPOSS. Adding the iBuPOSS to the BisEMA matrix leads

to a modification of the crystallite size, passing from 1150 Å for the neat POSS to 293 Å for

the BisEMA-10 wt % iBuPOSS. Then, increasing the iBuPOSS content in the BisEMA-

iBuPOSS networks leads to an increase of the crystallite size with a linear fashion from 293 Å

(BisEMA-10 wt % iBuPOSS) to 506 Å (BisEMA-30 wt % iBuPOSS). This could mean that,

in systems containing a low amount of POSS, the crystallites are partially “solubilized” by the

BisEMA probably during the polymerization process, as observed by Matějka et al. [Mat04]

for POSS-epoxy systems.

Generally speaking, the presence of these crystallites inside the networks happens because of

the poor solubility of the iBuPOSS with the neat BisEMA (that is, before the polymerization),

as indicated by the opacity of the reactive mixtures of BisEMA and more than 5 wt % of

iBuPOSS. This in turn means that some of the methacrylate functionalities borne by the POSS

are not accessible for the reaction of polymerization, as already proposed in the §3.5. In



principle, it could exist a correlation in between the decrease of the conversion of the

methacrylate double bonds and the increase of the crystalline phase with the POSS content, as

can be seen by the increase of the intensity of the crystalline peak at 2θ~8° shown in figure 3-

12. In figure 3-14 it is possible to visualize this correlation:

10 20 30

iBuPOSS wt (%)

Inte

grat

ed In

tens

ity (a

.u.)

80

85

90

95

100

Con

vers

ion

(%)

Figure 3-14 Variation of the integrated intensity of the crystalline peak at 2θ~8° (full squares ■ ) and the

conversion of methacrylate functionalities (full circles ●) relative to BisEMA-iBuPOSS networks. The data are

relative to dog-bone samples UV cured and thermal post-cured.

As can be seen, the two phenomena of increase of the crystalline intensity and the decrease of

the conversion of the methacrylate double bonds seem to vary in an opposite fashion with the

POSS content: as more the POSS content is (above 10 wt %), as more the crystallites are (i.e.

higher intensity of the crystalline peak at 2θ~8°) and as less is the conversion of the

methacrylate double bonds because some of them are “trapped” in the crystals of POSS and

cannot react.

On the other hand, the networks with the reactive solvent, CHMA, show a much higher

dispersion of POSS, as can be seen in figure 3-15:



5 10 15 20 25 300% iBuPOSS2.5% iBuPOSS5% iBuPOSS

10% iBuPOSS

20% iBuPOSS30% iBuPOSS

100% iBuPOSS

2θ=18.9°d-spacing=4.7 Å

2θ=8.05°d-spacing=11.0 Å

2θ (°)

Inte

nsity

(a.u

.)

neat iBuPOSS BisEMA-CHMA-30 wt % IBuPOSS BisEMA-CHMA-20 wt % IBuPOSS BisEMA-CHMA-10 wt % IBuPOSS BisEMA-CHMA-5 wt % IBuPOSS BisEMA-CHMA-2.5 wt % IBuPOSS BisEMA-CHMA

Figure 3-15 WAXS pattern of BisEMA-CHMA-iBuPOSS networks (UV cured and thermally postcured). All

the patterns have been normalized with respect to the absorption coefficient of the sample, except the one of the

neat iBuPOSS (for reasons of graphic scaling). The curves are offset for clarity.

The WAXS pattern of BisEMA-CHMA matrix reveals an amorphous structure and is very

similar to the pattern of neat BisEMA matrix, with the only remarkable exception of the

presence of a second halo centered at 2θ~8°. The presence of this halo may be due to

intermolecular interactions in between the polymer chains, as shown already by Miller et al.

for other amorphous acrylate-methacrylate systems [Mil84].

According to the WAXS analysis, increasing the POSS content up to 10 % does not lead to

the formation of a crystalline phase inside the networks. The patterns up to 10 % iBuPOSS

look very similar to the ones of the BisEMA-CHMA, but the halo centred at 2θ~8° is

increased in intensity. This can be due to the presence either of another amorphous phase or of

very small crystallites formed by the iBuPOSS molecules, but it is unclear which one of the

two is the reason of the development of this peak.

For the systems BisEMA-CHMA containing 30 wt % iBuPOSS, it can be seen that the halo at

2θ~8° increases in intensity and become narrower, forming a real crystalline peak insisting on

the top of an amorphous halo. Once separated this crystalline peak, it is possible to quantify

its FWHM as 0.6° 2θ (before the removal of the instrumental broadening), which corresponds

to a crystallite size of 350 Å. In the system BisEMA-CHMA-30 % iBuPOSS the POSS

crystallites are smaller than in the systems without reactive solvent (506 Å) and become close

to the crystallites size of BisEMA-POSS with a POSS weight fraction in the range 10-20 %.



Resuming, the BisEMA-iBuPOSS networks show the presence of a crystalline phase due to

the low solubility of POSS into the BisEMA above a POSS content of 5 wt %. The POSS

crystallite size increase with the POSS content. When the reactive solvent is used, the

networks BisEMA-CHMA-iBuPOSS show a better dispersion of the POSS. The WAXS

patterns reveal that in the networks BisEMA-CHMA with less than 20 wt % iBuPOSS, these

hybrid nanoobjects do not aggregate in crystallites, therefore they are likely to be well

dispersed to a nanometric level. Furthermore, the CHMA alters the WAXS pattern of the

BisEMA with establishing intermolecular interactions, resulting in the presence of a halo at

2θ~8° [Mil84].

3.3.3 Comparison of the organization of iBuPOSS and CyPOSS within a

dimethacrylate-based network

The aim of this section is to compare the influence of the POSS type, that is iBuPOSS and

CyPOSS, on their organization into the final networks. First of all, as already mentioned, the

CyPOSS are less soluble than the iBuPOSS into the BisEMA. Therefore the comparison will

be made on a mixture BisEMA-CHMA (50/50). The WAXS patterns are reported in figure 3-

16 for the systems containing an amount of iBuPOSS and CyPOSS equal to 5 and 10 wt %:

5 10 15 20 25 30

10% CyPOSS10% iBuPOSS

5% CyPOSS5% iBuPOSS

0% POSS

2θ (°)

Inte

nsity

(a.u

.)

Figure 3-16 WAXS patterns of BisEMA-CHMA (1:1 in weight) networks containing different types and weight

fractions of MonoPOSS: (▬) 0 wt % POSS, (▬) 5 wt % iBuPOSS, (▬) 5 wt % CyPOSS, (▬) 10 wt %

iBuPOSS, (▬) 10 wt % CyPOSS. The samples analysed were bulk samples thermally cured at 70°C/8hrs – post

cured at 130°C/2hrs . The data are offset for clarity.



The WAXS patterns of the two systems containing 5 wt % of both kind of POSS are very

similar, without any hint of the presence of a crystalline phase inside the networks. But at a

concentration of 10 wt % MonoPOSS, the iBuPOSS are still not aggregated in crystallites,

while the network BisEMA-CHMA-10 % CyPOSS shows a semicrystalline pattern, with a

pronounced peak at 2θ~8°. This allows one to conclude that, up to 10 wt % POSS, the

iBuPOSS are better dispersible in the BisEMA-CHMA matrix than the CyPOSS.

Mostly for this reason, the networks with CyPOSS will be the object of a less deep analysis,

while the iBuPOSS and the OctaPOSS will be at the centre of most part of this chapter. The

CyPOSS-containing systems have been effectively characterized only about surface

properties.

3.3.4 BisEMA-OctaPOSS and BisEMA-CHMA-OctaPOSS networks

In figure 3-17 and 3-18 are reported the WAXS patterns of the systems BisEMA-OctaPOSS

and BisEMA-CHMA-OctaPOSS:

5 10 15 20 25 30

100% OctaPOSShomopolymerised

30% OctaPOSS

20% OctaPOSS

10% OctaPOSS5% OctaPOSS2.5% OctaPOSS0% OctaPOSS

OctaPOSS homopolymerised BisEMA-30 wt % OctaPOSS BisEMA-20 wt % OctaPOSS BisEMA-10 wt % OctaPOSS BisEMA-5 wt % OctaPOSS BisEMA-2.5 wt % OctaPOSS BisEMA

Inte

nsity

(a.u

.)

2θ (°)

Figure 3-17 WAXS pattern of BisEMA-OctaPOSS networks (UV cured and thermally postcured). All the

pattern have been normalized with respect to the absorption coefficient of the sample, except the one of the

homopolymerized OctaPOSS (for reasons of graphic scaling). The curves are offset for clarity.



5 10 15 20 25 30

100% OctaPOSShomopolymerised

30% OctaPOSS20% OctaPOSS10% OctaPOSS5% OctaPOSS2.5% OctaPOSS0% OctaPOSS

OctaPOSS homopolymerised BisEMA-CHMA-30 wt % OctaPOSS BisEMA-CHMA-20 wt % OctaPOSS BisEMA-CHMA-10 wt % OctaPOSS BisEMA-CHMA-5 wt % OctaPOSS BisEMA-CHMA-2.5 wt % OctaPOSS BisEMA-CHMA

Inte

nsity

(a.u

.)

2θ (°)

Figure 3-18 WAXS pattern of BisEMA-CHMA-OctaPOSS networks (UV cured and thermally postcured). All

the pattern have been normalized with respect to the absorption coefficient of the sample, except the one of the

homopolymerized OctaPOSS (for reasons of graphic scaling). The curves are offset for clarity.

First of all, it should be recalled that the OctaPOSS by themselves are amorphous (see

Chapter II). The WAXS patterns of all the OctaPOSS containing networks display clearly an

amorphous character. It is clearly remarkable the absence of any peak in the 2θ range 5-10°,

even at high POSS content, as has been the case for the monofunctional-POSS. On the other

hand, there is not a large influence of the OctaPOSS on the amorphous halo typical of the

BisEMA and the BisEMA-CHMA matrices.

In the light of the very high miscibility of the OctaPOSS with both the matrices, it could be

stated that the OctaPOSS molecules are homogenously dispersed into the reactive mixtures.

When the polymerization starts, each OctaPOSS molecule has the possibility to react with

more than one double bond. The reaction of two double bonds per molecule over the eight

belonging to the POSS is enough to include the clusters into the network, therefore reducing

the mobility and preventing phase-separation to occur and a further possible organization of

POSS into domains. This chemical bond between a OctaPOSS molecule and the

dimethacrylate-based resin would hinder the phase-separation.

The patterns of the networks have been compared with the one of the OctaPOSS

homopolymerized because, due to the large number of methacrylate functionalities borne by

each OctaPOSS molecule, it is more likely that, in case of phase separation these hybrid



nanoobjects would react, forming therefore a phase of OctaPOSS homopolymer. The

OctaPOSS homopolymerized form a structure characterized by the presence of a broad peak

(or, better, of a narrow halo) centred at 2θ=6.89° and with a FWHM of 1.60°. Using again the

Scherrer equation, it is possible to see that this peak corresponds to a d-spacing of 12.8 Å with

a length of ~ 50 Å, that is the size of approximately two to three OctaPOSS molecules. In the

light that no such a peak is observable in all the OctaPOSS-based networks, even at high

POSS content, it could be stated that the OctaPOSS are very well dispersed in the final

systems, probably at the molecular scale.

3.4 Morphology of POSS-dimethacrylate networks from µm to nm scale

The WAXS has revealed large differences in between the structure of the systems synthesized

with iBuPOSS with or without the reactive solvent CHMA. On the other hand, despite the

WAXS has suggested that the OctaPOSS are very well dispersed, there is still the need of

having a confirmation of the extent of the POSS dispersion in all the cases at a larger scale.

For this purpose, electron microscopy as SEM and TEM have been used. The results of this

analysis will be presented separately for the networks based on iBuPOSS and OctaPOSS. All

the systems analysed have been UV cured and thermally postcured.

3.4.1 Morphology of iBuPOSS-containing networks

The SEM micrographs have been taken from the fracture surface of the very same samples

used for mechanical testing. Therefore, in most part of them, the features created by the crack

propagation are visible. These features, along with their physical meaning, will be also shown

later in this chapter. In this section the SEM will be used mostly as a powerful tool to exploit

the microstructure of these hybrid networks.

Dimethacrylate-based resins are brittle materials, and their fracture surface is very sensitive to

the average functionality number of the monomer mixtures composing the networks, this in

turn depending by the eventual presence of a monomethacrylate as a solvent, like in the work

of Cook [Coo91]. The more the solvent is, the more gross plastic flow will be induced on

small regions, this causing the presence of features known in the literature as river-like

patterns [Hul99]. A clear visualization of this phenomenon is visible in figure 3-19:



Figure 3-19 SEM micrographs of (A,B) BisEMA and (C,D) BisEMA-CHMA. The surface analysed was the

section of the sample broken during tensile testing and subsequently covered with gold.

In figure 3-19 are displayed the fracture surface micrograph of the two neat matrices:

BisEMA and BisEMA-CHMA. It is to be noted that both matrices present a very smooth

region (which is much smaller for BisEMA-CHMA than for BisEMA) with no features or a

very limited amount of them, as the few lines present in figure 3-19 (B). This zone is called

“mirror” zone [Hul99] and its intrinsic characteristic to be featureless makes it a good spot to

check for the existence of microstructures due to the POSS presence.

A B

C D



- BisEMA-based networks

The fracture surface of the sample BisEMA-5 wt % iBuPOSS is displayed in figure 3-20:

Figure 3-20 SEM micrographs of (A,B) BisEMA-5 wt % iBuPOSS at different magnifications. The surface

analysed was the section of the sample broken during tensile testing and subsequently covered with gold.

This SEM picture shows the presence of a second phase, i.e. the POSS crystallites whose

presence were evidenced by WAXS. The lines on the sides of each aggregate are due to the

crack propagation and they will be better explained later in this chapter. As can be seen from

the comparison between the figure 3-20 (A) and (B), the POSS aggregates present a very wide

distribution of sizes, from domain ~ 1 µm sized to “blocks” of 10-20 µm [Bre05], like the one

at the top of figure 3-20 (A).

Looking at higher magnification with the TEM on microtomed section of the sample, these

small aggregates can be better visualized, as shown in figure 3-21:

Figure 3-21 TEM micrographs of BisEMA-5 wt % iBuPOSS at different magnifications.

A B

A B



In the TEM micrographs, the POSS domains are dark, as a result of their higher electron

density with respect to the organic matrices. Both in SEM and in TEM the POSS domains are

angular-shaped, and their orientation is random. The TEM did not detect any consistent

presence of iBuPOSS objects in the range of nm.

Therefore, no nanostructuration of POSS have been observed, as was the case for POSS in

other thermoplastics polymers, like polyolefins [Zhe02a] or polynorbornene [Jeo00], or even

in thermoset resins, like epoxy-POSS networks [Mat04]. The lack of nanostructuration can be

addressed to the poor solubility of POSS in the organic monomers, this making present the

iBuPOSS crystals in the reactive mixture since the very beginning of the polymerization. The

polymerization will therefore mostly “freeze” the systems in the initial state-structure.

Comparing the iBuPOSS aggregates in figure 3-20 (therefore once incorporated in the

BisEMA matrix) with the ones in figure 2-6 (referred to neat iBuPOSS observed with POM),

it is possible to see that the sizes of the POSS objects in the two cases are in the same range.

In the light of the dispersion of the iBuPOSS at the micrometer range, it should be used the

term “microcomposites” to correctly identify these systems. In fact, despite a measurement of

the gel time has not been feasible because of the very fast curing process (completed in ~ 120

sec), it is very likely that the gel point is reached in few seconds, also in the light of the need

of a low conversion to attain it (in the range 10-15 % of the conversion). Therefore the

iBuPOSS do not have time to be dissolved in the BisEMA matrix with the aid of the increased

temperature due to the action of polymerization and the heating of the sample due to the UV

lamp.

As was seen via WAXS, increasing the POSS content leads to an increase of the crystallite

size and of the crystalline intensity. Due to the poor solubility of the iBuPOSS in the BisEMA

matrix, it is foreseeable an increase of the POSS aggregation. To have a confirmation, SEM

and TEM have been used to analyse the samples with the highest content of iBuPOSS,

BisEMA-30 wt % iBuPOSS, and the results are shown in figure 3-22 and 3-23:



Figure 3-22 SEM micrographs of BisEMA-30 wt % iBuPOSS at different magnifications. The surface analyzed

was the section of the sample broken during tensile testing and subsequently covered with gold.

In figure 3-22 (B) it is clearly visible that the iBuPOSS crystals practically cover all the

surface, making the morphology deeply inhomogeneous. In this case the size distribution of

the POSS aggregates ranges in the same interval of BisEMA-2.5 wt % iBuPOSS, that is ~

1÷20 µm. The shape of the aggregates is again angular and there is no specific orientation of

these objects. The main difference, apart from the much higher number of crystallites, is the

presence of objects with size in the nanometer range, as shown by TEM in figure 3-23:

A B

C D



Figure 3-23 TEM micrographs of BisEMA-30 wt % iBuPOSS at different magnifications.

As is visible in figure 3-23 (B), in the networks BisEMA-30 wt % iBuPOSS there are

aggregates in the size range 50-200 nm. These aggregates, despite having not clearly resolved

borders, are spherically shaped.

The reason of the presence of nano-aggregates of iBuPOSS only at high POSS concentration

is unclear. It may be that these aggregates are composed of POSS molecules initially

solubilized in the BisEMA matrix (less than 1%) and then pushed to segregate during the

polymerization process.

Due to the high concentration of POSS moieties in the system, the size attained by the

aggregates falls in the resolution range of the TEM, which can be not the case for the

networks with only 5 wt % iBuPOSS in BisEMA, where the size of these nanoaggregates

(perhaps caused by phase separation) should in principle be smaller.

A B



- BisEMA-CHMA based networks

The networks BisEMA-CHMA-iBuPOSS have been shown by WAXS to have a much better

dispersion of POSS in the network. In fact, no crystalline phase has been detected for an

iBuPOSS content lower than 20 wt %. The SEM micrographs relative to the sample BisEMA-

CHMA-5 wt % iBuPOSS are displayed in figure 3-24 (A):

Figure 3-24 SEM (A) and TEM (B) micrographs of BisEMA-CHMA-5% iBuPOSS at different magnifications.

The surface analyzed with SEM was the section of the sample broken during tensile testing and subsequently

covered with gold.

Apart from the linear features due to crack propagation, no POSS aggregates were visible at

the SEM resolution. It should be remembered that the reactive mixture of this network was

transparent, a clear indication that the three components were miscible. To better quantify the

dispersion of the POSS objects the TEM has been used, and the results are shown in figure 3-

24 (B). The TEM confirmed the excellent dispersion of the iBuPOSS in the BisEMA-CHMA

network for a POSS content of 5 wt %. In fact, only few domains with a size in the range of

tens of nanometer are visible, despite the TEM is not able to resolve them well.

Comparing the micrographs of the networks BisEMA-5 wt % iBuPOSS and BisEMA-

CHMA-5 wt % iBuPOSS, it is clear that the POSS are much better dispersed in the networks

when the reactive solvent is used. The possible phase separation due to polymerization, seen

by Bizet for similar systems thermally cured (see Chapitre III-Partie C in [Biz04]), is hindered

by the rate of the reaction (realized under UV irradiation) a competing physical phenomenon.

A B



Increasing the iBuPOSS content up to 30 wt % leads to a lack of solubility of the hybrid

objects in the resin. The reactive mixture BisEMA-CHMA-30 wt % iBuPOSS is opaque, hint

of a partial non solubility of the POSS, and at the end of the reaction the samples are

completely white, hint of heterogeneous materials. This network structure phase separation is

visible also in the SEM pictures of BisEMA-CHMA-30 wt % iBuPOSS in figure 3-25:

Figure 3-25 SEM micrographs of BisEMA-CHMA-30 wt % iBuPOSS at different magnifications. The surface

analyzed was the section of the sample broken during tensile testing and subsequently covered with gold.

The network BisEMA-CHMA-30 wt % iBuPOSS shows a fractured surface deeply

inhomogeneous, even with some holes clearly visible in figure 3-25 (A). These holes are

probably due to air bubbles entrapped inside the network before the polymerization because

of the higher viscosity of the system. The opacity of the network BisEMA-CHMA-30 wt %

iBuPOSS in figure 3-25 is, according to certain authors [Ats82], an indication of a multiphase

material. Looking closer to the sample it is possible to see the presence of POSS-rich, sphere-

like aggregates with a size lower than 1 µm, clustering in domains with sizes in the range 1-20

µm.

A B

C D



The systems BisEMA-CHMA-iBuPOSS, especially up to a POSS concentration of 10 wt %,

can be possible addressed as nanocomposites, as the hybrid second-phase is in the range of

nanometers even at the very high content of POSS of 30 % in weight.

3.4.2 Morphology of OctaPOSS-containing networks

Generally, as highlighted by the WAXS analysis, the OctaPOSS seem to be well dispersed in

the networks, probably at a molecular scale, as suggested by the lack of any peak due to

aggregation of two or three POSS molecules. Therefore, it has been a hard task for the two

techniques of electron microscopy used to give a visualization of the POSS objects so finely

dispersed. SEM has been unable to detect the presence of any POSS aggregate, even at high

resolution. The results of the SEM analysis on the mirror zone of the fracture surface of some

BisEMA-OctaPOSS and BisEMA-CHMA-OctaPOSS samples are shown in figure 3-26:

Figure 3-26 SEM micrographs of: (A,B) BisEMA-5 wt % OctaPOSS at different magnifications;

(C) BisEMA-30 wt % OctaPOSS; (D) BisEMA-CHMA-30 wt % OctaPOSS. The surface analyzed was the

section of the sample broken during tensile testing and subsequently covered with gold.

A B

C D



As is visible in figure 3-26, especially at high magnification (in figure 3-26 (B, C, D)), no

evidence of OctaPOSS aggregation has been detected by the SEM analysis in the µm range.

The few features visible are either debris of the fracture process (figure 3-26 (A)).However,

due to normal instrument limitation, the SEM cannot have access to a nanometer resolution,

like the TEM can do. To check for OctaPOSS aggregation in the nm range, TEM analysis has

been performed as well, and the results are shown in figure 3-27:

Figure 3-27 TEM micrographs of: (A) BisEMA-5 wt % OctaPOSS; (B) BisEMA-CHMA-5 wt % OctaPOSS;

(C) BisEMA-30 wt % OctaPOSS; (D) BisEMA-CHMA-30 wt % OctaPOSS.

The TEM micrographs detect some darker zones with sizes up to 100 nm. Due to the high

magnification and the non-optimal resolution of the images, it is very difficult to state that

these features are real OctaPOSS aggregates and not artifacts of the TEM analysis (as in

Duchet et al. [Duc03]). Furthermore, comparing them with the images at figure 3-24 relative

to BisEMA-CHMA-5 wt % iBuPOSS, another system well dissolved, it is possible to

A B

C D



conclude that, very likely, the features observed in figure 3-27 are due to artifacts of TEM

analysis.

This optimal dispersion of OctaPOSS moieties is due to (a) the miscibility of the OctaPOSS

with both the resins and (b) the high possibility of polymerization of these hybrid nanoobjects

via one of the multiple methacrylate group present on each POSS molecule. The latter is the

cause of an early bonding of the OctaPOSS to the network chains, with the subsequent

hindering of the POSS aggregation-crystallization. Therefore, the OctaPOSS-networks are

originally miscible and the reaction of these objects with the matrix chains prevent any phase

separation phenomena, thus ensuring POSS dispersion to the molecular level.

In the light of the dispersion to the molecular scale of the POSS in all the OctaPOSS-

containing networks, it would be possible to define these systems as hybrid networks.

3.4.3 Structure-morphology of POSS-modified polymethacrylate networks

This section is intended to summarize the most important results of the structure-morphology

analysis via WAXS-SEM-TEM for each group of POSS-dimethacrylate network:

BisEMA-iBuPOSS: The dispersion of the POSS in these systems is the poorest among all the

systems analyzed. µ-sized aggregates (1-20 µm) are present even at a iBuPOSS concentration

of 5 wt %, while the WAXS has revealed the existence of a crystalline phase for all the POSS

weight fractions above 5 wt %. The crystallite size varies linearly with the POSS content from

293 Å (10 wt % iBuPOSS) to 1150 Å (neat iBuPOSS). Increasing the POSS content does not

vary the size range of the µ-sized aggregates, while increasing the integrated intensity of the

crystalline reflections in WAXS (that is, increasing the amount of crystals present in the

systems). It is reasonable to speak about these systems as “microcomposites”.

BisEMA-CHMA-iBuPOSS: The presence of the reactive solvent, along with the speed of the

UV polymerization, allows the iBuPOSS to be better dispersed in the matrix. Up to 20 wt %

the WAXS has not detected any iBuPOSS crystallization, while the TEM has shown round-

shaped aggregates in the range of tens of nanometers. For the networks BisEMA-CHMA with

30 wt % iBuPOSS, massive phase separation (along with the presence of a crystalline phase)

has been detected, with the separated spherical domains measuring less than 1µm in diameter.

For these systems therefore it makes sense to speak about “nanocomposites”.



BisEMA-OctaPOSS and BisEMA-CHMA-OctaPOSS: Similar considerations may be

drawn for both the systems containing OctaPOSS. The OctaPOSS have been shown to be

molecularly dispersed in both the matrices, even at the very high weight fraction of 30 wt %.

For these systems it is possible to use the definition “hybrid networks”.

To better visualize what was stated above, in figure 3-28 are schematized the morphologies of

four POSS-dimethacrylate networks, taken as example:

A B

C D

Figure 3-28 Schematic representation of the morphologies of POSS-dimethacrylate networks:

(A) BisEMA-2.5 wt % iBuPOSS, (B) BisEMA-10 % wt iBuPOSS, (C) BisEMA-CHMA-10 wt % iBuPOSS,

(D) BisEMA-10 wt % OctaPOSS. The symbol = indicates unreacted C=C bonds borne by the POSS.

The POSS are represented as cube either pendant from the polymer chains (figure 3-29 (A),

(B) and (C)) or as crosslinking points (figure 3-29 (D)). The aggregation of iBuPOSS in

crystals is shown in figure 3-29 (B). The (heterogeneous) dimethacrylate network is

represented ideally as homogenous for better clarity. The presence of unreacted C=C bonds is



evidenced in the systems showing low conversion, that is, BisEMA-10 wt % iBuPOSS and

BisEMA-10 wt % OctaPOSS.

3.5 Solid state properties

In this section will be presented the solid state properties of POSS-dimethacrylate networks

studied in this research. In particular, the properties studied have been the mechanical and

thermomechanical ones. The aim of this section is also to draw structure-properties

relationships. The existing literature has been studied in order to give a correct explanations of

the phenomena encountered.

3.5.1 Bibliography – Relation between network morphology and properties

As explained before, the formation of microgels at the beginning of the polymerization

process leads to dimethacrylate networks having a deeply heterogeneous structure. The

heterogeneity present in this type of networks are due to spatial fluctuation of both

crosslinking density (for instance, the core of a microgel is ideally much more crosslinked

than the shell) and chemical composition (for instance, in case of copolymers, in the pool of

unreacted/partially reacted monomer there could be variations of stechiometric ratio between

the monomers). The chemical structure is also affected by the presence of defects, like cycles

or pending chains (that is, molecules of monomers reacted only partially).

The final situation is a very heterogeneous morphology: macrogels formed by clustering of

several microgels plus “pools” of unreacted monomer both in between and inside the

microgels [Bow90], plus defects in the chemical structure. A representation of the nano-

morphology of a network synthesized via free radical polymerization can be found in figure

3-29 [Rey02]:



Figure 3-29 Schematic representation of the heterogenous structure of networks synthesized via radical

polymerization. The microgels are visible in light grey, and the black parts are “pools” of unreacted monomers

[Rey02].

A more direct visualization of the above-described morphology of dimethacrylate resins can

be found in the figure 3-30:

Figure 3-30 AFM image of a fractured surface of cured BisEMA.

The presence of the microgels give rise to a morphology which can be called “nodular”

[Rey02], clearly visible in figure 3-30.

A quantification of the extent of the heterogeneity comes from the use of DMTA analysis, in

particular from the broadening of the tanδ peak as a function of temperature [Biz04, Coo03,

Kan97, Kan01, Kan98, Sim01]. The intrinsic heterogeneity of the dimethacrylate networks

leads to the presence of chains with different relaxation times. Therefore, when in the nearby

of the glass transition, each of the chains will relax with a different time scale. The more the

heterogeneities are, the broader the distribution of relaxation times is. This will be reflected

immediately in the full width at half maximum (FWHM) of the peak of tanδ: the more

heterogeneous is the system, the higher the FWHM is.



The heterogeneous structure of dimethacrylate networks has a number of consequences on

their behaviour in the solid state, for instance concerning the modulus (glassy and rubbery),

glass transition temperature and fracture behavior.

The influence of the structural features on the properties will be here summarized.

- Thermomechanical properties

Thermomechanical properties have been studied analyzing the Tα, E’ rubbery, the maximum

value and the FWHM of the tanδ peak associated to the α-transition, i.e. glass transition. The

α-transition is the mechanical transition associated with the glass transition in a dynamical

thermo-mechanical experiments [Fer70]. Therefore, any variation on the Tα reflect a variation

on the Tg.

Several factors may influence the above reported four parameters in thermoset networks. The

most importants factors are the followings:

Tα is increased by increasing the crosslinking density [Spe92], the cohesive energy density

and the intrachain steric hindrance or by a reduction of the segmental mobility [Van97].

Despite popular, it would be better not include in this list the free volume as one of the factors

affecting the Tα because this theory is not well accepted by some authors (see for instance

Jones in [Jon02]) and also because its application to the case of POSS-containing polymers

may bring a too simplicistic explanation of the variation on Tα induced by the POSS presence

[Pit03].

E’rubbery is increased by the crosslinking density [Spe92], by the presence of rigid groups

inside the main polymer chain composing the network [Sid03, Van97] and by the presence of

hard objects [Gér01].

The FWHM and the maximum of the tanδ peak associated to the α-transition are respectively

a measurement of the network heterogeneity (especially in dimethacrylate networks [Coo03,

Rey02]) and the overall mobility of the polymer chains [Spe92]. The heterogeneity of a

dimethacrylate network is usually very high, and it is usually further increased by the

presence of monomers with more functionalities than the monomer composing the network

[Rey02]. The main factor affecting the overall mobility of the polymer chain in a network is

again the crosslinking density, even if the presence of bulky side groups pendant from the

polymer chains can decrease the overall mobility as well [Spe92].



As can be clearly seen in the list reported above, the crosslinking density plays a pivotal role

in influencing the thermomechanical properties of the dimethacrylate-based networks. To

determine the crosslinking density of a dimethacrylate network, unfortunately, due to the high

complexity of these systems, it is not possible to use the rubber elasticity theory [Kan98].

Therefore nothing will be said about molar mass between crosslinks CM , even if the rubbery

modulus could be taken as an indication of the average crosslinking density in the network, a

procedure already present in the literature [Rey02].

- Mechanical properties

The mechanical properties of dimethacrylate network are strongly depending both on the

chemical structure of monomers, and on the structural features of the network.

The most important mechanical properties analyzed studied have been the tensile modulus

and strength, which is in turn related to the mechanisms of crack propagation in the systems.

Both the properties, except when otherwise specified, are to be intended as measured in the

glassy state via tensile testing.

The Young modulus, E, in the glassy state of a thermoset system is related essentially to the

cohesive energy density of the network [Pas02]. This in turn is due to the existence of

covalent bonds and non-covalent interactions in between the molecules, plus the influence of

secondary sub-glassy transitions. The presence in a general monomer of groups able to

establish polar interactions increases usually the values of E [Van97]. Another influence may

come from the presence in the polymer backbone of rigid groups, like phenyl rings [Van97],

which in principle increase the stiffness of the network, or long alkyl chains, able to soften the

material [Sid03]. The crosslinking density cannot be clearly correlated with the E in the glassy

state of the networks. Therefore, the influence of the POSS on E will be discussed in terms of

the POSS influence on the interactions inside the polymer, in a similar fashion to what will be

reported in the Chapter 4 for MMA-co-POSS linear copolymers.

In heterogeneous networks, as has been shown to be the case for the dimethacrylate-based

ones studied here, the presence of defects is unavoidable and plays a pivotal role in explaining



the fracture properties of these materials. In fact, it is nowadays accepted that in a

dimethacrylate resin, the less the defects (as, for instance, pools of sol species [Coo91]) are,

the higher the strength is [Bow91, Coo91, Gan02, Pal01, Rey02]. As mentioned before, the

FWHM of the tanδ variation corresponding to the glass transition is an indication of the

heterogeneity of the network.

The tensile strength in networks is related to the presence of defects in the system which may

act as points of stress concentration and therefore as starting points for the cracks propagation

[Coo91, Kan01]. The more these defects are, the more probable is the nucleation of a critical

flaw, and hence the initiation of the crack, leading to catastrophic failure. Thus, the strength of

the material is lowered. Certain authors [Kar97, Rey02] go so far as to correlate directly the

breadth of the tanδ peak for the α-transition to the strength: the smaller this FWHM is, the

smaller the heterogeneity of the network is, this leading to higher strengths.

The way to reduce the number of defects may pass through the reduction of the average

crosslinking density through the choice of the monomers (giving rise to smaller microgels and

macrogels [Rey00], therefore generating less pools of unreacted monomers), the adoption of

slower curing procedures (to avoid cyclization of the macromolecules [Gan02]) or the use of a

post-curing procedure [Coo91] to reach a maximum conversion. The latter is the option used

in this research to easily maximize the extent of the curing procedure, and its efficacy lies in

the removal of the entrapment of the radicals, which allows them to further react and reduce

therefore the number of sol-species in the system.

The importance of these sol-species for the tensile strength of a dimethacrylate matrix is

clearly visualized by Rey et al. in the figure 3-31:



Figure 3-31 Tentative schematic representation of crack propagation in a network formed by microgels

agglomeration. It has to be noted that the boundaries are probably not sharp but a gradient of crosslinking density

(after Rey et al. [Rey02] and Bowman et al. [Bow90]). In black are represented the pools of unreacted

monomers already reported in figure 3-29.

The crack propagates through the points with lower crosslinking density (like the boundaries

of the microgels or the pools of unreacted monomers) considered as weakest points of the

network, while the core of the microgels is considered inaccessible to the crack propagation.

Therefore, to increase the strength of a dimethacrylate resin, the point of weakness should be

minimized via optimization of the conversion.

As will be shown in the following of this chapter, this objective will be achieved by varying

the composition of the networks, both using POSS and an organic monomer as comparison.

Due to the lower reactivity of iBuPOSS with respect to the organic monomers and to their

lack of solubility in the BisEMA, a big influence of the POSS on the amount of unreacted-

partially reacted monomer, hence on the fracture resistance, is expected. For this reason, Near

Infrared Spectroscopy has been used to determine the conversion of the final systems, with

the aim to check for the existence of a very high extent of double-bond reaction. In fact, the

fracture resistance of a dimethacrylate resin is very sensitive to the presence of unreacted

monomer [Coo91], therefore it has to be ensured a very high conversion of double bonds in

order to be allowed to compare the fracture behavior of different networks.



3.5.2 Dynamical Mechanical Thermal Analysis on POSS-containing networks

Dynamical mechanical thermal analysis (DMTA) has been used to characterise the POSS-

containing networks. This analysis has been done on the networks BisEMA-iBuPOSS,

BisEMA-CHMA-iBuPOSS, BisEMA-OctaPOSS and BisEMA-CHMA-OctaPOSS, for POSS

concentrations of 0, 1, 2.5, 5 and 10 wt %. The samples tested were UV cured and thermally

postcured. Other experimental details are reported in the annexes.

To better understand the influence of POSS on the thermomechanical response of POSS-

based networks, the viscoelastic properties and the more interesting features of the DMTA

analysis of the two neat matrices, BisEMA and BisEMA-CHMA, will be shown firstly.

The tanδ curves of the two matrices used are reported in figure 3-32:

-100 -50 0 50 100 150 2000,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8101.4 °C

108.0 °C

-100 -90 -80 -70 -60 -500,01

0,02

0,03

0,04

0,05

0,06

Tanδ

T (°C)

BisEMA BisEMA-CHMA

tanδ

T (°C)

Figure 3-32 tanδ curves of BisEMA (full squares ■) and BisEMA-CHMA (empty circles ○) matrices. In the

small frame are reported the tanδ curves in the range of temperatures -100 ÷ -50°C. (Frequency. 1 Hz).

The BisEMA matrix shows one distinct transition at T=108.0 °C. This is the α-transition,

associated to the glass transition. Another weak relaxation is visible at lower T, and it is the β-

transition which reaches its maximum at ~ -100°C due to the rotation of 2-3 ethoxy groups in

the dimethacrylate chain [Rey02].



The BisEMA-CHMA matrix presents two distinct transitions: the main one (α-transition) is

centred at ~101.4°C and the second one (β-transition) reaches its peak at ~80°C. The α-

transition is associated to the glass transition while the β-transition is due to a conformational

change of the cyclohexyl groups, from boat to chair [Fro66].

Some interesting features appear comparing the two tanδ curves. First of all, the Tα of

BisEMA is ~7°C higher than the one of BisEMA-CHMA. This is clearly explicable in terms

of the presence in the BisEMA-CHMA network of the reactive solvent, which reduces the

average functionality of the monomer blend (from the value of 4 for the BisEMA to the value

of 2.7 for the BisEMA-CHMA), thus reducing the average crosslinking density, hence

reducing the Tg, accompanied by a decrease of the E’rubbery, as can be seen in figure 3-33:

-50 -25 0 25 50 75 100 125 1501E7

1E8

1E9

1E7

1E8

1E9

E' (P

a)

T (°C)

Figure 3-33 E’ curves for BisEMA (full squares ■) and BisEMA-CHMA (empty circles ○). (Frequency: 1 Hz)

E’rubbery is visibly lower when CHMA is used as comonomer. In fact, the CHMA reduces the

crosslinking density of the dimethacrylate networks, hence causing a decrease of the E’rubbery

from the value of 77.0 MPa to 21.1 MPa.

Another effect of the presence of the reactive solvent in the network is the strong increase of

the tanδmax, due to an increase of the overall mobility of the polymer chains, and a reduction

of the FWHM of the Tα peak from 48°C (BisEMA) to 30°C (BisEMA-CHMA). The latter, as

already stated, is proportional to the degree of heterogeneity present in the dimethacrylate



systems: loosely crosslinked parts (i.e. more mobile parts) relax at lower T, while the densely

crosslinked regions (i.e. less mobile parts) relax at higher temperature. The higher the

heterogeneity is, the higher the distribution of relaxation times (or temperature) is and hence

the broader the tanδ peak will be.

Therefore, in the light of a smaller FWHM of the tanδ peak for BisEMA-CHMA than for

BisEMA, it is possible to state that the CHMA decreases the intrinsic heterogeneity of the

dimethacrylate networks. This could happen because the presence of the CHMA during the

polymerization increase the diffusion of reactive species via the reduction of the viscosity of

the not fully polymerized systems. The presence of not fully reacted monomer or of loosely

crosslinked regions will be therefore reduced, this resulting in a decrease of the heterogeneity.

The results of DMTA will be now presented as a function of the type of POSS, and at the end

of each sub-section some general conclusions about the thermomechanical properties of

POSS-dimethacrylate networks will be drawn.

- Effect of the presence of iBuPOSS: BisEMA-iBuPOSS and BisEMA-CHMA-

iBuPOSS networks

The figure 3-34 reports the values of E’ in the rubbery state (that is, measured at a

temperature 50 °C above the Tα [Biz04]) as a function of POSS concentration for the

iBuPOSS-containing networks. E’(Tα+50°C) will be hereafter called E’rubbery.



0 2 4 6 8 10

16

18

20

2240

50

60

70

80

16

18

20

2240

50

60

70

80

E'(T

α+50

°C)

(MPa

)

iBuPOSS wt (%)

Figure 3-34 Values of E’ measured at T = Tα + 50°C as a function of iBuPOSS content in BisEMA (full squares

■) and BisEMA-CHMA (empty circles ○) matrices. No data were present in the region of break of the y-axis.

(Frequency: 1 Hz).

In all cases, a decrease of E’rubbery is observed as the amount of iBuPOSS is increased.

However the reason of this decrease may not be the same for the two series of networks.

In the simplest case of the BisEMA-CHMA-iBuPOSS networks, in which the POSS clusters

are well dispersed and the double-bonds conversion is the same for all these networks, the

POSS play the role of chain extender.

In the more complex case of BisEMA-iBuPOSS networks, two different regions can be

distinguished in the figure 3-34, at low (up to 2.5 wt % POSS) and high (5 and 10 % wt

POSS) iBuPOSS content.

At low content no crystallites were evidenced in the network by WAXS analysis. The role of

the iBuPOSS in this case is similar to the one in case of BisEMA-CHMA-iBuPOSS networks,

that is, the POSS play the role of chain extender.

At high iBuPOSS content most of the POSS form crystalline aggregates and these POSS

domains do not participate in the network build-up. Therefore, the average crosslinking

density is lower, and as a consequence the E’rubbery is lower. One may think that these



crystalline aggregates could be considered as fillers, and lead to an increase of E’rubbery, but it

should be remembered that the iBuPOSS melt at ~110°C, well below the temperature at

which the E’rubbery have been measured (usually in the range 150 – 160 °C). The presence of a

liquid phase inside the networks, which is supposed to decrease the E’rubbery, can explain the

drop of the E’rubbery for a POSS content of 5 wt %.

The proportionally more marked decrease of the E’rubbery for the BisEMA matrix than for the

BisEMA-CHMA is due probably to the different crosslinking densities of the two neat

matrices, that is, the iBuPOSS decrease the average crosslinking density proportionally more

for the more crosslinked matrix BisEMA.

The Tα of the networks are not particularly affected by the reduction of the crosslinking

density caused by the POSS presence, as can be seen in figure 3-35:

0 2 4 6 8 1090

95

100

105

110

90

95

100

105

110

Tα (°

C)

iBuPOSS wt (%)

Figure 3-35 Tα of the networks containing iBuPOSS: BisEMA (full squares ■) and BisEMA-CHMA (empty

circles ○) matrices. (Frequency: 1 Hz).

The small reduction of the values of Tα, equal to ~4 °C for 10 wt % iBuPOSS in both the

matrices, may be considered inside the range of the experimental error in the determination of

the peak of the tanδ curves, therefore it would be preferable to speak about the lack of an

overall effect of the POSS on the Tα of the networks.



The effect of the reduction of the crosslinking density on the Tα may be counterbalanced by

the presence of the bulky POSS cubic cage in a position pendant from the polymer chains, a

factor which in principle should retard the movement of the polymer chains [Lic94, Mat99]

and therefore cause a delay of the α-transition, with the overall lack of large effect of the

POSS on the values of Tα of both the matrices.

Also the overall mobility of the polymer chains seems unaffected by the iBuPOSS presence,

as shown by the values of tanδmax as a function of iBuPOSS content in the two matrices,

shown in figure 3-36:

0 2 4 6 8 100,20

0,22

0,24

0,65

0,70

0,75

0,80

0,20

0,22

0,24

0,65

0,70

0,75

0,80

iBuPOSS wt (%)

ta

nδm

ax

Figure 3-36 tanδmax values as a function of iBuPOSS content for both the matrices: BisEMA (full squares ■)

and BisEMA-CHMA (empty circles ○) matrices. No data were present in the region of break of the y-axis.

(Frequency: 1 Hz).

The explanation of this lack of POSS effect may be again found in the balancing of the effects

due to the reduction of the crosslinking density (lowering Tα and increasing the tanδmax) and

to the steric hindrance originated by the presence of the bulky cubic cage in a position

pendant from the main polymer chain (increasing Tα and decreasing the tanδmax).

The dispersion of the iBuPOSS objects have not shown a clear influence on the

thermomechanical properties analyzed up to this point, that is the E’rubbery, the Tα and the

tanδmax. A different situation happens when the heterogeneity of the networks, in terms of the



values of the FWHM of the tanδ peak corresponding to the α-transition, is studied as a

function of the POSS content, as shown in figure 3-37:

0 2 4 6 8 10

25

30

35

50

55

60

25

30

35

50

55

60

tanδ

FW

HM

(°C

)

iBuPOSS wt (%)

Figure 3-37 Values of the full width at half maximum (FWHM) of the tanδ peak associated with the α-transition

as a function of iBuPOSS content for both the matrices: BisEMA (full squares ■) and BisEMA-CHMA (empty

circles ○) matrices. No data were present in the region of break of the y-axis. (Frequency: 1 Hz).

The FWHM of the BisEMA-iBuPOSS networks increases with the amount of POSS,

especially above 2.5 wt %, then a plateau seems to be reached. Therefore it could be

considered that, up to 2.5 wt % of POSS, the networks obtained have about the same degree

of structural heterogeneity as the neat BisEMA matrix. Above 2.5 wt % POSS the

heterogeneity of the networks is steadily increased. An explanation of this two-regime

behaviour can be found in the amount of residual C=C functionalities present in the network,

which is an increasing function of the POSS content. When the amount of residual C=C

functionalities steadily increases (as is the case for a content of iBuPOSS higher than 5 wt %),

also the amount of loosely crosslinked regions increases, this resulting in an increase of the

heterogeneity. This increased heterogeneity will be shown in the next section to have a deep

impact on the tensile strength.

In the case of BisEMA-CHMA matrix, the iBuPOSS are well dispersed in the networks, up to

the tens of nm scale as seen in the section 3.7. Therefore, the reaction of the methacrylate

functionalities borne by the POSS is not hindered by the POSS aggregations in crystals, as



happens for the iBuPOSS in the BisEMA networks. As shown already, NIR spectroscopy

confirms that most of the POSS methacrylate functionalities have reacted. The heterogeneity

of the networks is, thus, not altered by the POSS presence, this resulting in the similar values

of the FWHM of the tanδ peak of the α-transition as a function of the iBuPOSS content in

BisEMA-CHMA matrix, as clearly visible in figure 3-37.

Summarizing briefly, the iBuPOSS (for a content up to 10 wt %) appears to reduce the

average crosslinking density of the dimethacrylate-based networks, thus reducing the

E’rubbery, while not altering the Tα and the tanδmax because of the simultaneous presence of

the steric hindrance of the cubic cage to the movement of the polymer chain. The dispersion

of the iBuPOSS plays a pivotal role in the heterogeneity of the network, as seen from the

value of the FWHM of the tanδ peak associated to the α-transition: as better the dispersion of

the iBuPOSS moieties is, as lower the increase of the heterogeneity of the network due to the

POSS presence.

- Effect of the presence of OctaPOSS: BisEMA-OctaPOSS and BisEMA-CHMA-

OctaPOSS networks

The figure 3-38 reports the values of E’rubbery as a function of POSS concentration for the

OctaPOSS-containing networks:

0 2 4 6 8 10

20

40

60

80

100

120

20

40

60

80

100

120

E' (

Tα+5

0°C

) (M

Pa)

OctaPOSS wt (%)

Figure 3-38 Values of E’ measured at T = Tα + 50°C as a function of OctaPOSS content in BisEMA (full

squares ■) and BisEMA-CHMA (empty circles ○) matrices. (Frequency: 1 Hz).



The role of crosslinking agent represented by the highly-functional OctaPOSS monomer

results in the increase of the E’rubbery with the POSS content. The presence of the OctaPOSS in

the hybrid copolymers clearly increases the average crosslinking density of the networks, in a

fashion quasi-linear with respect to the POSS content.

For 10 wt % OctaPOSS content, the E’rubbery of BisEMA-based networks passes from 77 to

115 MPa, while the BisEMA-CHMA networks exhibit a more than three-fold increase, with

E’rubbery “jumping” from 21.1 to 76 MPa. This more marked increase for the BisEMA-CHMA

matrix may be explicable in terms of the higher change of the average crosslinking density

due to the OctaPOSS in this matrix than in the more crosslinked BisEMA.

The optimal dispersion influences positively the crosslinking effect of the OctaPOSS in both

matrices making all the methacrylate bonds theoretically available for polymerization, this

resulting in values of conversion for the hybrid networks very close to the neat matrices. The

polymerization of all (or better, the very most part of) the double bonds exploit the

crosslinking capability of the OctaPOSS moieties, thus resulting in an increase of the

E’rubbery for all the networks.

The increase of the average crosslinking density due to the OctaPOSS presence is directly

reflected in the variation of the Tα, as shown in figure 3-39:

0 2 4 6 8 10100

105

110

115

120

100

105

110

115

120

Tα (°

C)

OctaPOSS wt (%)

Figure 3-39 Tα of the networks containing OctaPOSS: BisEMA (full squares ■) and BisEMA-CHMA (empty

circles ○) matrices. (Frequency: 1 Hz).



The Tα (and hence the glass transition temperature) increases pseudo-linearly with the

OctaPOSS content in the BisEMA-based networks from 108 to 116 °C. On the other hand, the

BisEMA-CHMA networks show a steady increase of the Tα from 101°C (neat BisEMA-

CHMA) to 107 °C (BisEMA-CHMA-1 wt % OctaPOSS). Above 1 wt % as a content of

OctaPOSS the increase is much less marked and presents a quasi-linear trend. This two-

regimes behavior is not explainable only in terms of the increasing of the average crosslinking

density.

On the other hand, in the case of the tanδmax the results are foreseeable in terms of the

increasing of the average crosslinking density due to the OctaPOSS presence. The values of

tanδmax as a function of the OctaPOSS concentration for both the families of hybrid networks

are reported in figure 3-40:

0 2 4 6 8 100,10

0,15

0,20

0,5

0,6

0,7

0,8

0,10

0,15

0,20

0,5

0,6

0,7

0,8

tanδ

max

OctaPOSS wt (%)

Figure 3-40 tanδmax values as a function of OctaPOSS content for both the matrices: BisEMA (full squares ■)

and BisEMA-CHMA (empty circles ○) matrices. No data were present in the region of break of the y-axis.

(Frequency: 1 Hz).

The overall mobility of the polymer chains is decreased by the presence of the hybrid

crosslinking agent OctaPOSS in a similar way in both the matrices. The BisEMA-CHMA-

OctaPOSS networks display a higher decrease because of the higher initial mobility in the

BisEMA-CHMA network with respect to the BisEMA one.



Also the FWHM of the tanδ peak associated with the α-transition varies proportionally to the

OctaPOSS content for both the matrices, as can be seen in figure 3-41:

0 2 4 6 8 10

30

35

40

45

50

55

60

65

70

30

35

40

45

50

55

60

65

70

tanδ

FW

HM

(°C

)

OctaPOSS wt (%)

Figure 3-41 Values of the full width at half maximum (FWHM) of the tanδ peak associated with the α-transition

as a function of OctaPOSS content for both the matrices: BisEMA (full squares ■) and BisEMA-CHMA (empty

circles ○) matrices. No data were present in the region of break of the y-axis. (Frequency: 1 Hz).

The OctaPOSS increases the heterogeneity of the network, in a similar fashion to the existing

literature [All98, Bar00, Biz04, Coo03, Kan98, Rey02, Sim01].

The effect of OctaPOSS is more marked for the BisEMA-based networks likely because these

systems present a higher number of unreacted methacrylate double bonds, as can be seen from

the conversion as a function of OctaPOSS content in figure 3-9.

The simultaneous decrease in overall mobility and increase of the heterogeneity of the

networks with the content of the multifunctional OctaPOSS is similar in proportion to what is

present in the existing literature for the copolymerization of highly functional monomers with

dimethacrylate resins (i.e. DVB in BisEMA [Rey02]).

Summarising briefly, the main effect of the OctaPOSS up to 10 wt % is increasing the average

crosslinking density of the dimethacrylate networks. This results in an increase of the E’rubbery

and of the Tα for both the matrices. The overall mobility of the polymer chains is decreased

by the OctaPOSS as can be predicted by the increase in the crosslinking density. On the other



hand the heterogeneity of the networks increase with the OctaPOSS content. The two matrices

are influenced in a similar way by the POSS presence.

3.5.3 Mechanical behavior of Dimethacrylate-POSS networks

Tensile testing have been used to asses the influence of POSS type and concentration on the

Young Modulus, E, and the tensile strength. This analysis has been done on the networks

BisEMA-iBuPOSS, BisEMA-CHMA-iBuPOSS, BisEMA-OctaPOSS and BisEMA-CHMA-

OctaPOSS, for POSS concentrations of 0, 2.5, 5, 10, 20 and 30 wt %. The samples tested

were mini dog-bone samples UV cured and thermally postcured. Other experimental details

are reported in the annexes.

All the materials exhibit brittle-elastic behavior under tensile testing. The typical stress-

deformation curve is reported in figure 3-42:

0 1 2 3 4 5 6

0

10

20

30

40

Strength at break

Stre

ss (M

Pa)

Deformation (%)

Figure 3-42 Stress-deformation curves for a typical dimethacrylate-POSS network. (The particular curve is

referred to the network BisEMA-2.5 wt % iBuPOSS).

In figure 3-43 are visible the values of E in iBuPOSS-containing networks as a function of

POSS concentration in both matrices:



0 5 10 15 20 25 30

0,8

1,2

1,6

2,0

2,4

0,8

1,2

1,6

2,0

2,4

E (G

Pa)

iBuPOSS wt (%)

Figure 3-43 Young Moduli E of BisEMA-iBuPOSS (full squares ■ ▬)

and BisEMA-CHMA-iBuPOSS networks. (empty circles ○ ---).

Firstly it should be noted that the presence of the reactive solvent CHMA increases the

Young’s modulus, which passes from 1.52 GPa for the neat BisEMA to 2.13 GPa for the neat

BisEMA-CHMA. This increase was already reported for similar systems, that is styrene-

BisEMA [Rey02]. This effect, called “antiplasticization” (i.e. decrease of Tg and increase of

the Young modulus in the glassy state), can be related to the higher extent of the β-transition

when the CHMA is present, as visible in the figure 3-32, which in principle should increase

the cohesive energy density of the BisEMA-CHMA networks with respect to the BisEMA

ones.

The iBuPOSS decreases the Young’s moduli of both the matrices, even though with an

apparently different trend, up to a maximum percentage of decrease of 45 % for BisEMA-30

wt % iBuPOSS and 40 % for BisEMA-CHMA-20 wt % iBuPOSS. The explanation of this

decrease, already observed for other dimethacrylate-POSS systems [Li05], does not directly

lay in the already shown reduction of the crosslinking density, because the glassy modulus is

not connected to the crosslinking density in a way similar to which the rubbery modulus is

[Pas02]. Generally, Eglassy is proportional to the cohesive energy density, the packing density

and the molecular mobility [Pas02]. The influence of iBuPOSS on any of these parameter is

difficult to be determined exactly. In any case, it may be noted that Bizet [Biz04] found that



the copolymerization of iBuPOSS with polymethylmethacrylate (PMMA) decreases the

cohesive energy density of the final random copolymers with respect to the neat PMMA, thus

reducing the E’glassy. It may be that a similar event happens in the dimethacrylate-based

networks. This would represent a justification of the reduction with the iBuPOSS content of

the E of the network.

The presence of the OctaPOSS in the networks increases slightly the Young modulus of

BisEMA-based networks while leaving effectively unaltered the one of the BisEMA-CHMA

systems. This twofold behavior is visible in figure 3-44:

0 5 10 15 20 25 30

1,4

1,6

1,8

2,0

2,2

2,4

2,6

1,4

1,6

1,8

2,0

2,2

2,4

2,6

OctaPOSS wt (%)

E (G

Pa)

Figure 3-44 Young Moduli E of BisEMA-OctaPOSS (full squares ■ ▬)

and BisEMA-CHMA-OctaPOSS networks. (empty circles ○ ---).

The (small) effect of the OctaPOSS in both matrices is similar to what already present in the

literature for other polymeric systems, like polyimide-POSS [Hua03], vinyl ester-POSS

[Li02] or epoxy-POSS systems [Li01b]. It should be outlined that the OctaPOSS now is

included as crosslinking point in the chains and their dispersion in the hybrid dimethacrylate

networks is optimal, probably to the molecular level.

Comparing the influence of the two types of POSS on the values of the Young modulus (in

the glassy state) of the dimethacrylate systems here studied and comparing them to the

existing literature it is possible to outline some common points.



The presence of monofunctional-POSS in thermoset networks as pendant unit decrease the

glassy E (or the E’glassy) of epoxy resins (see Figure 1 in [Str04]), dicyclopentadiene networks

[Con04] and dimethacrylate resins [Li05, Pit03]. The above mentioned authors do not give

any sound explanation of this phenomenon, but it may be possible to infer that the presence of

the bulky cubic cage in a position pendant from the main polymer chain decreases the CED

(thus reducing the E) in a similar way as in thermoplastic polymers with POSS units pending

from the main chain [Bha00, Biz04].

The copolymerization of octafunctional-POSS in thermoset resins give rise instead to all the

three possible influence of POSS on the E values, that is an increase, a decrease or a

substantial lack of effect. In any case, both the decrease (Figure 4 in [Str04], [Cho03, Con04,

Hua04]) and the increase [Hua04, Li02, Ni04] are small. The copolymerization of the

OctaPOSS with thermoset resins, resulting usually in an increase of the average crosslinking

density, seems to be not sufficient on its own to cause a dramatic increase of the Young

modulus. According to the work of Huang et al. [Hua04] and Choi et al. [Cho03], a large

effect on the rigidity of the OctaPOSS molecules (and thus, on the final influence on the

Young modulus) comes from the nature of the interphase around the POSS inorganic core.

This is in agreement with the assumption that, for instance in dimethacrylate networks

[Sid03], a higher Young modulus is depending more on the rigidity of the molecules of the

multifunctional monomers than on the crosslinking density of the final network [Pas02].

It could be therefore concluded that the small effect of the OctaPOSS on the networks Young

moduli is probably due both to similar rigidity of the two molecules and to the substantial lack

of effect of the average crosslinking density (the latter increased by the highly functional

OctaPOSS) on the glassy modulus.

In figures 3-44 and 3-45 the mean tensile strengths with the standard deviations respectively

of all the iBuPOSS-containing and the OctaPOSS-containing networks may be seen:



0 5 10 15 20 25 30

10

20

30

40

50

10

20

30

40

50

Ten

sile

Stre

ngth

(MPa

)

iBuPOSS wt (%)

Figure 3-45 Mean tensile strength (± standard deviation) of iBuPOSS-containing networks as function of the

weight fraction of POSS: BisEMA (full squares ■) and BisEMA-CHMA (empty circles ○) matrices.

0 5 10 15 20 25 30

30

40

50

60

30

40

50

60

Ten

sile

Str

engt

h (M

Pa)

OctaPOSS wt (%)

Figure 3-46 Mean tensile strength (± standard deviation) of OctaPOSS-containing networks as function of the

weight fraction of POSS: BisEMA (full squares ■) and BisEMA-CHMA (empty circles ○) matrices

The tensile strength has been analysed assuming a Normal distribution of the values.

Therefore the one-way analysis of the variance (ANOVA) has been applied along with

complementary t-Test. In the present study all the ANOVA calculation has been performed at

0.05 significance level. Four different groups have been considered (that is, BisEMA-

iBuPOSS, BisEMA-CHMA-iBuPOSS, BisEMA-OctaPOSS and BisEMA-CHMA-



OctaPOSS) and the ANOVA has been applied on each of these four groups to determine

wheter or not the POSS change the tensile strength. The null-hypothesis, that is that the means

are equal, is rejected for all the four different groups, i.e. the addition of POSS changes the

tensile strength.

To determine which are the materials likely to have same tensile strength, we have performed

ANOVA only for the systems with weight fraction of POSS up to 5 %. The results are shown

in the table 3-3:

Table 3-3. One way ANOVA for networks with 0, 2.5 and 5 wt % POSS wt

at 0.05 level of significancy.

Material F Fcrit

BisEMA-iBuPOSS 2,365207 3,982308

BisEMA-CHMA-iBuPOSS 3,749246 3,554561

BisEMA-OctaPOSS 1,393994 3,88529

BisEMA-CHMA-OctaPOSS 10,40198 3,88529

The null-hypothesis cannot be rejected for the BisEMA-iBuPOSS and the BisEMA-

OctaPOSS, that is, the tensile strengths of these materials do not vary significantly adding up

to 5 wt % POSS weight fraction. For the BisEMA-iBuPOSS, this result means that, adding

the POSS, there is no diminution of the tensile strength value. This effect was outlined as a

problem in the literature counterbalancing the improvement of other properties (i.e. reduction

of the shrinkage [Gao01]). For the BisEMA-OctaPOSS, the result of the ANOVA test states

that there is not the increasing of the tensile strength up to 5 wt % POSS, which could be

hypothesized looking only to the values in figure 3-47. On the other hand, a t-Test performed

between the BisEMA matrix and the BisEMA-5 wt % OctaPOSS revealed that these two

values are different (t=1.999, tcrit=1.859), i.e. there is an improvement of the tensile strength

of ca. 10 % adding 5 % OctaPOSS wt. The tensile strength increases for the systems

BisEMA-CHMA-OctaPOSS up to 5 % POSS wt, then it stabilizes on a plateau value of ca. 43

MPa up to 30 wt % POSS (see figure 3-45). In particular, the tensile strength appears to

increase by 18 % through the adding of 5 wt% OctaPOSS to the BisEMA-CHMA matrix.

This increasing has been confirmed by a t-Test (t=8.583, tcrit=1.859). Another t-Test has been



used to determine that the difference of the tensile strength between BisEMA and BisEMA-

CHMA (respectively, 40.2±2.8 MPa and 47.2±2.6 MPa) is real (t=4.201, tcrit=1.943).

The positive effect of the CHMA on the tensile strength may be due to the fact that the

average crosslinking density is decreased by the reactive solvent, which acts as a chain

extender, along with a possible localized plastic flow (induced by the presence of the CHMA)

which decreases the characteristic brittleness of the dimethacrylate matrices. These effects are

also explicable in terms of the reduction of the average functionality number of the monomers

blends “f”, which passes from 4 to 2.47 when the reactive solvent is added to the matrix. The

influence of the variation of the average functionality number on the fractured surfaces has

been investigated with SEM on poly(methyl-methacrylate) (PMMA) and poly(glycolethylene-

dimethacrylate) (PEGDM)[Coo91]. In figure 3-19 (A-B), the BisEMA shows an extended

mirror-like surface, indicating slow crack growth. On the contrary, the BisEMA-CHMA

(Figure 3-19 C-D) shows a mirror-mist-hackle surface, with the crack starting from the upper

right corner. The presence of a rougher surface indicates the existence of gross plastic flow on

small regions [Hul99], which is supposed to be able to increase the tensile strength of a

material [Con04].

In BisEMA-iBuPOSS, at low iBuPOSS concentration, the tensile strength is not affected by

the POSS, because the conversion does not change, thus the presence of less crosslinked zone

is not more probable than in the neat matrix. At higher concentration, the tensile strength is

decreased. This decrease is probably connected to the decrease of the conversion, which

causes the presence of pools of monomer (see Figure 3-7). This decrease is in agreement with

the existing literature about very similar systems, that is methacrylate-POSS and BisGMA

[Gao01].

In BisEMA-CHMA-iBuPOSS, the depression effect of the iBuPOSS on the tensile strength

cannot be explained by the presence of these pools of unreacted monomer, because the

conversion remains always high. It is thought that another mechanism could be operating, that

is related to the phase separation (as evidenced by the SEM analysis, Figure 3-25). The

connection between this phenomenon and the trend of the tensile strength is still unclear.

In the BisEMA and in the BisEMA-CHMA systems containing OctaPOSS, the initial increase

of the tensile strength can be attributed to the presence of more highly crosslinked points

inside the systems, as well as the higher stiffness of the Si-O-Si cube, which now participates



directly inside the network chains. This in turn produces a beneficial effect on the tensile

strength hindering the presence of loosely crosslinked regions (the above-mentioned pools).

The further decrease is explicable again in terms of decreasing of the conversion. This effect

is in agreement with the existing literature about very similar systems, that is OctaPOSS in a

mixture of dimethacrylates [Fon04], where the tensile strength was found to increase up to 10

wt % of POSS then decrease to values lower than the neat matrix.

As can be seen, no correlation between the tensile strength of the dimethacrylate network and

its corresponding FWHM of the tanδ peak (associated to the α-transition), representative of

the network heterogeinity, has been proposed. In fact, for the systems analysed here, there is

not any apparent correlation in between the two phenomena. For instance, while the FWHM

of the BisEMA and BisEMA-CHMA networks increases continuously with the POSS content,

the tensile strength up to 5 wt % OctaPOSS show a real increase and then a decrease.

A possible explanation of the independence between the FWHM of the tanδ peak of the α-

transition and the tensile strength may be that the increase in the heterogeneity observed is not

due to the presence of pools of sol-species but to a cyclization of the double bonds borne by

the multifunctionalPOSS molecules. The latter will lead to a broadening in the distribution of

the relaxation times [Gan02]. This explanation is in agreement with the maintenance of high

values of conversion (see Figure 3-9) for a OctaPOSS content up to 5 wt % in both matrices.

This cyclization will not originate loosely crosslinked zones, and the crosslinking action of

the OctaPOSS molecules will reduce the number of these pools already present in the neat

matrices, thus increasing the tensile strength.

When the conversion will start to decrease (that is, for a OctaPOSS weight fraction above 10

wt %), the presence of these pools of unreactead- partially reacted monomer will be more

probable, and this gives an explanation for the decrease of the tensile strength above 10 wt %

OctaPOSS.



3.6 Surface properties of POSS-containing networks: Surface Tension and

Hardness via Nanoindentation

The surface properties have been determined on 100µm thick films UV cured and thermally

postcured. The degree of conversion of each system cured as thick film is the same of the one

presented for homologous networks processed as dog-bone sample (see Figure 3-7 ÷ 3-9).

The films have been prepared on clean glass plates. The reactive mixtures have been poured

on the glass plate and spread homogeneously with the aid of a bar coater. The thicknesses of

the final films have been assessed with an incertitude range of ±0.5 µm with a laser

interferometer. Each film appeared smooth and homogenous on the surface, while preserving

a good transparency. The networks analysed were: BisEMA-OctaPOSS, BisEMA-CHMA-

iBuPOSS and BisEMA-CHMA-CyPOSS. The results are reported in the next two sections.

3.6.1 Surface Tension of POSS-containing networks as films

In this section, the terms surface energy and surface tension will be used with the same

meaning, that is the excess energy per unit area due to the existence of the free surface

[Van97]. In figures 3-46 and 3-47 are reported the values of the surface tension for the

systems BisEMA-CHMA-iBuPOSS and BisEMA-CHMA-CyPOSS:

0

10

20

30

40

50

0

10

20

30

40

50

10%iBuPOSS

5%iBuPOSS

2.5%iBuPOSS

1%iBuPOSS

BisEMA-CHMA(50/50)

Surf

ace

Tens

ion

(mJ/

m2 )

Total Energy Dispersive Contribution Polar Contribution

Figure 3-47 Surface Tension of BisEMA-CHMA-iBuPOSS networks (divided in total, dispersive and polar

Contribution) as function of POSS content (Owens Wendt’s method).



0

10

20

30

40

50

0

10

20

30

40

50

10%CyPOSS

5%CyPOSS

2.5%CyPOSS

1%CyPOSS

Su

rfac

e T

ensi

on (m

J/m

2 )

BisEMA-CHMA(50/50)


Figure 3-48 Surface Tension of BisEMA-CHMA-CyPOSS networks (divided in total, dispersive and polar


As can be noted, the introduction of only 1 wt % POSS drops the surface energy of ~ 20-25

%, afterwards the energy does not change massively with the POSS content.

To explain this behavior a hypothesis has been formulated. This hypothesis is based on the

amphiphilic nature of a MonoPOSS molecules, like the iBuPOSS and the CyPOSS. The

simultaneous presence of a polar group (methacrylate) and seven hydrophobic, non-polar

groups (isobutyl or cyclohexyl), which make up a real hydrophobic crown, as depicted in

figure 3-49:

Figure 3-49 Amphiphilic structure of the CyPOSS. Similar sketch can be drawn for the iBuPOSS.

O

Si OSi

O

SiOSi

O

SiO

Si

O

SiO

Si

O

O O

O

O

O

Non-Polar Part

Polar Part



The assumption made by Bizet [Biz04] is that the POSS migrate on the free surface of the

films “in being” as soon as the reactive mixture is poured on the glass plate. This would

happen because of their little compatibility with the reactive mixture. Furthermore, it is

believed by Bizet that the methacrylate arm is oriented downwards (i.e. toward the bulk of the

films), leaving the hydrophobic crown on the surface of the films. This assumption is

schematized in figure 3-50:

Figure 3-50 Schematic draw of the POSS organization in the reactive mixture before the polymerization.

The organization of the POSS as a layer on the surface happens in the reactive mixture once

distributed on the glass plate. The UV polymerization simply freezes this organization.

The presence of a layer of POSS on the surface, even at very low content, steadily modifies

the surface tension of the films.

The difference in between the effect of the two POSS, iBuPOSS and CyPOSS (that is, the

apparent higher effect of CyPOSS in changing the surface tension of the films) can be

addressed to the lower solubility of the CyPOSS, which in principle leads to a higher

migration. This is in agreement with the results about the surface energy of the networks

BisEMA-OctaPOSS, shown in figure 3-51:

Glass plate

air

Reactive mixture Methacrylate arm

Hydrophobic crown



0

10

20

30

40

50

60

0

10

20

30

40

50

60

15% OctaPOSS10% OctaPOSS5% OctaPOSSBisEMA

Surf

ace

Tens

ion

(mJ/

m2 )


Figure 3-51 Surface Tension of BisEMA-OctaPOSS networks (divided in total, dispersive and polar


The high miscibility of OctaPOSS with BisEMA does not supply the driving force required

by the OctaPOSS to migrate. Furthermore, the OctaPOSS do not have the typical amphiphilic

nature of the MonoPOSS because the presence of only methacrylate groups gives to these

molecules a polar character.

The difference in the surface organization of the POSS has a direct consequence on the

surface hardness of these films, which will be illustrated in the next section.

3.6.2 Surface hardness of POSS-containing films

The nanoindentation has been used to assess the hardness of the films of POSS-networks.

Nanoindentation has been performed on a Digital Instruments MultiMode AFM, equipped

with a diamond cantilever tip covered with a reflecting layer of gold. The radius of the tip was

50 nm (value supplied by the producer).

Due to a large array of parameters influencing the nanoindentation analysis, the results are

most of the times very depending on the conditions of the analysis [Aim94, Bri89, Du01,

Kla05, Mam05a, Mam05b, Mil03, Tsu01]. Hence, the nanoindentation hardly gives the real

values of the hardness or of the Young modulus, giving rather pseudo-values. Therefore in

this study it has been preferred to normalise all the data obtained for the networks with the

value obtained for the respective matrices BisEMA and BisEMA-CHMA. In this way it



would not be possible to determine the absolute values of the hardness and Young modulus,

but only the influence of POSS on this property. On the other hand, all the problems

connected with the nanoindentation analysis (like tip geometry influence on the properties

determination [Du01]) will be simply minimized with the procedure of normalization.

Therefore will be used the definition normalized Pseudo-Hardness and Pseudo-Young-

modulus. Other details of the nanoindentation analysis can be found in the Annexes.

The choice has been done on the basis of the data from the surface energy, trying to analyse

the systems which promized larger influence of the POSS on the surface mechanical

properties.

The effective indentation depth was usually comprized in the range 100-150 nm. Generally,

the shape of the force-displacement curve does not display the effect of the substrate (i.e. a

marked increase near the zero-displacement of the force required for indentation), as is the

usual case in the literature for nanoindentation analysis on thin film-coated wafers [Hu03].

The general force-displacement curve can be seen in figure 3-52:

0,0 50,0 100,0 150,0 200,0

0,0

30,0

60,0

Forc

e (µ

N)

Vertical displacement of the tip (nm)

Figure 3-52 Typical force-displacement curve of the nanoindentation analysis on 100 µm thick films. (The

particular curve is referred to the network BisEMA).

Unfortunately, from the force-displacement curves it has not been possible to determine the

thickness of the POSS layer on the top of the films.



The results of the Pseudo-Young moduli have been judged not necessary to be reported here

because their trend is very similar to the one determined via tensile testing on bulky samples,

presented in the section 3.5.3.

The results of the Pseudo-Hardness for the systems reported in Table 3, normalized to the

respective matrices, are shown in Table 3-4:

Table 3-4 Pseudo-Hardness normalized to the values of the neat matrices as a function of POSS concentration

for the systems reported in Table 3-4.

wt % POSS Pseudo-H normalized to that of

the corresponding matrix

5 1.01 ± 0.08 BisEMA-OctaPOSS

10 1.04 ± 0.13

2.5 1.02 ± 0.18 BisEMA-CHMA-iBuPOSS

10 1.07 ± 0.08

2.5 1.11 ± 0.12 BisEMA-CHMA-CyPOSS

5 1.12 ± 0.10

The ratio of the hardness of the two matrices (not reported in table 4) indicates that the

BisEMA-CHMA is ~20 % harder than the neat BisEMA. This result is not surprising when

considering the difference in the two Young moduli measured via tensile testing (~ 30 %

higher for the BisEMA-CHMA) and the general direct proportion between the hardness of a

polymer and its Young modulus [Van97].

All the three POSS give rise to a slight increase of the pseudo-Hardness of the respective

matrices.

For the BisEMA-OctaPOSS the increase is the less marked and reflects the trend of the E as

function of the POSS content shown in figure 3-43.

The iBuPOSS in a weight fraction of 2.5 % causes a small increase of the hardness,

accompanied by the highest scattering of data among the nanoindentation measurements of



these films. On the other hand, increasing more the content of POSS up to 10 wt %, there is a

further increase of the hardness.

The CyPOSS shows a sudden increase of the hardness (with respect to the neat BisEMA-

CHMA matrix) for a POSS weight fraction of 2.5 % and then the value seem unaffected by a

further increase of the POSS concentration.

The explanation of the different trends observed for the iBuPOSS and the CyPOSS lies in the

surface migration of the POSS moieties. The iBuPOSS has been shown to migrate

progressively with the POSS content, and this may explain the progressive increase of the

hardness passing from 2.5 to 10 wt % POSS. On the other hand, the CyPOSS seems to

migrate massively even for the lower POSS content (see Figure 3-47). The CyPOSS will

therefore form a layer on the surface of the films at lower weight fraction than the isobutyl-

POSS. This layer will be formed essentially of POSS molecules and will be more stiff than

most of the polymers, according to the work of Foerster et al. [Foe04].

In any case, the impact of the CyPOSS on the surface hardness may also be larger because of

the difference in stiffness among the organic substituents on the POSS cage, that is, isobutyl

and cyclohexyl.

The increase in the surface hardness are somehow less marked than the ones reported in the

literature for other polymeric systems based on silsesquioxanes [Har03, Hu03]. A possible

explanation can be found in the higher percentage of organic constituents on all the three

POSS used in this research than in the silsesquioxanes studied in the above-mentioned papers.

The organic constituents could play the role of soft interphase around a more rigid core

[Hua04], this resulting in a minor impact of the POSS on the hardness of the polymer

systems.



3.7 Conclusions

Dimethacrylate-POSS networks, based on two types of MonoPOSS and a type of OctaPOSS

in conjuction with two different thermoset matrices, have been succesfully processed via UV

curing. Thermal postcuring has been used in order to maximise the conversion of the

methacrylate double bonds.

Solubility (of the MonoPOSS) and miscibility (of the OctaPOSS) has been a key parameter to

ensure the dispersion of the POSS in the final networks. The MonoPOSS have been shown to

be more difficult to disperse in the reactive mixtures than the OctaPOSS: the reason of this

difference lays in the different groups borne by the POSS molecules, with the methacrylate

groups of the OctaPOSS clearly more effective than the isobutyl groups of the iBuPOSS, in

turn more efficient then the cyclohexyl groups of the CyPOSS.

For this reason, the very most part of the analysis has been done on the systems with

iBuPOSS and OctaPOSS, leaving the CyPOSS only for the surface properties

characterization. To further improve the solubility of the iBuPOSS in the reactive mixtures a

reactive solvent, the cyclohexylmethacrylate (CHMA), has been used. The two matrices used

were the neat BisEMA (average functionality number of the reactive mixture equal to 4) and

the BisEMA-CHMA (in a ration 1:1 in weight, average functionality number of the reactive

mixture equal to 2.7).

The conversion of the methacryl double bonds has been shown to depend on the aggregation

state of the POSS in the reactive mixtures (a non-optimal dispersion leads to the trapping of

radicals in between POSS molecules, thus, not allowing them to react) and on the lower

reactivity of the POSS molecules (especially marked for the OctaPOSS). In general,

increasing the POSS content leads to a decreasing of the conversion.

Few outlines can be drawn from this research. Firstly, the two types of POSS (iBuPOSS and

OctaPOSS) have very different effects on the final bulk properties of the networks. While the

OctaPOSS increase slightly the Eglassy, the tensile strength, the Tg and the E’rubbery of both the

matrices (even for a very low amount of POSS, i.e. 2.5 wt %), the iBuPOSS decrease the

Eglassy markedly, the tensile strength and the E’rubbery, while not affecting much the Tg. The

main reason for these changes is the different functionality of the POSS used. In fact, while

the OctaPOSS is a crosslinking agent, the iBuPOSS is a chain extender, and the POSS objects

are as dangling objects. The OctaPOSS will therefore increase the average crosslinking

density while the MonoPOSS will tend to decrease it. Along with the rigidity of the Si-O



cubic cage, the effect on the crosslinking density can explain most of the changes of the

mechanical-thermomechanical properties induced by the presence of these hybrid objects. The

decrease of the Eglassy due to the presence of the MonoPOSS cannot be succesfully explained

only in terms of the crosslinking density. It could be that the cohesive energy density of the

networks is decreased by the presence of the POSS, in a similar way as in the literature for

methacrylate-POSS systems. The bulky size of the cubic cage seems to affect the overall

mobility of the polymer chains (indicated by the maximum of the tanδ peak) for the

iBuPOSS-containing network.

Secondly, the dispersion of the iBuPOSS is not particularly affecting the mechanical-

thermomechanical properties of the networks. The only parameter seemingly affected by the

differences in the iBuPOSS dispersion is the heterogeneity of the networks. When the

dispersion of the iBuPOSS is not optimal, the heterogeneity of the networks is increased. This

lack of influence of the dispersion may induce to think that the iBuPOSS are not to be

considered as “classic” fillers (like silica or clay), rather that their role is the one of a co-

monomer.

Thirdly, the surface properties are affected by the POSS segregation on the top of the films

before the polymerization. The MonoPOSS, due to their lower miscibility associated with the

non-polar isobutyl groups, segregate when the OctaPOSS do not. This has an immediate

influence on the surface tension and on the hardness. The MonoPOSS decrease the surface

tension and increase the hardness while the effect of the OctaPOSS (as it remains in the bulk)

is rather marginal. It could be deduced that, once the MonoPOSS are segregated on the

surface, their amphiphilic nature (polar methacrylate arms and apolar organic groups) is

exploited and their inorganic core is the characteristic which makes the POSS similar to silica,

this resulting in an increase of the hardness.

No nanostructuring has been attained in any POSS-dimethacrylate system and the influence of

the POSS on the properties of the dimethacrylate network has been experimentally shown to

be very limited, as in the existing literature about POSS-thermoset systems. The structures

formed by the POSS in the dimethacrylate networks were effectively an issue of the POSS

dispersion before the polymerization. Even if most mechanisms driving the POSS-

dimethacrylate properties have been elucidated, there are still some unclear aspects, for

instance, it has not been possible to evaluate the impact of the POSS on the microgels



morphologies of the dimethacrylate networks, an important feature to control when properties

change is the aim of the study.

Therefore, in the quest for POSS-nanostructuration and in the successive comprehension of

the properties change due to it, POSS-based linear methacrylate copolymers have been

synthesised and studied. In fact, according to the literature, the POSS may easily self-

assembly in nanostructures when in a linear copolymer, and these nanostructures can enhance

the increase of the properties due to the POSS. Linear copolymers of iBuPOSS and CyPOSS

with MMA will be the object of the next chapter.

Date post:	23-Mar-2021
Category:	Documents
Upload:	others
View:	3 times
Download:	0 times

Chapter 3 Dimethacrylate-POSS networks: processing,...

Documents