Chemical Bonding between Phenolic Resins and Polyhedral Oligomeric Silsesquioxanes (POSS) in...

Chemical Bonding between Phenolic Resins and PolyhedralOligomeric Silsesquioxanes (POSS) in Inorganic–Organic

Hybrid Nanocomposites

Charles U. Pittman Jr.,1,2

Gui Zhi Li,1and Ho Souk Cho

1

Three classes of inorganic–organic hybrid phenolic resin/polyhedral oligomeric silsesquioxane

(POSS) nanocomposites were synthesized. Multifunctional dichloromethylsilylethylheptai-sobutyl-POSS (POSS-1), trisilanolheptaphenyl-POSS (POSS-2), and poly(phenylsilsesquiox-ane) uncured POSS (POSS-3) were employed. Nonfunctional POSS-4 (octaisobuty1-POSS)

was blended into the uncured phenolic resin and cured under the same conditions used for theother three nanocomposite classes. Weight ratios of 99/1, 97/3, 95/5 and 90/10 were preparedfor the POSS-1, 2 and 4 series and 99/1, 97/3 and 95/5 ratios for the POSS-3 nanocomposites.

POSS-1 incorporation into this phenolic resin network increases Tg and broadens the tandpeak (DMTA) range. Tg and E¢ values at T>Tg both increase with higher POSS-1 content. Incontrast, incorporating 5 wt% of POSS-2 into the phenolic resin network lowers Tg to 193from 213�C for the neat phenolic resin. All values of E¢ for POSS-2 composites were higher,

than those of the phenolic control in both glassy and rubbery regions. The Tg values of the 1and 10% POSS-2 systems were higher. Incorporating 10 wt% of POSS-1 or POSS-2 improvedthe heat distorsion temperature and moduli (E¢=123 and 201 GPa at 265�C, respectively,versus 56 GPa for the pure phenolic resin). Increases in E¢ for T>Tg and T<Tg were alsoobserved for all POSS-3 nanocomposites. However, the E¢ at T>Tg and the Tg values of thePOSS-4 composites were lower than those of the control resin. Octaisobutyl POSS-4 cannot

form chemical bonds to the resin and could be extracted from its composites with THF. POSSderivatives were not present in residues extracted by THF from the phenolic resins containingPOSS-1, 2 or 3, because each of these derivatives were chemically bound within the phenolic

resin. Subsequent heating cycles produce much larger increases in Tg and E¢ values in therubbery region for the POSS-1, 2 and 3 composites than for the neat phenolic resin or for thePOSS-4 systems.

KEY WORDS: Synthesis; viscoelastic properties; phenolic resin/polyhedral oligomeric silsesquioxane(POSS) nanocomposites; inorganic–organic hybrid nanomaterials.

1. INTRODUCTION

Inorganic–organic hybrid polymer nanocompos-ites are a relatively new class of materials with

ultrafine phase dimensions typically in the range of1–100 nm [1–3]. These nanocomposites often exhibitunexpectedly improved properties when compared totheir micro- and macrocomposite analogs or to thepristine parent polymer matrix [1]. Polymer–inorganichybrid nanocomposites have attracted significantattention in the past a few years [1–5] and extensivereviews are available [4]. Phenolic thermoset resins arecommercially important and widely employed in a

1 Department of Chemistry, Mississippi State University, Box

9573, Mississippi State, MS 39762, USA.2 To whom correspondence should be addressed.

E-mail: [email protected]

Journal of Inorganic and Organometallic Polymers and Materials, Vol. 16, No. 1, March 2006 (� 2006)DOI: 10.1007/s10904-006-9028-9

431574-1443/06/0300-0043/0 � 2006 Springer Science+Business Media, Inc.

huge variety of applications [6]. Recently, phenolicresin/silica hybrid composites prepared by the sol–gelmethod [7] and phenolic resin/clay nanocomposites[8–10] have been studied. Mechanical properties andthermal stability of these systems have been improved.However, the silica phases generated by the sol–gelmethod and well-dispersed clay tactoids and plateletshave much larger dimensions than the [SiO1.5]n (n=8,10, 12) cages of polyhedral oligomeric silsesquioxane(POSS) chemicals.

Dispersed silica or clay phases improve theflammability resistance of phenolic resins by promot-ing char formation and inhibiting gas transportthrough the material. Molecular dispersion of POSSmacromers creates a far larger surface contact areabetween the phenolic and inorganic phases than theequivalent weight fraction of either silica particles or‘‘nanoclays’’. Therefore, modification of phenolicresins by incorporating POSS derivatives is of greatinterest.

POSS chemicals are typically from 1 to 3 nm indiameter. Their (SiO1.5)n inorganic cage framework,composed of silicon and oxygen, is externally covered(and solublized) by organic substituents. One or moreof these substituents can contain reactive functionalgroups, which can be employed to copolymerize thePOSS species with other common monomers. Cornersubstituents on the silicon atoms of POSS can makethe nanostructured molecules compatible with poly-mers or monomers, offering a unique opportunity forpreparing hybrid organic–inorganic nanocompositeswith the inorganic phase truly molecularly dispersedwithin the matrix [11, 12]. The development oforganic–inorganic hybrid POSS nanocomposites hasaccelerated in the past a few years [11–21].

Incorporation of POSS cages into polymers viacopolymerization, grafting or blending has pro-duced increases in use temperature, oxidation resis-tance, surface hardening and improved mechanicalproperties, as well as reductions in flammabilityand heat evolution for a wide range of thermo-plastic and some thermoset systems, i.e., methacry-lates [13], styrenes [14, 15], norbornenes [17, 22],ethylenes [23], siloxanes [24], epoxies [16, 25], andvinyl esters (VEs) [26] etc.

Multifunctional POSS chemicals have beenchemically incorporated into thermoset resins. Weincorporated an octa-functional POSS,[(C6H5CHCHO)4(Si8O12)(CH@CHC6H5)4], with fourb-substituted styrenes and four epoxidized b-styrylfunctions, into both epoxy [25] and VE resins [26].These POSS units were molecularly dispersed in the

epoxy network [25]. The glass transition temperatureranges for these epoxy/POSS nanocomposite werebroadened, but the Tg (the tand peak temperaturefrom DMTA curves) remained unchanged. Incorpo-rating this multifunctional POSS ( £ 10wt%) into VEnetworks had almost no influence on Tg or the glasstransition region [26]. The storage moduli, E¢, of bothepoxy/POSS and VE/POSS composites in the rubberyregion were higher than those of the neat epoxy or VEresins, respectively, reflecting their improved heatdistorsion values [25, 26]. Multifunctional vinyl-POSScage mixtures (POSS cages of 8, 10 and 12 Si atoms)were used to modify Derakane 441–400 VE resins [27],improving both thermal and fire-retardant properties.However, little change was observed for tensilestrengths, tensile moduli, elongations, flexuralstrengths and flexural moduli [27].

Laine et al. [25, 29] cured two multifunctionalepoxy-POSS derivatives, octakis(glycidyldimethylsil-oxy)octasilsesquioxane (OG) and octaethylcyclohex-enyl-epoxide octasilsesquioxane (OC) withdiaminodiphenylmethane (DDM). The glass transi-tion was barely noticeable for OG/DDM compositeat N=0.5 (N=NH2:epoxy group) [28, 29]. Mya et al.used octadimethylsiloxybutyl-epoxide-POSS (OB) tomodify Ciba’s epoxy resin (araldite, LY-5210, amixture of aromatic and aliphatic diepoxides) [30].Their thermal stability at OB £ 10 mol% didn’tchange. Above 20 mo1% OB, no Tg was observedfor the OB/epoxy nanocomposites. These nanocom-posites exhibited higher storage moduli in the rub-bery region which increased with an increase of OBcontent [30].

Coughlin incorporated well-dispersed norborne-nylethylisobutyl-POSS and trisnorbornenylethyli-sobutyl-POSS into dicyclopentadiene thermosetresins by copolymerization [31]. However, thermaland mechanical reinforcement improvements werenot achieved. The mechanical properties, in tensionand compression, decreased as POSS concentrationsincreased from 0 to 10 and 20 wt% [31]. Clearly,incorporating functional POSS into thermoplastic orthermoset resins, does not always raise Tg or improvemechanical and thermal properties. Substituents onthe POSS cage, crosslink density, the POSS molefraction, the monomer sequence distributions andother variables unpredictably affect how the visco-elastic, thermal and mechanical properties of ther-moplastics and thermosets are modified.

Three functional POSS macromers and onenonfunctional POSS (Scheme 1) were incorporatedinto a commercial resole phenolic resin (Hitco 134A)

44 Pittman, Li, and Cho

in this paper. The viscoelastic properties of thesenanocomposites were determined by dynamicmechanical thermal analysis (DMTA). The influenceon the viscoelastic properties of POSS bonding intothe phenolic matrix is reported.

2. EXPERIMENTAL

2.1. Specimen Preparation

Dichloromethylsilylethyl heptaisobutyl-POSS(POSS-1, Mw: 958.56 g/mol), trisilanol heptaphenyl-POSS (POSS-2, Mw: 931:34 g/mol), uncuredpoly(phenylsilsesquioxane) (POSS-3, Mw: variable),and octaisobutyl-POSS (POSS-4, Mw: 873.60 g/mol)were purchased from HybridTM Plastics Co. All fourPOSS derivatives dissolved in tetrahydrofuran(THF). The commercial phenolic resole resin, Hitco134A, containing 30 wt% isopropanol, was pur-chased from Ashland Specialty Chemical Co.

The Hitco 134A phenolic resin is a resole resincontaining 63–67 wt% phenolformaldehyde oligo-

mers with an additional 10 wt% of phenol present.It contains 3–7 wt% of diethanolamine, 13–17 wt%of isoprophanol and 1.1 wt% formaldehyde. Since allof the POSS blending took place in THF solutions,the resin’s viscosity was not an issue during thedispersion stage.

Specified amounts of each POSS were dissolvedin THF, producing transparent 5 wt% POSS in THFsolutions. Each POSS solution was added to thephenolic resin to produce resin/POSS weight ratiosof 99/1, 97/3, 95/5, or 90/10 as transparent solutions.These solutions were put into trays and the solventwas removed in a vacuum oven (500–600 mmHg) atlow temperature ( £ 70�C). The resulting viscousmixtures were refrigerated and frozen into glassysolids. Each solid was broken and ground intopowder. These powders were press-cured in alumi-num molds under a pressure of 3.5 MPa (508 psi)using the following temperature protocol: 83�C/10–15 min, 87�C/45 min, 149�C/110 min, and 155�C/180 min. The samples were post-cured at 250�C for2 h. Phenolic resin/POSS nanocomposites with com-positions (wt/wt) of 99/1, 97/3, 95/5 and 90/10 were

Si

SiO

Si

SiO

O Si

O

OSi

SiO

O

OSi

O

OO

O

R

R

R R

R

R

R

SiMe

Cl

Cl Si

SiO

Si

SiO

O Si

O

OSi

SiO

O

OH

OH

OHO

O

R

R

R R

R

R

R

Si

SiO

Si

SiO

O Si

O

OSi

SiO

O

OSi

O

OO

O

R

R

R R

R

R

R

R

R=isobutyl

POSS-1

R=phenyl

POSS-2

POSS-3 POSS-4

Si

Si

SiO

Si

SiO

O Si

O

OSi

Si O

OSi

O

OO

O

R

R

R R

R

R

R

R

O Si O O

O O

Si SiOO O

R

RR

R

Si

OH

R

]

][

[

R=phenyl

R=isobutyl

Scheme 1. Chemical structures of the four POSS derivatives employed in this study.

45Chemical Bonding between Phenolic Resins and Polyhedral Oligomeric Silsesquioxanes

prepared for POSS-1 and 2. Phenolic resin/POSS-3nanocomposites with compositions of 99/1, 97/3 and95/5 were made. POSS-4, without reactive functionalgroups, dissolved in THF and a transparent solutionwas obtained. This solution was then added tophenolic resin and to give a translucent mixture. Atranslucent viscous mixture was obtained after cast-ing into trays and solvent removal, indicating POSS-4 was not completely soluble in the uncured resin.POSS-4/phenolic resin blends (99/1, 97/3, 95/5 and90/10 wt/wt) were prepared in this way and thencured using the same curing conditions with thosefor POSS-1, 2 and 3 nanocomposites. A purephenolic resin control sample was also produced bythe same protocol as those used for the POSSnanocomposites.

2.2. Measurements

The dynamic storage modulus, E ¢, and lossfactor (tand) were determined in the dual-levelbending mode using a Polymer Laboratories DMTAMK3 instrument. Small amplitude bending oscilla-tions (both 1 and 10 Hz) at a gap setting of 8.00 mmwere carried out from 35�C to 270–300�C at a heatingrate of 2�C/min. Sample sizes of 1.7–3.4 mm thick,5.5–7.1 mm wide and 38 mm long were used.

Phenolic resin and phenolic resin/POSS nano-composite densities were measured using an Elec-tronic Densimeter (ED-120T) at 25�C.

Specimens of every nanocomposite (0.67–1.48 g)were immersed into THF at room temperature for42 days. Only the 10 wt% POSS-4 composite sampleexhibited many cracks on its surface. The othernanocomposite samples and pure phenolic resinexhibited little weight increase due to solvent swellingafter 42 days in THF. Very small amounts ofungelled copolymers were extracted into these THFsolutions. Selected samples were cryoground intosmall particles and extracted with THF, DMF and(at 80�C) phenol. After coating the extract onto KBrplates and removal of solvent, IR spectra wereobtained on an FT-IR instrument (MIDAC Corpo-ration). The remaining insoluble phenolic resin andits POSS nanocomposites after extraction were alsoexamined by FT-IR.

A JEM-100 CXII transmission electron micro-scope (TEM) (JEOL USA Inc.) operated at 60 KVwas used to characterize morphology of the selectedPOSS-1, 2 and 3 nanocomposites and POSS-4/phe-nolic resin blend. Specimens were microtomed to 70–90 nm and set on copper grid.

3. RESULTS AND DISCUSSION

3.1. Synthesis of the Nanocomposites

The chemical structures of the four POSS deriv-atives are shown in Scheme 1. The resole phenolicresin/POSS-1, 2, 3 and 4 nanocomposites were curedthermally and post-cured at 250�C (2 h). POSS-4,with only isobutyl functions, could not chemicallyreact with phenolic components during curing. Thus,it forms simple blends. Phenolic resin/POSS-1nanocomposite synthesis is represented in Scheme 2.Chemical incorporation of POSS-1 was accomplishedby condensation between hydroxyl groups in theuncured phenolic resin (phenolic or methylol hydrox-yls) and Si–Cl groups attached to POSS-1. This wasconfirmed by independent reactions of POSS-1 inexcess phenol at 40�C which generated the diphenoxyderivative by loss of two equivalents of HCl. Thisconverted the –CH2CH2SiCl2Me side chain to –CH2CH2Si(O-Ph)2CH3 groups, demonstrating thatPOSS-1 has reacted with the phenolic resin longbefore the phenolic cure advances during nanocom-posite synthesis.

POSS-2 and POSS-3 contain Si–OH functionalgroups which cannot react as quickly at mild condi-tions with methylol or phenolic hydroxyl groups asthe –SiMeCl2 groups in POSS-1 can. Thus, thesequence of curing reactions which bond POSSderivatives 2 or 3 into the phenolic resin would bedifferent from that of POSS-1 (Scheme 2). Scheme 3shows the synthesis of phenolic resin/POSS-2 nano-composite. Reactions between the Si–OH functionsof POSS-3 and the phenolic resin will be essentiallythe same as those of POSS-2. Thus, no structuralequation is shown for phenolic resin/POSS-3 nano-composites. Because POSS-2 or 3 were well dispersedin the viscous uncured phenolic resin, POSS to POSSmacromer encounters are reduced (low mole % slowssecond order rates) slowing any self-condensations.Furthermore, self-condensation of POSS-2 does notoccur below �250�C.

Trisilanolheptaphenyl POSS-2 does react withthe phenolic and methylol hydroxyl groups to splitout water under mild conditions, but the relative ratesversus phenolic curing is not known. Therefore,POSS-2 was independently reacted with a large excess(50:1 ratio) of the model phenolic compound 2,6-dimethyl-4-hydroxymethylphenol. This molecule wasselected because all positions ortho and para to thehydroxyl function are blocked, retarding rapidmolecular weight


g g p p / p

+ +CH2OH

OHCH2OH

CH2OCH2

OH OH

CH2OH

CH2

OH

HOCH2

OH

CH2

CH2OH

OH

POSS-1

etc.

1-10wt%

10wt%

Cureprotocal

CH2

OH

HOCH2

CH2

OH

CH2

CH2

OH

CH2

OH

CH2CH2

OH

CH2

CH2OH

OH

CH2

CH2

OH

CH2CH2

OH

OH

OH

CH2

CH2

OHCH2

OHCH2OH

CH2H2C

Si

SiO

Si

SiO

O Si

O

OSi

SiO

O

OSi

O

OO

O

R

R

R R

R

R

R

SiMe

CH2

OHOCH2

O

CH2

HOCH2

Si

SiO

Si

SiO

O Si

O

OSi

SiO

O

OSi

O

OO

O

R R

R

R

R

R

Si

CH2

OCH2

Me

OH

CH2 CH2

OR

CH2

OH

CH2

CH2CH2

CH2

OH

CH2

OH

CH2

CH2 OH

5

6

83oC/10-15min, 87oC/45min,149oC/110min and 155oC/180minat pressure of 3.5 MPa (508psi)

Postcuring 250oC/120min

Scheme 2. Synthesis of phenolic resin/POSS-1 nanocomposites.

+ + POSS-2

1-10wt%10wt%

Cureprotocal

CH2

OH

HOCH2

CH2

OH

CH2

CH2OH

OH

CH2

OH

CH2CH2

OH

CH2

CH2OH

OH

CH2

CH2

5 6

Si HO

Si

SiO

O Si

O

OSi

SiO

O

O

Si

OO

O

O

R R

R

R

R

R

CH2

OHCH2

OH

CH2CH2

OH

OH

CH2

CH2

OHCH2

OHCH2OH

CH2CH2

OH

CH2

HOCH2

CH2

OHCH2

OH

CH2 CH2

OH

CH2

OH

CH2

CH2CH2

CH2

OH

CH2

OH

CH2

CH2 OH

Si

SiO

Si

SiO

O Si

O

OSi

SiO

O

O

O

OHO

O

R

R

R R

R

R

R

HO

CH2

R

CH2OH

83oC/10-15min, 87oC/45min,149oC/110min and 155oC/180minat pressure of 3.5 MPa (508psi)

Postcuring 250oC/120min

Scheme 3. Synthesis of phenolic resin/POSS-2 nanocomposites.


build-up. On heating to 80�C for 1 h, a mixture ofcompounds containing both a single POSS cage andaromatic rings was obtained. Thus, POSS-2 and thephenolic resin react together much more readily thanPOSS-2 self-condensation.

3.2. Viscoelastic Properties of Phenolic Resin/POSS

Nanocomposites

The bending storage moduli, E¢, versus temper-ature curves at 1 Hz (from DMTA) for the neatphenolic resin and the phenolic resin/POSS-1nanocomposites are given in Fig. 1 and Table I.The E¢ values of all POSS-1 nanocomposites arehigher than those of the neat phenolic resin in therubbery region (T >Tg). Furthermore, these E¢values increase almost linearly with an increase inPOSS loading. The E¢ values of the 1, 3 and 5 wt%POSS-1 nanocomposites are also higher than those ofthe neat phenolic resin in the glassy region (T<Tg).The neat phenolic resin and the phenolic resin/POSS-1 1, 3, 5 and 10 wt% nanocomposites at 40�C exhibitE¢ values (Table I) of 1.4, 1.7, 1.8, 2.0 and 1.3 GPa,respectively. The corresponding E¢ values at 265�C(>Tg) were E¢=56 MPa (phenolic resin) and 66, 61,80 and 124 MPa (1, 3, 5 and 10 wt% POSS-1,respectively). The E¢ value for the 90/10 nanocom-posite is about twice of that for the neat phenolicresin at 265�C. The POSS-1 nanocomposites have

higher heat distortion temperatures than the phenolicresin and they increase as POSS-1 loadings go from 3to 10 wt%.

Curing 1 into phenolic resin broadens the tandpeak (1 Hz) and the intensities decrease with anincrease in POSS-1 loading (Fig. 2). The Tg values,defined as the tand peak temperature, are 213, 210,211, 217 and 245�C for the neat phenolic resin andPOSS-1 1, 3, 5 and 10 wt% nanocomposites, respec-tively. Only 10 wt% POSS-1 causes a major changein Tg (32�C increase).

The bending storage moduli, E ¢, versus temper-ature curves at 1 Hz for the neat phenolic resin and thePOSS-2 (1, 3, 5 and 10 wt%) composites are shown inFig. 3. All four POSS-2 loadings raise the E¢ values inboth glassy and rubbery regions. The bending storagemoduli at 40�C (<Tg) are 1.4 (phenolic resin) and 2.0,2.0, 2.2 and 1.5 GPa for 1, 3, 5 and 10 wt% POSS-2,respectively. At 265�C (>Tg), the E¢ values for thesesamples were 56, 106, 89, 141 and 201 MPa, respec-tively. The 10 wt% POSS-2 nanocomposite displays a3.6 times greater E¢ value at 265�C than the neatphenolic resin. The 1, 5 and 10 wt% POSS-2 nano-composites have broader and less intense bending tandpeaks in the glass transition region versus the neatphenolic resin (Fig. 4), while the 3 wt% POSS-2nanocomposite exhibits a somewhat more intenseand broader peak. The Tg values (tand peak temper-atures) are 213, 217, 213, 222 and 254�C, for the neat

g g p p / p

7.5

8

8.5

9

9.5

20 60 100 140 180 220 260 300

Temperature (°C)

Ben

ding

logE

' (P

a)

0wt% POSS-1

1wt% POSS-1

3wt% POSS-1

5wt% POSS-1

10wt% POSS-1

Fig. 1. Bending E ¢ versus temperature curves at 1 Hz for phenolic resin/POSS-1 nanocomposites.


phenolic resin and the 1, 3, 5 and 10 wt% POSS-2loadings, respectively. The 10 wt% POSS-2 nanocom-posite exhibits a 41�C higher Tg than that of the

control resin. Clearly, incorporating 10 wt% POSS-2into the phenolic resin greatly improves itsTg and hightemperature mechanical properties.

Table I. Tg and E¢ Values at 40 and 265�C of the Phenolic Resin Control (PR) and Phenolic Resin (PR)/POSS-1, 2, 3 and 4 Composites in the

First, Second and Third Heating Cycles

Composite type POSS (wt%) Heating cycles Tg (�C) E¢ at 40�C (GPa) E¢ at 265�C (MPa) Duplicated experimentsa E¢ at 265�C

Phenolic resin (PR) 0 1 213 1.4 56

2 234 1.7 103

3 257 1.7 199

PRIPOSS-1 1 1 210 1.7 66 (65)b

2 230 2.1 115 (211)b

3 257 2.1 217

PRIPOSS-1 3 1 211 1.8 61

2 238 2.0 121

3 263 2.2 280

PR/POSS-1 5 1 217 2.0 80

2 235 2.4 141

3 252 2.6 264

PR/POSS-1 10 1 245 1.3 124 (134,141)

2 273 1.6 279

3 >300 1.6 594 (612,549)

PR/POSS-2 1 1 217 2.0 106

2 250 2.3 253

3 271 2.2 502

PR/POSS-2 3 1 213 2.0 89 (101,95)

2 241 2.3 218

3 270 2.3 401 (410,462)

PR/POSS-2 5 1 222 2.2 141 (136,144)

2 234 2.8 269

3 247 2.9 413 (391,428)

PR/POSS-2 10 1 254 1.5 201

2 280 1.6 581

3 >300 1.5 853

PR/POSS-3 1 1 203 1.9 62

2 219 2.2 89

3 232 2.3 134

PR/POSS-3 3 1 224 1.4 70

2 255 1.7 193

3 296 1.7 456

PR/POSS-3 5 1 223 1.5 75 (81,84)

2 261 1.6 221

3 >298 1.6 471 (459,487)

PR/POSS-4 1 1 187 1.7 47

2 195 2.0 62

3 207 2.0 73

PR/POSS-4 3 1 195 1.4 36

2 216 1.5 55

3 234 1.5 88

PR/POSS-4 5 1 197 1.6 55 (49,52)

2 222 1.7 125

3 257 1.7 248 (220,231)

PR/POSS-4 10 1 212 1.0 43

2 235 1.2 118

3 263 1.3 310

aTwo additional composite samples were each independently resynthesized and cured by the identical procedure. The reproducibility of the E¢values of the first and third heatings are shown here.bOnly a single additional synthesis was performed on this sample.


All POSS-3 (1, 3 and 5 wt%) nanocompositeshave higher E¢ values (1 Hz) than those of thephenolic resin in the rubbery region. These valuesincrease continuously with an increase of POSS-3content (Fig. 5) at 265�C from 56 (phenolic resin) to62 (1 wt%), 70 (3 wt%), and 75 MPa (5 wt%).Below Tg, (40�C) the 3 and 5 wt% POSS-3 nano-composites exhibit E¢ values similar to those of theneat phenolic resin (1.4–1.5 GPa) while the 1 wt%POSS-3 nanocomposite has higher E¢ values(1.9 GPa). The bending tand peak intensities

decreased with rising POSS-3 contents (Fig. 6). TheTg values did not vary drastically in this series(Table I).

Octaisobutyl POSS-4 has no reactive functionalgroups to participate in the phenolic cure. Further-more, the peripheral isobutyl group cannot partici-pate in p-stacking interactions with the resin’s phenylrings. POSS-4 was blended into the phenolic resinand cured in order to compare the effect of chemicalincorporation (POSS-1, 2 and 3) with physicalblending on viscoelastic properties. The DMTA

g p / / p , , g y

0

0.1

0.2

0.3

0.4

20 60 100 140 180 220 260 300

Ben

ding

tanδ

0wt% POSS-1

1wt% POSS-1

3wt% POSS-1

5wt% POSS-1

10wt% POSS-1

Temperature (°C)Fig. 2. Bending tand versus temperature curves at 1 Hz for phenolic resin/POSS-1 nanocomposites.

7.5

8

8.5

9

9.5

20 60 100 140 180 220 260

Ben

ding

logE

'(P

a)

0wt% POSS-2

1wt% POSS-2

3wt% POSS-2

5wt% POSS-2

10wt% POSS-2

Temperature (°C)Fig. 3. Bending E¢ versus temperature curves at 1 Hz for phenolic resin/POSS-2 nanocomposites.


curves for 1, 3, 5 and 10 wt% POSS-4 composites areshown in Figs. 7 and 8. E¢ values of the 1, 3 and5 wt% POSS-4 composites are slightly higher in theglassy region (T<Tg) than those of the neat phenolicresin (Fig. 7 and Table I), but the 10 wt% POSS-4composite’s E¢ values drop to 1.0 GPa (versus1.46 GPa for the control resin) at T<Tg (40�C).

However, in the rubbery region (T>Tg), all POSS-4composites exhibit lower bending storage moduli(36–55 MPa at 265�C) than the phenolic resin56 MPa). This phenomenon contrasts sharply withthe behavior of the POSS-1, 2 and 3 nanocomposites,where POSS is chemically bonded into phenolic resin(Schemes 2 and 3).

0

0.1

0.2

0.3

0.4

20 60 100 140 180 220 260

Ben

ding

tanδ

0wt% POSS-2

1wt% POSS-2

3wt% POSS-2

5wt% POSS-2

10wt% POSS-2


7.5

8

8.5

9

9.5

20 60 100 140 180 220 260

Ben

ding

logE

'(P

a)

0wt% POSS-3

1wt% POSS-3

3wt% POSS-3

5wt% POSS-3

Temperature (°C)Fig. 5. Bending E¢ versus temperature curves at 1 Hz for phenolic resin/POSS-3 nanocomposites.


The Tg values of these POSS-4 compositesincrease slightly with POSS loading, but are lowerthan that of the neat phenolic resin. Furthermore, thetand peak intensities for the POSS-4 compositesare higher than that of the phenolic resin (Fig. 8).Thus, unlike POSS derivatives 1, 2 and 3, bending inPOSS-4 does not enhance the viscoelastic properties orraise Tg.

POSS-1, 2 and 3 react with reactive groups in theuncured phenolic resin, incorporating POSS into the

phenolic resin’s crosslink network by chemical bonds.No POSS particles were observed in phenolic resin/POSS-1 (3, 5, 10 wt%) nanocomposites by TEM at amagnification of 20,000. Similarly, no particles wereobserved in 10 wt% POSS-2 or 5 wt% POSS-3nanocomposites. Thus, POSS-1, 2 or 3 were compat-ibly dispersed in their phenolic nanocomposites.However, POSS-4 formed phase-separated nano- ormicro-particles observed by TEM (Fig. 9) and con-focal microscopy studies. The POSS-rich particle in

0

0.1

0.2

0.3

0.4

20 60 100 140 180 220 260

0wt% POSS-3

1wt% POSS-3

3wt% POSS-3

5wt% POSS-3

Ben

ding

tanδ


7.5

8

8.5

9

9.5

20 60 100 140 180 220 260

Ben

ding

logE

'(P

a)

0wt% POSS-4

1wt% POSS-4

3wt% POSS-4

5wt% POSS-4

10wt% POSS-4

Temperature (°C)Fig. 7. Bending E ¢ versus temperature curves at 1 Hz for phenolic resin/POSS-4 composites.


Figure 9(a) is about 0.5 lm in diameter. Some smallirregular particles are observed in Fig. 9(b), where thesmall dark particle is about 0.04 lm. POSS-4 blendsexhibited lower heat distortion temperatures andpoorer mechanical properties.

3.3. Solvent Extraction Studies

THF extraction readily removed POSS-4 fromits composites. The amount extracted increased with

an increase of POSS-4 loadings from 3 to 10 wt%(Table II). In contrast, POSS derivatives 1, 2 and 3

were not removed from their nanocomposites byTHE extraction. No POSS monomers or POSS-containing linear copolymers could be extracted fromthe phenolic/POSS-1, 2 and 3 nanocomposites withcompositions of 99/1 and 97/3. IR spectra of the THFextracts from the 95/5 phenolic resin/POSS (1, 2 or 3)nanocomposites exhibited very weak absorptions at

0

0.1

0.2

0.3

0.4

0.5

20 60 100 140 180 220 260

0wt% POSS-4

1wt% POSS-4

3w% POSS-4

5wt% POSS-4

10wt% POSS-4B

endi

ng ta

nδ

Temperature (°C)Fig. 8. Bending tand versus temperature curves at 1 Hz for phenolic resin/POSS-4 composites.

0.5µm

(a)

0.25µm

(b)

Fig. 9. TEM micrographs for phenolic resin/POSS-4 95/5 composite.


about 1100–1135 cm )1. These are attributed to verysmall amounts of the very strong Si–O stretchingbands within POSS units [32, 33]. Even at high(10 wt%) POSS content, only traces of POSS-con-taining species were extracted. POSS monomerscould not be separated from these THF-solubleresidues because these extracts were ungelled phenolicpolymeric species, chemically bound to POSS.

The insoluble solid residues (crosslinked net-work) from the POSS-1, 2 and 3 samples exhibitedSi–O absorptions in their IR spectra, demonstratingthese POSS monomers were present within the resins,after THF extraction. This 1100–1135 cm)1 absorp-tion becomes increasingly stronger as POSS-1 or 2

loadings increased to 10 wt%. However, no Si–Oabsorptions were observed from the POSS-4 com-posite after THF extraction, showing that extractionhad moved all or most of the blended POSS-4.

A reviewer suggested that good H-bonding or thelarge size of POSS monomers might slow or preventextraction of POSS-1, 2 and 3. Thus, nanocompositesof 5 wt% POSS-1, 2 and 3 were ground after coolingin liquid nitrogen and aliquots of each were extractedat room temperature with THF, DMF and at 80�Cwith phenol. These extractions did not remove POSSfrom the resins, providing further evidence for thechemical bonding of POSS to the matrix.

The chemical structures of POSS-1, 2 and 3 aredifferent, so their reactivities with phenolic resinduring curing would be different. The POSS cagesincorporated into the crosslink network in the POSS-1nanocomposite are pendant. However, open-cagePOSS-2 contains three acidic O Si OH)( 3 groups.

If two or all three OH groups react, the POSS-2framework would be incorporated into the network asa crosslink center. POSS-3 contains the same type ofacidic O Si OH)( 3 groups as trifunctional POSS-2.However, they are attached along the ladder-likebackbone of POSS-3 and the Si–OH to Si ratio for 2and 3 are different. POSS-3 is not a discrete singlestructure, but is a distribution of various molecularweight molecules. All of these factors would havesome influence on the extent of cure, free-volume andviscoelasticity of the resulting phenolic resin/POSSnanocomposites.

The weight percentages of total solid, which canbe extracted by THF from the phenolic resin and itsPOSS-1, 2, 3 and 4 composites are listed in Table I.The percentage of material extracted from POSS-1, 2and 3 nanocomposites increased with an increase inPOSS loadings. This may result from perturbationsin the mechanism and extent of cure due to pHchanges, the more acidic nature of the Si–OH groupsor other factors. However, the residues extractedcontained very little POSS. The extraction percent-ages from the phenolic resin/POSS-4 composites,containing 1, 3, 5, and 10 wt% POSS, rise quicklywith a rise of POSS-4 loading and are due tosubstantial loss of 4.

3.4. Effect of Thermal History on Viscoelastic

Properties of Phenolic Resin/POSS

Nanocomposites

Thermal history effected the viscoelastic proper-ties of both the neat phenolic resin and its POSS

Table II. Tg, Bending Storage Moduli, Density and Percentages Extracted by THF of the Phenolic Resin Control (PR) and the Phenolic

Resin (PR)/POSS-1, 2, 3 and 4 Composites

Composite type POSS (wt%) Tg (�c) E¢ at 40�C (GPa) E¢ at 265�C (MPa) Density q (g/cm3) Extraction percentage (%)

PR 0 213 1.4 56 1.195 0.9

PR/POSS-1 1 210 1.7 66 1.205 0.7

PR/POSS-1 3 211 1.8 61 1.212 0.8

PR/POSS-1 5 217 2.0 80 1.229 1.6

PR/POSS-1 10 245 1.3 124 1.221 2.6

PR/POSS-2 1 217 2.0 106 1.248 0.7

PR/POSS-2 3 213 2.0 89 1.237 0.7

PR/POSS-2 5 222 2.2 141 1.235 5.5

PR/POSS-2 10 254 1.5 201 1.215 8.4

PR/POSS-3 1 203 1.9 62 1.226 0.6

PR/POSS-3 3 224 1.4 70 1.201 1.0

PR/POSS-3 5 223 1.5 75 1.189 2.5

PR/POSS-4 1 187 1.7 47 1.248 0.5

PR/POSS-4 3 195 1.4 36 1.249 1.5

PR/POSS-4 5 197 1.6 55 1.248 5.4

PR/POSS-4 10 212 1.0 43 1.229 14.3


nanocomposites. DMTA curves of the first, second,and third heating cycles for the neat phenolic resinare shown in Fig. 10. Tg values increased going fromthe first to the third heating cycle: 213, 234 and257�C, respectively, and the tand peak intensitiesdropped with successive heating cycles. E¢ increasedin the rubbery region (T>Tg) during in the secondand third heating cycles. The E¢ values in the glassyregion also increased during the second heating andthen varied little in the third heating (values at 40 and265�C are listed in Table II). The phenolic resin’s E¢value at 265�C in the third cycle is 199 MPa, about3.5 times greater than its value in the first cycle(56 MPa). Thus, the phenolic resin underwent furthercrosslinking during DMTA heating to 300�C.

POSS-1, 2 or 3 nanocomposites all exhibedhigher storage moduli values and heat distortiontemperatures on their second and third heatingcycles, consistent with further curing. Typicalexamples are shown in Figs. 11–13. The 10 wt%POSS-1 nanocomposite exhibited a sharp increasein Tg from 245�C to �273�C and >300�C in thesecond and third heating cycles, respectively(Fig. 11). The E¢ values at 265�C greatly improvedfrom 124 MPa in the first cycle to 594 MPa in thethird cycle, a 4.8 fold increase. The pure phenolicresin’s E¢ value at 265�C was 199 MPa in the thirdheating cycle, only one-third that of the 10 wt%POSS-1 nanocomposite. The presence of POSSaccentuates the property changes achieved in thesecond and third heating cycles.

The 10 wt% POSS-2 nanocomposite also exhib-its large Tg and E¢ (T>Tg) improvements duringsubsequent heating cycles (Fig. 12). Its Tg increasesfrom 254�C to �280�C in the second cycle, accom-panied by a large decrease in tand intensity. In thethird heating cycle, the Tg is much higher than 300�C.The E¢ value at 265�C in the third heating was853 MPa, much higher than that of the 10 wt%POSS-1 nanocomposite (594 MPa) or the phenoliccontrol (199 MPa) after the same treatment. The10 wt% POSS-2 sample has a higher E¢ at 40�C in thefirst heating cycle than either the phenolic controlresin or the 10 wt% POSS-1 sample. E¢ values in theglassy region from 100 to 200�C substantiallyincreased in the second and third heating cycles(Fig. 12). Heating the phenolic resin/POSS-2 nano-composites has a larger effect on the viscoelasticproperties than does heating the POSS-1/phenolicsystems. Perhaps acidic SiOH groups in 2 maymodify the high temperature curing chemistry bypromoting further acid-catalyzed resin curing. Alter-natively POSS-2 may increasingly serve as crosslinksites when subjected to higher temperatures.

POSS-3 also induced enhancements in Tg and E¢values on heating (Fig. 13, Table I). The 5 wt%POSS-3 nanocomposite’s Tg values rose from 223 to261 and �300�C for the three heating cycles, respec-tively. This sample’s high temperature E¢ values(265�C) greatly increased (75.3, 220.8 and471.3 MPa) in the first, second and third heatingcycles, respectively. The Tg enhancements upon

7.5

8

8.5

9

9.5

20 60 100 140 180 220 260 300

Ben

ding

logE

' (P

a)

0

0.1

0.2

0.3

0.4

E' first heatingE' second heatingE' third heatingtanδ first heatingtanδ second heatingtanδ third heating

Temperature (°C)

Ben

ding

tanδ

Fig. 10. DMTA curves of the neat phenolic resin in the first, second, and third heating cycles.


heating were substantially less than those caused by10 wt% POSS-2. The decrease in the tand peakintensity was greater on successive heatings for the5 wt% POSS-3 then for 10 wt% POSS-1.

The DMTA curves for the first, second and thirdheating cycles of the 10 wt% POSS-4 composite(Fig. 14) exhibited increasing Tg values (212, 235 and263 �C). This improvement is similar to that observedfor the neat phenolic resin. The E¢ values in therubbery region for this 10 wt% POSS-4 compositewere also improved by thermal history. The E¢ value

at 265�C in the third cycle is 310 MPa, which ishigher than that of phenolic resin (199 MPa), butmuch lower that those from the third heating of the10 wt% POSS-1 and 2 and 5 wt% POSS-3 compos-ites (Table II). The 1, 3 and 5 wt% POSS-4/phenolicresin composites gave lower Tg values than the neatphenolic resin for the corresponding heating cycles. Itis clear that the unfuctionalized POSS-4 is noteffective at enhancing the heat distorsion temperatureexcept after several heating cycles. Using 1 and3 wt% of POSS-4 sharply degrades the E¢ values in


Temperature (°C)

7.5

8

8.5

9

9.5

20 60 100 140 180 220 260 300

Ben

ding

logE

' (P

a)

0

0.1

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Ben

ding

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Fig. 12. DMTA curves of the phenolic resin/POSS-2 90/10 nanocomposite in the first, second, and third heating cycles.


Temperature (°C)

7.5

8

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20 60 100 140 180 220 260 300

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' (P

a)

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Ben

ding

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the rubbery region (265�C) to values far below thoseof the phenolic control in all three heating cycles(Table II). In contrast, the POSS-1, 2 and 3 compos-ites all exhibited significantly improved Tg and E¢values at elevated temperatures versus the neatphenolic resin during all heating cycles. Chemicalbonding of POSS moieties into the phenolic resinenhances E¢ and Tg far more than adding unfunc-

tionalized POSS-4, both prior to and after thermaltreatments.

4. CONCLUSIONS

Three multifunctional POSS macromers, POSS-1,POSS-2 and POSS-3, were chemically incorporated


Temperature (°C)

7.5

8

8.5

9

9.5

20 60 100 140 180 220 260 300

Ben

ding

logE

' (P

a)

0

0.1

0.2

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0.4

Ben

ding

tanδ



Temperature (°C)

7.5

8

8.5

9

9.5

20 60 100 140 180 220 260 300

Ben

ding

logE

' (P

a)

0

0.1

0.2

0.3

0.4

0.5

Ben

ding

tanδ

Fig. 14. DMTA curves of the phenolic resin/POSS-4 90/10 composite in the first, second, and third heating cycles.


into the phenolic resin crosslinked networks bythermal curing. The phenolic resin/POSS-1 nano-composites with 1, 3, 5 and 10 wt% POSS exhibitprogressively higher Tg and E¢ values in the rubberyregion versus those of the neat phenolic resin. TheirTg values and the E¢ values at T>Tg increase almostin proportion with the increase in POSS-1 or 2

loading. The incorporation of l0 wt% of eitherPOSS-1 or POSS-2 into the phenolic resin leads tohigh Tg and storage moduli (T>Tg). The improve-ments in Tg and E¢ values in the rubbery region werealso obtained for the phenolic resin/POSS-3 nano-composites containing 3 or 5 wt% POSS. However,no improvements in viscoelastic properties or thermalstability of the phenolic resin were observed byblending incorporating the unfunctionalized POSS-4into the phenolic resin. The structural differences inthe substituents and the POSS structures influence theproperty improvements of the nanocomposites con-taining POSS-1, 2 or 3. THF extraction of the POSS-1, 2 and 3 nanocomposites removed no measurablePOSS residues from the 99/1 and 97/3 samples. Onlytraces of POSS-containing residues were obtainedfrom the nanocomposites containing 5 or 10 wt% ofthese three functional POSS monomers. However,POSS-4, which is not chemically bound to thephenolic resin, is easily extracted. Heating leads toimprovements of the Tg and E¢ values in the rubberyregion for the neat phenolic resin and all phenolicresin/POSS composites. However, the phenolic resin/POSS nanocomposites containing functional POSS-1,2 and 3, exhibited much more prominent viscoelasticimprovements than those of the neat phenolic resin orthe phenolic resin/POSS-4 composites. POSS-2 wasthe most effective POSS derivative at enhancing thehigh temperature properties by heating, possiblybecause it promotes acid-catalyzed curing reactionsor by acting directly as a crosslinking site.

ACKNOWLEDGMENTS

This work was supported by the Air Force Officeof Scientific Research, grant no. F496200210260, andby the National Science Foundation, grant no.EPSO132618.

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