Morphology, Crystallization, and Mechanical Property Study of Polypropylene and In-situ fibrillated Polyethylene

Terephthalate Composite

by

Yu Guang Chen

A thesis submitted in conformity with the requirements for the degree of Master of Science

Department of Mechanical and Industrial Engineering University of Toronto

© Copyright by Yu Guang Chen 2018

ii

Morphology, Crystallization, and Mechanical Property Study of

Polypropylene and In-situ fibrillated Polyethylene Terephthalate

Composite

Yu Guang Chen

Master of Science

Department of Mechanical & Industrial Engineering

University of Toronto

2018

Abstract

This project covers a complete in-depth study of crystallization behavior of PP and in-situ

fibrillated PET composites. Two kinds of PET, APET and CPET, were examined. SEM and DSC

experiments were carried out to exam fiber morphology and crystallization kinetics. PET

nanofiber was manufactured through a scalable melt spinning technology, and around 250 nm

PET fiber was obtained. Isothermal and non-isothermal DSC experiments were performed. With

the presence of PET nanofiber, dramatic improvement of PP’s crystallization kinetics was

observed. Both Avrami and Mo’s method indicated the crystallization complexity and

crystallization rate were increased. Reprocessing of fibrillated material into pellet showed

moderate setback effect in PET’s promotional effect in PP crystallization. Also, the mechanical

property was measured to show the influence of changed crystal structure. Increase in both the

modulus and strength of the composites was observed.

iii

Acknowledgments

I would like to express my deepest appreciation to my supervisor and mentor, professor

Chul B. Park, who provided me with essential guidance throughout my two years master study.

His passionate teaching made me interested and even excited about the polymer science and

technology. This work would have not been possible without his continuous help, guidance and

supervision. Through the two years working with him, I was truly impressed by his work ethic,

enthusiasm, integrity. He is not only a supervisor to me, but also a model I want to imitate in my

future study and career.

I want to thank professor Hani E. Naguib for using DSC equipment in Smart Polymers &

Composite Lab(SAPL). I would also like to express my thanks to all the lab members in

Microcellular Plastics Manufacturing Laboratory (MPML) during year 2016 to 2018. Thank you

for the support and help along the way.

iv

Table of Content

Acknowledgments...................................................................................................................... iii

Table of Content ........................................................................................................................ iv

List of Figures ............................................................................................................................ vi

List of Tables ........................................................................................................................... viii

Introduction ............................................................................................................... 1

Preamble ........................................................................................................................... 1

Polymer crystallization ..................................................................................................... 2

1.2.1 Crystal nucleation ......................................................................................................... 2

1.2.2 Growth .......................................................................................................................... 6

1.2.3 Degree of crystallinity .................................................................................................. 7

Crystallization of polypropylene ...................................................................................... 7

Research objectives .......................................................................................................... 9

Theoretical Background and Literature Review ..................................................... 10

Crystallization of polypropylene with inorganic filler ................................................... 10

2.1.1 Crystallization of polypropylene with particulate filler ............................................. 10

2.1.2 Crystallization with fibrillated filler ........................................................................... 12

In-situ fibrillation of immiscible polymer composites ................................................... 13

Material and Experimental ...................................................................................... 16

Material .......................................................................................................................... 16

Experimental .................................................................................................................. 16

3.2.1 Mixing of polypropylene and polyethylene terephthalate .......................................... 16

3.2.2 In-situ fibrillation of PET nanofibril (adding processing cond) ................................. 17

3.2.3 Reprocessing the fiber into pellet (adding processing cond) ...................................... 17

Characterization ............................................................................................................. 18

v

3.3.1 SEM ............................................................................................................................ 18

3.3.2 DSC ............................................................................................................................ 18

3.3.3 Mechanical property ................................................................................................... 18

Results and discussion ............................................................................................ 19

Morphology characterization ......................................................................................... 19

4.1.1 Morphology of PP and amorphous PET (APET) ....................................................... 19

4.1.2 Morphology of PP and crystallisable PET (CPET) .................................................... 22

Crystallization kinetics ................................................................................................... 27

4.2.1 Isothermal DSC experiments ...................................................................................... 27

4.2.2 Non-isothermal crystallization ................................................................................... 34

Mechanical property characterization ............................................................................ 45

Conclusion .............................................................................................................. 48

Reference ................................................................................................................ 50

vi

List of Figures

Figure 1 Schematic representation of energy barrier with respect to radius r ................................ 4

Figure 2 Graphical illustration of rectangular chain folded nucleus............................................... 4

Figure 3 Schematic drawing of PP spherulitic crystal [29] ............................................................ 8

Figure 4 Spherulite morphology of PP crystals[30] ........................................................................ 8

Figure 5 SEM morphology of polymer crystal under strong shear flow [40] ................................. 9

Figure 6 Schematic drawing of shish kebab crystal structure [34] ................................................. 9

Figure 7 Tensile modulus vs. filler loading[45] ............................................................................ 11

Figure 8 Exothermic crystallization peaks for different filler loaded PP[45] ............................... 11

Figure 9 DSC heat flow results of pure PP & CNF reinforce PP [46].......................................... 13

Figure 10 Avrami plot of pure PP&CNF reinforced PP[46] ........................................................ 13

Figure 11 PET fiber morphology with stretch ratio 16.1:1[49] .................................................... 14

Figure 12 polarized microscopy photo of crystal structure of a) neat PP b) PP/PET microfiber

composite[49] ................................................................................................................................ 15

Figure 13 a) Non isothermal DSC heat flow trace of PP/ PET composites with different PET

content; b) Relative crystallinity of PP/PET composite vs time with different PET content[52] 15

Figure 14 Twin screw extruder used for PP PET compounding .................................................. 16

Figure 15 Schematics of melt spinning system............................................................................. 17

Figure 16 Compounded PP/APET cryogenically fractured cross section area morphology ........ 20

Figure 17 Spun APET nanofiber morphology with different PET loading .................................. 21

Figure 18 Reprocessed PP/ APET (95/5) nanofiber composite APET fiber morphology ............ 21

Figure 19 Amorphous PET fiber diameter measurements ............................................................ 22

Figure 20 Compounded PP/CPET cryogenically fractured cross section area morphology ........ 24

Figure 21 Spun CPET nanofiber morphology with different PET loading .................................. 25

Figure 22 Reprocessed PP/ CPET (95/5) nanofiber composite CPET fiber morphology ............ 26

Figure 23 DSC isothermal crystallization at 135 ⁰C heat a) flow curve of neat PP and PP with 5

wt% APET composite; b) relative crystallinity vs time; c) Avrami plot ...................................... 29

Figure 24 DSC isothermal crystallization at 130 ⁰C heat a) flow curve of neat PP and PP with 5

wt% APET composite; b) relative crystallinity vs time; c) Avrami plot ...................................... 31

vii

Figure 25 DSC isothermal crystallization at 130 ⁰C heat a) flow curve of neat PP, PP with 5 wt%

CPET and PP with 15 wt% CPET composites; b) relative crystallinity vs time; c) Avrami plot 33

Figure 26 Reprocessed PP/PET composite heat flow results at different cooling rates ............... 34

Figure 27 Non-isothermal DSC heat flow vs temperature at cooling rate of 10 ⁰C/min .............. 35

Figure 28 neat PP non-isothermal crystallization a) heat flow vs temperature; b) relative

crystallinity vs time; c) relative crystallinity vs temperature; d) Avrami plot .............................. 37

Figure 29 Two-stage fitted Avrami plot for neat PP..................................................................... 38

Figure 30 Ozawa plot ln(-ln(1-Xt)) vs. lnФ for neat PP ............................................................... 40

Figure 31 Neat PP ln Ф vs. ln t plot at relative crystallinity of 20, 40, 60, and 80% ................... 41

Figure 32 Crystallization halftimes for different PP/ PET composites at 10⁰C/min cooling rate 45

Figure 33 Stress strain curve of PP/APET fibrillated and reprocessed composite ....................... 46

Figure 34 PP/ PET composite tensile modulus vs. PET loading level ......................................... 46

Figure 35 PP/ PET composite tensile strength vs. PET loading level .......................................... 46

viii

List of Tables

Table 1 Diameters of APET droplets, fibers, and reprocessed fibers with varying PET content . 22

Table 2 Diameters of APET droplets, fibers, and reprocessed fibers with varying PET content . 26

Table 3 Summary of Avrami parameters and crystallization halftime of PP/APET nanofiber

composite crystallized at 135 ⁰C ................................................................................................... 29

Table 4 Summary of Avrami parameters and crystallization halftime of PP/APET nanofiber

composite crystallized at 130 ⁰C ................................................................................................... 31

Table 5 Summary of Avrami parameters and crystallization halftime of PP/CPET nanofiber

composite crystallized at 130 ⁰C ................................................................................................... 33

Table 6 Summary of Avrami parameters and crystallization halftime of PP/APET nanofiber

composite in non-isothermal crystallization ................................................................................. 38

Table 7 Summary of Mo's parameters of PP/APET nanofiber composite in non-isothermal

crystallization ................................................................................................................................ 42

Table 8 Summary of Avrami parameters and crystallization halftime of PP/CPET nanofiber

composite in non-isothermal crystallization ................................................................................. 43

Table 9 Summary of Mo's parameters of PP/APET nanofiber composite in non-isothermal

crystallization ................................................................................................................................ 44

1

Introduction

Preamble

Polymeric materials are materials composed of long chain molecules with high molecular

weight[1]. Historical records show that natural polymer like rubber first existed in human

activity around 1500’s. Archeologist found balls made from natural rubber and being used in ball

games in Mesoamerican culture [2]. In 1839, Charles Goodyear developed volcanized rubber by

adding sulfur to natural rubber and processing it with heat [3]. This practice greatly improved

rubbers durability, and the technique is still applied in automobile tire manufacturing today. In

early 20th century, success in polymerization theory made synthetic polymer possible [4]. Well

known polymers such as polystyrene, nylon, and polyethylene were developed one after another

over the first half page of 20th century. Since then, an extensive amount of research has been

conducted upon polymeric material, and polymer has gained increasing popularity in our day to

day application.

Polymers are normally superior to metal and ceramic in terms of weight, cost, and processability.

It is also better than metal in corrosion resistivity [5], and better than ceramic in shape forming

ability and ductility [6], [7]. On the other hand, polymers do have drawbacks, which include low

heat deflection temperature [8], low strength and stiffness, and low melting temperatures [9].

Therefore, reinforcement of polymeric material has been a popular topic among researchers and

industries over the past several decades. Polymer blending and polymer composites are most

widely used processing techniques in polymer reinforcement [10]–[12]. People typically blend

two or more polymers or polymer and other inorganic or organic fillers together, and the blends

or composite will have superior properties of all the constituents [13]. Glass fiber or carbon fiber

are two common examples of fillers used in modern reinforcing process. As technology

advances, nanoparticles and nanofibril composite became popular candidates in polymer

reinforcement [14], [15]. Dramatic improvements in mechanical property have been achieved.

Polymer property tuning through altering its own structuring is another branch of polymer

reinforcing study. Cross linking and branching are the most common ways to chemically modify

polymer chain structure [16], [17]. They can improve polymer melt strength and induce strain

hardening activity [18]. The other way of changing polymer molecular structure is changing its

2

crystallization behavior [19]. This also involves adding fillers, though in much smaller scale

comparing to composite and blends. In this case, fillers act as polymer crystal nucleating agent to

induce crystallization. Through this process, eventual polymer product can have either much

higher crystallinity or much higher number of nuclei. Either result can improve its final property.

In this work, a thorough crystallization study of an innovatively made polypropylene and

polyethylene terephthalate nanofibril composite was conducted. The scope of this project covers

sample preparation, characterization, analysis, and performance test. Detailed research plan will

be discussed in the following sub chapters.

Polymer crystallization

Crystallization is a crucial area in studying polymeric material. It, to a large extent, determines

the mechanical, thermal, chemical, and optical property of the material [20]. Traditionally,

crystallization refers to formation of a new thermal dynamic phase within a homogeneous

system, such as vapor condensation and solidification from melt. Crystallization of polymer

refers to the formation of closely pact polymer chains during the cooling process from polymer

melt to solid [21]. Crystallisable polymer is normally referred to as semi crystalline polymer. The

degree of crystallinity is measured by crystallinity. The crystallinity of typical semi crystalline

polymer ranges from 10 to 80 percent. In terms of determining material property, not only

crystallinity, but also crystal size and orientation play important roles [21]. Therefore, in the

following sections, details about crystal nucleation, growth, crystallinity and crystallization

kinetics will be discussed.

Crystal nucleation

Crystal nucleation starts with parallel arrangement of polymer chains or their segments.

Depending on its energy state preference, the parallel chains may either dissociate or grow

further. For the structure to grow, the grain size needs to exceed a critical radius. That is the

basic initiation mechanism of crystal nucleation. [21] Nucleation can be further divided into

homogeneous nucleation and heterogeneous nucleation. Both will be discussed in the following

sections.

3

1.2.1.1 Homogeneous nucleation

Homogeneous nucleation is a rare form of nucleation. It happens much less often than

heterogeneous nucleation, though its mechanism is simpler. Gladys Ronca [22] did a very

thorough review of both homogeneous nucleation and heterogeneous nucleation in his PhD

thesis. According to Ronca’s description in his thesis, homogeneous nucleation involves

“spontaneous aggregation of polymer chains” [22]. This aggregation starts reversibly, and forms

an embryo, until it reaches a critical size; further chain addition to the embryo will result in an

irreversible growth [22].

The probability of the formation of embryo is proportional to the Gibb’s free energy of its

formation. For a spherical embryo case, the standard Gibb’s free energy required to form an

spherical embryo can be expressed using the following equation [22]

∆𝐺ℎ𝑜𝑚 = −4

3𝜋𝑟3∆𝐺𝑣 + 4𝜋𝑟2𝜎 (1)

∆𝐺𝑣 is the difference between standard Gibb’s free energy per unit volume of the matrix phase

and the nucleated phase. 𝜎 is the surface free energy of the interface between nucleated phase

and melt phase. The growth of the nucleus is considered sustainable when the Gibb’s free energy

is negative, which agrees with the fact that the phase transformation taking place to reduce the

energy state of the entire system [22]. The graphical representation of this expression is in Figure

1 below. By differentiating equation (1) with respect to radius 𝑟, the following equation is

obtained.

𝑑

𝑑𝑟∆𝐺ℎ𝑜𝑚 = −4𝜋𝑟2∆𝐺𝑣 + 8𝜋𝑟𝜎 (2)

Setting equation (2) equal to zero, the critical radius 𝑟∗

𝑟∗ =2𝜎

∆𝐺𝑣 (3)

Substituting equation (3) in into equation (1) gives the critical Gibb’s free energy for

homogeneous nucleation as follow

∆𝐺ℎ𝑜𝑚∗ =

16𝜋

3∙

𝜎3

(∆𝐺𝑣)2 (4)

4

Figure 1 Schematic representation of energy barrier with respect to radius r

To account for the asymmetric polymer crystal nucleus geometry, Hoffman and Lauritzen [23]

and Price [24] developed a rectangular model to describe the crystal nucleus where polymer

chains fold on itself and chain segments are connected through short loops. The schematic is

illustrated in Figure 2 below.

Figure 2 Graphical illustration of rectangular chain folded nucleus

In this case, the Gibb’s free energy for nucleation is expressed as follow

∆𝐺ℎ𝑜𝑚 = −𝑎𝑏𝑙∆𝐺𝑣 + 2𝑎𝑙𝜎 + 2𝑏𝑙𝜎 + 2𝑎𝑏𝜎𝑒 (5)

Similar to the spherical model, ∆𝐺𝑣 represents the bulk free energy difference per unit volume

between matrix phase and nucleated phase. 𝜎 is the surface free energy per unit area for AC and

BC surfaces, and 𝜎𝑒 is the surface free energy per unit area for AB surface. By equating the first

5

derivatives with respect to 𝑎, 𝑏, and 𝑙 to zero, critical dimensions 𝑎∗, 𝑏∗, and 𝑙∗can be calculated

as follow

𝑎∗ = 𝑏∗ =4𝜎

∆𝐺𝑣 (6)

𝑙∗ =4𝜎𝑒∆𝐺𝑣

(7)

Plug in the critical dimensions from equation (6) and (7) into equation (5). The free energy for

nucleation can then be calculated as follow

∆𝐺ℎ𝑜𝑚∗ =

32𝜎2𝜎𝑒(∆𝐺𝑣)2

(8)

Given above information, the rate of nucleation can then be derived. Turnbull and

Fisher[25] developed an expression for rate of homogeneous nucleation based on Boltzman’s

law. The expression is as follow

𝐼ℎ𝑜𝑚 = 𝐼0exp[−𝐸𝐷 + ∆𝐺ℎ𝑜𝑚

∗

𝑘𝑇] (9)

𝐼0 =𝑁𝑘𝑇

ℎ (10)

𝐸𝐷 is the free energy of activation determining diffusion within short range along phase

boundary. ∆𝐺ℎ𝑜𝑚∗ is the critical energy for homogeneous crystal nucleation. 𝑘 is Boltzmann’s

constant. 𝑇 is temperature, and h is Plank’s constant.

1.2.1.2 Heterogeneous nucleation

Crystal nucleation is also strongly affected by additives or impurities. Foreign surfaces from

these additives can facilitate crystal nucleation by reducing the required free energy. A very

similar model as proposed by Price [24] can be applied here as well. Instead of the having all

surfaces of the chain folded cube in contact with melt surface, one of the surfaces is in contact

with the foreign particle surface. Following the homogeneous discussion case, the model shown

in figure 2 can be used again. A different free energy calculation method needs to be adopted for

the nucleus surfaces which had been put into contact with the foreign particle. The expression of

the free energy for such a nucleus to form can be written as below

∆𝐺ℎ𝑒𝑡 = −𝑎𝑏𝑙 ∙ ∆𝐺𝑣 + 2𝑎𝑏𝜎𝑒 + 2𝑏𝑙𝜎 + 𝑎𝑙∆𝜎 (11)

6

For this heterogeneous case, ∆𝜎 is defined as the difference of interfacial free energy between

one surface contacting the melt and one surface contacting the foreign particle. The value is

calculated through the following equation

∆𝜎 = 𝜎 + (𝜎𝑠𝑐 − 𝜎𝑠𝑚) (12)

Where 𝜎𝑠𝑐 and 𝜎𝑠𝑚 represents the specific interfacial energy between the foreign particle surface

and crystal nucleus surface and specific interfacial energy between the foreign particle surface

and polymer melt surface. 𝜎 and 𝜎𝑒 also defined as crystal nucleus side and end surface free

energy. The other quantities are defined the same as in equation (5). By comparing equations (5)

and (11), we can find that the total free energy required for crystal formation is decreased by

replacing the term 2𝜎 by ∆𝜎. As long as ∆𝜎 is less than 2𝜎, the additives will have a positive

effect on nucleation. Plugging in the critical dimensions into equation (11), the critical energy

needed for heterogeneous nucleation can then be derived as follow.

∆𝐺ℎ𝑒𝑡∗ =

16𝜎𝜎𝑒∆𝜎

(∆𝐺𝑣)2 (13)

Further improvement in crystallization can be achieved by increase the irregularities of the

additive surfaces. Research results have shown that nucleating agent with rough surface has

improved effectiveness on crystal nucleation[26]. However, this is beyond the scope of this

thesis.

Using similar argument as of homogeneous nucleation case, the nucleation rate for

heterogeneous nucleation can be derived as follow

𝐼ℎ𝑒𝑡 = 𝐼0exp[−𝐸𝐷 + ∆𝐺ℎ𝑒𝑡

∗

𝑘𝑇] (14)

All the quantities in equation (14) are defined the same as equation (9), and the critical

nucleation energy for homogeneous nucleation is replaced by heterogeneous nucleation.

Growth

Following nucleation is another vital process in crystallization, crystal growth. As the

dimensions of crystal nucleus exceed the critical dimensions, the nucleation step is considered

complete. Further addition of polymer chain to the crystal nuclei makes the crystal grow in size.

7

Crystal growth typically happens at a temperature below the melting temperature and above the

glass transition temperature. From a polymer physics point of view, too high temperature frees

the polymer from aligning against each other, while too low temperature freezes the chain

motion.

Degree of crystallinity

Semi-crystalline polymer has both amorphous region and crystal region. Degree of crystallinity

is introduced to quantify the composition of crystalline phase. Typical semi-crystalline polymer

has a degree of crystallinity between 10 to 80 percent[27]. To a large extent, crystallinity

determines the mechanical and thermal property of the material[20]. The same polymers with

higher crystallinity exhibit better mechanical properties, such as higher Young’s modulus and

higher yield strength. Also, crystals can even exist at temperatures higher than the melting point

of the material [28]. Throughout this thesis, crystal and crystallinity and their influences on

material property will be an important parameter being discussed. It is also an important

motivation of this study.

Crystallization of polypropylene

PP is the major resin used in this study. Therefore, it is beneficial to understand the basics of its

crystallization mechanism. In general, neat PP have a low crystallization rate. Under quiescent

and free crystallization, PP crystals exhibit a spherulitic morphology. Researchers have

categorized these spherulites into four different categories based on the birefringence reading and

preferred growing temperature[29]. The actual morphology and schematic drawing of the crystal

are shown in Figure 4 and 4. As figure 4 shows, the close packed lamella structures are crystals

and they grow in the direction where temperature drops. The lose polymer chain ends and

randomly arranged chains are amorphous region.

8

Figure 3 Schematic drawing of PP spherulitic crystal [29]

Figure 4 Spherulite morphology of PP crystals[30]

Using wide angle X-ray diffraction (WAXD) technology, crystals can be further categorized into

α, β, and γ crystals based on the diffraction pattern of different surfaces [31]. For a typical free

crystallized PP sample, α phase shows three peaks under WAXD which corresponds to (110),

(130), and (040) planes; while β phase shows one peak due to (300) plane. A γ form was also

observed, but it can only be obtained in bulk under high pressure condition [32]. The mechanical

property of α phase crystal are stable, high modulus and high strength. The mechanical property

of β phase crystal are excellent impact strength and improved elongation at break [33]. Most

crystals generated under quiescent condition are α type crystals. In order to generate β type

crystals, β-nucleating agent or shear during processing are required.

Under certain flow or stretched condition, PP crystals can exhibit a different morphology.

According to Keller and Kolnaar [34], a shish kebab crystal structure can be generated under

strong flow condition. Figure 5 and 6 are SEM photo of polymer crystallized under strong flow

condition and schematic drawing of shish kebab crystal structure. In terms of mechanical

property, shish kebab structure acts as a self-reinforced structure to the polymer, which can

simultaneously improve the impact strength and Yong’s modulus [35], [36]. Injection molding is

a desirable process to obtain this structure, however, the shish kebab structure generated through

normal injection molding is very limited. Modified injection molding such as shear controlled

orientation in injection molding [37], oscillatory shear injection molding[38], and vibrate

injection molding[39].

9

Figure 5 SEM morphology of polymer crystal under strong shear flow [40]

Figure 6 Schematic drawing of shish kebab crystal structure [34]

Research objectives

This project aims at studying the crystallization behavior of PP and in-situ fibrillated PET

nanofiber composite. Specifically, three goals are targeted. First is to obtain PP with finely

dispersed and in-situ fibrillated PET nanofiber structure. Second is to study the crystallization

behavior of PP/PET nanofiber composite through varying nanofiber content. Lastly, the final

mechanical property of the material will be examined and the correlation between property and

structure will be investigated.

10

Theoretical Background and Literature Review

Tuning material property and crystallization behavior through adding filler or making in-situ

fibrillated composite is not new concept. In this section, works relevant to this field, which have

done by previous researchers are summarized and some representative results will be presented.

Also, some characterization methods and analysis approach will be in this section.

Crystallization of polypropylene with inorganic filler

In the entire filler market, inorganic fillers have a dominating position. Studies on organic fillers

just emerged in recent years, and it is mainly for weight and environment concerns[41]. Among

all the inorganic fillers, calcium carbonate(CaCO3), talc, and glass fiber are the most popular

ones [42]. Depending on the filler type and material application, different fillers are featured with

different reinforcing effects. However, in general, these fillers are added to improve the

mechanical strength [15], crystallization kinetics [43], and heat deflection properties[44]. In the

following sections, works related to different types of fillers will be summarized and results will

be presented.

Crystallization of polypropylene with particulate filler

Particulate fillers, namely, are fillers of particle or powder forms, such as CaCO3, Si3N4, and talc

etc. Talc filled polypropylene is one of the most commonly used polymer composites in the

world, and numerous studies regarding to the composited properties can be found.

Leong et al. [45] did a thorough study on the effects of mechanical property and crystallization

behavior on talc, kaolin, and CaCO3. All the fillers were compounded with PP using twin screw

extruder. The author then did thermal analysis and mechanical property characterization on the

samples. Representative results were shown below in Figure 7 and Figure 8.

11

Figure 7 Tensile modulus vs. filler loading[45]

Figure 8 Exothermic crystallization peaks for different filler loaded PP[45]

As shown in Figure 8 Exothermic crystallization peaks for different filler loaded PP[45], both

onset temperature and crystallization temperature of PP increased with addition of nanoparticle

fillers. The abbreviations K30, UCC30, and T30 represents kaolin, untreated CaCO3, and talc

respectively. Though, the rate of nucleation seems unaffected given the peak width. These results

correspond to the heterogeneous nucleation effect of these foreign particles. Figure 7 show the

effect of different filler on tensile modulus of filler loaded PP at different filler content. Almost

linear increase of modulus can be observed with increasing amount of filler loading. Leong

12

attributes this to two main causes. Firstly, PP is substituted by filler with much stronger tensile

modulus. Secondly, the widely spread fillers constrained the molecule motion of PP. In addition,

for the case of talc-filled PP, the author also believes that strong nucleation effect can change the

crystal form from β spherulites into α spherulites, which are more stable and stiffer [45]. Beside

tensile modulus, other mechanical properties are also affected by the fillers. For tensile strength,

talc-filled PP exhibits an increase trend up to a talc volume fraction of 0.15, and then start to

drop. Kaolin has negligible influence on the tensile strength, while CaCO3 have negative

influence on tensile strength. The flexural property showed a similar trend to tensile strength, but

the turning point is much earlier. For all the fillers, the flexural strength increased up to 0.04

volume fraction and start to drop beyond that point[45]. As for impact strength, talc and kaolin

filled PP composites showed a drastic drop after 0.06 volume fraction of filler loading, while

CaCO3 had barely no impact at all.

Crystallization with fibrillated filler

Fiber is another major category of reinforcement filler for PP. Typical fibers in this category are

short or long glass fiber and carbon fiber etc. Similar to particulate fillers, fibrillated filler can

also improve the mechanical property. Sandler et al. [46] studied crystallization behavior and

mechanical properties of carbon nanofiber (CNF) reinforced PP. The differential scanning

calorimetry (DSC) heat flow results during cooling stage and Avrami plot were shown below in

Figure 9 and 10. From the narrowed and heightened crystallization peak after addition of CNF,

Sandler concluded that CNF effectively improved crystallization kinetics of PP. Since the final

crystallinity was not affected compared to pure PP case, Sandler suggested that CNF only acted

as heterogeneous nucleation sites but had no influence on crystal structure. Lozano and Barrera

[47] reported similar findings in their work.

13

Figure 9 DSC heat flow results of pure PP & CNF

reinforce PP [46]

Figure 10 Avrami plot of pure PP&CNF reinforced PP[46]

In-situ fibrillation of immiscible polymer composites

In-situ fibrillation through immiscible polymer blends is a novel technology to improve the

crystallization behavior and mechanical properties. The process is to blend two immiscible

polymers together, and use certain approaches such as melt spinning or cold drawing to obtain a

desired morphology of one of the constituent phase. Compared to previously mentioned two

categories of fillers, particulate filler and fiber filler, in-situ fibrillation has numerous advantages

including cost effectiveness, processing steps, and weight to property ratio. Furthermore,

considering the nature of the material, in situ fibrillated composites usually easy to recycle and

creates less environmental concerns.

Li et al. [48]–[52] was one of the pioneer researchers in this field. He explored micro-fibrillated

composites with material compositions such as PET/PE, PET/iPP, PC/HDPE, and PET/HDPE

etc. In all these cases, PET and PC were chosen as second phase reinforcement material, PE, iPP,

and HDPE serve as matrix. Though, in Li’s study, fibers are in a micro scale with diameters

ranging from 0.6 to 2 µm. Rizvi et al. [53], [54] used PTFE and PET as fillers in PP and also

obtained in-situ fibrillated nanofiber composite with PP as matrix. He adopted an improved

stretching approach and obtained fiber with diameter below 200 nm. They all successfully

modified mechanical, rheological, or viscoelastic behavior of the matrix polymers.

In a series of Li’s study[48], [49], [52], the promotional effect of PET microfiber on PP

crystallization were proved. First, PP and PET, with varying weight percentage were

compounded through a single screw extruder. The extrudate was then passed through a slit die

14

and hot stretched with stretching ratios of 4 and 16.1. PET fibers with 2 µm and 0.6 to 0.9 µm in

diameter were obtained with stretching ratio of 4, 7.9 and 16.1. The morphology of PET

microfiber after PP being selectively removed with solvent is shown below in Figure 11. As the

figure shows, clearly defined fibrillated PET domain were obtained. The crystallization

behaviors of both stretched and neat PP sample were then characterized under polarized

microscopy. Figure 12 shows the polarized optical microscopy (POM) photos of neat PP and

PP/PET microfiber composite. Crystallization procedure was performed at 130 ⁰C. Figure 12 a)

shows neat PP crystals, and figure 12b) shows PP crystals with the presence of PET fibers.

Within observed region, perfectly grew large α type spherulites are observed in figure a).

Contrarily, figure 12 b) shows a majority of trans crystalline structure. Li argues that crystal

growth is constrained due to PET fibers being too close to each other. This argument agrees with

Moon’s finding [55]. The crystallization kinetics was characterized using DSC. The heat flow

trace and calculated relative crystallinity curve are shown in Figure 13a) and b). Similar to the

results of particulate and fibrillated samples, in situ fibrillated PP composites also demonstrated

improved crystallization kinetics. As the result shows, a big jump in crystallization temperature

was detected in low filler loading level (5% PET content). With further addition of PET, the

change in crystallization temperature is negligible. However, Figure 13b) shows a continuous

decrease of crystallization time with PET loading level. This is another evidence of promoted

crystallization process. This effect seems also plateau at loading level beyond 15%.

Figure 11 PET fiber morphology with stretch ratio 16.1:1[49]

15

Figure 12 polarized microscopy photo of crystal structure of a) neat PP b) PP/PET microfiber composite[49]

Figure 13 a) Non isothermal DSC heat flow trace of PP/ PET composites with different PET content; b) Relative crystallinity of

PP/PET composite vs time with different PET content[52]

a b

16

Material and Experimental

Material

The matrix polymer used in this experiment was PP homo-polymer. PP was donated by

Braskem, commercially available as PP H521. It has a melting temperature of 160 ⁰C and a melt

flow index (MFI) of 3.6 g/10 min at 230 ⁰C/2.16 kg. Two kinds of PET were used as fillers. The

first one was bottle grade amorphous PET with trade name SKYPET® BR8040. This was

supplied by SK chemical. It has a melting point of 236 ⁰C. The second PET used was

crystallisable PET, and it is supplied by Eastman Chemical Company. This product

commercially available as EastmamTM Polyester F61HC. It has a melting point of 255 ⁰C.

Experimental

Mixing of polypropylene and polyethylene terephthalate

To begin with, PP and PET was compounded using a co-rotating twin-screw extruder. Before

compounding, the materials were dried in vacuum oven at 70 ⁰C for more than 3 hours. The

extruder used for compounding was Leistriz ZSE 27 MAXX. It has a screw outer diameter of

28.3 mm, and a length of 1100 mm. The ratio of screw outer diameter to inner diameter is 1.66.

To achieve good dispersion, a mixing temperature above both polymers need to be applied.

Therefore,10 heating zones along the screw were set to 90, 130, 150, 190, 250, 250, 270, 270,

270, 270, and 255 ⁰C. The screw speed was set to 100 rpm. A photo of the extruder is attached in

Figure 14 below. Two feeders were used to simultaneously feed PP and PET. PP and PET were

compounded at weight ratios of 99/1, 97/3, 95/5, 93/7, and 85/15. The extrudate was then passed

through a cold-water bath at room temperature, and granulated into pellets.

Figure 14 Twin screw extruder used for PP PET compounding

17

In-situ fibrillation of PET nanofibril (adding processing cond)

The next step was in-situ fibrillation of compounded PP/PET blends. This was done by a melt

spinning machine. The machine is composed of a single screw extruder and a spinning unit. PP/

PET blends were first passed through the single screw extruder and get melted. At the end of the

screw, the melt will pass through numerous capillary dies and form thin strand macro fibers. The

temperature profile of the heating sections from hopper to spinneret die is 240, 250, 260, 260,

270, and 270 ⁰C. The die has a diameter of 600 micrometre. Below the die, a rotating drum will

further stretch the fiber at a drawing speed of 1000 m/min. A schematic drawing of the melt

spinning system is shown below in Figure 15.

Figure 15 Schematics of melt spinning system

Reprocessing the fiber into pellet (adding processing cond)

From an application perspective, in situ fibrillated PP/ PET composite needs to be reprocessed

into pellet. This step was done using a counter-rotating twin screw extruder. To avoid disruption

of PET fiber morphology, the processing temperature was chosen to be below the melting point

of PET and above the melting point of PP. Also, short screw was chosen to minimize residence

time and shear.

18

Characterization

SEM

SEM is used to characterize blends and fiber morphology. The model used was JEOL 6060. For

PP/ PET blends, blended samples were cryogenically fractured to expose the cross section. For

in-situ fibrillated composites, PP was removed by dissolving the composite in boiling xylene for

1 hours. The remaining fiber was then sputter coated and characterized under SEM.

DSC

The crystallization kinetics was characterized using DSC. The equipment used is TA Instruments

Q2000 DSC. Nitrogen atmosphere was applied to minimize material degradation. Both

isothermal DSC and non-isothermal DSC were performed. For both isothermal DSC and non-

isothermal DSC, the sample was first heat to 200 ⁰C at 10 ⁰C /min and kept for 5 min to remove

any thermal history. For isothermal DSC, the sample was rapidly cooled to 130 ⁰C and 135 ⁰C to

start isothermal crystallization. Then, the sample was kept at crystallization temperature for 40

min to allow maximum crystallization. For non-isothermal DSC, the sample was cooled from

200 ⁰C to ambient temperature at different cooling rates of 5, 10, 15, and 20 ⁰C/min.

Mechanical property

The tensile modulus and tensile strength of the composites were measured. The samples with

different material composition will be compression molded into dog-bone shape for mechanical

testing. The testing equipment used was Instron Universal Tester, model 3367. The testing

method followed was ASTM-D 638 [56].

19

Results and discussion

Morphology characterization

4.1.1 Morphology of PP and amorphous PET (APET)

Figure 16 below shows the phase morphology of compounded PP/ APET blends. As the figure

shows, APET, as the minor phase, was deformed under shear into discrete spherical domains and

evenly dispersed in PP. This morphology resembles the island-in-the-sea[57] morphology

proposed by previous Japanese scholar. Clearly defined APET droplet domain can be observed.

This was resulted from poor interfacial interaction between the two polymers. At the

cryogenically fractured surface, spherical dents were observed. These were believed to be

remaining marks of APET droplets. After in situ fibrillation. The samples were dissolved in

xylene to completely remove PP, and APET nanofiber were collected for morphology

characterization. The SEM photos was shown in Figure 17. As Figure 17 showed, well defined

fiber domain was observed. A fiber diameter measurement was conducted using Image J

software. The statistics of the fiber diameter is shown in Figure 18. A slight increasing trend can

be observed as APET loading increased. This phenomenon was expected, because of APET

coalescence during either blending or fiber spinning steps at higher loading level. Figure 18

showed the APET fiber morphology after being reprocessed into pellet. The processing

temperature was 175 ⁰C. Also, the fiber was obtained after PP being removed using xylene

solvent. After reprocessing, fiber entanglement and sintering occurred. This was due to the high

shear force applied to the material during reprocessing. Though, the processing temperature was

chosen below the melting point of APET, the shear force seemed being transferred onto PET

nanofiber through PP matrix. Additionally, the 175 ⁰C is well above the glass transition

temperature of PET, which make the deformation of PET nanofiber unavoidable. Table 1

summarizes APET droplets domain, spun nanofiber dimensions, and reprocessed fiber

dimensions.

20

Figure 16 Compounded PP/APET cryogenically fractured cross section area morphology

21

Figure 17 Spun APET nanofiber morphology with different PET loading

Figure 18 Reprocessed PP/ APET (95/5) nanofiber composite APET fiber morphology

22

Figure 19 Amorphous PET diameter measurements

Table 1 Diameters of APET droplets, fibers, and reprocessed fibers with varying PET content

APET content

(wt%)

APET droplet

diameter (µm)

APET nanofiber

diameter (nm)

Reprocessed

APET nanofiber

diameter (nm)

1 2.3 208 231

3 2.5 242 267

5 2.8 275 295

7 3.1 257 297

15 3.2 295 330

4.1.2 Morphology of PP and crystallisable PET (CPET)

Figure 20 shows the phase morphology of compounded PP/ CPET blends. Like the APET case,

CPET was also deformed into spherical domain and evenly dispersed into PP matrix. Figure 21

shows the CPET nanofiber morphology after xylene etching. Comparing to APET nanofiber

23

morphology, CPET exhibited a much more aligned orientation. This is probably due to CPET’s

higher crystalline structure, which helped the nanofiber to maintain its shape during xylene

etching. Figure 22 shows a typical CPET fiber morphology of the reprocessed PP/ CPET

nanofiber composite. Similar results as APET nanofiber was obtained. However, less sintering

and agglomeration was found in CPET case. This was attributed to its higher melting

temperature and its higher crystalline structure compare to APET. Table 2 summarizes the

dimension information of CPET phase in blends, spun fiber, and reprocessed fiber stage.

Comparing to APET fiber case, CPET had a much interesting diameter distribution. For the

blends, CPET droplets diameters are almost the same as APET. For CPET fibers, however, much

larger diameter was observed compare to APET fiber. Given the fact that the same stretching

ratio was applied in both cases, the author believes that CPET’s larger diameter was caused by

fast cooling and solidification during drawing process. Again, this is due to CPET’s higher

crystallinity. During stretching, CPET quickly crystallized and further stretching can no longer

decrease its diameter. This can also explain the much less diameter change for reprocessed

CPET. Higher crystallinity helped the PET to maintain its spun morphology, so that only very

limited swelling was obtained.

24

PP/PET 99/1 PP/PET 97/3

PP/PET 95/5 PP/PET 93/7

PP/PET 85/15

Figure 20 Compounded PP/CPET cryogenically fractured cross section area morphology

25

PP/PET 99/1 PP/PET 97/3

PP/PET 95/5 PP/PET 93/7

PP/PET 85/15

Figure 21 Spun CPET nanofiber morphology with different PET loading

26

Figure 22 Reprocessed PP/ CPET (95/5) nanofiber composite CPET fiber morphology

Table 2 Diameters of APET droplets, fibers, and reprocessed fibers with varying PET content

CPET content

(wt%)

CPET droplet

diameter (µm)

CPET nanofiber

diameter (nm)

Reprocessed

CPET nanofiber

diameter (nm)

1 % 2.2 223 230

3 % 2.7 267 271

5 % 2.8 284 295

7 % 3.5 291 310

15% 3.7 335 348

27

Crystallization kinetics

4.2.1 Isothermal DSC experiments

PP/APET

Figure 23 a) shows the heat flow trace for isothermal crystallization at 135 ⁰C of both neat PP

and PP/APET nanofiber composite. The PET loading in this case was 5 wt%. The red curve

represents the material after fibrillation, and the green curve represents the material after

reprocessing. By integrating the entire crystallization peak with respect to time, the relative

crystallinity with respect to time can be calculated using equation 15 below. 𝑡0 is the moment

when crystallization starts and 𝑡∞ is the time crystallization ends. Following this calculation, the

relative crystallinity versus time curve can be obtained as Figure 23b) below.

From this figure, a significant decrease in crystallization time of PP is observed after the addition

of PET. That is to say, the crystallization kinetics is greatly improved. To further elaborate the

effect of PET nanofiber in crystallization, Avrami equation [58] is adopted to quantitatively

examine the results. As Avrami’s proposed model of phase change in crystallization, the

mathematical representation can be expressed as in equation 16.

𝑋𝑡 = 1 − 𝑒−𝑘𝑡𝑛 (16)

Where 𝑋𝑡 represents the relative crystallinity, 𝑘 represents combined crystallization rate constant

including both nucleation and growth, and 𝑛 represents Avrami exponent. Through math

operation, this equation can be written in the form of equation 17 below.

ln[−𝑙𝑛(1 − 𝑋𝑡)] = 𝑙𝑛𝑘 + 𝑛𝑙𝑛𝑡 (17)

By plotting ln[−𝑙𝑛(1 − 𝑋𝑡)] with respect to 𝑙𝑛𝑡, the value of 𝑙𝑛𝑘 and 𝑛 can then be extrapolated

from y axis intercept and the slope respectively. The Avrami plot of the above mentioned three

samples are plotted in Figure 23 c) below. From the Avrami equation, crystallization half time

can be calculated by setting 𝑋𝑡 =1

2. Therefore, crystallization half time can be derived as

equation 18 below.

𝑋𝑡 = ∫𝑑𝐻

𝑑𝑡𝑑𝑡

𝑡

𝑡0

∫𝑑𝐻

𝑑𝑡𝑑𝑡

𝑡∞

𝑡0

⁄ (15)

28

𝑡12⁄= (

𝑙𝑛2

𝑘)1/𝑛

(18)

In summary, the crystallization rate constant k and Avrami exponent for each case is summarized

in Table 4 below. As it can be seen from both the plot and the table, in situ fibrillated composites

showed a dramatically improved crystallization kinetics compared to neat polypropylene. There

was a one order of magnitude increase in crystallization rate constant. Also, the calculated

crystallization halftime decreased from 494 s for neat PP to 283 s for fibrillated composite. This

result was in consistency with the work done by Rizvi et. al [59]. The improved crystallization

kinetics should be attributed to the heterogeneous nucleation effect of PET nanofibers. For the

spun PP/PET fiber composite case, the fully stretched PET fiber with large aspect ratio (length to

diameter) creates large number of nucleation sites for PP matrix. However, as the fiber being

reprocessed into pellet, the promotional effect of PET fiber on PP crystallization was disrupted.

The crystallization half time increased to 373 s. Although, compared to neat PP material, which

is 494 s, the improvement is still considerable. This set back can be explained by the morphology

alteration illustrated in Figure 18. During reprocessing, both heat and shear being applied to PET

fiber cause it to break, entangle, and sinter. The aspect ratio, as a result, is lowered.

29

a b

c

Figure 23 DSC isothermal crystallization at 135 ⁰C heat a) flow curve of neat PP and PP with 5 wt% APET composite; b)

relative crystallinity vs time; c) Avrami plot

Table 3 Summary of Avrami parameters and crystallization halftime of PP/APET nanofiber composite crystallized at 135 ⁰C

A notable problem with the isothermal DSC experiments was that the crystallization peaks were

very low due to high crystallization temperature. Therefore, another set of isothermal DSC

experiment was repeated at an isothermal crystallization temperature of 130 ⁰C. The results are

shown in Figure 24. Also, the extracted crystallization rate constant, Avrami exponent, and

crystallization halftime are summarized in Table 4. In this case, the crystallization kinetics were

Sample k n 𝑡12⁄(s)

Neat PP 6.8 x 10-7 2.23 494

PP/APET fibrillated composite 2.8 x 10-6 2.20 283

PP/APET reprocessed composite 1.2 x 10-6 2.24 373

30

dramatically improved in all three cases due to lower crystallization temperature. This was also

reflected on the rate constant k. A noticeable difference for 130 ⁰C case is that the set back effect

due to reprocessing the fiber composite on crystallization kinetics is not as significant. This is

mainly due to the governing mechanism of crystallization. At an elevated temperature,

crystallization is difficult to initiate due to smaller value of change in Gibb’s free energy (△G).

Therefore, the crystallization kinetics is more sensitive to the change in surface free energy. By

introducing a foreign surface (PET nanofiber), the lowered interfacial free energy has a greater

effect on total value of change in Gibb’s free energy. That sensitivity, being reflected on the DSC

result, translates into greater improvement in crystallization halftime (~42% reduction in

fibrillated composite compared to neat PP) and relatively obvious setback effect in crystallization

halftime for reprocessed composited (~32% increase in reprocessed composite compared to

fibrillated composite). At low crystallization temperature (130 ⁰C), a 42% reduction in

crystallization can still be observed for PP/PET fibrillated composite. However, the setback

effect for reprocessing is only 3.3% (1.6s).

31

a b

c

Figure 24 DSC isothermal crystallization at 130 ⁰C heat a) flow curve of neat PP and PP with 5 wt% APET composite; b)

relative crystallinity vs time; c) Avrami plot

Table 4 Summary of Avrami parameters and crystallization halftime of PP/APET nanofiber composite crystallized at 130 ⁰C

Sample k n 𝑡12⁄(s)

Neat PP 1.67 x 10-5 2.41 82.5

PP/PET fibrillated composite 8.11 x 10-5 2.34 47.9

PP/PET reprocessed composite 5.95 x 10-5 2.40 49.5

32

PP/CPET

Figure 25 shows the DSC results of PP/CPET fiber composite. For the CPET filled composite,

two CPET loading levels, 5 and 15 wt% were investigated. The Avrami parameters and

crystallization halftimes are summarized in table 5. As Figure 25 a) and b) indicate,

crystallization kinetics of PP/PET composites improved dramatically compared to neat PP. With

5 and 15 wt% CPET loading, the crystallization halftimes were 24 and 22.6 seconds respectively.

Comparing to neat PP, the decreases in crystallization halftime were approximately 73% and

74%. This was an even greater improvement when compared with PP/APET composite. To

explain the further decrease of crystallization compared to APET composite, the author

speculates that it is due to the difference of interfacial behavior between PP and CPET and PP

and APET. Stronger heterogeneous nucleation effect was found in CPET compared to APET.

The other attributions to this phenomenon could be the intrinsic composition of CPET. The

crystallizable PET was used as purchased. Since the CPET may have been modified with crystal

nucleating agent to enhance crystallinity. The added crystal nucleating agent may have reacted

with PP matrix and altered its crystallization behavior.

As for the effect of PET loading level on PP crystallization, a positive correlation seemed to be

existing. Though, the marginal effect was very low. For fibrillated composites, the crystallization

halftime only reduced from 24 seconds to 22.6 seconds as PET loading increased from 5 wt% to

15 wt%. This, to the author’s opinion, is due to the theoretical limitation of enhancing

crystallization through foreign additives. In PP/PET composite, PP’s crystallization is affected

by PET nanofiber primarily on initial nucleation stage. Since the fiber is in nanoscale and the

aspect ratio was significant. Therefore, a big jump in crystallization kinetics can be seen with

addition of only a moderate loading level of PET. As the loading level increased, the intrinsic

property of the matrix material posed a limit on the nucleation behavior. That was why the

crystallization halftime leveled off towards high filler loading. Similar results were observed in

other composite material systems [60], [61].

33

a b

c

Figure 25 DSC isothermal crystallization at 130 ⁰C heat a) flow curve of neat PP, PP with 5 wt% CPET and PP with 15 wt%

CPET composites; b) relative crystallinity vs time; c) Avrami plot

Table 5 Summary of Avrami parameters and crystallization halftime of PP/CPET nanofiber composite crystallized at 130 ⁰C

Sample k n 𝑡12⁄(s)

Neat PP 0.272 2.41 88.4

5% CPET fibrillated composite 6.62 2.34 24.0

5% CPET Reprocessed 4.31 2.46 28.6

15% CPET fibrillated composite 7.61 2.46 22.6

15% CPET Reprocessed 4.35 2.56 29.3

34

4.2.2 Non-isothermal crystallization

PP/APET

For PP and APET composite, experiments were carried out with 5 wt% APET loading. All

samples, including neat PP, PP/APET fibrillated composite, and PP/APET reprocessed

composite, were crystallized under cooling rates of 5, 10, 15, and 20 ⁰C/min.

Figure 26 shows the reprocessed heat flow results of PP/APET composite crystallizing at

different cooling rate. As the figure shows, the crystallization peak shifted toward lower

temperature as cooling rate increases. Neat PP and PP/APET fibrillated composite showed

similar trend. This was a typical phenomenon for semi-crystalline polymer. With low

temperature drop rate, the molecule chains of the polymer have longer time to react and

rearrange, therefore, a preferable energy state can be reached at a comparatively high

temperature. Contrarily, with high cooling rate, polymer chains quickly freeze and crystallization

peak temperature, consequently shift to lower temperature.

Figure 26 Reprocessed PP/PET composite heat flow results at different cooling rates

35

Figure 27 below shows the heat flow results of all three samples at a cooling rate of 10⁰C/min.

The peaks represent the crystallization processes. As it can be told from the peaks, both onset

temperature and finishing temperature of crystallization were increased with addition of PET

nanofiber. The crystallization peak temperatures (Tp) for neat PP, fibrillated composite and

reprocessed composite are 122, 128, and 126 ⁰C respectively. This result indicated that addition

of PET nanofiber effectively lowered the activation energy required for crystallization.

Figure 27 Non-isothermal DSC heat flow vs temperature at cooling rate of 10 ⁰C/min

Three methods were adopted to analyze the non-isothermal DSC results. They were Avrami [58],

[62], Ozawa’s[63], [64], and Mo’s[65], [66] methods.

To begin with, Avrami’s model of phase change in crystallization is still applied the same as

isothermal crystallization. However, to account for temperature change, the model is corrected

using Jeziorny [67] correction. In order distinguish from isothermal crystallization model, the

new Avrami model and Jeziorny correction are given as in equation 19 and 20 below.

ln[−𝑙𝑛(1 − 𝑋𝑡)] = 𝑙𝑛𝑍𝑡 + 𝑛𝑙𝑛𝑡 (19)

ln 𝑍𝑐 =ln 𝑍𝑡𝑅

(20)

36

Where 𝑋𝑡 still represents relative crystallinity, and 𝑍𝑐 represents the combined crystallization

rate constant. R is the cooling rate in DSC experiments. The relative crystallinity was obtained

by integrating the crystallization peak and calculating using equation 15. By plotting

ln[−𝑙𝑛(1 − 𝑋𝑡)] against 𝑙𝑛𝑡, ln 𝑍𝑡 and 𝑛 can then be extrapolated through linear regression.

Figure 28 a) shows the heat flow curves of neat PP at different DSC cooling rates. Figure 28 b)

shows the calculated relative crystallinity curves. With obtained crystallinity curve, the double

log Avrami plot can then be constructed as Figure 28 d). Since severe nonlinearity was seen in

the curves, a two-stage fitting was applied to better study the crystallization kinetics. The fitted

curves are shown in Figure 29. For each cooling rate, the first section is referred to as primary

crystallization stage, and the second section is referred to as secondary crystallization stage. The

extrapolated Avrami parameters, along with crystallization halftimes and Tp’s were summarized

in table 5 below.

37

d

a b

c

Figure 28 neat PP non-isothermal crystallization a) heat flow vs temperature; b) relative crystallinity vs time; c) relative

crystallinity vs temperature; d) Avrami plot

38

Figure 29 Two-stage fitted Avrami plot for neat PP

Table 6 Summary of Avrami parameters and crystallization halftime of PP/APET nanofiber composite in non-isothermal

crystallization

Sample Cooling rate

(oC/min)

Primary stage Secondary stage 𝑡1/2

(min)

Tp

(oC) 𝑛1 𝑍𝑐1 𝑛2 𝑍𝑐2

Neat PP

5 3.2 0.88 2.5 1.0 0.98 124

10 3.0 1.15 3.2 1.0 0.49 123

15 2.5 1.14 2.7 1.0 0.37 118

20 2.3 1.15 2.6 1.0 0.29 117

PP/ APET 95/5

Fibrillated

composite

5 3.8 1.27 2.3 1.2 0.66 128

10 3.6 1.15 2.6 1.1 0.40 128

15 2.5 1.20 2.0 1.2 0.29 126

20 2.4 1.16 2.0 1.1 0.25 124

PP/ APET 95/5

Reprocessed

composite

5 3.4 0.94 2.4 0.94 0.97 128

10 3.4 1.28 2.5 1.3 0.50 127

15 2.3 1.18 2.2 1.2 0.29 125

20 2.4 1.15 2.1 1.1 0.28 123

39

The tabulated data provides some essential information about crystallization kinetics. At primary

crystallization stage, an increase in Avrami exponent, 𝑛1, can be observed for PP/APET

composites with cooling rate of 5 and 10 ⁰C/min. Given the fact that physical interpretation of n

is the dimension of growth of crystals, larger value of 𝑛 corresponds to a more complex mode of

crystallization. Since the primary stage of crystallization is dominated by initial nucleation. It is

reasonable to believe that the heterogeneous nucleation effect of PET caused the increase in

complexity of crystallization mode. With increased cooling rate, however, temperature quickly

drops below the desired crystallization or nucleation temperature, and the promotional effect of

heterogeneous nucleation got suppressed. That explains the similar 𝑛1values for all the samples

with 15 and 20 ⁰C/min cooling rate. Unlike isothermal crystallization, the rate constant Zc didn’t

show much variation with addition of PET nanofiber. Across all the cooling rates, on average,

only slight increase of Zc can be seen. This can be attribute to the driven force of crystallization,

which was the temperature drop in non-isothermal crystallization. Compare to isothermal

crystallization, which had a spontaneous feature, non-isothermal crystallization was mainly

driven by temperature drop. Therefore, the additive had a much smaller effect.

During the secondary crystallization stage, an interesting fact about 𝑛2 values can be observed.

For all cooling rates, the 𝑛2 values for neat PP was larger than the corresponding values for

PP/APET fiber composites. Since secondary crystallization stage was dominated by secondary

nucleation and growth. The author believes that this phenomenon was caused by intrinsic

property of PP. During primary stage, crystal nucleation is much more active in PP/PET

composites than neat PP. Therefore, most potential crystal nucleation sites were already occupied

when PP reaches secondary crystallization stage. Additionally, due to previous active nucleation,

crystal growth may be hindered by impingement from neighboring crystals. That’s why PP/PET

composites showed less complex mode of crystallization from the analysis. As for crystallization

rate constant, 𝑍𝑐2’s were only slightly smaller than 𝑍𝑐1’s, and barely no change of 𝑍𝑐2 was

observed with addition of PET fiber. This should also be attributed to the driven mechanism of

non-isothermal crystallization.

Another popular approach to analyze non-isothermal crystallization kinetics is Ozawa’s

equation. The model can be expressed by equation 21 below.

40

ln[−𝑙𝑛(1 − 𝑋𝑡)] = ln𝐾(𝑇) − 𝑚𝑙𝑛Ф (21)

where 𝐾(𝑇) is the Ozawa crystallization rate constant, 𝑋𝑡 is the relative crystallinity, Ф is the

cooling rate, and m is the Ozawa exponent. The values of 𝐾(𝑇) and 𝑚 can be derived through

plotting ln[−𝑙𝑛(1 − 𝑋𝑡)] against 𝑙𝑛Ф at a given temperature. The typical plot looks like Figure

30 below.

Figure 30 Ozawa plot ln(-ln(1-Xt)) vs. lnФ for neat PP

As Figure 30 shows, severe nonlinearity disapproves the application of Ozawa’s model in this

case. This could be attributed the selection of cooling rates, which were too different to form

continuity.

The third method being employed is Mo’s method. Mo[65], [66] and his colleague

combined Avrami and Ozawa’s methods and concluded a new model to describe kinetics of non-

isothermal crystallization. The final form of Mo’s model can be expressed as follow:

lnФ = ln𝐹(𝑇) − 𝛼 ln 𝑡 (22)

Where 𝐹(𝑇) = [𝐾(𝑇)/𝑍𝑡]1/𝑚, and 𝛼 = 𝑛/𝑚. In this model, 𝐹(𝑇) represents the cooling rate

needed for the system at unit crystallization time to reach a certain crystallinity. That is to say,

the smaller F(T) is, the higher crystallization rate is.

41

With Mo’s method, crystallization time and cooling rate data corresponding to relative

crystallinity of 20, 40, 60, and 80% were extracted from Figure 28 b) and plotted in Figure 31.

Figure 31 Neat PP ln Ф vs. ln t plot at relative crystallinity of 20, 40, 60, and 80%

The data points at different relative crystallinity were linear fitted, and F(T) and α can be

calculated accordingly. A complete result summary of Mo’s parameters for all PP/APET

composite samples are listed in Table 7 below.

From Figure 31, it can be seen that Mo’s method provides the best fitting to the DSC result.

Given the practical meaning of F(T), fibrillated PP/APET composite showed the most active

crystallization kinetics. Throughout the entire crystallization process, lowest cooling rate is

required for the fibrillated composite. As for reprocessed composite, the F(T) value still showed

considerable reduction compared to neat PP. However, these F(T) values are not comparable to

fibrillated composites. This should be attributed to the change in APET nanofiber morphology.

Additionally, an increasing trend of F(T) values were observed with increasing crystallinity

level. This result coincides with previous stepped Avrami analysis, since crystallization kinetics

got lower toward the end of crystallization stage.

42

Table 7 Summary of Mo's parameters of PP/APET nanofiber composite in non-isothermal crystallization

Sample Relative

Crystallinity (%) F(T) α

Neat PP

20 2.93 1.29

40 4.17 1.24

60 5.31 1.20

80 6.69 1.08

PP/ APET 95/5

Fibrillated

composite

20 0.67 1.83

40 0.22 3.03

60 1.01 2.39

80 1.51 2.58

PP/ APET 95/5

Reprocessed

composite

20 2.74 1.32

40 3.44 2.53

60 3.89 2.57

80 5.10 3.01

PP/CPET

Following the same procedure, non-isothermal DSC results of PP/CPET composites were

characterized. The crystallization parameters obtained from Avrami and Mo’s methods are

summarized in table 8 and 9 below.

43

Table 8 Summary of Avrami parameters and crystallization halftime of PP/CPET nanofiber composite in non-isothermal

crystallization

Sample Cooling rate

(oC/min)

Primary stage Secondary stage 𝑡1/2

(min)

Tp

(oC) 𝑛1 𝑍𝑐1 𝑛2 𝑍𝑐2

Neat PP

5 3.2 0.88 2.5 1.0 0.98 124

10 3.0 1.15 3.2 1.0 0.49 123

15 2.5 1.14 2.7 1.0 0.37 118

20 2.3 1.15 2.6 1.0 0.29 117

PP/ CPET 95/5

Fibrillated

composite

5 3.5 1.28 1.9 1.1 0.65 129

10 3.6 1.20 1.6 1.2 0.33 128

15 2.4 1.16 2.1 1.3 0.27 127

20 2.3 1.17 1.9 1.2 0.22 126

PP/ CPET 95/5

Reprocessed

composite

5 3.4 1.22 2.4 0.94 0.71 129

10 3.4 1.30 2.5 1.2 0.35 128

15 2.3 1.18 2.2 1.1 0.29 127

20 2.4 1.17 2.1 1.1 0.24 126

PP/ CPET 85/15

Fibrillated

composite

5 3.4 1.22 2.0 1.3 0.49 130

10 3.5 1.32 2.2 1.3 0.24 130

15 2.6 1.17 2.2 1.2 0.23 129

20 2.4 1.19 1.7 1.1 0.21 128

PP/ CPET 85/15

Reprocessed

composite

5 3.4 1.20 2.0 1.1 0.53 130

10 3.0 1.32 2.1 1.0 0.33 129

15 2.5 1.18 1.9 1.1 0.27 129

20 2.3 1.19 2.1 1.2 0.24 128

44

Table 9 Summary of Mo's parameters of PP/CPET nanofiber composite in non-isothermal crystallization

Sample Relative

Crystallinity (%) F(T) α

Neat PP

20 2.93 1.29

40 4.17 1.24

60 5.31 1.20

80 6.69 1.08

PP/ CPET 95/5

Fibrillated

composite

20 0.304 2.10

40 1.30 1.66

60 2.03 1.62

80 2.94 1.64

PP/ CPET 95/5

Reprocessed

composite

20 0.845 1.65

40 1.24 1.78

60 1.59 2.01

80 2.35 2.19

PP/ CPET 85/15

Fibrillated

composite

20 0.31 4.84

40 0.55 4.57

60 0.74 5.96

80 1.46 2.58

PP/ CPET 85/15

Reprocessed

composite

20 0.469 2.17

40 0.705 2.44

60 1.01 2.77

80 1.81 3.01

To a large extent, PP/ CPET composites crystallization exhibited a similar trend as PP/ APET

composites. In stepped Avrami analysis, more complex crystallization mode was observed with

low cooling rates during primary crystallization stage. This corresponded to increase in 𝑛1

values. From Mo’s parameters, the F(T) values for PP/CPET composites were also much smaller

than corresponding F(T) values for neat PP. Increasing the loading level of CPET had no further

influence in Avrami parameters. However, further decrease in 𝑡1/2 and increase in Tp were

45

observed. Decrease in 𝑡1/2 values can also be interpreted from Mo’s parameters. Since F(T)

values represents cooling rate needed at unit crystallization time when the material reaches a

certain crystallinity. Lower F(T) values correspond to higher crystallization rates and shorter

crystallization time. To better visualize and compare crystallization time of neat PP and different

PP/ PET composites, Figure 32 was created below to visualized different crystallization

halftimes of different composites. As it can be seen, PP/PET composites always exhibit shorter

crystallization halftimes than neat PP regardless the kinds of PET. CPET showed better

promotional effects in crystallization of the composites than APET. As it was discussed in

isothermal crystallization, this was probably due to the interfacial behavior between PP and

CPET. Another finding is that reprocessing always hurt PET’s promotional effect in PP’s

crystallization. This matches the isothermal crystallization results and should be attributed to the

change in PET fiber morphology during reprocessing.

Figure 32 Crystallization halftimes for different PP/ PET composites at 10⁰C/min cooling rate

Mechanical property characterization

Figure 33 shows the stress strain curve obtain from Instron Universal Tester for PP/APET

composites. An obvious improvement of stress strain behavior can be observed compared with

neat PP. In the elastic portion, the curve of APET nanofiber filled PP became steeper which

reflected improved tensile modulus. The ultimate yield strength, which corresponded to the

yielding point also improved significantly with inclusion of PET nanofiber. Figure 34 and Figure

35 quantitatively summarize the tensile modulus and tensile strength of both APET and CPET

nanofiber filled composites. As the figures show, remarkable increase of both properties were

46

achieved. With 15 weight percent of CPET fiber, the tensile modulus and tensile strength of

fibrillated composite increased 50% and 30% respectively. For fibrillated composites, CPET and

APET composites have similar effect on tensile modulus, whereas CPET outperform APET in

tensile strength. Another interesting fact being seen from the results was that reprocessed

composites always perform worse than fibrillated material. This result is predictable since

previous SEM experiments demonstrated the abruption of fiber morphology during reprocessing.

Figure 33 Stress strain curve of PP/APET fibrillated and reprocessed composite

Figure 34 PP/ PET composite tensile modulus vs. PET

loading level

Figure 35 PP/ PET composite tensile strength vs. PET

loading level

47

Rule of mixture and Halpin-Tsai’s equation are two popular methods used to predict composite

material properties given the constituent material properties. Specifically, rule of mixture is good

for strength prediction and Halpin-Tsai’s equation is good for modulus prediction. The

mathematical forms of the two methods are given in equation 23 and 24 below.

𝑃𝑐 = 𝑃𝑚(1 − 𝑣𝑓) + 𝑃𝑓𝑣𝑓 (23)

𝑃𝑐 = 𝑃𝑚 (1 + ℥𝜂𝑓

1 − 𝜂𝑓) ; 𝜂 =

𝑃𝑓𝑃𝑚

− 1

𝑃𝑓𝑃𝑚

+ ℥

(24)

Equation 23 is Rule of mixture. Where 𝑃𝑐, 𝑃𝑚, and 𝑃𝑓 represents the composite, matrix, and filler

property values respectively. 𝑣𝑓 represents the volume fraction of the filler. Equation 24 is

Halpin-Tsai’s equation. 𝑃𝑐, 𝑃𝑚, and 𝑃𝑓 represents the same physical quantities as Rule of

mixture. ℥ is an empirical value taken given constituent materials. Evidence has shown that ℥

value of 2 provides promising prediction of tensile modulus [68]. In this study, the tensile

modulus and strength were calculated according to Halpin-Tsai’s equation and rule of mixture to

compare with experimental results. The calculated results are shown in table 10 below.

Table 10 Calculated tensile modulus and strength of PP/ PET nanofiber composites

Properties PP/PET 95/5 composite PP/PET 85/15 composite

Tensile Modulus (MPa) 1003 1073

Tensile Strength (MPa) 49.5 50.2

Given above calculation results, the experimental results seem rather unproportionable.

However, to the author’s opinion, this can be attributed to two reasons. First, PET phase has been

fibrillated into 200 nm nanofiber. Nano-sized filler are proven to have better mechanical property

reinforcing effect than traditional fibers [69], [70]. Therefore, traditional composite model may

fail to account for this improvement. More importantly, the widely dispersed nanofiber

significantly promoted the crystallization kinetics of PP. From this perspective, the crystal

structure of PP has changed compare to neat PP. Therefore, the change in crystal structure can

justify the additional increase in mechanical properties.

48

Conclusion

This project covers a complete crystallization study of PP/ PET in-situ fibrillated nanofiber

composite. Through the a scalable in-situ fibrillation process, around 250 nm PET nanofiber

were obtained inside PP matrix. Compounded blends, fibrillated composite and reprocessed

composite are all characterized under SEM. SEM photos clearly demonstrate the change in PET

domain morphology. Evenly dispersed spherical droplets, finely fibrillated fibers, and fiber with

disrupted morphology were observed in blends, fibrillated composites and reprocessed pellets

respectively. From diameter measurements, APET had a smaller diameter than CPET in both

fibrillated composite and reprocessed composite. That was believed to be the result of higher

crystallinity of CPET, which caused fast crystallization and solidification during stretching. Also,

this could explain the smaller morphology change during reprocessing compare to APET

nanofiber.

DSC experiments revealed the crystallization kinetic of both neat PP and PP/ PET nanofiber

composites. In isothermal DSC, PP/ PET nanofiber composites exhibited a much more active

crystallization kinetics than neat PP. Nearly one order of magnitude increase in crystallization

rate constant was observed. Potentially due to better interfacial behavior between CPET and PP,

better results were obtained with CPET nanofiber than APET nanofiber. Avrami and Mo’s

method also showed improved crystallization kinetics in non-isothermal DSC experiments.

Again, CPET outperformed APET in crystallization promotional effect. Additionally, CPET had

better sustained its promotional effect in crystallization kinetics after reprocessing. This result

coincided with the morphology observation with SEM, where reprocessed PP/ CPET nanofiber

composite had a better fiber morphology than PP/ APET nanofiber composite.

Mechanical property characterization showed unproportionable increase of tensile modulus and

strength for PP/ PET filled composites compared to neat PP. The experimental results

significantly deviated from mathematical prediction. This should be attributed to the nanoscale

PET fiber and improved crystal structure of PP/ PET nanofiber composite. First, the reinforcing

capability of PET nanofiber was much better than traditional fiber due to its aspect ratio and

surface area. Secondly, PET nanofiber dramatically improved PP’s crystallization kinetics,

which led to a better PP crystal structure. This improved crystal structure acted as a self

49

reinforcing element to the composite. Therefore, much better mechanical property was achieved

due to these synergic effects.

50

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