Crystallization of Polymer Blend Nanocomposites

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Crystallization of Polymer Blend Nanocomposites

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Sonalee Das, Sushanta K. Samal, Smita Mohanty, Sanjay K. Nayak Central Institute of Plastics Engineering and Technology (CIPET), Bhubaneswar, India

1. INTRODUCTION The global market of engineering plastics, according to various sources, has exceeded 19.6 million metric tons in 2013 and is expected to increase to the tune of 29.1 million metric tons by 2020. Multiple solutions offered by polymeric materials have opened up new dimensions of application with engineering plastics. Currently, blending two or more polymers has emerged as an established route for designing new polymeric materials tailored with desired superior attributes as compared with a single polymeric system [1,2]. However, superior properties in a multicomponent polymeric blend depend on the miscibility of the different components. Most of the polymeric blend pairs are integrally immiscible because of the difference in their polarity, intermolecular interactions, solubility parameter, and low entropy of mixing. These factors result in phase-separated morphology because of significant incompatibility of various components present in the blend matrix. Moreover, the presence of immiscible blends leads to limited application because of poor adhesion strength at the interface, which is required to attain a stable morphology. Consequently, to get an optimum blend property and long-term stability, phase segregation should be prevented and the morphology of immiscible blends should be stabilized through suitable compatibilization techniques [1,2]. Various approaches have been adopted to enhance the miscibility of polymer blends, including: 1. 2. 3. 4. 5. 6.

Incorporation of block copolymer, graft copolymer In situ polymerization Reactive compatibilization Addition of compatibilizer Addition of specific interactive groups Incorporation of micro- and nanofillers

Reinforcement of nanofiller within the immiscible polymer blends to tailor their properties has been an appealing route for creating high-performance materials. Nanofiller-reinforced polymer blends have been considered as an inspiring route for manipulating a new type of polymeric material that attains synergism between different components in the polymer blend and delivers superior performance for Crystallization in Multiphase Polymer Systems. http://dx.doi.org/10.1016/B978-0-12-809453-2.00011-6 Copyright © 2018 Elsevier Inc. All rights reserved.

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CHAPTER 11 Crystallization of Polymer Blend Nanocomposites

high-end applications [3e13]. The concept of using nanofillers as compatibilizer or nucleating agent is basically to improve the selected properties (e.g., mechanical, thermal, chemical) and stabilize the blend morphology. However, it was noticed that the properties of polymer blend nanocomposites depend on various factors such as the morphology of the blend, nature of the various components, orientation of the nanofillers, localization of the nanoparticles, and dispersion of the nanoparticles within blended components. The polymer blend nanocomposite retained its own distinctive behavior but varied in different systems because of the complex nature of the multiple components. The present chapter provides an in-depth insight to apprehend the role of nanofillers on the crystallization of the polymer blend systems.

2. FUNDAMENTALS OF CRYSTALLIZATION 2.1 POLYMER CRYSTALLIZATION

Polymer crystallization has become an important area of research interest ever since the concept of chain folding was introduced by Stroks in 1938 and later validated by Andrew Keller [14] and Fisher [15]. Crystallization is a first-order transition encountered by the thermal analysis of polymer and is a process in which molecular chains are aligned and folded together to form an ordered region, which is best known as lamellae. Nucleation of crystal growth occurs at various nuclei, which grow out in a three-dimensional radical fashion from each nucleus, and is termed as spherulites. During the crystallization process a change in density, symmetry, and phase transition takes place, which determines the behavior of the end products. The initial crystallization is from an entangled melt, which can be thought of as a kind of separation process between crystallizable and noncrystallizable polymer chains. This process eventually forms crystalline and amorphous regions in the final crystallized polymer. Crystallization commonly proceeds by nucleation of a fiber-like structure followed by lamellar structure formation. The crystallization in polymers can take place by three fundamental ways: 1. Crystallization during polymerization 2. Crystallization induced by orientation 3. Crystallization under quiescent condition

2.2 CRYSTALLIZATION IN POLYMER BLENDS The formation of a homogeneous polymer blend is associated with the negative free energy of mixing. Miscibility of polymer blend in the amorphous state depends on the value of the FloryeHuggins interaction parameter. Phase-separated polymer morphology can be ensured by increasing or decreasing the temperature because of the dependence of interaction parameter on temperature. A lower or upper (UCST) critical solution temperature can be observed with the increase or decrease in temperature.

2. Fundamentals of Crystallization

In a binary polymer blend, one or both components crystallize, which may be miscible or immiscible in the amorphous state, and phase separation may occur along with crystallization. When only one component crystallizes and the others are rejected, crystallization results by diffusion of the noncrystallizing component into the surrounding amorphous phase, which results in a change in the glass transition temperature (Tg) of this phase and consequently affects the rate of crystallization.

2.3 CRYSTALLIZATION IN POLYMER BLEND NANOCOMPOSITES A perceptive scientific understanding of crystallization mechanisms in nanofillerreinforced polymer blends is very important to control and manipulate the desired properties. In a semicrystalline polymer, the crystallization rate can either increase or decrease with the incorporation of nanoparticles [16] because of the possible effects of nanoparticles in crystal nucleation. The concentration of the nanoparticle also affects crystallization, e.g., the crystallization rate decreases with higher nanoclay loading [17] and also, besides the influence on nucleation, it has been reported that nanofiller imparts a retarding effect on the crystal growth of polymer matrices [18,19]. Lipatov [20] has shown that the addition of solid particles to an immiscible polymer blend may lead to compatibilization. This effect can be explained by the change in free energy of mixing of three components and is expressed as: DGmix ¼ DGAS þ DGBS þ DGAB

(11.1)

where A and B are two polymers and S is the solid particles, DGAS and DGBS are the free energies of interaction of polymers A and B with the solid particles S, respectively. DGAB is the free energy of mixing of the two polymers A and B. When DGmix 2 wt%) the nucleation growth process is retarded. Thus the synergistic effect of higher nucleation sites with limited crystal growth resulted in producing fine-grained crystals. Hence, a low filler concentration promotes heterogeneous nucleation, resulting in an increase in the nucleation rate and the overall crystallization kinetics. Moreover, the value of Avrami exponent “n” and “k,” which dictate the nucleation mechanism, geometry, and growth rate, were in the range of two for a starch-PCL blend. This indicates 2-D linear crystal growth rate, with crystals nucleating athermally. However, for the nanocomposite it was in the range of three, which indicates the 3-D growth. Generally, n ¼ 2 indicates a circular diskeshaped growth, whereas n ¼ 3 indicates spherical growth. Li et al. [61] studied the nonisothermal crystallization behavior of CB-filled PP and PP/epoxy composites. Isothermal kinetic studies were performed using DSC to analyze the crystallization kinetics of the blend nanocomposite using the Avrami equation. HoffmanneLauritzen equation was utilized to study the crystallization thermodynamics and kinetics of the polymer materials. The author reported that all the PP/CB composites exhibited higher K(T) and lower t1/2 than that of pure PP, indicating faster crystallization in the former. This can be attributed to the nucleation influence of CB on the crystallization rate of PP. With an increment in the CB content K(T) increases, whereas t1/2 decreases. At a lower CB content, i.e., 5.3 phr, the alteration in K(T) and t1/2 is minimal, whereas at a CB content of 10.0 phr, there is significant increase in K(T) and decrease in t1/2. However, when the CB content is increased from 10.0 to 17.7 phr, K(T) is slightly increased but t1/2 remains almost unchanged, confirming a compromise between the nucleation effect of CB and the restrictions caused by its own particles on the diffusion of crystallizing macromolecules. From the observation the Avrami exponent value of “n” was found to be three, indicating tridimensional spherulitic growth. The authors reported that the incorporation of CB into PP/epoxy blend resulted in the preferential distribution of CB in the epoxy phase. As a consequence, the spherical shape of epoxy particles was changed into an elongated structure, with a reduced nucleation effect of epoxy particles. The value of free energy for chain folding “sc” calculated using the HoffmanneLauritzen equation was found to be lower for PP/epoxy/CB composites as compared with neat PP and the parameter se decreases with the increase in CB content. The lower se values in the PP/CB composites further confirm the nucleation effect of CB on PP crystallization. The se value also remains unchanged as observed for t1/2 values when the CB content increased from 10.0 to 17.7 phr. This could be attributed to the compromise between the nucleation effect and the restrictions of

4. Crystallization Kinetics in Polymer Blend Nanocomposite

CB particles on the crystallization of PP. Thus PP/epoxy/CB composites indicated maximum “K”, minimum “t1/2,” and se values, respectively, as compared with neat counterparts. Dehaghani et al. [87] studied the microstructure and nonisothermal crystallization behavior of PP/PLA-blended clay hybrid nanocomposites using DSC and wide-angle X-ray scattering (WAXD). Although PP possesses excellent thermal, mechanical, chemical, and optical properties, it has poor oxygen barrier properties. Blending of PP with PLA results in improving the barrier properties because the later has lower oxygen permeability. Although PLA possesses significant oxygen barrier properties, it suffers from a slow crystallization rate, resulting in higher mass transport phenomena. Thus the overall crystallization rate of PLA can be improved through blending with a nucleating agent and plasticizers. Hence, in the present study the author blended PP with PLA reinforced with clay to enhance the crystallization rate and the overall barrier properties. The thermal behavior, crystallization studies, and multiple phase transitions were evaluated by DSC by using a heatingecoolingeheating cycle from 20 to 200 C. The author determined the crystallization enthalpy (DHc), cold crystallization enthalpy (DHcc), melting enthalpy (DHm), and degree of crystallinity. The DSC cooling curve of PP exhibited a single exothermic narrow peak at 116 C as a result of crystallization. On the other hand, PLA shows a glass transition temperature (Tg) of 60.7 C. Further peaks at 121 C and a melting peak (Tm) at 152.7 C were also observed in the case of PLA. However, no exothermic peak was observed in the case of PLA. This reveals no occurrence of crystallization phenomenon during the nonisothermal quenching process suggesting difficulty of melt crystallization of PLA. Blending of PP with PLA led to considerable alteration in the thermal behavior of the blends as compared with the pure counterparts. The PP-rich blend indicated an insignificant Tg corresponding to the PLA phase, and a Tm corresponding to the PP phase at 163.5 C. On the other hand, the PLA-rich blend indicated a marginal decrease in Tg and a considerable decrease of about 10 C in the cold crystallization temperature (Tc) as compared with pure PLA. This observation was due to the defective crystallization of PLA in the presence of PP droplets. With incorporation of C30B within the PP/PLA blend it was observed that there was an overall decrease in the cold crystallization (Tcc) value, implying the nucleating effect of C30B for effective crystallization. The nucleating effect was also reflected by an increase in the degree of crystallinity (Xc) values of the PP-rich blend with addition of 5% C15A. However, the crystallization process and Xc of PP/PLA/clay nanocomposite are reduced in the presence of a compatibilizer. This might be due to the disappearance of nucleating efficiency caused by the compatibilizer, which encapsulates the nanoparticles, resulting in kinetic restriction on the crystallization phenomenon. Thus the abovementioned result conveys that the value of nonisothermal crystallization temperature depends upon the type of compatibilizer used. Moreover, PLA crystals in the PLArich system were also formed during the heating and cooling process. Thus the observed increase in crystallinity can lead to an improvement in the barrier properties of the blend nanocomposite. WAXS study was also conducted to investigate the

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morphological features such as crystalline form and spherulites of the polymer nanocomposite blend. The DebyeeScherrer formula was utilized to determine the dspacing of the blend nanocomposite. The Scherrer equation is only applicable if the particle size is equal to 200 nm or less. The crystallite size of each nanocomposite blend could be calculated from the broadening of the WAXS peak. All the samples, except PLA, revealed five distinct a-crystalline peaks at 2q values of 14.0, 16.8, 18.5, 21.8, and 21.9 degrees, corresponding to 110, 040, 130, 111, and 041 planes of monoclinic a-crystalline phase of PP, respectively. The authors observed a narrowing down of the peaks in the PP-rich blend as compared with that of pure PP, which was due to an increase in the size of the crystals. On the other hand, the broadening of the peaks in the PLA-rich blend indicated a decrease in the size of the crystals. Using the Scherrer equation it was revealed that the average dimensions of the crystallites increased with an overall increase in the filler concentration for both the blended systems. This observation can be explained on the basis of the nucleating effect of the nanofillers. Also, it was revealed that the interlayer distance values of Cloisite 15A and C30B within PP-rich and PLA-rich blends were comparatively high as compared with the neat counterparts. However, the interlayer distance of silicate layers within the PP-rich system was found to be higher than that in the PLArich system. This observation can be attributed to the lower affinity of PLA toward the cationic modifier in C30B and C15A surface. Also, the nonpolar nature of the PLA matrix hinders the uniform dispersion of nanoclays. Thus from the earlier results it was concluded that the PP-rich system exhibited a greater degree of crystallinity as compared with the PLA-rich one. Thus the PP-rich system showed high barrier properties, which can be utilized for modified atmosphere packaging with potential biodegradability behavior. The author also used scanning electron microscopy (SEM) to analyze the nonisothermal crystallization behavior of PP/PLAblended clay hybrid nanocomposites. SEM was also utilized to analyze the presence of spherulites and to calculate the volume (Rv) and number average radii (Rn) and polydispersity of the particles. Discrete PLA spherical domains almost uniformly dispersed in the matrices of the blends were observed. However, the size of these domains was greater in the PLA-rich blend as compared with that of the PP-rich one. Moreover, the effect of compatibilization and incorporation of clay nanoparticles on the morphologies of PP-rich and PLA-rich blends was also investigated. It was found that, with the incorporation of C30B and C15A, the size of the dispersed phase decreases, whereas the polydispersity of the droplet size increases with compatibilization (Fig. 11.7). Similarly, Gomari et al. [88] used WAXD to study the crystallization of PA6/poly(ethyleneco-1-butene)-graft-maleic anhydride/organoclay nanocomposites. The author observed that with the addition of organoclay the degree of crystallinity “Xc” of PA6 reduced. Moreover, it was also observed that the organoclay with higher polarity and exfoliated structure, i.e., C30B produced the lowest value of “Xc” in comparison with C20A and C15A. The DSC thermogram of the blends and nanocomposites indicated no crystallization peak for the EB-g-MAH phase during the cooling stage, which was due to the strong interactions of PA6 and EB-g-MAH.


FIGURE 11.7 Scanning electron micrographs of cryofractured surfaces of compatibilized blends and nanocomposites: (A) PP/PLA/PTW 75/25/5, (B) PP/PLA/PTW 25/75/5, (C) PP/PLA/ Cloisite 15A 75/25/5, (D) PP/PLA/Cloisite 30B 25/75/5, (E) PP/PLA/Cloisite 15A/PTW 75/ 25/5/5, (F) PP/PLA/Cloisite 30B/PTW 25/75/5/5 [87].

In the heating scans, transition of crystal form from “a” to “g” crystals of PA6 was observed after incorporation of both elastomeric phase and organoclay. WAXD micrographs corroborate the findings of DSC by confirming the formation of g crystals at 2q ¼ 11, 22, and 23 degrees, respectively, and lowering of “a” peak intensity with the incorporation of organoclay. Similarly, Feng et al. [28] studied the crystallization of CNTs induced PVDF/ PMMA blend system using the WAXD technique. The author used polarized optical microscopy to study the crystallization of CNTs-induced PVDF/PMMA blend system by analyzing its morphology. During the nonisothermal process it was

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observed that incorporation of CNTs resulted in an enhancement of the crystallizability of PVDF in the blend system. This was evident through the increase in Tc and Tonset up to 120 C and 126.6 C as observed from the DSC curve. The results obtained from the isothermal crystallization process confirmed that CNTs can induce concentration fluctuation in the sample. As a result, different types of spherulites can be formed, i.e., the banded spherulites and compact spherulites; these can be observed through polarized optical microscopy images and are depicted in Fig. 11.8. Wang et al. [89] studied the nonisothermal crystallization behavior of PP/PA6/ MMT nanocomposites. To enhance the compatibility of PA6 and PP, maleic anhydride-g-polypropylene (MPP) compatibilizer was added. Furthermore, o-MMT

FIGURE 11.8 Polarized optical microscopy images showing the crystallization morphology of binary PVDF/PMMA/CNT system (A1eA3) and ternary PVDF/PMMA/CNT (B1eB3) [28].


was also utilized to improve the properties of the blend drastically. In the current study, the crystallization kinetic behavior of the PP/PA6 blend nanocomposite was thoroughly examined using X-ray diffraction (XRD), SEM, and DSC methods. Moreover, the theoretical method proposed by Mo and coworkers was used to analyze the nonisothermal crystallization kinetics of the synthesized nanocomposite. Mo and coworkers proposed a new model for analyzing the nonisothermal crystallization kinetics by using the Avrami and Ozawa equations. The authors proposed that the combined OzawaeAvrami model can be a more convenient method to analyze the nonisothermal crystallization process of the nanocomposite. The experimental results revealed good agreement with Mo’s theoretical prediction as indicated through the plot of log R versus log t. It was also observed that the introduction of MMT decreases the value of Mo exponent “a,” which refers to the ratio between Avrami exponent “n” and Ozawa exponent “m.” Moreover, the simultaneous incorporation of MMT and MPP lowers the parameter F(T), which indicates a higher crystallization rate. The XRD technique was used to determine the extent of silicate dispersion in the prepared PP/PA6 layered silicate nanocomposite. The interlamellar distance of MMT within the nanocomposite can be obtained from the position of d001 and diffraction peak. The presence of a broad intense peak of d001 at 2q ¼ 3.77 degrees corresponds to organophillic MMT with basal spacing of 2.34 nm. Moreover, the less intense shoulder and broad peaks at 2q ¼ 2.41 and 2.03 degrees with d001 spacing of 3.66 and 4.35 nm, respectively, found separately in PP/MMT and PP/MMT/MPP samples clearly reveal the intercalation or partial intercalation of MMT. This signifies that the compatibilizer MPP promotes the organophillic MMT to form the intercalated structure with a greater degree of d-spacing. This phenomenon can be attributed to the presence of grafted polar maleic anhydride groups in MPP chains favoring the promotion of interspace between the silicate galleries. Furthermore, XRD was utilized to study the crystallization behavior of the PA6/PP blend nanocomposite, which indicated the presence of both a-PP and g-PA6 diffraction peaks. The b-PP form also occurs in the injection samples, which disappears after annealing at 150 C or in hot-pressed samples. This indicates that a higher temperature results in acute segmental mobility of PP with antiparallel arrangement of chains in the crystal region. With the introduction of MMT or MPP to the PP/PA system no new diffraction peaks were observed. However, the intensities of the peaks at 21.1 and 21.8 degrees increase with the addition of MMT, which overlaps with the g-form of PA6. From the above-mentioned observation, the author suggested the crystallization of PA6 was improved through the inclusion of MMT. Furthermore, SEM was used to analyze the influence of MMT on the morphology of the PA6/PP blend nanocomposite. The incompatibility within PP and PA6 was observed through the presence of two different phases, wherein PP constitutes the continuous phase and PA6 constitutes the fibrillar structure. The compatibilization effect of MMP was found to be effective with lower loading of PP and higher loading of PA6. This might be due to the higher dispersion of MMT in the PA6 phase. However, when MPP and MMT were both added onto the PP/PA6 blends, the PA6 fibrillar structure disappeared and a more homogeneous morphology

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was observed. This homogenous morphology can be attributed to the compatibilization effect of exfoliated MMT through the formation of hydrogen bonds within the maleic anhydride groups of the MPP and alkylammonium groups of the organophilic MMT intercalant. Thus the presence of MPP compatibilizer located within the interphase may act as a “bridge” between PA6 and PP phases thereby enhancing the compatibility of the blend. DSC endothermal and exothermal curves were used to study the thermal properties and crystallization behavior of PP/PA6 and nanocomposites. The author observed an increment of Tc of PA6 by 2e3 C on introducing MPP or MMT into the PP/PA6 system, whereas the Tc of PP underwent no obvious change. However, when both MMT and MPP were incorporated simultaneously, the Tc of PP and PA6 increased by 6.6 and 4.2 C, respectively. Moreover a new crystallization peak was observed at 162.5 C representing the PP-g-PA6 copolymer phase. Thus the synergistic effect of MPP and MMT resulted in promoting better compatibility of PP with PA6. Hence, from these findings it was concluded that the simultaneous incorporation of MPP and MMT into the PP/PA6 system contributed to a significant reduction of the crystallization time. Wang et al. [90] analyzed the nonisothermal crystallization behavior of PET/ PTT/MMT nanocomposites using DSC. Both Tonset and Tp of PET/PTT/MMT nanocomposite in crystallization exotherms shifted to a higher temperature with an increase in the MMT content. This was due to the heterogeneous nucleating effect of MMT. However, with an increase in the cooling rate, Tonset and Tp of PET/ PTT/MMT nanocomposite shifted to a lower temperature. This observation might be due to an increase in the polymer melt viscosity, resulting in hindrance of molecular mobility. Consequently, this slow segment motion affected the formation of perfect crystals at a high cooling rate in the polymer nanocomposite. Also, an increase in the cooling rate resulted in shortening the crystallization time, thereby forming imperfect crystals.

5. CONCLUSIONS Polymer blend nanocomposites are versatile materials offering synergism in properties for a wide array of phenomenal applications. Crystallization in blends greatly affects the overall properties of the systems. Nanofillers at lower concentrations substitute the primary nuclei, thus competing with the confined crystallization. Furthermore, higher concentrations of nanofillers lead to a retarding effect owing to diffusion constraints. Thus it can be concluded that incorporation of nanofiller within the polymer blends can substantially affect the crystallization behavior. Depending on the fillerepolymer interaction a new crystal may be developed at the vicinity of the filler, which may not be present in case of the virgin polymer blends. More specifically, in certain cases there are examples of decreased spherulitic growth or crystallization temperature of polymer blends in the presence of nanofiller. Also, as elaborated in the chapter, nanofillers contribute to heterogeneous nucleation in the polymeric system.

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