Bio-based poly(lactide)/ethylene-co-vinyl acetate thermoplastic vulcanizates by dynamic crosslinking: structure vs. property

Abstract

Bio-based thermoplastic vulcanizates (TPV) from poly(lactide) (PLA) and ethylene-co-vinyl acetate rubber (EVA) were fabricated using dicumyl peroxide (DCP) as a curing agent; it is the first time that the application of PLA in elastic materials is demonstrated. A two-stage competing reaction mechanism as a function of peroxide content is revealed via gel analysis. The crosslinking of EVA is dominant at low DCP content (<1 wt%), which levels off when the DCP content exceeds 1 wt%. The gel fractions of the EVA and PLA phases can be tuned by the DCP content and fabrication technique. A desirable phase inversion of the PLA/EVA blends due to selective dynamic curing was monitored by both atomic force microscopy (AFM) and dynamic mechanical analysis (DMA). Consequently, PLA/EVA thermoplastic vulcanizates with high strength, high elongation at break, low tensile set and intermediate hardness were obtained. Moreover, their mechanical properties can be tuned by the DCP content and plasticization. The correlation between their structures and properties was investigated.

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1. Introduction

The widespread use of traditional rubber and plastic has received considerable attention in recent years due to their unsustainability and environmental pollution. The use of bio-based and biodegradable polymers provides a possible solution to the abovementioned issues.^1,2 Well-known bio-based and degradable plastics include poly(hydroxyalkonates) (PHA), poly(lactide) (PLA) and thermoplastic starch (TPS).^3–5

PLA, which has high strength, high transparency and relatively low cost, is derived from bio-resources. It is regarded as one of the most promising bio-based polymers.⁴ However, PLA is too brittle at room temperature (elongation at break <10%, impact toughness <3 kJ m⁻²) for applications where plastic deformation at high stress levels is required.⁶ As a result, many approaches, such as plasticization, blending and copolymerization, have been attempted to improve its toughness and flexibility.^7–9 Plasticizers, e.g. oligomeric lactic acid, citrate esters and poly(ethylene glycol), can improve the flexibility and ductility of PLA, but lead to leaching into the environment and phase separation during their storage.¹⁰ Another effective approach is blending PLA with rubbery polymers such as ethylene-co-vinyl acetate copolymer (EVA),⁶ natural rubber (NR),¹¹ ethylene-co-octane copolymer (POE)¹² and poly(ether-b-amide) (PEBA).^13–15 The introduction of PEBA can enhance the flexibility and toughness of PLA; however, this enhancement is limited due to immiscibility and weak interfacial adhesion.^14,15

Compared with plastics, fewer bio-based elastomers or rubbers, except for the well-known natural rubber, have been commercially available due to the chain stiffness and relatively high glass transition temperature (T_g) of the bio-based polymers. Thermoplastic elastomer (TPE) is a new class of copolymer or polymer blend that exhibits rubber-like behavior, but can be melt-processed like thermoplastics. The well-known TPEs are styrene-b-(ethylene-co-butylene)-b-styrene (SEBS) copolymer and poly(propylene)/poly(ethylene-propylene-diene) (PP/EPDM) blends.^16–19 The combination of these properties results from a two-phase structure: a soft phase confers rubbery properties in the solid state, whereas a hard phase provides crosslinks and retains melt processability. Thermoplastic vulcanizates (TPV) is a highly engineered class of TPE, comprising a cross-linked elastomeric dispersed phase and a melt processable plastic matrix. TPV is usually fabricated via dynamic vulcanization, where mixing and cross-linking are carried out simultaneously.

In addition to their typical elastomeric behavior, TPVs also possess certain advantages in comparison to the traditional rubber vulcanizates, such as reusability after disposal, short processing time and energy savings.^20,21 To date, the most commercialized TPV are PP/EPDM compounds, which have been widely used in the auto industry.^22,23 Recently, EVA-based TPVs were also reported in the literature, e.g., PP/EVA, poly(ethylene)/EVA and polyamide/EVA thermoplastic vulcanizates.^24–26

EVA is a commodity copolymer that can be either thermoplastic or rubber depending on the vinyl acetate (VAc) content. It shows typical rubbery behavior when the VAc content is between 40 and 90 wt% (LEVAPREN® EVM, Lanxess), with excellent flexibility, transparency, weather resistance and oil resistance, as well as good affinity with fillers and pigments. EVA can be cured by peroxide.²⁷ Currently, EVA is mainly derived from petroleum; however, it can also be made from bio-resources because both bio-based ethylene (e.g. Braskem, Brazil) and vinyl acetate (e.g. Wacker, Germany) have already been commercialized based on the newly developed bioethanol technology. In our previous study, compatible PLA/EVA blends with high strength and super (low temperature) toughness were obtained by physical compounding.⁶ Therefore, PLA and EVA rubber might be an ideal combination for bio-based TPVs. To the authors' knowledge, bio-based TPVs have rarely been reported.

The prime objective of this study is to provide a simple route to prepare bio-based thermoplastic vulcanizates by the dynamic vulcanization of PLA/EVA blends in the presence of dicumyl peroxide. To reveal the relationship between the structures and properties of these vulcanizates, crosslinking of each phase, phase morphology, phase inversion, rheological behavior and (dynamic) mechanical properties were systematically investigated. The present study not only provides a fundamental investigation on PLA/EVA-based TPVs, but may also broaden the application ranges of both PLA and EVA rubber.

2. Experimental section

2.1 Materials

Poly(lactide) (PLA, Ingeo 2003D) was purchased from Nature Works LLC, U.S.A, with a melt flow index (MFI) of 3.25 g per 10 min (190 °C, 2.16 kg). The content of L-lactide in the PLA is approximately 96%. Rubber grade EVA (Levapren® EVM500) with a Mooney viscosity of 27 ± 4 MU (ML (1 + 4) 100 °C) was supplied by Lanxess Chemical Co., Ltd., Qingdao, China. The average vinyl acetate (VAc) content in the EVA is 50 wt% chloroform (AR). Dicumyl peroxide (DCP, purity ≥ 99.5%) was supplied by Sinopharm Group Chemical Reagent Co., Ltd., China. Acetyl tributyl citrate (ATBC, purity ≥ 99.5%) was purchased from Guangzhou Chemical Industry Co. Ltd., China. All the chemicals were used as received.

2.2 Sample preparation

Prior to blending, PLA and EVA were dried overnight in a vacuum oven at 80 °C and 60 °C, respectively. To crosslink the EVA phase rather than the PLA phase, DCP was premixed with EVA at 80 °C in an internal mixer (RM-200, HABO Electrical Appliance Manufacturing Company, China) for 8 min at a rotor speed of 30 rpm to form a homogeneous EVA/DCP rubber compound. The rubber compound was then blended with molten PLA at 170 °C using the same device for 8 min at a rotor speed of 50 rpm, and dynamic crosslink occurred. Because DCP content is very crucial to the structure and properties of the TPV, different amounts of DCP (0–3 wt% based on EVA) were investigated, while the weight ratio of PLA/EVA was fixed at 40/60 (w/w). Plasticized TPV was obtained by feeding ATBC at the end of the abovementioned process and continuing mixing for 5 minutes. All the samples were finally compression molded into sheets (1 mm in thickness) at 170 °C for 3 min. The compression-molded samples were used for characterizations. The abovementioned preparation routes are summarized in Scheme 1.

Scheme 1 Preparation routes of PLA/EVA-based TPV.

2.3 Characterizations

Crosslink structure analysis. The crosslink structures of the TPV were studied by swelling equilibrium experiments. Specimens (2 × 1 × 0.5 mm³) of each sample were accurately weighed (M₀) and then immersed in chloroform for 72 hours to reach a swelling equilibrium. The swollen specimens were removed, wiped and weighed to an accuracy of 0.1 mg at a given time (M₁). The swollen specimens were then dried in a vacuum oven at 60 °C until the mass change was less than 0.1 mg. The dried residuals, defined as “gel” in this work, were then weighed to an accuracy of 0.1 mg (M₂). The gel content (W_R) of the TPV was calculated via W_R = M₂/M₀ × 100%. Three specimens of each sample were measured, and the average values are presented. All the experiments were performed at 30 °C.

With assumptions of the volume additivity of the gel and the absorbed solvent during swelling, the gel swelling ratio (S) of the TPV was calculated using eqn (1).²⁸

where ρ_PLA and ρ_EVA are the density of PLA and the solvent, respectively, while a and b are the mass fractions of PLA and EVA in the gel, respectively. M_S is the weight of the absorbed solvent, i.e. M_S = M₁ − M₂.

Thermal gravimetric analysis (TGA). TGA (1100SF, Mettler-Toledo International Trade Co., Ltd. Switzerland) was used to evaluate the thermal decomposition behavior and the composition of the TPV gel. The TGA measurements were performed from 20 to 600 °C at 10 °C min⁻¹ in a nitrogen atmosphere.

Rheological behavior. Dynamic rheological experiments were carried out on a DHR-2 rheometer (TA Instruments, USA) in a plate–plate configuration (25 mm in diameter and 1 mm in gap) at 170 °C. The samples were tested in a frequency-sweep mode (from 100 to 0.01 Hz) with an optimal strain of 1%. The optimal strain was pre-determined from a strain-sweep experiment to ensure that the measurements were performed in the linear viscoelastic strain range.

Dynamic mechanical analysis (DMA). DMA (Q800, TA Instruments, USA) was carried out in tensile-film mode to measure the dynamic mechanical properties and thermal behavior of the TPV. The specimens (15 × 5.3 × 0.5 mm³) were tested under a nitrogen atmosphere from −60 °C to 120 °C at a temperature ramp of 3°C min⁻¹. The frequency and amplitude were set as 1 Hz and 20 μm, respectively. The storage modulus and loss modulus were recorded as a function of temperature.

Atomic force microscopy (AFM). AFM (Nanonavi E-Sweep SPM, Seiko Instruments, Japan) was used to study the phase morphology and phase inversion of the TPV as a function of DCP content. The AFM was operated in the tapping mode under an air atmosphere. The TPV samples were cryo-microtomed, and the resulting ultrasmooth surfaces were subjected to AFM characterization.

Mechanical properties. The tensile properties of the TPVs were measured using a universal tensile tester (Instron 5967, USA) according to the GBT529-2008 standard at a tensile speed of 200 mm min⁻¹. The dimension of the parallel section of the tensile bar was 25 × 4 × 1 mm³. Five specimens of each sample were tested, and the averaged results are presented. The tensile permanent set (S_p) of each specimen was measured according to S_p = (L₁ − L₀)/L₀ × 100%, where L₀ is the original length of the parallel section, while L₁ is the final length of the stretched parallel section, which was measured 24 hours after fracture. The hardness was measured using a Shore D hardness tester (LX-D, Qianzhou Measuring Instrument, Wuxi, China) according to the GB/T 531.1-2008 standard. All the mechanical tests were performed at room temperature.

3. Results and discussion

3.1 Crosslink-structure analysis of the PLA/EVA-based TPV

A crosslinked network (gel) was generated after the addition of dicumyl peroxide (DCP) to the PLA/EVA blends, i.e., TPV. The structure and composition of the gel are very important to the phase morphology and mechanical properties of the TPV; thus, these properties were first studied using swelling equilibrium experiments, as described in Section 2.3.

The gel swelling ratios (S) of the TPV were measured as a function of DCP content, and the results are shown in Fig. 1. The S values gradually decreased from 42 to 23 as the DCP content increased from 0.5 wt% to 3.0 wt%. These results indicate an increase in the crosslink density of the gel with DCP content, which is more pronounced at low DCP contents (0.5–1.0 wt%). However, the crosslink density of the gel is low because the S values are larger than 20 even at a DCP content of 3.0 wt%.

Fig. 1 The swelling ratio (S) of the PLA/EVA-based TPV as a function of DCP content.

Although the DCP was premixed in EVA, a certain amount of DCP can still migrate into the PLA phase during subsequent compounding. It is known that both EVA and PLA can crosslink with peroxide via a free-radical reaction mechanism.^29,30 Such reactions at the interface would increase the compatibility of the polymer blends, resulting in enhanced mechanical properties of the TPV.^31–33 Therefore, the gel of the TPV likely consists of both PLA and EVA components. The composition of the gel not only affects the morphology and properties, but also the processability of the TPV.

The decomposition behavior and composition of the gel were characterized using thermal gravimetric analysis (TGA), as shown in Fig. 2. EVA exhibited a two-step decomposition behavior related to the side chain (CH₃COO–, around 335 °C) and the main chain (–CH₂–CH₂–, around 445 °C), respectively,³² while PLA decomposed in a single step (around 340 °C). It can be observed from Fig. 2 that the residual mass of the gel at 400 °C monotonically decreased with increasing DCP content, which demonstrated an increase in the PLA fraction of the gel.

Fig. 2 TGA curves of EVA, PLA and the gel of the TPV.

The quantity of each component in the gel and the gel fraction of each component can be calculated based on the overall gel fraction and the TGA analysis. The results are shown in Fig. 3. Apparently, the gel fraction of the EVA phase is always higher than that of the PLA phase at the examined DCP range due to the premixing technique, while the overall gel fraction of the TPV is in between. Interestingly, a two-stage curing behavior as a function of DCP content was observed. In stage I (DCP ≤ 1 wt%), the gel fraction of the EVA phase linearly increased with DCP content, whereas the gel fraction of the PLA phase remained at a low level (<10 wt%). On the contrary, the gel fraction of the EVA phase leveled-off in stage II (DCP ≥ 1.5 wt%), while the gel fraction of the PLA phase linearly increased with the DCP content. These results clearly indicate that it is mainly the EVA phase that is crosslinked at low DCP content. DCP becomes “saturated” in the EVA phase when it exceeds 1 wt%, and the excess amount of DCP could migrate into the PLA phase during mixing, leading to the undesirable crosslinking of the PLA phase. To serve as a commodity TPV material, a higher gel fraction of the rubber phase (EVA) and a lower gel fraction of the plastic phase (PLA) are preferable.³⁴ Thus, the ideal DCP content should be around 1 wt%.

Fig. 3 Gel fraction of each component in the PLA/EVA-based TPV as a function of DCP content.

3.2 Rheological behavior of the PLA/EVA-based TPV

Rheology has been proven to be a powerful technique to investigate the structures of polymers and polymer blends. The storage modulus (G′) and the complex viscosity (η*) of the PLA/EVA-based TPV are shown as a function of DCP content and frequency (Fig. 4a and b). The G′ and η* are affected by the three dimensional network that results from dynamic crosslinking. Consequently, this leads to a sharp rise in both G′ and η* with DCP content at a frequency (ω) less than 1 Hz, indicating an increased melt elasticity of the TPV. When the frequency approaches 0.01 Hz, the slope of log(G′) versus log(ω) decreases from 0.5 to 0.15 with DCP content up to 3.0 wt% (Fig. 4a); meanwhile, the slope of log(η*) vs. log(ω) is reduced from −0.45 to −0.85 (Fig. 4b). Apparently, the TPV showed a solid-like behavior at the low frequency zone due to the presence of networks. These results show good consistency with the gel analysis. It should be noted that all the samples show similar viscosities at the high frequency zone, which means that the TPV retains its processability after dynamic crosslinking.

Fig. 4 (a) Storage modulus and (b) complex viscosity of the PLA/EVA-based TPV as a function of frequency and DCP content.

3.3 Phase morphology of the PLA/EVA-based TPV

It is well-known that the mechanical properties of multiphase polymer blends depend largely upon their morphologies; thus, AFM was used to identify the phase structures of the dynamically vulcanized EVA/PLA blends with different DCP content, as shown in Fig. 5. The dark region in the phase-angle images (Fig. 5a–c) corresponds to the EVA phase, while the bright region corresponds to the PLA phase. A contrast inversion occurred in the height images (Fig. 5a′–c′).

Fig. 5 AFM images of the PLA/EVA-based TPVs with different DCP content. (a, a′) 0.0, (b, b′) 0.5 and (c, c′) 1.0 wt%. (a–c) are phase images while (a′–c′) are height images.

Apparently, a typical sea-island structure is observed in the blend without DCP, where PLA is the fine dispersed phase (Fig. 5a and a′). As expected, a phase inversion occurred when 1.0 wt% of DCP was added (Fig. 5c and c′). A co-continuous-like morphology appears to be present in the images Fig. 5b and b′, which is regarded as a transition state of the phase inversion with DCP content. The phase inversion is mainly due to the increased viscosity of the EVA phase, caused by dynamic crosslinking. During dynamic vulcanization, the EVA rubber phase was dominantly crosslinked and immobilized; thus, it could be further broken into micron-sized particles under the applied shear field.³⁵ The particle size is associated with the crosslink extent, the viscosity ratio of the two phases, the shear rate and the processing time.³⁶ In addition, no significant interfacial debonding is observed from the AFM images, indicating fine wetting and interactions between the EVA and PLA phases.

3.4 Dynamic mechanical properties of the PLA/EVA-based TPV

The dynamic mechanical properties of the PLA/EVA blends with and without DCP were studied using dynamic mechanical analysis (DMA). The storage modulus (E′) and loss modulus (E′′) as a function of temperature are shown in Fig. 6. The peak temperatures of E′′ are referred to as glass transition temperatures (T_g) in this work.

Fig. 6 (a) Storage modulus (E′) and (b) loss modulus (E′′) of the PLA/EVA-based TPV as a function of temperature and DCP.

In general, polymer chains and segments are rigid below the T_g of the polymer; thus, both the samples show high E′ values (>2000 MPa) at low temperatures (<T_g-EVA ≈ −28 °C), as shown in Fig. 6a. The E′ of the physical PLA/EVA blend (0 wt% DCP) was reduced by 2 orders of magnitude when the temperature was increased to the T_g-EVA. This result indicates that EVA is the matrix, while PLA is the dispersed phase. In contrast, the E′ of the PLA/EVA-based TPV (cured with 1 wt% of DCP) remained above 500 MPa up to the T_g-PLA (≈65 °C), indicating a continuous PLA phase in the PLA/EVA-based TPV. Evidently, a phase inversion of the PLA/EVA blends occurred after the dynamic crosslinking, which is consistent with the morphology observation (AFM images). It was also noticed that the E′ of the PLA/EVA-based TPV increased at around 100 °C, which is due to the cold crystallization (T_cc) of the PLA phase.³⁷ The loss modulus as a function of temperature is shown in Fig. 6b. Only one E′′ peak (corresponding to the T_g-EVA) is observed in the physical PLA/EVA blend, suggesting that the fine dispersed PLA domains have less effect on the E′′ in comparison with the EVA matrix. In addition, the T_g-PLA response is clearly visible in the PLA/EVA-based TPV due to the phase inversion.

3.5 Mechanical properties

3.5.1 Effect of crosslink agent. The stress–strain curves and mechanical properties of the EVA/PLA-based TPV as a function of DCP content are presented in Fig. 7a–c. PLA is a brittle polymeric material with low elongation at break (ε_b ≈ 4%) and high strength (σ_t ≈ 60–70 MPa),⁶ while the σ_t and ε_b of the EVA rubber are around 5 MPa and 800%, respectively. The PLA/EVA (40/60) blend in which EVA is the matrix showed the typical tensile behavior of an uncured rubber, i.e. high ε_b (≈600%) but low σ_t (8 MPa) and tensile modulus (Fig. 7a). After the addition of DCP, the stress–strain behavior changed obviously. Notably, a much higher tensile modulus and more pronounced strain hardening were observed for the TPV in comparison with the physical PLA/EVA blend. It can be seen from Fig. 7b that the σ_t gradually increased from 8 MPa to 20 MPa with DCP up to 1 wt%, and decreased with a further increase in DCP content. The stress at 100% elongation (σ_100%) exhibited a similar trend to the σ_t. Moreover, the ε_b decreased monotonically with the DCP content, which is in agreement with the mechanical behavior of cured rubber.²⁶ Tensile set (T_S) and hardness are important for TPV. Low T_S indicates high elastic recovery for a rubbery material. The tensile set of the PLA/EVA blend is 90%, which was reduced to around 30% after the addition of 0.5–3.0 wt% DCP. On the other hand, the hardness of the PLA/EVA blend was enhanced from 14 Shore D to around 50 Shore D after the addition of DCP due to the phase inversion. Apparently, the mechanical behavior is strongly associated with the above discussed crosslink structure and the phase morphology of the PLA/EVA-based TPV.

Fig. 7 Mechanical behavior of the PLA/EVA-based TPV as a function of DCP content: (a) stress–strain curves, (b) tensile strength, stress at 100% elongation and elongation at break and (c) tensile set and hardness.

3.5.2 Effect of plasticization. As discussed above, good mechanical properties were obtained for PLA/EVA-based TPV with a DCP content of 1.0 wt% such as high tensile strength, reasonable elongation at break and relatively low tensile set. To enhance the flexibility and reduce the hardness of the TPV (1.0 wt% DCP), a plasticizer, i.e., acetyl tributyl citrate (ATBC), was incorporated into the TPV. The mechanical properties of the TPV with varying amounts of ATBC are shown in Fig. 8. As expected, the tensile strength and hardness of the TPV gradually decreased with increasing ATBC content. Moreover, the elongation at break was increased from 330% to 500%. It should be noted that a larger tensile set is obtained as well due to the higher elongation at break and the plasticization of the PLA matrix.

Fig. 8 Mechanical properties of the PLA/EVA-based TPV with varying amounts of ATBC: (a) tensile strength, stress at 100% elongation and elongation at break and (b) tensile set and hardness.

To summarize, the mechanical properties of the PLA/EVA-based TPV can be tuned by the DCP content and utilization of plasticizer.

4. Conclusions

A series of PLA/EVA-based TPV were successfully prepared via two-step mixing and dynamic vulcanization in the presence of dicumyl peroxide (DCP). Due to the two-step mixing technique, the gel fraction of the EVA phase was always higher than that of the PLA. Crosslinking of the EVA was dominant when the DCP content was lower than 1 wt%, while the gel fraction of the PLA phase increased linearly when the DCP content exceeded 1 wt%. A desirable phase inversion occurred in the PLA/EVA (40/60) blend after the addition of a small amount of DCP (0.5–3.0 wt%), which was confirmed by AFM observation and dynamical mechanical analysis. The mechanical properties reveal that PLA/EVA-based TPV with high strength, high elongation at break, intermediate hardness and relatively low tensile set can be obtained by tuning the DCP concentration. The optimum DCP content, in terms of mechanical properties, is proposed to be around 1.0 wt%. In addition, the mechanical properties of the TPV can be further tuned by plasticization. The development of this new bio-based PLA/EVA-based TPV may broaden the application ranges of both PLA and EVA rubber.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (51303067) and the Natural Science Foundation of Jiangsu Province (BK20130147).

References

1. I. Armentano, N. Bitinisand and E. Fortunati, Prog. Polym. Sci., 2013, 38, 1720–1747.
2. R. Auras, B. Harte and S. Selke, Macromol. Biosci., 2004, 4, 835–864.
3. R. M. Rasal, A. V. Janorkar and D. E. Hirt, Prog. Polym. Sci., 2010, 35, 338–356.
4. G. Y. A. Tan, C. L. Chen and L. Li, Polymers, 2014, 6, 706–754.
5. H. Liu, F. Xie and L. Yu, Prog. Polym. Sci., 2009, 34, 1348–1368.
6. P. Ma, D. G. Hristova-Bogaerds and J. G. P. Goossens, Eur. Polym. J., 2012, 48, 146–154.
7. K. S. Anderson, K. M. Schreck and M. A. Hillmyer, Polym. Rev., 2008, 48, 85–108.
8. H. Liu and J. Zhang, J. Polym. Sci., Part B: Polym. Phys., 2011, 49, 1051–1083.
9. G. Kfoury, J. Raquez, F. Hassouna, J. Odent, V. Toniazzo, D. Ruch and P. Dubois, Front. Chem., 2013, 1, 1–45.
10. J. F. Zhang and X. Z. Sun, Polym. Int., 2004, 53, 716–722.
11. D. Yuan, K. Chen and C. Xu, Carbohydr. Polym., 2014, 113, 438–445.
12. D. Li, B. Shentu and Z. Weng, J. Macromol. Sci., Part B: Phys., 2011, 50, 2050–2059.
13. W. Zhang, L. Chen and Y. Zhang, Polymer, 2009, 50, 1311–1315.
14. L. Han, C. Han and L. Dong, Polym. Int., 2013, 62, 295–303.
15. L. Han, C. Han and L. Dong, Polym. Compos., 2013, 34, 122–130.
16. A. Mirzadeh, P. G. Lafleur and M. R. Kamal, Polym. Eng. Sci., 2010, 50, 2131–2142.
17. Y. Kikuchi, T. Fukui and T. Okada, Polym. Eng. Sci., 1991, 31, 1029–1032.
18. J. P. Sheth, J. N. Xu and G. L. Wilkes, Polymer, 2003, 44, 743–756.
19. P. Dey, K. Naskar and B. Dash, RSC Adv., 2014, 4, 35879–35895.
20. M. Bousmina and R. Muller, Rheol. Acta, 1996, 35, 369–381.
21. S. Abdou-Sabet, R. C. Puydak and C. P. Rader, Rubber Chem. Technol., 1996, 69, 476–494.
22. K. Naskar and J. W. M. Noordermeer, J. Elastomers Plast., 2006, 38, 163–180.
23. K. Naskar, U. Gohs, U. Wagenknecht and G. Heinrich, eXPRESS Polym. Lett., 2009, 3, 677–683.
24. C. Joubert, P. Cassagnau and A. Michel, Polym. Eng. Sci., 2002, 42, 2222–2233.
25. O. Bianchi, A. J. Zattera and L. B. Canto, J. Elastomers Plast., 2010, 4, 561–575.
26. W. Wu, C. Wan and Y. Zhang, J. Appl. Polym. Sci., 2013, 130, 338–344.
27. A. Šturcová, G. R. Davies and S. J. Eichhorn, Biomacromolecules, 2005, 6, 1055–1061.
28. M. Amin, G. M. Nasr, G. Attia and A. S. Gomaa, Mater. Lett., 1996, 28, 207–213.
29. W. Dong, P. Ma and S. Wang, Polym. Degrad. Stab., 2013, 98, 1549–1555.
30. O. Bianchi, J. D. N. Martins and R. Fiorio, Polym. Test., 2011, 30, 616–624.
31. P. Ma, D. G. Hristova-Bogaerds and P. J. Lemstra, Macromol. Mater. Eng., 2012, 297, 402–410.
32. R. Wang, S. Wang and Y. Zhang, Polym. Eng. Sci., 2009, 49, 26–33.
33. A. Y. Coran and R. Patel, Rubber Chem. Technol., 1981, 54, 892–903.
34. P. Ma, D. G. Hristova-Bogaerds and P. Schmit, Macromol. Res., 2012, 20, 1054–1062.
35. R. R. Babu and K. Naskar, Adv. Rubber. Compos., 2011, 239, 219–248.
36. A. K. Bhowmick and T. Inoue, J. Appl. Polym. Sci., 1993, 49, 1893–1900.
37. P. Song, G. Chen, Y. Chang and Z. Wei, Polymer, 2012, 53, 4300–4309.

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