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Influence of molten-state annealing on the phase structure and crystallization behaviour of high impact polypropylene copolymer
Polymer 52 (2011) 2956e2963
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Polymer journal homepage: www.elsevier.com/locate/polymer
Influence of molten-state annealing on the phase structure and crystallization behaviour of high impact polypropylene copolymer Ruifen Chen a, b, Yonggang Shangguan a, b, *, Chunhui Zhang a, b, Feng Chen a, b, Eileen Harkin-Jones c, Qiang Zheng a, b, * a b c
Key Laboratory of Macromolecular Synthesis and Functionalization, Ministry of Education, Hangzhou 310027, PR China Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, PR China Polymers Research Cluster, School of Mechanical and Aerospace Engineering, Queen’s University Belfast, Belfast BT9 5AH, UK
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Article history: Received 16 December 2010 Received in revised form 14 April 2011 Accepted 1 May 2011 Available online 6 May 2011
The phase structure evolution of high impact polypropylene copolymer (IPC) during molten-state annealing and its influence on crystallization behaviour were studied. An entirely different architecture of the IPC melt was observed after being annealed, and this architecture resulted in variations of the crystallization behaviour. In addition, it was found that the core-shell structure of the dispersed phase was completely destroyed and the sizes of the dispersed domains increased sharply after being annealed at 200 C for 200 min. Through examination of the coarseness of the phase morphology using phase contrast microscopy (PCM), it was found that a co-continuous structure and an abnormal ‘sea-island’ structure generally appeared with an increase in annealing time. The original matrix PP component appeared as a dispersed phase, whereas the copolymer components formed a continuous ‘sea-island’ structure. This change is ascribed to the large tension induced by solidification at the phase interface and the great content difference between the components. When the temperature was reduced the structure reverted to its original form. With increasing annealing time, the spherulite profiles became more defined and the spherulite birefringence changed from vague to clear. Overall crystallization rates and nucleation densities decreased, but the spherulite radial growth rates remained almost constant, indicating that molten-state annealing mainly affects the nucleation ability of IPC, due to a coarsened microstructure and decreased interface area. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: High impact polypropylene copolymer (IPC) Molten-state annealing Phase inversion
1. Introduction Over the past two decades high impact polypropylene copolymer (IPC) has attracted much attention due to its excellent toughness [1e3]. A typical IPC is prepared via a continuous polymerization system which includes homo-polymerization of propylene and subsequent copolymerization of propylene and ethylene in a gas-phase reactor with a special ZieglereNatta catalyst [4,5]. Much work has been published on IPC or so-called ‘PP inreactor alloy’ in recent years [6e13]. According to previous report [14], the composition of IPC is very complex consisting mainly of ethylene-propylene random copolymer (EPR), ethylene-propylene
* Corresponding authors. Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, PR China. Tel./fax: þ86 571 8795 2522. E-mail addresses: [email protected] (Y. Shangguan), [email protected] (Q. Zheng). 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.05.005
block copolymer with different segment lengths (EbP) and propylene homopolymer (PP). In addition, a micro-phase separated structure in IPC was observed, and a multilayered core-shell structure of the dispersed phase was proposed to describe its morphology and explain its high toughness [15,16]. It is well-known that PP/EPR blends prepared by melt or solution blending form an immiscible system in the molten-state and that phase separation occurs [17e19]. Considering that the PP and EPR components are the matrix and the main dispersed phase in IPC respectively, whether the micro-phase separated structure of IPC further coarsens during molten-state annealing or not is not clear. To date, little attention has been paid to the possible evolution in structure and morphology of IPC during molten-state annealing despite the fact that changes in structure will invariably lead to changes in performance. Chen Y et al. [20] studied the phase morphology evolution of hiPP upon thermal treatment after extrusion melt-compounding and they suggested that the meltprocessing conditions could affect the phase structure. Recently
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thermal degradation and crosslinking of IPC have been investigated by means of atomic force microscopy (AFM), temperature rising elution fractionation (TREF), differential scanning calorimeter (DSC), gel permeation chromatography (GPC) and dynamic rheological measurement, respectively [21e24]. Yet only changes in the macromolecular structure of IPC were considered, and the influence of annealing on architecture was not considered. It was found in our previous research [25] that the molten-state annealing of polypropylene catalloys had a remarkable effect on its crystallization behaviour, and it was suggested that these phenomena result from possible changes in the micro-phase structure. In the present research, we try to probe the influences of molten-state annealing on the architecture and phase morphology of IPC, and study the crystallization behaviour of IPC post annealing. In order to avoid possible degradation and crosslinking of IPC molecules during annealing, all the experiments were conducted under a nitrogen atmosphere. 2. Experimental 2.1. Materials The material used was a commercial high impact polypropylene copolymer in granular form (SP179, Sinopec Qilu Petrochemical Co. of China) with Mw ¼ 1.74 105 and Mn ¼ 4.39 104. 2.2. Fractionation of IPC The original IPC was fractionated by temperature-gradient extraction fractionation using n-Octane as the solvent. Firstly, 25 g of IPC pellets were completely dissolved in n-Octane at 125 C, and then the solution was subsequently cooled down to 50 C and held for 72 h when the EPR fraction was collected. Then, the remaining sample was extracted at 100 C for 72 h and the EbP fraction collected. The remaining fraction was isotactic polypropylene (iPP). An iPP/EPR binary blend was prepared by solution-mixing. Firstly, the iPP and EPR fractions were weighted according to the designated proportion, and then dissolved in boiling xylene. Subsequently, the solution was stirred thoroughly for 30 min to ensure complete mixing. This was followed by precipitation into ice cooled methanol and then washing. Finally, the blend was dried at ambient temperature for 48 h and subsequently dried in vacuum at 80 C for 24 h.
observed using SEM (S4800, Hitachi, Japan). An operating voltage of 5 kV was used. 2.4. Dynamic mechanical analysis Dynamic mechanical analysis (DMA) was carried out using a Q800, TA Instruments Corporation dynamic mechanical analyzer. The film specimens were prepared by compression molding at 170 C and characterized in the tensile mode at a heating rate of 3 C/min and a frequency (u) of 10 Hz. 2.5. Differential scanning calorimetry A modulated differential scanning calorimeter (Q100, TA Instruments Corporation) with N2 as purge gas was used to probe the isothermal and non-isothermal crystallization behaviour of IPC specimens. Pure indium and zinc were used as reference materials to calibrate both the temperature scale and the melting enthalpy before the specimens were tested. Firstly, the IPC specimens were heated to 200 C and held for different time periods before quenching down to 140 C to complete the isothermal crystallization. When the crystallization was completed, the samples were cooled down to 40 C at 30 C/min and subsequently their melting traces were recorded at a heating rate of 10 C/min. Similarly, the specimens annealed at 200 C for different time periods were cooled down to 40 C at 10 C/min and then reheated to 200 C at 10 C/min to examine the non-isothermal crystallization and melting behaviour. 3. Results and discussion 3.1. Phase morphology evolution during molten-state annealing As reported previously, IPC presents a phase-separated structure in which the dispersed phase appears as a multilayered coreshell structure. In Fig. 1 two tand peaks at 37 C and 14.5 C appear, corresponding to the glass transitions of the EPR and iPP, respectively [26]. A weak peak is observed at 113.5 C, which may result from the EbP fraction [26]. These results indicate the existence of a complex phase-separated microstructure in IPC. The inlay in Fig. 1 shows the morphology of the etched fracture surface of IPC. It can be seen that in the matrix there are some aggregated granules as well as the uniformly dispersed holes (0.5e2 mm). The dispersed particle is composed of not only EPR but also EbP [26], and the aggregate granules are ascribed to the aggregation of those
2.3. Morphology observation Phase contrast optical microscopy (PCM), polarized optical microscopy (POM) and scanning electron microscopy (SEM) were used to observe the morphology of the IPC and iPP/EPR blend specimens. PCM and POM observations were carried out using an optical microscope (BX51, Olympus) equipped with an Olympus camera and the temperature was monitored with a temperature controlled hot stage (THMS600, Linkam Co.). The specimens were sandwiched between two microscope cover slips for observation. PCM micrographs of the IPC specimens annealed at 200 C for different time (10, 30, 60, 90, 120, 200 min) were taken as the morphology evolved. All thermal treatments proceeded under nitrogen atmosphere to avoid possible degradation or crosslinking. Isothermal crystallization experiments were conducted at 140 C and POM was used to observe the morphologic development during crystallization. IPC and iPP/EPR specimens were annealed at 200 C for 200 min on the THMS600 hot stage before fracturing in liquid N2. The fracture surfaces were etched in xylene for 12 h at room temperature, and then coated with gold-palladium and
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Fig. 1. SEM image and temperature dependence of tand for original IPC.
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Fig. 2. SEM images and partial enlargement of the extracted cross-section surface of IPC after being annealed at 200 C for 200 min.
dispersed particles which could not be etched by xylene. Fig. 2 shows the phase morphology of IPC annealed at 200 C for 200 min. It can be seen that the core-shell structure of the dispersed phase in the IPC sample disappears and some remarkable changes take place: (i) the holes and aggregated granules disappear, (ii) dispersed domains with an irregular shape appear, and (iii) the sizes of the dispersed domains increase. After annealing in the molten-state, although the sizes of the dispersed domains become 5e10 times larger than those of the original IPC, the dispersion seems more uniform with the disappearance of holes and aggregated granules. In addition, the dispersed domains connect to one another and a network-like structure is observed as seen in the enlarged figure. These results indicate that annealing at 200 C leads to a definite change in architecture and morphology of IPC. To observe the morphology evolution of IPC in more depth, the morphology of iPP/EPR binary blends (prepared by the fractions of IPC), in which the weight ratio of the two components is the same as that in IPC, was investigated. Fig. 3a shows the SEM image of the original iPP/EPR sample. It is found that without the EbP component, the dispersed phase dimensions are much smaller than those of the IPC and are typically no more than 0.2 mm. Although it is proposed that the ethyleneepropylene copolymer disperses well in IPC as compared with conventional polypropylene/ethyleneepropylene rubber blends prepared by mechanical blending for the existence of EbP [27], the iPP/EPR sample prepared by solution blending has obviously a more uniform dispersion compared with IPC. Fig. 3b shows the morphology of the iPP/EPR sample subjected to an annealing at 200 C for 200 min. It can be seen that the EPR domains retain their shapes and only a slight change in the size appears. Most of the dispersed particles are no more than 0.5 mm. These results show that the development of the dispersed phase in iPP/EPR is clearly different from that of IPC as shown in Fig. 2 and it is necessary to monitor the morphology evolution of IPC during annealing to explore the reason for the above difference. Fig. 4 shows the PCM images of IPC during annealing at 200 C. It can be seen that the phase domains coarsen gradually, and the domain
size of the dispersed phase becomes larger with increasing annealing time. Due to the resolution limit of optical microscopy, the phase-separated structure is blurred until the annealing time reaches 30 min. As the annealing time increases, demixing is detected but the boundary between two domains is obscure as shown in Fig. 4b and c. When the annealing time reaches 90 min, a phase boundary can be distinguished, and a co-continuous structure appears. Considering that a co-continuous structure is one of the most important characteristics of spinodal demixing (SD), these results indicate that the coarseness development of IPC follows an SD mechanism. Fig. 4f presents a clear ‘sea-island’ structure indicating the disappearance of the co-continuous structure in the IPC sample. When the annealing time further increases, little change in morphology is observed indicating that an equilibrium phase structure has been reached. It is noted that little change of phase structure for the iPP/EPR samples can be observed via PCM, proving that this binary blend has a very slow rate of coarseness development, and the changes observed via SEM lie in the early stage of phase separation. Usually, a sea-island structure can be observed at the last stage of liquideliquid phase separation of a polymer blend with the continuous phase (sea) resulting from the majority content component, and the dispersed phase, from the minority component. However, it is surprising to find that the area of the dispersed phase (island) is much larger than that of ‘sea’ in the IPC annealed for 200 min. These results indicate that there exists an abnormal structure in the IPC samples, i.e. the dispersed phase (island) is PP, and the continuous phase (sea) is the ethyleneepropylene copolymer including the EPR and EbP components in the IPC. A similar phase inversion phenomenon in which the minor component forms the continuous phase during compounding has been reported [28] and this was ascribed to the lower viscosity of the minor component. However, in the present research the viscosity of EPR is much higher than that of PP as shown in Fig. 5. In the annealing process, two phases aggregate respectively and the domains coarsen. Because of the existence of EbP, the activity of the
Fig. 3. SEM images of extracted cross-section surface of (a) original iPP/EPR and (b) that after being annealed at 200 C for 200 min.
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Fig. 4. Phase contrast micrographs of IPC samples annealed at 200 C for different time periods. (a) 10 min, (b) 30 min, (c) 60 min, (d) 90 min, (e) 120 min, (f) 200 min.
dispersed phase in IPC is much stronger than that of the PP/EPR blend, and it makes phase inversion possible. In addition, once the EPR forms the continuous phase, it is hard to be destroyed due to its high viscosity in the molten-state. The appearance of the ‘sea-island’ phase inversion structure indicates that the original dispersed phase (ethyleneepropylene copolymer, including EPR and EbP) forms into a continuous phase and divides polypropylene into domains with different dimensions. Phase inversion happens if the blend composition and viscosity ratio change [29e31]; however it has rarely been reported that an annealing process can induce phase inversion of an immiscible polymer blend. Considering the results shown in Fig. 2, some interesting phenomena need to be noted, e.g. the continuous phase in the molten-state, ethylene-propylene copolymer converts to the dispersed phase when the IPC sample is cooled down to room temperature after being annealed for 200 min, whereas the dispersed phase, PP component in the molten-state, becomes the matrix phase again. Fig. 6 presents a schematic diagram of the phase morphology evolution of IPC. These results indicate that the copolymer domains cannot exist when temperature drops, which can be ascribed to the large tension induced by solidification at the phase interface and the great content difference between components.
Fig. 5. Relationship between complex viscosity and frequency of PP and EPR fractions in IPC at 200 C.
3.2. Influence of annealing on crystalline morphology Differences in the phase structure of a molten polymer blend may lead to different crystallization behaviour if the blend contains crystallizable polymer component. Since IPC samples exhibit different liquideliquid phase structures during annealing, it is important to investigate the effect of molten-state annealing on the crystallization behaviour of IPC. Fig. 7 shows the polarized optical micrographs of IPC samples crystallized isothermally at 140 C after being annealed for different time at 200 C. It can be seen that although the crystalline morphologies in all samples appear as spherulites, the spherulite profile becomes clear gradually and the corresponding Maltase cross seems more perfect with an increase in annealing time. Also, compared with the sample annealed for 10 min, the sample annealed for 200 min has a larger spherulite dimension. Since PP is the main crystallizable component, the above results indicate that the remaining portions from EbP and EPR molecules may affect the size and the birefringence of the PP crystal. Fig. 8 shows the crystallization micrographs of IPC at 140 C after being annealed for different time at 200 C. It is seen from the POM images that three samples exhibit distinct birefringence and spherulite density, and the birefringence of the PP sample is more perfect and distinct than those of the IPC samples. When observing two IPC samples annealed for different time there is a marked difference in birefringence. For the IPC sample annealed for 10 min, the incomplete birefringence mainly results from the vague Maltase cross. Contrarily, the birefringence of the IPC sample annealed for 200 min seems to be separated into many smaller disordered pieces although the spherulite profile looks very clear. It is well-known that during melt-crystallization of polymers, the spherulites are made up of a crystalline phase and an amorphous phase. In IPC samples containing uncrystallizable ethyleneepropylene copolymer, the amorphous phase in the spherulite should be a mixture of uncrystallizable copolymer and PP homopolymer, depending on the degree of phase separation. As is shown by bright field images of IPC samples in Fig. 8, the sample annealed for 10 min presents a tiny phase-separated state in the spherulite region, and there is no obvious domain of amorphous ethyleneepropylene copolymer, and consequently a vague Maltase cross can be observed. On the contrary, obvious domains can be found in the image of the IPC sample annealed for 200 min. Because the area of crystalline homopolymer covers the amorphous homopolymer, it is suggested that the continuous domain in the spherulite region
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Fig. 6. Schematic of phase morphology evolution of IPC annealed for 200 min at 200 C during cooling. (a) in the melt state, (b) at solidification.
Fig. 7. Polarized optical micrographs of IPC samples crystallized isothermally at 140 C after being annealed at 200 C for different time periods. (a)10 min, (b)30 min, (c) 60 min, (d) 90 min, (e)120 min, (f)200 min.
should be attributed to the amorphous copolymer and the ‘pieces’ are crystalline homopolymer, respectively. Due to the formation of two kinds of domains, the crystallization in IPC sample annealed for 200 min may happen in a relatively pure homopolymer region, unlike the sample annealed for 10 min, in which crystallization occurs in a mixture of homopolymer and copolymer. As a result, the
spherulite profile of the sample annealed for 200 min is relatively distinct but the inner morphology of the spherulite appears broken and discontinuous. Based on the above results and analysis, the incomplete Maltase Cross is thought to reflect that there may exists random copolymers between lamellas or fibrils of the PP spherulites, which then affect
Fig. 8. Bright field views and Polarized optical micrographs of fractionated iPP (a, a0 ) and IPC samples crystallized at 140 C after being annealed at 200 C for 10 min (b, b0 ) and 200 min (c, c0 ) respectively.
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Fig. 9. Development of relative crystallinity of IPC samples crystallized isothermally at 140 C after being annealed for different time at 200 C.
the birefringence of lamella along the radial and tangential directions. It is noted that the situation where the second component enters into lamellas of the crystalline polymer usually happens in miscible crystalline polymer blends. The above experimental results should be attributed to EbP to a large extent. As a compatiblizer, EbP can enhance the adhesion properties between the dispersed phase and the matrix. It should also be noted that serious aggregation lessens the interfacial area which may affect the crystallization, especially the nucleation stage, for IPC samples. This will be discussed in the following section.
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Fig. 11. Nucleation density of IPC crystallized isothermally at 140 C for 55 min after being annealed for different time periods at 200 C.
As mentioned above, the molten IPC exhibits different phase structures with an increase in annealing time, so it is necessary to investigate the crystallization behaviour of IPC which has been annealed in the molten-state for different time periods. Fig. 9 presents the crystallinity evolution of IPC samples crystallized isothermally at 140 C after being annealed at 200 C for different time. It can be seen that both the total crystallization time and halftime of crystallization, t1/2, of IPC generally increases with an
increase in annealing time. The isothermal crystallization of IPC at other temperatures behaves in a similar manner (not shown). These results show that molten-state annealing of IPC results in a decrease in the overall crystallization rate, a result that may be attributed to the phase separation of IPC induced by annealing. Since crystallization involves both nucleation and crystal growth, it is important to determine how these processes are individually affected by molten-state annealing. Fig. 10 shows the spherulite radius evolution of IPC crystallized isothermally at 140 C. It can be seen that for all IPC samples a linear spherulite growth is obtained and that growth rates are similar at a value of 1.1 mm/min for all samples. Since spherulite growths of both miscible and immiscible blends have been extensively studied and a linear spherulite growth is one of the characteeristics of an immiscible blend, these results suggest that the amorphous copolymer never goes into the lamella and only occurs between the lamellae. In other words, the phase separation in IPC is unlikely to affect the mobility of the propylene chains. This result also indicates that the reduction in crystallization rate of IPC as annealing time increases is not due to any effect on spherulite growth rate and therefore must be due to an effect on the nucleation process.
Fig. 10. Spherulite growth rates of IPC crystallized isothermally at 140 C after being annealed for different time periods at 200 C.
Fig. 12. Exotherms of IPC cooled down at a rate of 10 C/min after being annealed at 200 C for 10 min and 200 min respectively.
3.3. Influence of annealing on crystallization behaviour
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Fig. 13. Crystallization peak temperatures (Tc) of IPC cooled down at a rate of 10 C/min after being annealed for different time at 200 C.
For all of the IPC samples, the value of t1/2 is more than 55 min as shown by Fig. 9, hence, the morphologies of IPC samples crystallized isothermally for 55 min at 140 C are chosen to make a comparison of crystallization density. Fig. 11 shows the POM images of all IPC samples crystallized for 55 min and their statistical nucleation densities, which are the average of five tests. It can be observed that the nucleation density of IPC decreases gradually when the annealing time increases. Since the crystallization mechanism of iPP/EPR is characterized by heterogeneous nucleation [32], and taking into account the fact that the phase separation happened during annealing, the decrease in nucleation density is attributed to the decrease of interfacial area. Non-isothermal crystallization tests also provide proof that nucleation events are being influenced if the crystallization temperature is changed. Fig. 12 shows the exotherms of IPC crystallized at a cooling rate of 10 C/min. Two crystallization peaks appear, one at about 95 C and the other, at 118 C. These peaks correspond to the crystallizations of iPP and PE, respectively. It can be seen that both peaks shift to a lower temperature with the increasing annealing time. Fig. 13 shows the crystallization peak temperature (Tc) for IPC samples annealed for different time at 200 C. The Tc of PP drops by almost 3 C when annealing time increases from 10 to 200 min, while that of PE drops no more than 1 C. The change of Tc suggests that increasing annealing time leads to a decrease in nucleation ability. As PP is the majority component and it constitutes almost 70% in the sample, it will be affected greatly by changes in the dispersed phase compared with the other components. PCM micrographs show that the dispersed domains conglomerate, the two phases invert and the size increases greatly with increasing time. Also since the crystallization of the PP/EPR system is dominated by heterogeneous nucleation, the above results are attributed to the decrease in interfacial area induced by phase separation and in turn the reduction of nucleation points (Fig. 13). 4. Conclusion It has been found through morphological observations that the molten-state annealing of IPC has a significant effect on its architecture. When the IPC is annealed at 200 C for 200 min, the core-shell structure of the dispersed phase is destroyed completely, while the size increases sharply. Also, a network-like structure could be observed. There exists an obvious coarsening of the phase morphology, and with the increase of annealing time, a co-continuous structure and an abnormal ‘sea-island’ structure appear in which the original matrix PP component becomes to the dispersed phase. In comparison with the morphology of fractionated iPP/EPR binary blends which have the same weight ratio as IPC, it is found that unlike IPC, the former has a very slow
coarsening and there is no obvious change in phase structure during 200 min annealing. In addition, molten-state annealing greatly affects the crystalline morphology and crystallization behaviour. Although the spherulite profile generally becomes more well defined and crystal perfection increases with increasing annealing time, obvious differences in spherulite birefringence can be found depending on the phase structure of the IPC melt. With increasing annealing time, the spherulite radial growth rates remain almost constant while the nucleation density and thus overall crystallization rate decreases. It is suggested that a decrease in the nucleation ability of IPC with increasing annealing time is due to the coarsened microstructure and decreased in interface area. Acknowledgements This work was supported by National Basic Research Program of China (No. 2005CB623800), National Nature Science Foundation of China (No. 50603023), Nature Science Foundation of Zhejiang Province (No. Y4100314) and the Fundamental Research Funds for the Central Universities. References [1] Debling JA, Zacca JJ, Ray WH. Chemical Engineering Science 1997;52(12): 1969e2001. [2] Hermanová S, Tochácek J, Jancár J, Kalfus J. Polymer Degradation and Stability 2009;94(10):1722e7. [3] Mirabella JFM. Polymer 1993;34(8):1729e35. [4] Debling JA, Ray WH. Journal of Applied Polymer Science 2001;81(13): 3085e106. [5] Prasetya A, Liu L, Litster J, Watanabe F, Mitsutani K, Ko GH. Chemical Engineering Science 1999;54(15e16):3263e71. [6] Dong Q, Wang XF, Fu ZS, Xu JT, Fan ZQ. Polymer 2007;48(20):5905e16. [7] Cai HJ, Luo XL, Chen XX, Ma DZ, Wang JM, Tan HS. Journal of Applied Polymer Science 1999;71(1):103e13. [8] Cai HJ, Luo XL, Ma DZ, Wang JM, Tan HS. Journal of Applied Polymer Science 1999;71(1):93e101. [9] Li RB, Zhang XQ, Zhao Y, Hu XT, Zhao XT, Wang DJ. Polymer 2009;50(21): 5124e33. [10] Chen Y, Chen Y, Chen W, Yang D. Polymer 2006;47(19):6808e13. [11] Cui NN, Ke YC, Lu ZX, Wu CH, Hu YL. Journal of Applied Polymer Science 2006; 100(6):4804e10. [12] Song SJ, Wu PY, Feng JC, Ye MX, Yang YL. Polymer 2009;50(1):286e95. [13] Timothy FM, Djallel B, Shigeyuki M, Toshihiko S. Polymer Reaction Engineering 2003;11:177e97. [14] Tan HS, Li L, Chen ZN, Song YH, Zheng Q. Polymer 2005;46(10):3522e7. [15] Chen Y, Chen Y, Chen W, Yang DC. Journal of Applied Polymer Science 2008; 108(4):2379e85. [16] Zacur R, Goizueta G, Capiati N. Polymer Engineering and Science 2000;40(8): 1921e30. [17] D’Orazio L, Mancarella C, Martuscelli E, Cecchin G, Corrieri R. Polymer 1999; 40(10):2745e57. [18] Inaba N, Sato K, Suzuki S, Hashimoto T. Macromolecules 1986;19(6): 1690e5.
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Influence of molten-state annealing on the phase structure and crystallization behaviour of high impact polypropylene copolymer Ruifen Chen a, b, Yonggang Shangguan a, b, *, Chunhui Zhang a, b, Feng Chen a, b, Eileen Harkin-Jones c, Qiang Zheng a, b, * a b c
Key Laboratory of Macromolecular Synthesis and Functionalization, Ministry of Education, Hangzhou 310027, PR China Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, PR China Polymers Research Cluster, School of Mechanical and Aerospace Engineering, Queen’s University Belfast, Belfast BT9 5AH, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2010 Received in revised form 14 April 2011 Accepted 1 May 2011 Available online 6 May 2011
The phase structure evolution of high impact polypropylene copolymer (IPC) during molten-state annealing and its influence on crystallization behaviour were studied. An entirely different architecture of the IPC melt was observed after being annealed, and this architecture resulted in variations of the crystallization behaviour. In addition, it was found that the core-shell structure of the dispersed phase was completely destroyed and the sizes of the dispersed domains increased sharply after being annealed at 200 C for 200 min. Through examination of the coarseness of the phase morphology using phase contrast microscopy (PCM), it was found that a co-continuous structure and an abnormal ‘sea-island’ structure generally appeared with an increase in annealing time. The original matrix PP component appeared as a dispersed phase, whereas the copolymer components formed a continuous ‘sea-island’ structure. This change is ascribed to the large tension induced by solidification at the phase interface and the great content difference between the components. When the temperature was reduced the structure reverted to its original form. With increasing annealing time, the spherulite profiles became more defined and the spherulite birefringence changed from vague to clear. Overall crystallization rates and nucleation densities decreased, but the spherulite radial growth rates remained almost constant, indicating that molten-state annealing mainly affects the nucleation ability of IPC, due to a coarsened microstructure and decreased interface area. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: High impact polypropylene copolymer (IPC) Molten-state annealing Phase inversion
1. Introduction Over the past two decades high impact polypropylene copolymer (IPC) has attracted much attention due to its excellent toughness [1e3]. A typical IPC is prepared via a continuous polymerization system which includes homo-polymerization of propylene and subsequent copolymerization of propylene and ethylene in a gas-phase reactor with a special ZieglereNatta catalyst [4,5]. Much work has been published on IPC or so-called ‘PP inreactor alloy’ in recent years [6e13]. According to previous report [14], the composition of IPC is very complex consisting mainly of ethylene-propylene random copolymer (EPR), ethylene-propylene
* Corresponding authors. Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, PR China. Tel./fax: þ86 571 8795 2522. E-mail addresses: [email protected] (Y. Shangguan), [email protected] (Q. Zheng). 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.05.005
block copolymer with different segment lengths (EbP) and propylene homopolymer (PP). In addition, a micro-phase separated structure in IPC was observed, and a multilayered core-shell structure of the dispersed phase was proposed to describe its morphology and explain its high toughness [15,16]. It is well-known that PP/EPR blends prepared by melt or solution blending form an immiscible system in the molten-state and that phase separation occurs [17e19]. Considering that the PP and EPR components are the matrix and the main dispersed phase in IPC respectively, whether the micro-phase separated structure of IPC further coarsens during molten-state annealing or not is not clear. To date, little attention has been paid to the possible evolution in structure and morphology of IPC during molten-state annealing despite the fact that changes in structure will invariably lead to changes in performance. Chen Y et al. [20] studied the phase morphology evolution of hiPP upon thermal treatment after extrusion melt-compounding and they suggested that the meltprocessing conditions could affect the phase structure. Recently
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thermal degradation and crosslinking of IPC have been investigated by means of atomic force microscopy (AFM), temperature rising elution fractionation (TREF), differential scanning calorimeter (DSC), gel permeation chromatography (GPC) and dynamic rheological measurement, respectively [21e24]. Yet only changes in the macromolecular structure of IPC were considered, and the influence of annealing on architecture was not considered. It was found in our previous research [25] that the molten-state annealing of polypropylene catalloys had a remarkable effect on its crystallization behaviour, and it was suggested that these phenomena result from possible changes in the micro-phase structure. In the present research, we try to probe the influences of molten-state annealing on the architecture and phase morphology of IPC, and study the crystallization behaviour of IPC post annealing. In order to avoid possible degradation and crosslinking of IPC molecules during annealing, all the experiments were conducted under a nitrogen atmosphere. 2. Experimental 2.1. Materials The material used was a commercial high impact polypropylene copolymer in granular form (SP179, Sinopec Qilu Petrochemical Co. of China) with Mw ¼ 1.74 105 and Mn ¼ 4.39 104. 2.2. Fractionation of IPC The original IPC was fractionated by temperature-gradient extraction fractionation using n-Octane as the solvent. Firstly, 25 g of IPC pellets were completely dissolved in n-Octane at 125 C, and then the solution was subsequently cooled down to 50 C and held for 72 h when the EPR fraction was collected. Then, the remaining sample was extracted at 100 C for 72 h and the EbP fraction collected. The remaining fraction was isotactic polypropylene (iPP). An iPP/EPR binary blend was prepared by solution-mixing. Firstly, the iPP and EPR fractions were weighted according to the designated proportion, and then dissolved in boiling xylene. Subsequently, the solution was stirred thoroughly for 30 min to ensure complete mixing. This was followed by precipitation into ice cooled methanol and then washing. Finally, the blend was dried at ambient temperature for 48 h and subsequently dried in vacuum at 80 C for 24 h.
observed using SEM (S4800, Hitachi, Japan). An operating voltage of 5 kV was used. 2.4. Dynamic mechanical analysis Dynamic mechanical analysis (DMA) was carried out using a Q800, TA Instruments Corporation dynamic mechanical analyzer. The film specimens were prepared by compression molding at 170 C and characterized in the tensile mode at a heating rate of 3 C/min and a frequency (u) of 10 Hz. 2.5. Differential scanning calorimetry A modulated differential scanning calorimeter (Q100, TA Instruments Corporation) with N2 as purge gas was used to probe the isothermal and non-isothermal crystallization behaviour of IPC specimens. Pure indium and zinc were used as reference materials to calibrate both the temperature scale and the melting enthalpy before the specimens were tested. Firstly, the IPC specimens were heated to 200 C and held for different time periods before quenching down to 140 C to complete the isothermal crystallization. When the crystallization was completed, the samples were cooled down to 40 C at 30 C/min and subsequently their melting traces were recorded at a heating rate of 10 C/min. Similarly, the specimens annealed at 200 C for different time periods were cooled down to 40 C at 10 C/min and then reheated to 200 C at 10 C/min to examine the non-isothermal crystallization and melting behaviour. 3. Results and discussion 3.1. Phase morphology evolution during molten-state annealing As reported previously, IPC presents a phase-separated structure in which the dispersed phase appears as a multilayered coreshell structure. In Fig. 1 two tand peaks at 37 C and 14.5 C appear, corresponding to the glass transitions of the EPR and iPP, respectively [26]. A weak peak is observed at 113.5 C, which may result from the EbP fraction [26]. These results indicate the existence of a complex phase-separated microstructure in IPC. The inlay in Fig. 1 shows the morphology of the etched fracture surface of IPC. It can be seen that in the matrix there are some aggregated granules as well as the uniformly dispersed holes (0.5e2 mm). The dispersed particle is composed of not only EPR but also EbP [26], and the aggregate granules are ascribed to the aggregation of those
2.3. Morphology observation Phase contrast optical microscopy (PCM), polarized optical microscopy (POM) and scanning electron microscopy (SEM) were used to observe the morphology of the IPC and iPP/EPR blend specimens. PCM and POM observations were carried out using an optical microscope (BX51, Olympus) equipped with an Olympus camera and the temperature was monitored with a temperature controlled hot stage (THMS600, Linkam Co.). The specimens were sandwiched between two microscope cover slips for observation. PCM micrographs of the IPC specimens annealed at 200 C for different time (10, 30, 60, 90, 120, 200 min) were taken as the morphology evolved. All thermal treatments proceeded under nitrogen atmosphere to avoid possible degradation or crosslinking. Isothermal crystallization experiments were conducted at 140 C and POM was used to observe the morphologic development during crystallization. IPC and iPP/EPR specimens were annealed at 200 C for 200 min on the THMS600 hot stage before fracturing in liquid N2. The fracture surfaces were etched in xylene for 12 h at room temperature, and then coated with gold-palladium and
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Fig. 1. SEM image and temperature dependence of tand for original IPC.
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Fig. 2. SEM images and partial enlargement of the extracted cross-section surface of IPC after being annealed at 200 C for 200 min.
dispersed particles which could not be etched by xylene. Fig. 2 shows the phase morphology of IPC annealed at 200 C for 200 min. It can be seen that the core-shell structure of the dispersed phase in the IPC sample disappears and some remarkable changes take place: (i) the holes and aggregated granules disappear, (ii) dispersed domains with an irregular shape appear, and (iii) the sizes of the dispersed domains increase. After annealing in the molten-state, although the sizes of the dispersed domains become 5e10 times larger than those of the original IPC, the dispersion seems more uniform with the disappearance of holes and aggregated granules. In addition, the dispersed domains connect to one another and a network-like structure is observed as seen in the enlarged figure. These results indicate that annealing at 200 C leads to a definite change in architecture and morphology of IPC. To observe the morphology evolution of IPC in more depth, the morphology of iPP/EPR binary blends (prepared by the fractions of IPC), in which the weight ratio of the two components is the same as that in IPC, was investigated. Fig. 3a shows the SEM image of the original iPP/EPR sample. It is found that without the EbP component, the dispersed phase dimensions are much smaller than those of the IPC and are typically no more than 0.2 mm. Although it is proposed that the ethyleneepropylene copolymer disperses well in IPC as compared with conventional polypropylene/ethyleneepropylene rubber blends prepared by mechanical blending for the existence of EbP [27], the iPP/EPR sample prepared by solution blending has obviously a more uniform dispersion compared with IPC. Fig. 3b shows the morphology of the iPP/EPR sample subjected to an annealing at 200 C for 200 min. It can be seen that the EPR domains retain their shapes and only a slight change in the size appears. Most of the dispersed particles are no more than 0.5 mm. These results show that the development of the dispersed phase in iPP/EPR is clearly different from that of IPC as shown in Fig. 2 and it is necessary to monitor the morphology evolution of IPC during annealing to explore the reason for the above difference. Fig. 4 shows the PCM images of IPC during annealing at 200 C. It can be seen that the phase domains coarsen gradually, and the domain
size of the dispersed phase becomes larger with increasing annealing time. Due to the resolution limit of optical microscopy, the phase-separated structure is blurred until the annealing time reaches 30 min. As the annealing time increases, demixing is detected but the boundary between two domains is obscure as shown in Fig. 4b and c. When the annealing time reaches 90 min, a phase boundary can be distinguished, and a co-continuous structure appears. Considering that a co-continuous structure is one of the most important characteristics of spinodal demixing (SD), these results indicate that the coarseness development of IPC follows an SD mechanism. Fig. 4f presents a clear ‘sea-island’ structure indicating the disappearance of the co-continuous structure in the IPC sample. When the annealing time further increases, little change in morphology is observed indicating that an equilibrium phase structure has been reached. It is noted that little change of phase structure for the iPP/EPR samples can be observed via PCM, proving that this binary blend has a very slow rate of coarseness development, and the changes observed via SEM lie in the early stage of phase separation. Usually, a sea-island structure can be observed at the last stage of liquideliquid phase separation of a polymer blend with the continuous phase (sea) resulting from the majority content component, and the dispersed phase, from the minority component. However, it is surprising to find that the area of the dispersed phase (island) is much larger than that of ‘sea’ in the IPC annealed for 200 min. These results indicate that there exists an abnormal structure in the IPC samples, i.e. the dispersed phase (island) is PP, and the continuous phase (sea) is the ethyleneepropylene copolymer including the EPR and EbP components in the IPC. A similar phase inversion phenomenon in which the minor component forms the continuous phase during compounding has been reported [28] and this was ascribed to the lower viscosity of the minor component. However, in the present research the viscosity of EPR is much higher than that of PP as shown in Fig. 5. In the annealing process, two phases aggregate respectively and the domains coarsen. Because of the existence of EbP, the activity of the
Fig. 3. SEM images of extracted cross-section surface of (a) original iPP/EPR and (b) that after being annealed at 200 C for 200 min.
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Fig. 4. Phase contrast micrographs of IPC samples annealed at 200 C for different time periods. (a) 10 min, (b) 30 min, (c) 60 min, (d) 90 min, (e) 120 min, (f) 200 min.
dispersed phase in IPC is much stronger than that of the PP/EPR blend, and it makes phase inversion possible. In addition, once the EPR forms the continuous phase, it is hard to be destroyed due to its high viscosity in the molten-state. The appearance of the ‘sea-island’ phase inversion structure indicates that the original dispersed phase (ethyleneepropylene copolymer, including EPR and EbP) forms into a continuous phase and divides polypropylene into domains with different dimensions. Phase inversion happens if the blend composition and viscosity ratio change [29e31]; however it has rarely been reported that an annealing process can induce phase inversion of an immiscible polymer blend. Considering the results shown in Fig. 2, some interesting phenomena need to be noted, e.g. the continuous phase in the molten-state, ethylene-propylene copolymer converts to the dispersed phase when the IPC sample is cooled down to room temperature after being annealed for 200 min, whereas the dispersed phase, PP component in the molten-state, becomes the matrix phase again. Fig. 6 presents a schematic diagram of the phase morphology evolution of IPC. These results indicate that the copolymer domains cannot exist when temperature drops, which can be ascribed to the large tension induced by solidification at the phase interface and the great content difference between components.
Fig. 5. Relationship between complex viscosity and frequency of PP and EPR fractions in IPC at 200 C.
3.2. Influence of annealing on crystalline morphology Differences in the phase structure of a molten polymer blend may lead to different crystallization behaviour if the blend contains crystallizable polymer component. Since IPC samples exhibit different liquideliquid phase structures during annealing, it is important to investigate the effect of molten-state annealing on the crystallization behaviour of IPC. Fig. 7 shows the polarized optical micrographs of IPC samples crystallized isothermally at 140 C after being annealed for different time at 200 C. It can be seen that although the crystalline morphologies in all samples appear as spherulites, the spherulite profile becomes clear gradually and the corresponding Maltase cross seems more perfect with an increase in annealing time. Also, compared with the sample annealed for 10 min, the sample annealed for 200 min has a larger spherulite dimension. Since PP is the main crystallizable component, the above results indicate that the remaining portions from EbP and EPR molecules may affect the size and the birefringence of the PP crystal. Fig. 8 shows the crystallization micrographs of IPC at 140 C after being annealed for different time at 200 C. It is seen from the POM images that three samples exhibit distinct birefringence and spherulite density, and the birefringence of the PP sample is more perfect and distinct than those of the IPC samples. When observing two IPC samples annealed for different time there is a marked difference in birefringence. For the IPC sample annealed for 10 min, the incomplete birefringence mainly results from the vague Maltase cross. Contrarily, the birefringence of the IPC sample annealed for 200 min seems to be separated into many smaller disordered pieces although the spherulite profile looks very clear. It is well-known that during melt-crystallization of polymers, the spherulites are made up of a crystalline phase and an amorphous phase. In IPC samples containing uncrystallizable ethyleneepropylene copolymer, the amorphous phase in the spherulite should be a mixture of uncrystallizable copolymer and PP homopolymer, depending on the degree of phase separation. As is shown by bright field images of IPC samples in Fig. 8, the sample annealed for 10 min presents a tiny phase-separated state in the spherulite region, and there is no obvious domain of amorphous ethyleneepropylene copolymer, and consequently a vague Maltase cross can be observed. On the contrary, obvious domains can be found in the image of the IPC sample annealed for 200 min. Because the area of crystalline homopolymer covers the amorphous homopolymer, it is suggested that the continuous domain in the spherulite region
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Fig. 6. Schematic of phase morphology evolution of IPC annealed for 200 min at 200 C during cooling. (a) in the melt state, (b) at solidification.
Fig. 7. Polarized optical micrographs of IPC samples crystallized isothermally at 140 C after being annealed at 200 C for different time periods. (a)10 min, (b)30 min, (c) 60 min, (d) 90 min, (e)120 min, (f)200 min.
should be attributed to the amorphous copolymer and the ‘pieces’ are crystalline homopolymer, respectively. Due to the formation of two kinds of domains, the crystallization in IPC sample annealed for 200 min may happen in a relatively pure homopolymer region, unlike the sample annealed for 10 min, in which crystallization occurs in a mixture of homopolymer and copolymer. As a result, the
spherulite profile of the sample annealed for 200 min is relatively distinct but the inner morphology of the spherulite appears broken and discontinuous. Based on the above results and analysis, the incomplete Maltase Cross is thought to reflect that there may exists random copolymers between lamellas or fibrils of the PP spherulites, which then affect
Fig. 8. Bright field views and Polarized optical micrographs of fractionated iPP (a, a0 ) and IPC samples crystallized at 140 C after being annealed at 200 C for 10 min (b, b0 ) and 200 min (c, c0 ) respectively.
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Fig. 9. Development of relative crystallinity of IPC samples crystallized isothermally at 140 C after being annealed for different time at 200 C.
the birefringence of lamella along the radial and tangential directions. It is noted that the situation where the second component enters into lamellas of the crystalline polymer usually happens in miscible crystalline polymer blends. The above experimental results should be attributed to EbP to a large extent. As a compatiblizer, EbP can enhance the adhesion properties between the dispersed phase and the matrix. It should also be noted that serious aggregation lessens the interfacial area which may affect the crystallization, especially the nucleation stage, for IPC samples. This will be discussed in the following section.
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Fig. 11. Nucleation density of IPC crystallized isothermally at 140 C for 55 min after being annealed for different time periods at 200 C.
As mentioned above, the molten IPC exhibits different phase structures with an increase in annealing time, so it is necessary to investigate the crystallization behaviour of IPC which has been annealed in the molten-state for different time periods. Fig. 9 presents the crystallinity evolution of IPC samples crystallized isothermally at 140 C after being annealed at 200 C for different time. It can be seen that both the total crystallization time and halftime of crystallization, t1/2, of IPC generally increases with an
increase in annealing time. The isothermal crystallization of IPC at other temperatures behaves in a similar manner (not shown). These results show that molten-state annealing of IPC results in a decrease in the overall crystallization rate, a result that may be attributed to the phase separation of IPC induced by annealing. Since crystallization involves both nucleation and crystal growth, it is important to determine how these processes are individually affected by molten-state annealing. Fig. 10 shows the spherulite radius evolution of IPC crystallized isothermally at 140 C. It can be seen that for all IPC samples a linear spherulite growth is obtained and that growth rates are similar at a value of 1.1 mm/min for all samples. Since spherulite growths of both miscible and immiscible blends have been extensively studied and a linear spherulite growth is one of the characteeristics of an immiscible blend, these results suggest that the amorphous copolymer never goes into the lamella and only occurs between the lamellae. In other words, the phase separation in IPC is unlikely to affect the mobility of the propylene chains. This result also indicates that the reduction in crystallization rate of IPC as annealing time increases is not due to any effect on spherulite growth rate and therefore must be due to an effect on the nucleation process.
Fig. 10. Spherulite growth rates of IPC crystallized isothermally at 140 C after being annealed for different time periods at 200 C.
Fig. 12. Exotherms of IPC cooled down at a rate of 10 C/min after being annealed at 200 C for 10 min and 200 min respectively.
3.3. Influence of annealing on crystallization behaviour
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Fig. 13. Crystallization peak temperatures (Tc) of IPC cooled down at a rate of 10 C/min after being annealed for different time at 200 C.
For all of the IPC samples, the value of t1/2 is more than 55 min as shown by Fig. 9, hence, the morphologies of IPC samples crystallized isothermally for 55 min at 140 C are chosen to make a comparison of crystallization density. Fig. 11 shows the POM images of all IPC samples crystallized for 55 min and their statistical nucleation densities, which are the average of five tests. It can be observed that the nucleation density of IPC decreases gradually when the annealing time increases. Since the crystallization mechanism of iPP/EPR is characterized by heterogeneous nucleation [32], and taking into account the fact that the phase separation happened during annealing, the decrease in nucleation density is attributed to the decrease of interfacial area. Non-isothermal crystallization tests also provide proof that nucleation events are being influenced if the crystallization temperature is changed. Fig. 12 shows the exotherms of IPC crystallized at a cooling rate of 10 C/min. Two crystallization peaks appear, one at about 95 C and the other, at 118 C. These peaks correspond to the crystallizations of iPP and PE, respectively. It can be seen that both peaks shift to a lower temperature with the increasing annealing time. Fig. 13 shows the crystallization peak temperature (Tc) for IPC samples annealed for different time at 200 C. The Tc of PP drops by almost 3 C when annealing time increases from 10 to 200 min, while that of PE drops no more than 1 C. The change of Tc suggests that increasing annealing time leads to a decrease in nucleation ability. As PP is the majority component and it constitutes almost 70% in the sample, it will be affected greatly by changes in the dispersed phase compared with the other components. PCM micrographs show that the dispersed domains conglomerate, the two phases invert and the size increases greatly with increasing time. Also since the crystallization of the PP/EPR system is dominated by heterogeneous nucleation, the above results are attributed to the decrease in interfacial area induced by phase separation and in turn the reduction of nucleation points (Fig. 13). 4. Conclusion It has been found through morphological observations that the molten-state annealing of IPC has a significant effect on its architecture. When the IPC is annealed at 200 C for 200 min, the core-shell structure of the dispersed phase is destroyed completely, while the size increases sharply. Also, a network-like structure could be observed. There exists an obvious coarsening of the phase morphology, and with the increase of annealing time, a co-continuous structure and an abnormal ‘sea-island’ structure appear in which the original matrix PP component becomes to the dispersed phase. In comparison with the morphology of fractionated iPP/EPR binary blends which have the same weight ratio as IPC, it is found that unlike IPC, the former has a very slow
coarsening and there is no obvious change in phase structure during 200 min annealing. In addition, molten-state annealing greatly affects the crystalline morphology and crystallization behaviour. Although the spherulite profile generally becomes more well defined and crystal perfection increases with increasing annealing time, obvious differences in spherulite birefringence can be found depending on the phase structure of the IPC melt. With increasing annealing time, the spherulite radial growth rates remain almost constant while the nucleation density and thus overall crystallization rate decreases. It is suggested that a decrease in the nucleation ability of IPC with increasing annealing time is due to the coarsened microstructure and decreased in interface area. Acknowledgements This work was supported by National Basic Research Program of China (No. 2005CB623800), National Nature Science Foundation of China (No. 50603023), Nature Science Foundation of Zhejiang Province (No. Y4100314) and the Fundamental Research Funds for the Central Universities. References [1] Debling JA, Zacca JJ, Ray WH. Chemical Engineering Science 1997;52(12): 1969e2001. [2] Hermanová S, Tochácek J, Jancár J, Kalfus J. Polymer Degradation and Stability 2009;94(10):1722e7. [3] Mirabella JFM. Polymer 1993;34(8):1729e35. [4] Debling JA, Ray WH. Journal of Applied Polymer Science 2001;81(13): 3085e106. [5] Prasetya A, Liu L, Litster J, Watanabe F, Mitsutani K, Ko GH. Chemical Engineering Science 1999;54(15e16):3263e71. [6] Dong Q, Wang XF, Fu ZS, Xu JT, Fan ZQ. Polymer 2007;48(20):5905e16. [7] Cai HJ, Luo XL, Chen XX, Ma DZ, Wang JM, Tan HS. Journal of Applied Polymer Science 1999;71(1):103e13. [8] Cai HJ, Luo XL, Ma DZ, Wang JM, Tan HS. Journal of Applied Polymer Science 1999;71(1):93e101. [9] Li RB, Zhang XQ, Zhao Y, Hu XT, Zhao XT, Wang DJ. Polymer 2009;50(21): 5124e33. [10] Chen Y, Chen Y, Chen W, Yang D. Polymer 2006;47(19):6808e13. [11] Cui NN, Ke YC, Lu ZX, Wu CH, Hu YL. Journal of Applied Polymer Science 2006; 100(6):4804e10. [12] Song SJ, Wu PY, Feng JC, Ye MX, Yang YL. Polymer 2009;50(1):286e95. [13] Timothy FM, Djallel B, Shigeyuki M, Toshihiko S. Polymer Reaction Engineering 2003;11:177e97. [14] Tan HS, Li L, Chen ZN, Song YH, Zheng Q. Polymer 2005;46(10):3522e7. [15] Chen Y, Chen Y, Chen W, Yang DC. Journal of Applied Polymer Science 2008; 108(4):2379e85. [16] Zacur R, Goizueta G, Capiati N. Polymer Engineering and Science 2000;40(8): 1921e30. [17] D’Orazio L, Mancarella C, Martuscelli E, Cecchin G, Corrieri R. Polymer 1999; 40(10):2745e57. [18] Inaba N, Sato K, Suzuki S, Hashimoto T. Macromolecules 1986;19(6): 1690e5.
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