Detecting crystallization structure evolution of polypropylene injection-molded bar induced by nucleating agent.

INTRODUCTION

The microstructure of polymer injection-molded bars has been intensively researched in the last 10 years because the microstructure is very important for the end-use properties of the molded articles. During injection-molding processing, the wall temperature of the mold is generally much lower than that of the polymer melt, for example, the mold temperature is usually set at the room temperature. This induces a phenomenon that the solidifying process is always beginning in the skin of the molded bar and ending in the core zone. Macroscopically, the injection-molded bar has an apparent multilayer structure, that is, a skin-intermediate layer-core structure. In the different layers of an injection-molded bar, the bar shows different microstructures or morphologies (1-10).

On the other hand, because of the high shear stress during injection-molding processing, polymer chains generally extend along the flow direction when the melt is injected into the mould. The time for the relaxation of the extended polymer chains at the different zones of the bar is different: in the skin layer, extended polymer chains are solidified quickly and the article shows high orientation texture in this region; in the core zone, the solidifying rate is very low and the time is long enough for the relaxation of extended polymer chains before the melt is solidified completely, and in this condition no orientation texture but isotropic texture is observed (9), (11-15). For example, Shishi-kebab texture of semicrystalline polymer has been observed in the skin layer of an injection-molded bar, but spherulites or isotropic lamellae structure in the core zone (12), (14). Above all, the bar has a quite heterogeneous or anisotropic structure.

Nucleating agents are widely used in thermoplastic polymer processing because they have the abilities to induce polymer crystallization as a certain crystal phase, increase the crystallization rate of these semicrystalline polymers, reduce the cycle time, and improve the optical properties apparently (16), (17). Furthermore, nucleating agents are also used to strengthen the formation of the anisotropic structure of polymer melt in the shear field or during the injection molding processing (18-21).

As a part of serial work about the effect of nucleating agent on the crystallization behavior, morphology and mechanical properties of polymer and its blend, in this study, we research the crystallization structure evolution of PP injection-molded bar induced by nucleating agents. This study is different from our previous work, (1) in which the hierarchy structure of polypropylene random copolymer (PPR) injection-molded bar induced by sorbitol derivatives was studied in detail and the attention was paid on the variation of spherulites size and the corresponding crystallization mechanism, here our attention is paid on the formation of different crystal forms, such as [alpha]-phase and [beta]-phase PP, and their relative content variation in the different zones of PP injection-molded bar. To realize the subject, two different nucleating agents, [alpha]-phase nucleating agent DMDBS (l,3:2,4-bis (3,4-dimethylbenzylidene) sorbitol) and [beta]-phase nucleating agent TMB-5 (aryl amides compounds), and their compounds were introduced into PP, respectively. The crystal phases and their relative contents were characterized via WAXD and DSC systematically. It is known to all that [beta]-phase is thermodynamically metastable and high levels of [beta]-phase only can be achieved under special conditions, such as crystallization in shear fields (22-26) or in temperature gradients (27), (28) and particularly with specific nucleating agents (15), (26), (29-31). During the injection molding processing, the shear condition and temperature gradient are coexistent for the crystallization of PP injection-molded bar, and the addition of a few amounts of TMB-5 provides another condition for the crystallization of [beta]-phase. So, studies on the crystal phase evolution especially the variation of [beta]-phase PP at the different zones of the injection-molded bar have very important significance, and our attention is mainly focused on the variation of [beta]-phase content at the different zones of a bar in this study. Our results show that, whether for the pure PP or PP with any nucleating agent, the skin layers of the bars show the coexistence of [alpha]/[beta]-phase PP. However, for the TMB-5 nucleated PP, the content of [beta]-phase is dependent on the location of the injection-molded bar. Furthermore, it is interesting to observe the structural transformation of PP with compounding nucleating agents. PP/0.1DM/0.1TMB injection-molded bar shows the mainly [beta]-phase formation in the intermediate layers and the core zone, however, PP/0.2DM/0.2TMB bar shows the mainly [alpha]-phase formation in these locations.

EXPERIMENTAL

Materials

All the materials used in this study are commercially available. PP (F401, Langang Petrochemical, Lanzhou, China) with a melt flow rate (MFR) of 2.5 g/10min (230[degrees]C/2.16 kg) was used as the matrix polymer. The [alpha]-phase nucleating agent l,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS, Millad 3988) was produced by Milliken Chemical, Belgium. The [beta]-phase nucleating agent aryl amides compound (TMB-5) was supplied by Fine Chemicals Department of Shanxi Provincial Institute of Chemical Industry, China.

Sample Preparation

To achieve the best dispersion of the nucleating agent in PP, a two-step process was employed to prepare the blends. Namely, a master batch of 5 wt% nucleating agent in PP was first prepared through melt blending of a nucleating agent and PP; and then the master batch was melt blended with different contents of PP to obtain the corresponding binary blends and ternary blends. The as-prepared specimens are named as PP/xDM and PP/yTMB or PP/xDM/y TMB (x, y = 0.1 or 0.2), where x and y represents the mass percentage of DMDBS and TMB-5, respectively. The melt blending of the blends was carried out on a twin-screw extruder (TSSJ-25). During the extrusion, the screw speed was set as 120 rpm and the temperature was 150-200[degrees]C from hopper to die. After making droplets, the pellets were injection molded and the standard specimens (with thickness of 4 mm and width of 10.0 mm) were prepared using an injection-molding machine (K-TEC 40). During the injection molding processing, the temperature of the melt from hopper to nozzle was set as 190-215[degrees]C and the mould temperature was 25[degrees]C, and the injection pressure was 30 MPa.

Measurement

To characterize the crystallization structure evolution of PP injection-molded bar in detail, the bar was separated as different zones. Wide angle X-ray diffraction (WAXD, Panalytical X'pert PRO diffractometer with Nifiltered CuK[alpha] radiation) and Differential scanning calorimetry (DSC, Netzsch STA 449C Jupiter, Germany) were used to measure the crystal structures of PP in these different zones induced by nucleating agent. The schematic representation of the different zones cut from the injection-molded bar is shown in Fig. 1. For WAXD measurement, the bar was separated as four different layers, such as the skin layer (0 mm), the intermediate layer 1 (0.5 mm away from the skin layer), the intermediate layer 2 (1.0 mm away from the skin layer), and the core zone (2 mm away from the skin layer). The continuous scanning angle range used in this study was from 10[degrees] to 35[degrees] at 40 kV and 40 mA. The [beta]-phase fraction ([K.sub.[beta]]) in the samples was calculated from WAXD diffractograms according to the following relation (32):

[FIGURE 1 OMITTED]

[K.sub.[beta]] - [I.sub.300.sup.[beta]]/([I.sub.110.sup.[alpha]] + [I.sub.040.sup.[alpha]] + [I.sub.130 sup.[alpha]] + [I.sub.300.sup.[beta]) (1)

where [I.sub.110.sup.[alpha]], [I.sub.040.sup.[alpha]], and [I.sub.130.sup.[alpha]] are the integral intensities of the (110), (040), and (130) reflections of the [alpha]-phase, respectively; and [I.sub.300.sup.[beta]] is the integral intensity of (300) reflections of [beta]-phase.

For the DSC measurement, the bar was separated as three different zones, named the skin zone (0-0.5 mm), the intermediate zone (0.5-1.0 mm), and the core zone (1.0-2.0 mm). For each measurement, about 8.0 mg sample which was cut from the corresponding zones of the bar was heated directly from 30 to 200[degrees]]C at the heating rate of 10[degrees]C/min. On the other hand, the sample which was obtained from the extrusion processing was heated to 200[degrees]C quickly and maintained at this temperature for 10 min to erase the thermal history; then the sample was cooled down to 30[degrees]C at the cooling rate of 10[degrees]C/min, and then the sample was heated again to 200[degrees]C at the heating rate of 10[degrees]C/min to study the melting behavior of such crystallization structure formed during the cooling process. All the DSC measurements were carried out in helium atmosphere.

RESULTS AND DISCUSSION

WAXD Results

The crystallization structures of pure PP at different zones of a bar are characterized via WAXD firstly and the results are shown in Fig. 2. One can observe that, for the sample cut from the skin layer, the diffraction patterns at 2[theta] = 14.0 [degrees], 16.7 [degrees], and 18.4[degrees] are the principle reflections of the [alpha]-phase of PP corresponding to the crystal plane of (110), (040), and (130), respectively, while 2 [theta] = 15.9[degrees] is the principle reflection of [beta]-phase corresponding to the crystal plane of (300). These reflections are considered as the marker peaks for [alpha]-phase and [beta]-phase of PP, respectively. The WAXD patterns show that [alpha]-phase and [beta]-phase grow in the skin layer simultaneously. According to the (32), the [beta]-phase content in the skin layer is calculated as 21.6%. The formation of high content of [beta]-phase is mainly due to the high shear stress inducing PP crystallization in [beta]-phase. For the samples cut from the intermediate layers and core zone, only the diffraction patterns of [alpha]-phase can be observed, which suggests that in these zones, PP crystallizes almost in [alpha]-phase. has been proved in other researchers' work (33).

[FIGURE 2 OMITTED]

Sorbitol derivatives are thought to be some of the most efficient [alpha]-phase nucleating agents for PP crystallization because they crystallize into nanofibrillar structures in the polymer melt through self-organization (34), (35). In this study, 0.1 and 0.2 wt% are also introduced into PP matrix to obtain the injection-molded bars. Undoubtedly, PP crystallizes exclusively in [alpha]-phase, no [beta]-phase formation in the intermediate layers or in the core zone. However, for the sample cut from the skin layer of bar, one also can observe a weak reflection of [beta]-phase at 2 [theta] = 15.9[degrees] and the [beta]-phase content is calculated as 9.2% and 6.6% for PP/0.1DM and PP/0.2DM, respectively. The WAXD patterns are not shown. Here with the presence of DMDMS in the skin layer of bar, the crystallization of PP is mainly influenced by DMDBS and the shear condition. Because the solidifying rate is very high, most of PP chains crystallize through self-seeding way and in this condition, the influence of shear condition is still apparent and finally a few PP crystallize in [beta]-phase and almost PP crystallize in [alpha]-phase. With the increasing content of DMDBS, the effect of nucleating agent becomes more apparent and less PP crystallize through self-seeding way. That's why less [beta]-phase grow in the skin layer of PP/0.2DM bar.

Figure 3 shows the WAXD patterns of the samples cut form the different zones of PP/0.1 TMB and PP/0.2TMB bars, respectively. From Fig.3 one can see that the reflections of [alpha]-phase and [beta]-phase are very apparent for the sample cut from the skin layer. The [beta]-phase content is calculated as 26.4% for PP/0.1TMB and 28.1% for PP/0.2TMB, respectively. Making a comparison of [beta]-phase content in the skin layers between pure PP bar and TMB-5 nucleated PP bar, the presence of TMB-5 in PP increases the [beta]-phase content slightly. Obviously, in this region, the nucleation effect of TMB-5 for PP crystallization is very weak, and almost PP crystallize in [alpha]-phase through self-seeding way. For the sample cut from the intermediate layer 1 or intermediate layer 2, the reflection of [beta]-phase becomes very strong. For PP/0.1TMB bar, the [beta]-phase content is up to 85.5% in the intermediate layer 1 and 86.7% in the intermediate layer 2, respectively. For PP/0.2TMB bar, the corresponding [beta]-phase content is up to 86.4% and 81.9%. For the sample cut from the core zone, a strong reflection of [beta]-phase is still observed and the [beta]-phase content is 79.2% in PP/0.1TMB bar and 74.0% in PP/0.2TMB bar. Compared with the [beta]-phase content in the intermediate layers, the core zone shows lower content of [beta]-phase formation. Because the solidifying rate of the core zone is the lowest and the melt is more easily up to the equilibrium state than the melt in other regions before the melt is solidified completely, and in this condition, the observed crystallization structure is the most close to those obtained from the static melt. So, the remnant problem is how to understand the formation of higher [beta]-phase content in the intermediate layers. An's work has shown that the [beta]-phase content in the shear stress field depends on the shear rate, lower shear rate is in the favor of the formation of [beta]-phase (26). For an injection-molded bar, the relaxation of oriented PP chains in the intermediate layers maybe not finished completely and few oriented PP chains still exist, which can be considered as the introduction of lower shear rate to the melt. With the presence of [beta]-phase nucleating agent and the remnant shear rate effect, more PP crystallize in [beta]-phase in the intermediate layers than in the core zone. That's why higher content of [beta]-phase is observed in the intermediate layers.

[FIGURE 3 OMITTED]

Besides the addition of single nucleating agent, the compounding nucleating agents were introduced into the PP matrix too. Figure 4 shows the WAXD patterns of the samples cut from the different zones of PP/0.1DM/0.1TMB bar and PP/0.2DM/0.2TMB bar, respectively. Similarly, both [alpha]-phase and [beta]-phase are proved in the skin layers. The [beta]-phase content is 23.7% for PP/0.1DM/0.1TMB and 15.2% for PP/0.2DM/0.2TMB, close to those in PP/0.1TMB and PP/0.2DM, respectively. Furthermore, it is interesting to observe the variation of [beta]-phase content in the intermediate layers and in the core zones. PP/0.1DM/0.1TMB bar shows very high content of [beta]-phase in the intermediate layer 1, and [beta]-phase content decreases with the increasing distance away from the skin layer. On the other hand, one should notice that the [beta]-phase contents in these regions are very close to those in PP/0.1TMB bar in the same regions. However, PP/0.2DM/0.2TMB bar shows very low content of [beta]-phase in the intermediate layers and in the core zone. From the skin layer to the core zone, the [beta]-phase content decreases linearly. Such variation of [beta]-phase content in the different zones of PP included by compounding nucleating agents suggests that, in PP/0.1DM/0.1TMB bar, the nucleation role of TMB-5 is very apparent and the crystallization of PP mainly influenced by TMB-5; however in PP/0.2DM/0.2TMB bar, DMDBS shows more apparent nucleation effect for PP and the crystallization of PP mainly influenced by DMDBS rather than TMB-5. The different crystallization behaviors of PP induced by such compounding nucleating agents will be further discussed in the next section.

[FIGURE 4 OMITTED]

The relative [beta]-phase contents of PP and PP nucleated by different nucleating agents obtained from the different zones of injection-molded bars are shown in Table 1. According to such comparison one can clearly know the crystallization structure evolution of an injection-molded bar during injection molding processing.

TABLE 1. Distribution of [beta]-phase in the different zones
of PP injection-molded bar.

                       [beeta]-Phase PP content(%)

Samples           Skin  Intermediate  Intermediate  Core
                 layer     layer 1       layer 2    zone

PP               21.6         0             0          0
PP/0.1DM          9.2         0             0          0
PP/0.2DM          6.6         0             0          0
PP/0.1TMB        26.4      85.5          86.7       79.2
PP/0.2TMB        28.1      86.4          81.9       74.0
PP/0.1DM/0.1TMB  23.7      86.9          76.4       64.8
PP/0.2DM/0.2TMB  15.2      13.0          12.2       10.7

DSC Results

DSC is another way to characterize the crystallization structures formed during injection molding processing through heating the sample and studying the melting behavior of it. In this study, the samples were cut from the different zones of an injection-molded bar to further prove the crystallization structure evolution of a bar from the skin zone to the core zone.

Figure 5 shows the DSC heating curves of pure PP and PP with different contents of single nucleating agent. For pure PP sample which is obtained whether from the skin zone and from the intermediate zone or from the core zone, the heating curve shows only one endothermic peak of [alpha]-phase. The same phenomena are observed for PP/0.1 DM and PP/0.2DM. For PP/0.1TMB and PP/0.2TMB. the sample obtained from the skin zone shows two weak endothermic peaks at the temperature of about 140.0 and 148.4[degrees]C (marked as [beta.sub.1] and [beta.sub.2]), and these endothermic peaks are attributed to the melting of [beta]-phase PP. From the DSC heating curve one also can see that [alpha]-phase is the main crystallization structure in the skin zone. This result is in good agreement with that obtained from WAXD measurement. For the samples obtained from the intermediate zone and the core zone the endothermic peaks of [beta]-phase become stronger and the areas of the peaks become larger, meanwhile the endothermic peak areas of [alpha]-phase become smaller. This suggests the greatly increase of [beat]-phase content in such regions. Because the [beta]-phase is thermodynamically metastable state and the [alpha]-phase is thermodynamically stable one. During the heating process, the [beta] [right arrow] [alpha] recrystallization is easily to be observed from DSC heating curve (shown by arrows in Fig. 5d and e) (36), (37). Thus, it should be pointed out that the endothermic peak of [alpha]-phase does not show the real content of [alpha]-phase formed during injection molding processing.

[FIGURE 5 OMITTED]

The DSC heating curves of PP with compounding nucleating agents are shown in Fig. 6. For PP/0.1 DM/0.1TMB bar, whether for the skin zone and for the intermediate zone or for the core zone, the DSC heating curve of the sample is very similar to that obtained from PP/0.1TMB bar. Similarly, the DSC results of PP/0.2DM/0.2TMB are very close to the results of PP/0.2DM. The DSC results also prove that the crystallization behavior of PP/0.1DM/0.1TMB is close to that of PP/0.1TMB during injection molding processing, and PP/0.2DM/0.2TMB close to PP/0.2DM.

[FIGURE 6 OMITTED]

But from the DSC heating curves, one possibly has a question that why there are no endothermic peaks of [beta]-phase for the samples cut from the skin zones of pure PP and PP with DMDBS bars, especially for the pure PP bar, in which the WAXD result shows 21.6% [beta]-phase formation in the skin layer. Even if in the skin layers of PP/0.1TMB and PP/0.2TMB bars, the [beta]-phase content is up to 26.4% and 28.1%, respectively; however, only two very weak endothermic peaks of [beta]-phase can be observed from the DSC heating curve. Factually, the crystallization of PP in the skin layer is mainly influenced by the high shear stress and the high solidifying rate, most of PP crystallizes through the self-seeding way. So, in our opinion, the [beta]-phase induced by shear stress is very unstable and it is very easily to be transformed to [alpha]-phase at low temperature during the heating process. However, only the [beta]-phase induced by TMB-5 is relative stable and needs more enthalpy to melt.

Aim to make a clear comparison of the heating curves of such samples obtained from the different zones of the injection-molded bars and to further understand the effect of nucleating agent on the crystallization behavior of PP during injection molding processing, the heating curves of such samples are organized again in another way. Figures 7 and 8 show the heating curves of PP, PP/0.1 DM, PP/0.1TMB and PP/0.1 DM/0.1 TMB system and PP, PP/0.2DM, PP/0.2TMB, and PP/0.2DM/0.2TMB system, respectively. From these figures one can clearly know the similarity of melting behaviors between PP/0.1TMB and PP/0.1DM/0.1TMB, and PP/0.2DM and PP/0.2DM/0.2TMB.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Further Understanding of the Crystallization Behavior of PP With Compounding Nucleating Agents

The above results show that the crystallization behavior of PP/0.1DM/0.1TMB is very similar to that of PP/0.1TMB, and PP/0.2DM/0.2TMB similar to PP/0.2DM during the solidifying process. To further prove the results which were obtained from WAXD and DSC results, two different crystallization procedures, such as isothermal crystallization and nonisothermal crystallization, were designed in this section. The isothermal crystallization of PP with compounding nucleating agents, PP/0.1DM/0.1TMB and PP/0.2DM/0.2TMB, were carried out as follows: first, the sample was placed between two glass slices and was heated to 200[degrees]C and maintained at this temperature for 10 min to erase any thermal history; second, the sample was transferred quickly to a hot stage with setting temperature of 125[degrees]C and maintained at this temperature for enough time until the crystallization of PP was finished completely; finally, such isothermal crystallization sample was characterized by WAXD. Figure 9 shows the WAXD patterns of isothermal crystallized PP/ 0.1TMB and PP/0.2DM/0.2TMB samples. For the former sample, the diffraction pattern shows very intense reflection of [beta]-phase but weak reflection of [alpha]-phase. This means that the [beta]-phase content is very high and the crystallization of PP in this condition is mainly induced by TMB-5. For the latter sample, the diffraction pattern shows not only the reflection of [beta]-phase but also the reflection of [alpha]-phase, which means that in this condition both DMDBS and TMB-5 have apparent nucleation effect for PP crystallization.

[FIGURE 9 OMITTED]

The nonisothermal crystallization behaviors and the subsequent melting behaviors of PP with nucleating agents are shown in Fig. 10. According to the difference of the crystallization peak temperatures ([T.sub.p]) which are observed from (a) and (al), one can know that the nucleation effect of the compounds of 0.1 wt% DMDBS and 0.1 wt% TMB-5 is more apparent than that of 0.1 wt% DMDBS, but weaker than that of 0.1 wt% TMB-5 for PP crystallization, however the nucleation effect of the compounds of 0.2 wt% DMDBS and 0.2 wt% TMB-5 is close to that of 0.2 wt% DMDBS. Furthermore, the exothermic peak of PP/0.2DM/0.2TMB becomes sharper and more symmetrical than that of pp/0.2DM, which means that the crystallization of PP is induced not only by DMDBS but also by TMB-5. The crystallization enthalpy ([DELTA][H.sub.c]) values of virgin PP and nucleated PP are shown in Table 2. PP/0.1DM/0.1TMB and PP/0.2DM/0.2TMB present similar [DELTA][H.sub.c] compared with PP/0.1TMB and PP/0.2DM, respectively, indicating that the compounding nucleating agents has no apparent effect on crystallinity of PP compared with single nucleating agent, although TMB-5 mainly nucleated PP shows a slightly decrease of [DELTA][H.sub.c] compared with DMDBS mainly nucleated PP. The subsequent melting behaviors of such samples also prove that in PP/0.1DM/0.1TMB, the crystallization of PP is exclusively induced by TMB-5; however in PP/0.2DM/0.2TMB, the crystallization of PP is induced by DMDBS and TMB-5 simultaneously.

[FIGURE 10 OMITTED]

TABLE 2. The crystallization peak temperature ([T.sub.p]) and
crystallization enthalpy ([DELTA].[H.sub.c]) for PP with different
contents of nucleating agent.

Samples          [T.sub.p]([degrees]C)  [DELTA] [H.sub.c] (J/g)

PP                       109.1                    43.76
PP/0.1DM                 114.5                    43.99
PP/0.2DM                 125.9                    44.75
PP/0.1TMB                123.0                    40.15
PP/0.2TMB                124.0                    40.74
PP/0.1DM/0.1TMB          118.8                    40.27
PP/0.2DM/0.2TMB          126.0                    43.29

Generally, the nucleation effect of nucleating agent for PP crystallization is dependent on the nucleating agent content. For sorbitol derivative DMDBS, only the amount of DMDBS is up to a critical value, it has apparent nucleation effect for PP (17), (38). Our results prove that even if the TMB-5 content is only 0.1 wt%, the nucleation effect is very apparent. In the PP/0.1DM/0.1TMB, the nucleation effect of TMB-5 is much more apparent than that of DMDBS, thus the crystallization is exclusively induced by TMB-5 and most of PP crystallize in [beta]-phase. In PP/0.2DM/0.2TMB sample, the content of DMDBS is high enough and it shows more apparent nucleation effect for PP. In the isothermal condition, the crystallization of PP is induced by DMDBS and TMB-5 simultaneously. However, in the nonisothermal crystallization, the gradually decrease of melt temperature is in favor of the gel formation of DMDBS and finally induces the formation of DMDBS network, and the latter accelerates the PP crystallization greatly (1), (35). Thats why the [alpha]-phase is mainly observed in the intermediate layers and in the core zone of PP/0.2DM/0.2TMB injection-molded bar.

CONCLUSION

In summary, we detected the crystallization structure evolution of PP injection-molded bar in detail via WAXD and DSC. Our results show that in the skin layer of PP without or with any nucleating agent, [alpha]-phase and [beta]-phase are simultaneously observed. The crystallization structure is mainly influenced by shear stress and PP crystallizes mainly through self-seeding way in this region. In the intermediate layers and in the core zone, the presence of single nucleating agent influences the crystal phase formation greatly. The presence of compounding nucleating agents influences the crystallization structures of PP in the different zones greatly, which depends on the nucleation effect of each of them and the crystallization conditions. PP/0.1DM/0.1TMB shows large number of [beta]-phase formation in the intermediate layers and in the core zone, however [alpha]-phase is mainly observed in the same regions of PP/0.2DM/0.2TMB bar.

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Hongwei Bai, Yong Wang, Bo Song, Yanli Li, Li Liu

Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China

Correspondence to: Yong Wang; e-mail: [e-mail]

Contract grant sponsor: National Natural Science and Technology Foundation; contract grant numbers: 50403019; contract grant sponsor: Sichuan Youthful Science and Technology Foundation (P. R. China), 07ZQ026-003.

DOI 10.1002/pen.21125

Published online in Wiley InterScience (www.interscience.wiley.com). [C] 2008 Society of Plastics Engineers