Ultrasonic characterization of phase morphology of high density polyethylene/polyamide 6 blend melts.INTRODUCTION
The phase morphology of multiphase polymer blends, as well as its evolution, is a basic issue in polymer processing area (1). The size, shape, spatial distribution, and time evolution of the dispersed phase, which are the focus of the morphology control, decide to a large extent the macro-properties of the materials. Therefore, characterizing the morphology of multiphase polymer blends has been considered as the key to investigate the relationship of structure and properties.
Presently, the morphology characterization is mostly based on microscopy (e.g., scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom force microscopy (AFM)), from which the microstructure can be observed directly. However, all the methods mentioned above are mostly confined to off-line characterizing the samples in the solid phase. Apart from time-consuming, the analytic results are not always precise as the samples just come from a small portion of the whole specimen (2). Moreover, measurements usually require separate sample preparation (e.g., quenching the melts rapidly), which may significantly alter the original state due to the different thermo-physical property of each phase. Thus, research and development of the rapid in situ characterization techniques aiming at the phase morphology of blend melts have been deemed to be a necessary and meaningful work.
In recent years, optical-related technologies have been successfully applied to polymer processing for real-time monitoring the morphological evolution during the melt blending. Leukel et al. (3) placed an optical microscope in an extruder die for real-time image observation and proved that this method is effective for characterizing the morphology of polymer blends resulting from the extrusion process. Light scattering techniques, which are based on the interaction between the light and the polymer melts, have been used to predict the concentration and the particle sue of dispersed phase. A succession of representative studies were performed by the following research groups, i.e., Sheng et al. (4), (5), Schlatter et al. (6), and Hobbie et al (7), (8). However, the need of sapphire optical windows and the desire of good light transmission properties for materials greatly limited the application of these techniques.
Ultrasonic diagnosis, as a novel method with the favorable fast, nonintrusive, real-time and convenient characteristics, appears to be appealing in this aspect for the measurements can be conducted in highly concentrated or optically opaque dispersions (9). Parameters such as ultrasonic velocity (or transit time) and attenuation can provide useful information on the morphology of polymer blends. Experimental studies of ultrasound signal dependence on the compatibility of polymer blends have been widely performed in polymeric solution and solid phase. Hourston et al. (10-12) and Sing et al. (13-15) suggested that ultrasonic velocity varied linearly with the blend composition for miscible blends, while it deviated from linearity if the miscibility of polymer blends decreased. He et al. (16) used ultrasonic attenuation to investigate blend morphology, and the results showed that attenuation depended on the size of dispersed phase and a discontinuity of scattering attenuation was always observed as phase inversion occurred.
Few authors have tried to study blend melts using ultrasound due to the heavy absorption and strong interference of detection signal, as well as the high temperature which limits the application of the transducer. Thanks to the work of Jen's group (17), a novel ultrasound transducer equipped with a clad metallic buffer rod can be successfully applied to the polymer melts and recently a series of work have been carried out. Based on the fact that the alteration in molecular chain orientation could result in a change in the sonic velocity of polymer melt, Li et al. (18-20) in-line investigated the relaxation of orientation and disorientation of various polyolefins. Villanueva et al. (21) fixed the ultrasound sensors (US) onto the extruder exit, and real-time studied the effects of screw configuration and clay nature on the dispersion of nanoclays in a LDPE matrix. Other works such as in-line monitoring the composition ratio (22), the residence time distribution (23), and the polymer degradation (24) were also conducted. However, few attentions have been paid to the blend morphology and the attempt to associate the acoustic parameters with the phase morphology is rarely reported (25).
High density polyethylene (HDPE) and polyamide 6 (PA6) are semicrystalline plastics, which have versatile industrial applications. HDPE has excellent low temperature flexibility, low cost, and good resistance to moisture permeation (26), while PA6 shows high strength, favorable thermo-mechanical characteristics, and good resistance to oxygen permeation and hydrocarbons (27). Thus, it is conceivable that blends of HDPE and PA6 (HDPE/PA6) can synergically combine the merits of both components, provided that the blends are appropriately compatibilized and the corresponding phase morphology gets optimized.
In this work, the immiscible HDPE/PA6 blends with 10, 20, and 30 wt% of a dispersed PA6 phase were prepared. Different amounts of HDPE grafted with maleic anhydride (HDPE-g-MAH), as a reactive compatibilizer, were added to obtain the different PA6 phase size. The morphology of the samples was firstly characterized by the conventional SEM. Then ultrasonic diagnosis as a novel technique was performed to investigate the influences of the PA6 concentration and the HDPE-g-MAH content on the ultrasonic velocity and attenuation. This work aims to explore the relationship between the ultrasonic parameters and the phase morphology for the droplet-matrix blend melts, which lays the foundation for the potential application of in-line ultrasonic diagnosis in the polymer processing.
A commercial HDPE (5000S) with a density of 0.946 g/[cm.sup.3] and a melt How index of 1.0 g/10 min (190[degrees]C, 2.16 kg) was purchased from Lanzhou Petrochemical Co., China. PA6 (M3400) with a density of 1.14 g/[cm.sup.3] and a flow melt index of 6 g/10 min (230[degrees]C, 2.16 kg), was obtained from Guangdong Xinhui Meida-DMS Nylon Chips Co., China. HDPE-g-MAH with maleic anhydride content of 0.8 wt% and a (low melt index of 1.2 g/10 min (190[degrees]C, 2.16 kg) was supplied by Ningbo Nengzhiguang New Materials Technology Co., China.
Compatibilized and uncompatibilized HDPE/PA6 blends were prepared by melt mixing with an intermeshing corotating twin-screw extruder (Haake Rheomex, PTW16/25). The temperatures of different zones were set to 240[degrees]C, except that those of feeder and die were 190 and 235[degrees]C, respectively. The feeding and screw speed were separately 40 and 60 rpm. Prior to melt blending, the pellets of PA6 and HDPE-g-MAH were dried in a vacuum oven at 85[degrees]C for 24 h to avoid the effects of moisture. The extruded strands were cooled by air and then pelletized.
Three composition ratios (wt/wt) of HDPE/PA6 blends were 90/10, 80/20, and 70/30. For each blend composition, the concentrations of HDPE-g-MAH ranging from 0 to 10 wt% with respect to the HDPE/PA6 resin were used.
Samples for the morphological analysis were prepared through compression molding at 230[degrees]C. The phase morphology was examined by Philips XL30FEG SEM operated at an accelerating voltage of 10 kV. To create better contrast, samples were fractured in liquid nitrogen, etched by formic acid to remove the PA6 particles, then sputter-coated with Au before SEM observation.
The SEM micrographs were analyzed by the JEOL Smile View software. At least 300 diameters were measured per sample. The number average diameter ([D.sub.n]) was then calculated by:
[D.sub.n] = [[SIGMA].sub.i][N.sub.i][D.sub.i] / [[SIGMA].sub.i][N.sub.i], (1)
where D, and /V, were the diameter and number of particles, respectively.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
Ultrasonic Instrumentation and Measurement Principles
Ultrasonic measurements were performed on a homemade slit die which was fitted to the barrel exit of the capillary rheometer (RH7, Malvern). Details of the setup were illustrated in Fig. 1. Two flush-mounted US were diametrically opposed across the die to emit and receive longitudinal waves with a central frequency of 5.0 MHz. Two pressure transducers (P2, P3) with J-type thermocouples were used to measure the melt pressure and temperature ([T.sub.m]). A wrapped-on heating jacket with a piecewise proportion integral differential program was employed to control the die temperature ([T.sub.die), which ensured that the [T.sub.m] in the slit die was consistent with that in the capillary rheometer. All the sensors were connected to the GIM1 system (PACE Simulations) composed of an ultrasonic pulser-receiver, a sampling digital oscilloscope, and an automated data acquisition system. And by this device, we could simultaneously track the evolution of temperature, pressure, and ultrasonic velocity and attenuation of the polymer melt in the slit die. In this work, pellets were first held in the barrel of capillary rheometer for melting, and then the melts were forced into the slit die at a constant speed of 10 mm/min and finally stayed in the slit die. All the measurements were performed at 230[degrees]C after enough relaxation until the melt pressure was zero and the ultrasonic velocity was unchanged. Each sample was taken for five measurements, and the mean values of the sound velocity and attenuation were then obtained, respectively.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Figure 2 showed the schematic of ultrasound propagation in the polymer me 11 which was confined between two aligned steel buffer rods. The longitudinal waves with amplitude of [a.sub.0] traveled to the steel/polymer interface where part of its energy went through the molten polymer with a thickness d = 2 mm. The acoustic wave was then reflected back and forth between the two interfaces. This produces a series of echo signals, [A.sub.1], [A.sub.2] ..., exiting from the second interface and directed toward the receiving transducer (Fig. 3). Usually, the first and second echoes were used for calculation of the ultrasonic velocity and attenuation. By measuring the flying time ([t.sub.1] and [t.sub.2]) and the amplitude ([A.sub.1] and [A.sub.2]) of the two successive echoes, the ultrasonic velocity (C) and attenuation (a) in the polymer melt can be separately expressed as follows:
C = 2d/[t.sub.2] - [t.sub.1], (2)
[chi] = 20[log.sub.10]([A.sub.1] / [A.sub.2])/2d, (3)
RESULTS AND DISCUSSION
[FIGURE 5 OMITTED]
The SEM images of the 90/10, 80/20, and 70/30 HDPE/PA6 blends with different amounts of HDPE-g-MAH are presented in Figs. 4, 5, and 6, respectively. It can be seen that PA6 phase is dispersed in HDPE matrix in the form of spherical particles, which is known as the typical droplet-matrix morphology. As is expected, the phase morphology takes on increasingly refined structure with the increasing amount of HDPE-g-MAH due to the effect of reactive compatibilization. The statistic average sizes of PA6 particles as a function of HDPE-g-MAH content for the three compositions are depicted in Fig. 7. It can be seen that for all the compositions, the particle size first decrease sharply with the increase of the HDPE-g-MAH content, and then gradually levels off, following a typical behavior of an emulsion curve (28). The following exponential equation (29) provides a good estimate of the dependency of the particle size on the HDPE-g-MAH concentration.
[FIGURE 6 OMITTED]
([D.sub.n,c] - [D.sub.n,[infinity]]) / ([D.sub.n,0] - [D.sub.n,[infinity]]) = exp(-nc), (4)
where [D.sub.n,c] is the number average diameter for a concentration c of HDPE-g-MAH, [D.sub.n,0] is the number average diameter for a blend without HDPE-g-MAH, [D.sub.n,[infinity]] is a constant representative of the limiting particle size, and n is a constant that determines the efficiency of the HDPE-g-MAH as an emulsifier.
[FIGURE 7 OMITTED]
Table 1 presents the constants [D.sub.[infinity]] and n for Eq. 4. Ii can be seen that, with the increase of PA6 concentration, [D.sub.[infinity]] gets increased while n shows the opposite. It is suggested that it should add more amounts of compatibilizer for the blends with higher concentration of PA6, to ensure the same emulsifying effect. This can be confirmed by the fact that the particle size increases with the increasing PA6 concentration when the content of HDPE-g-MAH is fixed, which is also shown in Fig. 7.
TABLE 1. Fining parameters of Eq. 4. Blend compositions (HDPE/PA6) [D.sub.n,x] n 90/10 0.72 1.0 80/20 0.96 0.63 70/30 1.2 0..51
On the basis of the above SEM results, the relationship between the ultrasonic parameters, i.e., ultrasonic velocity and attenuation, and the phase morphology will be investigated in the following section. Before we conducted ultrasonic measurements for the HDPE/PA6 blends, the acoustic properties of neat HDPE and PA6 were first measured at 230[degrees]C, and the results were listed in Table 2. For such homogeneous polymer melts, the differences in both the velocity and the attenuation for HDPE and PA6 are mainly originated from the different molecular structure. However, as for the HDPE/PA6 blends, the blend composition and the phase size may have additional effects on the propagation of ultrasound wave considering the presence of the interface.
TABLE 2. Acoustic properties of HDPE and PA6 measured at 230[degrees] Material Velocity (m/s) Attenuation (dB/cm) HDPE 1016 4.95 PA6 1352 3.26
Figure 8 presents the ultrasonic velocity as a function of the HDPE-g-MAH content for the 90/10, 80/20, and 70/30 HDPE/PA6 blends. The velocities seem unchanged irrespective of the HDPE-g-MAH content for the three systems. According lo the results of the SEM observation, it is known that the particle size decreases with the increasing HDPE-g-MAH content. Therefore, the constant velocity means that ultrasonic velocity is insensitive to the variation of the particle size. On the other hand, Fig. 8 also shows that there are velocity differences among the three blend compositions, and the velocity is larger for the blends with more concentration of PA6. Further relationship between ultrasonic velocity and PA6 concentration are shown in Fie. 9. It can be observed that the ultra-sonic velocity of the blends increases linearly with the PA6 concentration up to 30 wt% in our experiment. In terms of the above discussion, it is suggested that the ultrasonic velocity could be used to investigate the concentration of the dispersed phase, but it is unavailable for predicting the phase size.
As it has shown that ultrasonic attenuation is more sensitive than velocity to characterize the compatibility and morphology in solid blend systems (16), it is expected that this sensitivity could remain in the blend melts as well. Figure 10 shows the relationship between the ultrasonic attenuation and the HDPE-g-MAH content for the 90/10, 80/20, and 70/30 HDPE/PA6 blends. It should be noted that the attenuation used here is also called the total attenuation. Contrary lo the behavior of ultrasonic velocity, it m evident that the total attenuation shows great dependence on the HDPE-g-MAH content for the three blend compositions. In general, the attenuation shows an initial sharp decline with the increasing HDPE-g-MAH content, and then gradually levels off, showing the similar behavior as that of the particle size. Further observations can be found that the changes of the total attenuation with the HDPE-g-MAH content are well fitted by the exponential equation as follows.
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
([[alpha].sub.total,c] - [[alpha].sub.total,[infinity]]) / ([[alpha].sub.total,0] - [[alpha].sub.total,[infinity]]) = exp(-[n.sub.1]c), (5)
where [[alpha].sub.total,c] (is the total attenuation for a concentration c of HDPE-g-MAH, [[alpha].sub.total,0] is the total attenuation for a blend without HDPE-g-MAH, [[alpha].sub.total,[infinity]] is a constant representative of the limiting total attenuation, and [n.sub.1] is a constant that determines the efficiency of HDPE-g-MAH to decrease the total attenuation.
[FIGURE 10 OMITTED]
Table 3 presents the constants [[alpha].sub.total,[infinity]] and [n.sub.1] for Eq. 5. It can be found that, with the increase of PA6 concentration, [[alpha].sub.total,[infinity]] gets increased while [n.sub.1] decreases, suggesting that the higher concentration of PA6 in the blend compositions, the more amounts of HDPE-g-MAH should be added to ensure the same reducing effect. This can also be confirmed in Fig. 10, which displays the fact that the total attenuation shows higher value in the blend with more PA6 concentration when the content of HDPE-g-MAH is fixed.
TABLE 3. Filling parameters of Eqs. 5 and 7. Blend compositions (HDPE/PA6) [[alpha].sub. [n.sub.1] [[alpha]. [n.sub.2] total, excess, [infinity]] [infinity]] 90/10 5.39 1.2 0.60 0.94 80/20 6.27 0.53 1.66 0.53 70/30 6.59 0.46 2.15 0.46
In terms of the similarity between the changes of the total attenuation and that of the particle size with the HDPE-g-MAH content, it is reasonable to speculate that the attenuation could indirectly reflect the change of the particle size of the dispersed phase. However, as mentioned above, the ultrasonic attenuation in Fig. 10 refers to the total allenualion ([[alpha].sub.total]), which is mainly originated from the intrinsic wave absorption ([[alpha].sub.intrinsic), as well as the scattering effect due to the presence of the inclusion ([[alpha].sub.excess]). [[alpha].sub.intrinsic] is simply the sum of the absorption of each individual component and can be expressed by [[alpha].sub.intrinsic] = (1 -[PHI])[[alpha].sub.PE] + [PHI][[alpha].sub.PA6], where [[alpha].sub.PE] and [[alpha].sub.PA6] are, respectively, the attenuation of the pure HDPE and PA6, and [PHI] is the weight fraction of PA6. Herein, [[alpha].sub.excess] is mainly concerned, because it depends largely on the inclusion size. The value of [[alpha].sub.excess] can be obtained by subtracting the intrinsic absorption from the total attenuation, and the equation is expressed as follows (25).
[[alpha].sub.excess] = [[alpha].sub.total] - [[alpha].sub.intrinsic] = [[alpha].sub.total] - [(1- [PHI])[[alpha].sub.PE] + [PHI][[alpha].sub.PA6]]. (6)
Figure 11 displays [[alpha].sub.excess] as a function of the HDPE-g-MAH content for the three blend compositions. Similar to the behavior of the total attenuation, the excess attenuation decrease sharply with the increase of HDPE-g-MAH content, then gradually level off. Also, the experimental data can be reasonably well described by the following exponential equation.
([[alpha].sub.excess,c] - [[alpha].sub.excess,[infinity]]) / ([[alpha].sub.excess,0] - [[alpha].sub.excess,[infinity]]) = exp(-[n.sub.2]c), (7)
where [[alpha].sub.excess,c] is the excess attenuation for a concentration c of HDPE-g-MAH, [[alpha].sub.excess,0] is the excess attenuation for a blend without HDPE-g-MAH, [[alpha].sub.excess,[infinity]] is a constant representative of the limiting excess attenuation, and [n.sub.2] is a constant that determines the efficiency of HDPE-g-MAH to decrease the excess attenuation.
[FIGURE 11 OMITTED]
[FIGURE 12 OMITTED]
The constants [[alpha].sub.excess,[infinity]] and [n.sub.2] for Eq. 7 are also displayed in Table 3. Consistent with the tendency of [[alpha].sub.excess,[infinity]] and [n.sub.1], [[alpha].sub.excess,[infinity]] and [n.sub.2] show the increase and decrease with the increasing concentration of PA6, respectively. Apart from the difference for the 90/10 HDPE/PA6 blends, [n.sub.1] and [n.sub.2] keep the same for the 80/20 and 70/30 HDPE/PA6 blends. This might be ascribed that the contribution from the scattering effect is not large enough for 90/10 HDPE/PA6 blends, while it plays the main role for the other blend compositions. Moreover, if comparing the [n.sub.2] with the n in Table 1, it is found that, for each blend composition, the constant [n.sub.2] is not entirely consistent with but more or less smaller than n. As we mentioned above, the constant [n.sub.2] or n determine the efficiency of the HDPE-g-MAH to decrease the excess attenuation or particle size, so it indicates that ultrasonic results show some delays in reflecting the efficiency of the HDPE-g-MAH compared with the SEM results. Possible reasons may be due to the difference in thermal history, causing a slight difference in the particle size for the samples used for SEM observation and ultrasonic measurement.
Although some differences in the particle size exist for the two characterization method, it is still encouraging to seek the relationship between the present particle size and the excess attenuation. By combination of Figs. 7 and 11, the quantitative relationship between [[alpha].sub.excess] and the particle size of dispersed phase can be obtained in Fig. 12. Figure 12 shows that [[alpha].sub.excess] increases linearly with the increase of the dispersed particle size for all the three blend compositions. By the linear fitting of experimental data, three equations can be obtained as follows, respectively.
For 90/10 HDPE/PA6 blends, [[alpha].sub.excess] = 0.18 + 0.6 x [D.sub.n] (8)
For 80/20 HDPE/PA6 blends, [[alpha].sub.excess] = 1.01 + 0.9 x [D.sub.n] (9)
For 70/30 HDPE/PA6 blends, [[alpha].sub.excess] = 1.25 + 1.0 x [D.sub.n] (10)
Thus, after in-line measuring [[alpha].sub.excess] of HDPE/PA6 blend melts, the particle size of the dispersed phase can be easily calculated using these linear equations. However, it needs to note that, the Eqs. 8-10 have not applied to other immiscible polymer blends up to present, and more experiments need to be conducted in the future to verify the generality of these equations.
In this work, a reliable ultrasonic method was first used to measure the concentration and particle size of the dispersed PA6 phase for PA6/HDPE blend melts. It showed that ultrasonic velocity was insensitive to the particle size but varied linearly with the blend composition in our experimental region. However, the decrease of the ultrasonic attenuation with the addition of HDPE-g-MAH suggested that the attenuation depended greatly on the particle size of the dispersed phase. Similar to the evolution of the particle size, the relationship between the excess attenuation and HDPE-g-MAH content could be reasonably well described by the exponential decay model. Further investigations revealed that there was a good linear relationship between the excess attenuation and the particle size of the dispersed phase, which made it promising for in-line monitoring the morphological evolution by measuring the attenuation of the blend melt during polymer processing.
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Shan Wang, (1) Congmei Lin, (2) Huimin Sun, (1) Fan Chen, (1) Jiang Li, (1) Shaoyun Guo (1)
(1.) The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China
(2.) Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang, Sichuan 621900, China
Correspondence to: Jiang Li; e-mail: email@example.com or Shaoyun Guo; e-mail: firstname.lastname@example.org
Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 50973075.
Published online in Whiley Online Library (wileyonlinelibrary.com).
[c] 2011 Society of Plastic Engineers