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A novel hyperbranched polyborazine and its reactive blend with benzoxazine thermosets.

INTRODUCTION

Recently, various ceramic materials and polymer composites have been used as thermal insulating and ablative materials (1-6). It is desirable that the organic--inorganic nanostructure in polymer composites can be converted into a uniform ceramic layer that may led to significant heat resistance to oxidation and mechanical erosion compared with the polymer composites without the incorporation of inorganic atom. Generally speaking, powder processing and sintering techniques are the most common methods of making ceramic materials but with the limitation to produce ceramics in processed forms. An alternative to conventional high temperature powder techniques has been great interest in development of polymer precursor to ceramic materials that enables ceramic formation under mild processed conditions (7), (8). Therefore, the precursor molecules can be designed to contain structure units of the residual inorganic materials derives. Owning to their outstanding thermal and mechanical properties, boron carbide and boron nitride are materials of particular interest than other boron-containing polymers (9) which grows intensively at the present time.

In the case of applications of ceramic precursors in polymer composites, besides the versatile functionality of the precursor polymer, a favorable processing property is also one of the important aspects to be considered. So, a hyperbranched polymer, as opposed to a linear architecture for conventional polymers, containing boron and nitrogen atoms in its backbone is of great interest. Hyper-branched polymers have distinct advantages over traditional linear polymers in terms of low solution and melt viscosity, a number of terminal functional groups, and excellent solubility due to their specific structural characteristics (10), (11). So, hyperbranched precursor polymer owns favorable processability, excellent thermal and oxidant resistance. On the other hand, the terminal functional groups of hyperbranched polymer endow it with excellent solubility in many common organic solvents as well as the opportunity to react with prepolymer of thermosets. Hyperbranched polymers have exhibited a remarkable toughening effect on both thermoset and thermoplastic polymer matrix composites without loss of thermome-chanical and processability (12-16). They can be adapted to different thermoset systems by changing terminal functional groups according to the nature of the thermosets. Furthermore, the solubility and reactivity of hyper-branched polymers can also be tailored using the terminal tailoring strategy (17-20). Therefore, hyperbranched precursor polymer that can react with the prepolymer of target thermoset either as a toughening agent or a curing agent without substantially decreasing the modulus and the glass transition temperature, or diminishing other desirable mechanical properties is highly valued.

Polybenzoxazine is a novel type of addition-cure phenolic system and it has gained immense interest due to its potential to overcome several shortcomings of the traditional resole-type phenolic resins while retaining such benefits as near zero volumetric upon cure, low water absorption, no strong acid catalysts required for curing, and no byproduct released during curing, etc. (21-27). However, polybenzoxazines require somewhat higher polymerization temperature and monomer-based polyben-zoxazines are brittle as is the case for all other thermosetting resins. This paper first describes the synthesis of a novel reactive hyperbranched polymer based on the boron carbonitride skeleton. It then discusses the influence of the resulting polymer on the curing and the thermal properties of benzoxazine thermosets. To achieve that, the structural characterization and the thermal properties of hyperbranched polyborazine as well as its blend with ben-zoxazine were studied in detail.

EXPERIMENTAL

Materials

Boron trichloride in 1 M (mol [L.sup.-1]) toluene solution was purchased from Beijing Multi Technology, and used without further purification. P-phenylene diamine (99.9%) was purchased from J&K Scientific. N-methy1-2-pyrroli-done was freshly distilled under nitrogen from sodium/ benzophenone prior to use. Ethanol, acetone and ethyl ether were used as received. Bis-benzoxazine (P-ddm) was supplied by Sichuan University, China (Scheme 1a).

Synthesis of Hyperbranched Polyborazine

The reaction was performed in a 99.999% ultra-high pure nitrogen atmosphere. A 500-mL flame-dried flask was equipped with a condenser and a Teflon stir bar. The outlet temperature of the condenser was kept between -30 to -20[degrees]C. 20 mL (0.04 M) boron trichloride in 1 M-toluene solution was introduced with a syringe and injected into a dropping funnel. Then, 8.6512 g (0.08 M) p-phenylene diamine dissolved in 250 mL N-methyl-pyrrolidone (with 5.0 wt% CaC[1.sub.2]) was injected to the flask. A huge amount of white smoke was found as the boron trichloride was added dropwise to the flask under an ice-water bath. The flask was continuously flushed with nitrogen gas to purge the byproduct of hydrogen chloride during the reaction. After completing the addition of boron trichloride, the mixture was kept at 80[degrees]C under stirring for 4-6 h. Then, the mixture was slowly increased to 180[degrees]C and the reaction was continued until no volatile HCI can be found at the absorption apparatus. The solvent was removed by vacuum, and the dark brown residue reacted at 220[degrees]C for 2-3 h before dissolved in anhydrous ethanol and precipitated in acetone. The above dissolve and precipitate procedures were repeated for three times, and the precipitant was extracted in acetone by Soxhlet for 48 h, which was then kept in a vacuum oven at 80[degrees]C for 24 h, yielding light brown powder (yield, 6.71 g). A proposed structure for the hyperbranched polyborazine is show in Scheme lb.

Pyrolysis of Hyperbranched Polyborazine

Polymer samples were pyrolyzed up to 1100[degrees]C in a high temperature tube furnace (GSL 1200X). About 2 g of sample was placed in a nickel crucible and heated from room temperature to 1100[degrees]C with a heating rate of 5[degrees]C [min.sup.-1] in nitrogen. Initially, the furnace tube was both flushed with 99.999% ultra-high pure nitrogen and repeatedly purged by a vacuum at room temperature. Then, the sample was pyrolyzed at 1100[degrees]C for 2 h, followed by cooling approximately at 2[degrees]C [min.sup.-1] to the environment temperature under the same gas flow as heating.

Preparation of Hyperbranched PolyborazineIP-ddm Blend

The blend containing 10 wt% hyperbranched polybora-zine was prepared as following. Hyperbranched polybora-zine was dissolved in ethanol, and it was subsequently mixed with the chloroform solution of P-ddm. The mixture was stirred at room temperature for 0.5 h until a transparent solution was formed. In the end, the residual solvent of the solution was removed by vacuum oven at 60[degrees]C. The hyperbranched polyborazine/P-ddm blend was cured at 130 and 160[degrees]C for 2 h at each temperature, followed by heating at 220[degrees]C for 4 h and 250[degrees]C for 2 h.

Characterizations

FIR spectra of pyrolyzed P-ddm composites were recorded by a Bruker Equinox 55 FTIR in the range 4000400 [cm.sup.-1] with a resolution of 1.0 [cm.sup.-1] at room temperature. The 1H- and 13C-NMR spectra were recorded on a Bruker Avarice 300 (300 MHz for (1) H, 753 MHz for (13) C) using [D.sub.2]0 as solvent and M[e.sub.4]Si as an internal reference. Size exclusion chromatography (SEC) measurements were carried out on a Dionex Ultimate 3000 DGLC ([H.sub.2]0 was utilized as the mobile phase at a flow rate of 1.0 mL [min.sup.-1] at 30[degrees]C) equipped with an Acclaim 120 PA C16 column. Molecular weights were determined using a universal calibration curve based on the data from the RI and DV detectors using narrow polystyrene standards. A differential scanning calorimetry (DSC) model Q1000 from TA Instruments was used to study the thermal properties of polyborazine and its blend with P-ddm. The experiments were carried out with a heating rate of 10[degrees]C [min.sup.-1] from room temperature to 600[degrees]C under nitrogen purge at 50 mL [min.sup.-1]. Glass transition temperature ([T.sub.g]) was determined from the second heating run using the point of inflection method. The sample was first heated from room temperature to 180[degrees]C at a heating rate of 10[degrees]C [min.sup.-1] and then through a cooling cycle after holding for 2 min at 180[degrees]C, to destroy any previous thermal history. After that, the sample was heated again to the end temperature at a rate of 10[degrees]C [min.sup.-1] (the second heating scan). [T.sub.g] was taken as the peak top of the differentiation of DSC curve of the second heating scan. Thermal decomposition behaviors of hyper-branched polyborazine and polyborazine/P-ddm thermosets were investigated on a TA Instrument of SDT Q600. Thermal degradation experiments were carried out in an inert atmosphere using a nitrogen purge at 50 mL [min.sup.-1] with a heating rate of 10[degrees]C [min.sup.-1] from room temperature to 1200[degrees]C. X-ray diffraction (XRD) pattern was recorded at room temperature by using a Bruker D8 equipment (k = 0.154 nm) to analyze both the structure evolution and the crystalline property of the pyrolyzed samples. All specimens were analyzed by copper radiation using a computer-controlled powder diffractom-eter under 30 kV and 20 mA between 15[degrees] and 80'. The scan speed is of 4[degrees] [min.sup.-1] X-ray photoelectron spectroscopy (XPS) measurements were performed at a PHI-5400 (America PE) system. The elemental composition of polyborazine (atomic ratio of C, N, B) was determined from the curve-fitting Cls, Nis, and Bis multiplex spectra, respectively.

RESULTS AND DISCUSSION

Synthesis and Pyrolysis of Hyperbranched Polyborazine

As described in Scheme lb, the synthesis route developed in the study is based on the polycondense between two types of groups: the amine group of p-phenylene diamine and the chlorine atom of the boron trichloride. In general, high solution concentration and longer reaction time lead to high molecular weight products. Therefore, the polycondensation was carried out in a relatively low concentration of 5 wt%. In addition, calcium chloride was used to improve the molecular weight of polyborazine. It was reported that calcium chloride can form complex with NMP, and hydrogen bonds occur between the complex and the terminal amine group of polyborazine, which impedes the interchain agglomeration of polyborazine. As a result, the solubility of polyborazine in NMP and its molecular weight are improved (28). At the initial stage of the reaction, the flask was filled with a large amount of white smoke due to the hydrogen chloride byproduct, and clusters of polyborazine oligomers were formed. As the reaction temperature elevated, the viscosity of solution was increased. A monomer with functionality of 3 will introduce branching in the polymer. Ultimately, a cross-linked macrostructure or network will form even at low fractional conversion. In addition, the initial molar ratio of monomers has a strong influence on the polymerization process and the structure of the resulting polymers. In the study, the monomer feeding ratio of p-phenylene diamine ([A.sub.2]) to boron trichloride ([B.sub.3]) is 2, no crosslinking was observed during polymerization. On the other hand, the chlorine atom of the boron trichloride can not completely react with amino groups even in the presence of excessive p-phenylene diamine due to the topological hindrance of the supposed polymer.

Polymer Solubility and Molecular Weight Determination

The polyborazine is readily soluble in common polar solvent such as [H.sub.2]0, ethanol, DMF, DMAC, NMP, and the mixtures of the above mentioned solvents, but it can not dissolve in nonpolar solvent. Self-aggregation of hyperbranched polyborazine through hydrogen bonds may lead to its excellent solubility in polar solvent. Figure 1 shows the typical refractive index (RI) chromatogram of hyperbranched polymer. The chromatogram shows a slight bimodality, exhibiting the existence of specie with high molecular weight along with the main molecular mass distribution. This is typical for [A.sub.2] + [B.sub.3] hyper-branched polymer system with a broad weight distribution. In addition, the weak intensity of refractive index demonstrates that the content of the high molecular weight specie is low. It should be mentioned that the high molecular weight fraction can be separated by the precipitation fractionation. The weight-average molecular weight and the polydispersity index of polyboraxzine are ranged from 1100 to 4500 and from 1.15 to 2.06, respectively.

Structural Characterizations of Hyperbranched Polyborazine

In the (1) H-NMR (Fig. 2a) spectrum, [D.sub.2]0 was used as solvent that normally causes all hydrogen atoms are exchanged with deuteriums, thus make these hydrogen signals disappear. The groups of signal around 7.0-7.3 ppm is suggestive of a doubly (para) substituted aromatic protons (29). In addition, the peaks ranged from 1.8 to 3.6 ppm were attributed to the aliphatic hydrogens of C[H.sub.3] and C[H.sub.2] that may come from the residual solvent of N-methyl-2-pyrrolidone in the polyborazine sample (30). 13C-NMR spectrum (Fig. 2b) also verified the presence of the above residue with signals around 55.8 and 18.2 ppm (31), and the representative s[p.sup.2] carbons around 172.7 and 166.2 ppm (32). The other peaks centered around 136.3 and 121.4 ppm are due to the aromatic carbon atoms of various functionalities.

The sample was dried in a vacuum oven at 180[degrees]C for 48 h before utilized or FTIR analysis in order to identify the structure of as-prepared polyborazine polymer. Figure 3 indicates the formation of the expected bonds in the polyborazine backbone. Asymmetric stretching bonds of N--H were observed at 3420 [cm.sup.-1]The sharp peak at 1620 [cm.sup.-1] demonstrated the bending vibration of N--H bond. The band at 1501 [cm.sup.-1]is assigned to CH bending of benzene. The FTIR data also indicate the formation of B-N bonds since the spectrum shows band at 1411 [cm.sup.-1] (3), (33). The absorptions around 1311 and 1110 [cm.sup.-1]are due to the stretching vibration of C--N bonds that were linked with primary and secondary amines (32), and the absorption at 820 [cm.sup.-1] is corresponds with a para-substitution pattern on the benzene ring. Broad absorption bands in the range of 2800-2900 [cm.sup.-1], 2600-2520 [cm.sup.-1], and 2390-2300 [cm.sup.-1] are attributed to different amine salts. It is revealed that the polyborazine was not completely purified. The following NMR analysis further confirmed the structure of polyborazine and the presence of the above mentioned impurity.

The elemental composition of polyborazine was obtained from XPS scans (see Fig. 4). Atomic percentages of the B1s, C1s, and N1s were calculated from the total areas of the XPS signals corresponding to the B1s, C1s, and N1s core levels, respectively. In addition to the peaks (C. N, and B) assigned to polyborazine, the polymer displays an additional 0 peak at 532.10 eV, evidence of the presence of [H.sub.2]0 absorbed on the surface. Based on element-sensitive ratios, a chemical formula such as [C.sub.11.63][N.sub.3.87][B.sub.1.0][H.sub.x]can be proposed for polyborazine, and the hydrogen content could not be identified due to the limitations of the analysis instrument.

Further work has been carried out to study the nature of chemical environment of polyborazine by XPS. The deconvolution of the Cls spectrum (see Fig. 4a) gives two peaks centered at 284.3 and 286.1 eV, respectively. The binding energy of 284.3 eV corresponds to the energy of the Cl s core level for C=C bonds, whereas the binding energy with peak at 286.1 eV is identified as originating from the C--N bonds. The B 1 s spectrum was fitted with single Gaussian curve (see Fig. 4c). The spectrum was deconvoluted into two component peaks. The peak centered at 189.1 eV is for B--N bonds of polyborazine. The peak at about 197 eV present in the B is spectrum is most probably due to chloride atom bonded with boron atom (34). In addition, the Nis spectrum (see Fig. 4b) confirms the interpretation of B 1 s and Cls spectra with the characteristic peaks of B--N and C--N bonds. The broadening of Nis spectrum indicates the presence of multiple bonds. The curve can be well fitted by deconvo-luted into three Lorentzian-Gaussian peaks centered at 398.9, 400.5, and 401.4 eV respectively. As has been reported (35), the peak centered at 398.9 eV can be attributed to the B--N bonds. The less electronegative B atoms resulted in the lower binding energy of N. The smaller component peak at 400.4 eV is assigned to the N--H bonds (36). Moreover, the high energy peak centered at 401.4 eV is due to nitrogen atoms bonded to the carbon atoms. The Ols XPS spectrum exhibits a peak centered at 532.1 eV, which is assigned to the hydroxyl bonds of absorbed water (see Fig. 4d) (37). The XPS results indicated that polyborazine sample have such characteristic bonds as B--N, B--Cl. C=C. C--N, and N--H. Therefore, all the bonds in the hypothetic structure verified by XPS are in accord with the FTIR and NMR analysis.

Thermal Properties of the Precursor Polyborazine

Glass transition temperature is one of the important factors that influencing the usage of a polymer. In general, the glass transition temperature of a hyperbranched polymer is highly dependent on its terminal functional groups in addition to backbone structure. The polybora-zine synthesized with the [A.sub.2]/[B.sub.3] ratio of 2 shows a [T.sub.g] of 146[degrees]C (see Fig. 5). There may be some hydrogen bonds between amines and Cl atoms of the linear units in hyper-branched polyborazine, which contributes to the [T.sub.g].

TG/DSC analysis was performed by utilizing the sample dried at 180[degrees]T for 48 h to investigate the influence of residue NMP/water on the thermal property of polyborazine (see Fig. 6). The polyborazine exhibits a gradual weight decrease as well as an endothermic absorption in the temperature range of 30-230[degrees]C, with the endothermic peak of 143[degrees]C. The weight loss in the above temperature range is due to the presence of water as polyborazine is prone to absorbing water owning to its amine groups and boron atoms in the skeleton. In addition, there is residual NMP in the polyborazine as verified by NMR, which results in the weight loss in early stage of polyborazine. The residual NMP was hardly removed due to the presence of the hydrogen bonding interaction of the carbonyl group with the NH group in polyborazine (38). While after polyborazine was dried at 180[degrees]C for 48 h in a vacuum oven, there is no weight loss in the above temperature range as indicated by thermogravimetric analysis (see Fig. 7). The second stage of weight loss is in the range of 230-420[degrees]C, with an endothermic peak at about 285[degrees]C. The thermal stability of hyperbranched polymer prepared by the [A.sub.2] + [B.sub.3] approach depends on the backbone as well as the content and the type of terminal functional groups which are closely related to the initial ratio of [A.sub.2]/[B.sub.3]. The presence of amine groups and the remaining chloride atoms of linear units in the skeleton can further polycondense during the degradation process, which may be the main reason that results in the second stage weight loss. Moreover, the collapsing of polyborazine by releasing gaseous products such as [CH.sub.4] and [H.sub.2], with an endothermic peak at 450[degrees]C, also contributes to the weight loss at 420-600[degrees]C [39].

The polyborazine has a char yield of 43.0% at 1200[degrees]C, with the 5% and 10% weight loss in the temperature of 344[degrees]C ([Td.sub.5]) and 371[degrees]C ([Td.sub.10]), respectively (see Fig. 7). DTG curve exhibited a two-stage weight loss process: the first takes place from about 230 to 425[degrees]C owning to a polycondensation between the remaining chloride atoms of linear units and the amine terminal groups, the second one which occurs from 425 to 605[degrees]C may be attributed to the skeleton collapse of polyborazine. It is notable that the weight loss over 800[degrees]C was very weak, and residue at 1200[degrees]C maybe a mixture of boron carbonitride and carbonaceous materials. This hypothesis can be verified by the following pyrolysis analysis.

Pyrolysis of Hyperbranched Polyborazine

FTIR spectra and XRD analysis provide more information about the structural changes of the pyrolyzed poly-borazine samples with the increasing of temperature. An infrared study on the structural evolution revealed a stepwise loss of functional groups (see Fig. 8). When the sample was pyrolyzed at 400[degrees]C, the absorption band of the C--H groups of benzene ring was weakened. For the polyborazine sample pyrolyzed at 600[degrees]C, the C--H peak almost disappeared, indicating that the polyborazine was transformed into inorganic materials above this temperature. The final pyrolyzed materials exhibit three strong peaks centered at 1625, 1395, and 1080 [cm.sup.-1], which are typical absorption peaks of compound containing C=C, C--N, and B--N bonds, respectively. While the band intensities of B--N and C--N at 1392-1400 and 1064-1092 [cm.sup.-1] were increased after pyrolysis from 400[degrees]C to 600[degrees]C and 1100[degrees]C as compared with the standard band at 1625 [cm.sup.-1]. The content ratio of B--N to C=C was increased from 0.17 to 0.22 and 0.26, while that of C--N to C=C was increased from 0.52 to 0.57 and 0.89 at 600 and 1100[degrees]C, respectively.

XRD patter of the pyrolyzed polyborazine under 1100[degrees]C is showed in Fig. 9. The broad peak indicates the crystallization of polyborazine is small at characteristic peaks of 2[Theata] = 24.3[degrees](002) and 43.2[degrees] (100) planes, revealing the presence of amorphous layer sequential characteristic. In addition, according to the FTIR spectra and the XRD pattern of polyborazine, the crystalline of carbonaceous during pyrolysis process is hindered by the presence of boron and nitrogen by forming boron carbonitride material. It was speculated that boron and nitride atoms may be substitutionally incorporated into graphite layer.

Curing and Thermal Property of Hyperbranced PolyhorazinelP-ddm Blend

Figure 10 exhibits the dynamic curing at 10'C min-I of neat P-ddm and the formulation containing 10 wt% hyperbranched polyborazine. As can be seen, curing of the blend takes place at a lower temperature than that of neat P-ddm, indicating that the terminal functional groups of hyperbranched polyborazine can catalyze the curing reaction of P-ddm. In addition, protonation of benzoxa-zine originating from the amine groups of hyperbranched polyborazine may occur at the oxygen atoms of benzoxa-zine ring, which results in methylamine-based benzoxa-zine (40).

Table 1 shows the reaction heat during the dynamic curing of P-ddm and its blend with hyperbranched poly-borazine. The blend formulation produces an exotherm of 128.6 J [g.sub.-1], which is far from that of P-ddm of 203.2 J [g.sub.-1]. The decreasing reaction rate is due to the lower ben-zoxazine ring content as compared with the neat P-ddm. On the other hand, the improving viscosity of blend may restrict the cure process as the degree of cure increased. In a word, both the dilution effect and the steric hindrance of hyperbranched polyborazine counteracted the acceleration owning to the terminal amine groups that are responsible for the delayed curing.

TABLE 1. Cure characteristics of P-ddm and hyperbranched
polyborazine/P-ddm blend.

Hyperbranched   [T.sub.i  [T.sub.p  [T.sub.e  [DELTA]H [
polyborazine     .sup.a]   .sup.b]   .sup.c]  (J/g).sup.d]
in P-ddm (wt%)

0 (pure P-ddm)       206       241       286         203.2

10                   163       218       269         128.6

(a.)[T.sub.i] indicates the exothermic initial temperature.
(b.)[T.sub.p] indicates the exothermic peak temperature.
(c.)[T.sub.e] indicates the exothermic end temperature.
(d.)[DELTA]H indicates the exothermic heat.


Thermal stabilities of polybenzoxazine and polyborazine/P-ddm thermosets were evaluated using TGA under nitrogen atmosphere (see Fig. 11). For P-ddm, the initial degradation with [T.sub.d5] happens at 395[degrees]C whereas for the blend formulation the To is increased to 415[degrees]C. It is notable that majority weight losses occur at 350-560[degrees]C for both of the thermosets, corresponding to the release of volatile gas and breaking of backbone. In addition, the polyborazine/P-ddm thermosets exhibits a faster weight loss tendency than that of polybenzoxazine in that the steric hindrance of hyperbranched polyborazine may low the cross-link density of P-ddm, and the decreased cross-link density is the main factor influencing the thermal stability before the temperature of 600[degrees]C (41). Another reason is that hyperbranched polyborazine will produce volatile hydrochloride and the terminal functional groups of hyperbranched polyborazine is less stable than the backbone structure during the pyrolysis process, which lead to the weight loss and the weakened thermal stability. No obvious weight loss was observed in the temperature of 800-1200[degrees]C as for the polyborazine/P-ddm thermosets, indicating that the carbonizing process is completed for the preceramic polymer modified thermosets. But it is worthy to note that the thermal stability was improved in the temperature of 950-1200[degrees]C as for the polybora-zine/P-ddm thermosets, exhibiting both a lower weight loss rate and a higher char yield. Therefore, the precursor polymer of hyperbranched polyborazine might form a superficial layer constituted of boron carbonitride compound that limits the heat transfer to the underlying material, which contributes to the outstanding thermal property of polyborazine/P-ddm thermosets.

CONCLUSIONS

A novel hyperbranched polyborazine, derived from the reaction of boron trichloride and p-phenylene diamine, is synthesized for a boron carbonitride precursor polymer. The hyperbranched polyborazine was characterized by FTIR, NMR, GPC, and XPS. Thermal study indicates a compound containing B--C--N bonds can be conveniently obtained during the pyrolysis process of polybora-zine. Moreover, it was found that the hyperbranched polyborazine can be used as a reactive modifier to lower the cure temperature of P-ddm. At the same time, the char yield and thermal stability of the polyborazine/P-ddm thermosets is improved, especially at the temperature higher than 800[degrees]C. The approach of introducing inorganic-containing polymer to the precursor polymer proves to be a promising strategy for achieving a high performance P-ddm thermosets. Detail study of the relationship between the curing condition and the morphological evolution of the thermosets will be reported in a subsequent article.

Correspondence to: Yuhong Liu; e-mail: liuyuh@mail.xjtu.edu.cn

Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 51103114.

DOI 10.1002/pen.23279

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C] 2012 Society of Plastics Engineers

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Yuhong Liu, Xiying Zhang, Xin Yang

Department of Chemical Engineering, School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China
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Author:Liu, Yuhong; Zhang, Xiying; Yang, Xin
Publication:Polymer Engineering and Science
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Date:Mar 1, 2013
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