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Curing behavior, thermal, and mechanical properties of epoxy resins cured with a novel liquid crystalline dicarboxylic acid curing agent.


Epoxy resins are a versatile group of thermoset polymers that have excellent properties, such as stiffness, strength, chemical resistance, low dielectric constant and loss, adhesiveness to substrates, and thermal and dimensional stability. Thus, they have been widely used in construction, electronics, adhesives, and coatings (1). An important application of epoxy resins is as adhesives for electronic components, which require excellent adhesion, low thermal expansion coefficient, and low dielectric constant and dielectric loss. Notably, the use multifunctional glycidyl amine-type epoxy resins such as N,N-diglycidyl-4-glycidyloxyaniline (TGAP) and 4.4'-methylenebis(N,N-diglycidylaniline) (TGDDM) has been increasing in high-performance applications (2), (3). Recently, the incorporation of liquid crystalline-like structures into epoxy networks has been extensively investigated since they exhibit many interesting properties such as good thermal stability, low dielectric constant and loss, and excellenl anisotropic mechanical behavior (4-9). Liquid crystalline thermosets (LCTs) were developed to improve the mechanical and thermal properties of common thermosets and to overcome the drawbacks of thermoplastic liquid crystalline (LC) polymers. Farren et al. (10) investigated the effects of mesogen concentration on the physical properties of resulted epoxy thermosets. They found that higher mesogen contents gave networks with increased glass transition temperatures ([T.sub.g]), elastic moduli, and thermal stability. These improvements were explained in terms of a decrease in polymer free volume by presence of high molecular packing LC domains. Thus, the chain motion of epoxy network was suppressed with increasing of mesogen content.

Mosl related studies have focused on the preparation and characterization of iiquid-crystalline-epoxy-resin-based LCTs with different mesogenic groups (7), (11)-(17). It is well known that curing agents can influence the formation of LC phase in epoxy networks. However, few reports have investigated the importance of curing agents and LCTs based on LC curing agents (LCCAs) (13), (18), (19).

Epoxy resins have mainly been crosslinked with primary aromatic diamines as the curing agents (7), (12), (14), (15), (18), (19). In some studies, catalytic crosslinking of epoxy resins has been done using trialkyloxonium tetrafluoroborates (20), iodonium salts (21), and imidazoles catalysts (22-25). The advantage of catalytic systems is that they require a small proportion of curing agent, e.g., <5 wt%, but allow the epoxy resin can be cured at lower temperature and shorter curing time than that of without using catalyst. Catalytic curing agents generally remain attached to the initial chain unit of the polymer, which is not part of the inner structure of the network. Therefore, appreciable distortion in the ordered mesogens in the mesophase is not produced. Moreover, the proportion of the catalyst can be selected so that I he crosslinkirig process can be carried out at different curing temperatures and rates. In addition, epoxy resins have also been cured with anhydride, diol. and dicarboxylic acid curing agents. Unfortunately, to our knowledge, few studies have reported on thermosels based on dicarboxylic acid LC curing agents. Osada el al. (13), (26) studied on thermosets of a diaromatic rings dicarboxylic acid LC curing agent and difunctional epoxy resins. However, they have not reported on thermal stability and mechanical properties of resulting thermosets. In addition, thermosets of this curing agent with multifunctional epoxy resins have not been investigated.

In this study, we synthesized a novel LC dicarboxylic acid curing agent (DACA), which is composed of 2,6-naphthalene and 1,4-phenylene mesogenic units. LCTs of DACA and epoxy resins with different functionalities were prepared, and 2-methyl imidazole (2MI) was used as a catalyst. The optimum mole ratio of DACA/epoxy and the proportion of 2MI catalyst were examined by different scanning calorimetry (DSC) techniques. The phase behavior and morphology of cured systems were evaluated using a dynamic mechanical thermal analyzer (DMA), field emission scanning electron microscopy (FESEM) methods. The thermal and mechanical properties of cured epoxy systems were also investigated with thermogravimetry analysis (TGA), DSC, DMA, and thermo-mechanical analysis (TMA) methods.



Terephthaloyl chloride (99%) (TPC), 6-hydroxy-2-napthoic acid (98%) (HNA). triethylainine (TEA), 2-methylimidazole (2MI), N,N-dimethylacetamide (99%) (DMAc), glycidyl end-capped poly(bisphenol-A-co-epichlorohydrin) (pDGEBA. [M.sub.n] = 377). N,N-diglycidyl-4-glycidyloxyaniline (TGAP), and 4,4'-meihylenebis(N,N-diglycidylani]ine) (TGDDM) were purchased from Sigma-Aldrich. All of the chemicals were used as received. The chemical structures of the studied compounds are shown in Fig. 1.

Synthesis of Dicarboxylic Acid Curing Agent

In a 100 ml three neck round flask fitted with a condenser. 3.71 g (19.6 mmol. 2 eq.) of 6-hydroxy-2-naphthoic acid was dissolved in 40 ml of N,N'-Dimethylacetamide (DMAc) solvent under nitrogen atmosphere. A clear solution was obtained, to which 1.99 g (19.6 mmol, 2 eq.) of triethylamine was added. Then, a mixture of 2 g (9.8 mmol. 1 eq.) of terephthaloyl chloride in 20 ml of DMAc solvent was added slowly in an ice bath. The reaction mixture was subject to continued stirring for 2 h. The ice bath was removed and stirring was continued at room temperature overnight. The reaction temperature was increased to 60[degrees]C and was maintained for 12 h. After cooling, the crude mixture was poured into 200 ml of methanol. The precipitate was collected by filtering and washed thoroughly with methanol to remove impurities. Finally, the while solid product was dried under vacuum at 60[degrees]C for 1 day. The yield was 4.6 g (92%).

Preparation and Curing of Dicarboxylic Acid/Epoxy Mixtures

A homogeneous mixture of dicarboxylic acid curing agent (DACA) and the epoxy resins (pDGEBA, TGAP, and TGDDM) at a specific ratio was made in DMAc solvent using a magnetic stirrer. Then, a specific amount of 2MI catalyst was added to this mixture. The obtained viscous mixture was poured into a preheated mold, and the solvent was removed under vacuum at 120[degrees]C. The sample was cured at 200[degrees]C for 2 h.


Fourier transform infrared (FT1R) spectra were recorded with a Nicolel 6700 (Thermo Scientific) with KBr for solid samples. Proton nuclear magnetic resonance ([[.sup.1]H-NMR], 500 MHz) spectra were recorded on a Unity Inova spectrometer, and deuterated dimethyl sulfoxide (DMSO-[d.sub.6] was used for sample preparation. The molecular weights of the copolymers and their distributions were measured by gel pemieation chromatography (GPC) using a Waters ModeL 410 instrument with a refractive index detector (Shodex, RI-101) and three Siyragel (KF-803L, KF-802.5, and KF-802) columns in series, at a flow rate of 1.0 ml/min (eluent: DMF: 40[degrees]C). Polystyrene standards were used to determine the molecular weights. DSC test was performed on a DSC-2910 (TA Instrument) over a temperature range of 30-350[degrees]C at a heating rate of 10[degrees]C/min. The weight loss behaviors of cured samples were monitored for the temperature range 50-600[degrees]C at a heating rale of 10[degrees]C/min in nitrogen using a TG209F3 (Netzch, Germany). DMA test was performed using a TADMA Q800 apparatus (TA instruments, USA) at temperatures ranging from room temperature to 300[degrees]C at a heating rate of 2[degrees]C/min at 1 Hz. The glass transition temperature ([T.sub.g]) is defined as the peak (maximum) temperature in the tan [delta] temperature plot. The sample dimensions were (35 [+ or -] 0.02) X (13 [+ or -] 0.02) X <3 [+ or -] 0.02) [mm.sup.3]. The coefficient of thermal expansion (CTE) values were determined by TMA with a Q400 apparatus (TA instruments) at a scanning rate of 2[degrees]C/min at temperatures ranging from room temperature to 300[degrees]C. The failure surface morphology was observed using a field emission scanning electron microscope (FESEM, JEOL JSM 6700F) at an acceleration voltage of 10.0 kV.


Synthesis and Characterizations of Dicarhoxylic Acid Curing Agent

Scheme 1 shows the synthetic procedure of DACA. The chemical structure of DACA was confirmed by [[.sup.1]H-NMR] analysis and the [[.sup.1]H-NMR] spectrum is shown in Fig. 2, The figure shows a signal between [delta] 7.64 and [delta] 8.70 corresponding to the aromatic protons of the DACA. In addition. The GPC trace of DACA displays a single narrow peak at average molecular weight and polydispersity index of 506 g/mol and 1.034, respectively (in Fig. 3). According to above investigations, the DACA was successfully synthesized.

Curing Reaction Between Dicarboxylic Acid Curing Agent and Epoxy Monomers

Dynamic DSC measurements were used to investigate the appropriate amount of DACA and 2MI catalyst for pDGEBA. Figure 4a shows the DSC curves of DACA/pDGEBA systems with DACA/pDGEBA mole ratios in the range of 0.2-0.8, using a 1 mol% amount of 2MI catalyst. All DSC diagrams show one small endothermic peak in the temperature range of 74-94[degrees]C. which indicates the melting of DACA irt the pDGEBA phase, and a broad exothermic peak in the temperature range of 164-193[degrees]C. The exothermic peak temperature ([T.sub.cur]) and measured heal of curing reaction ([DELTA]H) are plotted in Fig. 4b and c, respectively. Figure 4b shows that the [T.sub.cur] values increase with increase in DACA content. The pDGEBA is in a viscous liquid state at room temperature, whereas DACA is in solid state. As a result, an increase of DACA content will lead to increase in the viscosity of the DACA/pDGEBA mixture, which increase the energy required to overcome the motion of molecular chains and segments, thus [T.sub.cur] shifts to higher temperature. In Figure 4c, it is observed that [DELTA]H increases linearly, reaching maximum at a DACA/pDGEBA molar ratio of 0.5, and then decreases linearly. A higher [DELTA]H indicates that in the same period of time, a higher extent of cure, i.e., higher crosslinking density in the cured product, can be achieved. Therefore, as far as the extent of curing is concerned, a DACA/pDGEBA system with carboxyl group to epoxy group molar ratio of 0.5:1 is preferred.

Figure 4d shows the dynamic DSC curves for DACA/pDGEBA (0.5:1 molar ratio) systems without catalyst and in the presence of various amounts of 2MI catalyst. For the reaction system without catalyst, a small endothermic peak and (wo overlapped exothermic peaks appear at 177.9 and 217.5[degrees]C. and the total heal of reaction is 262.8 J/g. The endothermic peak indicates the melting of DACA in the pDGEBA phase. The presence of two exothermic peaks indicates the possibility of multiple curing reactions occurring previously or sequentially. Similar phenomena were also reported previously for epoxy systems with carboxyl or anhydride curing agents (27), (26), in which the first peak was attributed to estcrification reaction between epoxy groups and carboxyl groups, and the second to the etherification reaction of epoxy groups with hydroxyl groups, which were generated from the first reaction. With the addition of 2MI catalyst, the exothermic peaks shift to lower temperature, e.g., 162.8[degrees]C for 2% mol of 2MI, and the peaks sharpen and become narrower. This implies that 2MI could be an effective catalyst for the DACA/pDGEBA system. In addition, the total heal of reactions was increased from 187.2 to 195.5 J/g with increasing amounts of 2MI catalysi from 0.5 to 2 mol%. respectively. Although the curing reaction of DACA/pDGEBA/2MI systems can proceed at a lower temperatures, their heats of reaction (<195.5 J/g) are lower than that of the system without catalyst (262.8 J/g). This phenomenon is attributed to much more incomplete reaction at lower curing temperature.

To understand the curing reaction mechanism. Figure 5 shows FTIR spectra of the 1 mol% 2MI catalyzed DACA/pDGEBA system before curing and after curing at temperature increased stepwise from 140 to 200[degrees]C with intervals of 20[degrees]C. held at 30 min for each temperature. For the uncured mixture, a broad peak appears at about 3398 [cm.sup.-1] for the O-H stretching in the carhonyl group of DACA. The strong and sharp peaks at 1730 and 1635 [cm.sup.-1] are attributed to C=0 stretching in ester and carbonyl groups of DACA, respectively. The peak at 915 [cm.sup.-1] is characteristic of the epoxy group of pDGEBA. After curing for 30 min at 140[degrees]C, a broad peak at around 3467 [cm.sup.-1] appears that is assigned to the hydroxyl group formed due to the ring-opening reaction of epoxy group by catalyst or carboxylale anions. The peak intensity of carboxyl group was decreased during the curing process, but remained at up to 200[degrees]C, whereas the peak of epoxy disappeared at 160[degrees]C. This implies that the reaction of carboxyl groups and generated hydroxyl groups with the epoxy groups occurred simultaneously, which agrees well with appearance of overlapped-curing peaks in DSC spectra.

On the basis of the combination of DSC and FTIR analysis results, we can propose a catalyst-initiated reaction mechanism for the DACA/pDGEBA/2MI system (Scheme 2). The catalyst 2MI initiates the ring-opening reaction of the epoxy group of pDGEBA to form an oxygen anion, which subsequently picks up hydrogen from carboxyl in DACA to yield a carboxylate anion [reaction (1) of Scheme 2]. The reaction between carboxylate anions and epoxy group generates an ester group and alcoholate anions [reaction (2) of Scheme 2]. Then, the alcoholate anions can react either with epoxy group to generate higher molecular weight alcoholate anions [reaction (3) of Scheme 2] or with carboxyl group to form carboxylate anions [reaction (4) of Scheme 2], The repetition of this reaction leads to chain growth, and finally to the formation of a crosslinked network.

Thermal Stability of Cured Dicarhoxylic Acid Curing A gent/Epoxy Mixtures

The TGA method was used to investigate the effect of functionality of epoxy on the thermal stability of the cured DACA/epoxy/2MI systems. Figure 6 shows TGA thermograms of cured DACA/pDEGBA/2MI, DACA/ TGAP/2MI, and DACA/TGDDM/2MI samples with the functionality of epoxies are 2, 3, and 4, respectively. The temperature at 2% ([T.sub.2]) and 20% ([T.sub.20]) weight loss were used to compare the thermal stabilities of the blends. The [T.sub.2]s, [T.sub.20]s and char yields are summarized in Table 1. The [T.sub.2] values was found to decrease by 10[degrees]C with increasing functionality from 2 (in DACA/pDGEBA2MI system) to 3 (in DACA/TGAP/2MI system). However, it was increased at 11[degrees]C in the DACA/TGDDM/2MI system. For cured systems of pDGEBA and TGAP, increasing the epoxy functionality leads to increased crosslink density of the network, which causes much more incomplete reaction. Meanwhile, in the cured system of TGDDM. the thermal stability could be enhanced by significantly increasing the crosslink density. At higher weight loss, the crosslink density is the most important factor to determine the thermal stability, which contributes to increasing the [T.sub.20] values with increasing epoxy functionality.

The char yields dramatically increased with the increase of epoxy functionality, which is due to the increase of crosslink density. The char yields were 21.1, 60.4, and 66.9% for the cured systems of pDGEBA, TGAP, and TGDDM, respectively.

Thermomcchanical Properties of Cured Dicarboxylic Acid Curing Agent/Epaxy Mixtures

Figure 7 shows the dimension-temperature plots of cured DACA/epoxy mixtures at curing temperature of 200[degrees]C, over a temperature range of 25-250[degrees]C, and the corresponding CTE values are summarized in Table 2. The CTE values of cured DACA/pDGEBA. DACA/ TGAP, and DACA/TGDDM mixtures are 62.6, 59.6, and 61.3 ppm/[degrees]C, respectively, indicating no signilicant change with increasing epoxy functionality. The crosslink density of a cured DACA/TGAP mixture is higher than that of a cured DACA/pDGEBA mixture; thus, the thermal expansion was enhanced. In the cured DACA/ TGDDM mixture, however, the high crosslink density prevents the extent of reaction, leading to much more incomplete reaction, which causes an increase of its thermal expansion.

Dynamic Mechanical Properties of Cured Dicarboxylic Acid Curing Agent/Epoxy Mixtures

The DMA method provides the most significant information on the viscoelastic behavior of polymers as well as information on their thermal transitions. In this method, a sinusoidal force (stress, [sigma]) is applied to a material, and the resulting displacement (strain, [epsilon]) is measured. In principle, viscoelastic properties can be expressed in terms of the dynamic storage modulus (E'), dynamic loss modulus (E") and mechanical damping factor (tan [delta]). Mathematically, these properties are delined as follows:




TABLE 1. The thermal stabilises of cured DACA/epoxy systems.

Epoxy    Functionality  [T.sub.2]     [T.sub.20]        Char yield at
 resins  ([degrees]C)   ([degrees]C)  600[degrees]C(%)

pDGEBA               2           337               382           21.1

TGAP                 3           327               389           60.4

TGDDM                4           338               443           66.9

The storage modulus is associated with stiffness of a material and is related to the Young's modulus. The dynamic loss modulus is associated with internal friction and is sensitive to different kinds of molecular motions, relaxation processes, transitions, morphology, and other structural heterogeneities. Thus, the dynamic properties provide information at the molecular level to understand the polymer mechanical behavior.

Generally, the DMA method is effective for measuring the glass transition temperature ([T.sub.g]) of a thermosetting network, Here. [T.sub.g] is defined as the maximum temperature point in the tan [delta] temperature plot of the results from the DMA test. In addition, the sharpness of this peak may be used as a convenient indicator of the morphology of a material. For a compatible blend, the tan [delta] temperature plot generally shows a single sharp peak (29), whereas in the case of a partially compatible system, the plot shows a broad peak or multiple peaks (30).

Figure 8 shows the storage modulus E' and tan [delta] results of the cured DACA/epoxy samples at curing temperatures of 200[degrees]C. The cured DACA/pDGEBA mixture exhibits a single sharp tan [delta] peak, demonstrating that it might have a monophase structure. The cured DACA/ TGAP and DACA/TGDDM mixtures show a broad peak, which indicates that the phase separation might have taken place. The broadened or multiple

tan [delta] peaks in high functionality epoxy systems have been reported previously in several studies, which is due to the presence of epoxy oligomer in a high-density epoxy network (31), (32). The [T.sub.g] and E' values are summarized in Table 3. At temporal ures below [T.sub.g], the E' of the cured DACA/pDGEBA sample is higher than that of the cured DACA/TGAP and DACA/TGDDM samples, which can be attributed to the much greater amount of epoxy oligomers in the higher epoxy functionality network due to more incomplete curing reaction. At temperatures above [T.sub.g], however, the E' values increased with increasing epoxy functionality. This can be explained by the increased crosslink density of the epoxy network. This trend was also observed in [T.sub.g] values, which were significantly increased with increasing epoxy functionality.

TABLE 2. The thermal and dynamic mechanical properties of cured
DACA/epoxy systems.
                                   Storage modulus (Mpa)
Epoxy resins  Functionality  at 50[degrees]C  at 250[degrees]C

pDGEBA                    2             2952                15

TGAP                      3             2674               915

TGDDM                     4             2761               316

                       CTE tppm/[degrees]C)
Epoxy resins  30-150[degrees]C   160-250[degrees]C   [T.sub.g]([degrees]
                                                     C) (DMA)

pDGEBA        62.6 [+ or -] 0.3  201.0 [+ or -] 1.2                  164

TGAP          59.6 [+ or -] 0.2  120.2 [+ or -] 1.5                  212

TGDDM         61.3 [+ or -] 0.2  114.3 [+ or -] 1.4                  270

Morphology of Cured Dicarboxylic Acid Curing Agent/Epoxy Mixtures

To evaluate the morphology of cured DACA/epoxy systems. FESEM micrographs were acquired for the failure surfaces of the specimens, after the samples were broken in liquid nitrogen (Fig. 9). No phase separation was observed in all cured DACA/epoxy systems. Noticeably, the micrographs of all samples show ductile rough failure surfaces. This can be attributed to the presence of high rigidity and highly packed molecular segments of DACA. which act as crack-stoppers, and can change the direction of crack propagation (8). Thus, as shown in the FESEM images, more complicated crack propagations were observed with increases of epoxy functionality due to increasing crosslink density.


A novel DACA containing 2,6-napthalene and 1,4-phenylene mesogenic units was successfully synthesized. The curing behaviors and reaction mechanism of reaction between DACA and glycidyl-ether-based epoxy resin were determined by dynamic DSC and FTIR. The influence of 2MI catalyst on the curing reaction was also investigated. It was found that the curing of DACA and glycidyl-ether-based epoxy resin took place through a mullireaction process. In addition, the 2MI catalyst could effectively catalyze the curing reaction, which was described by the decreasing of curing temperature peak in DSC diagrams. Moreover, the cured epoxy of DACA and epoxy resins with various functionalities showed excellent thermal and mechanical properties, especially at elevated temperature range. The experimental results indicate that the synthesized DACA may be a promising curing agent for epoxy resins, which could be applied in hjgh-performance applications. However, further precise determination of optimal conditions for curing still requires more experimental work.


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Le Hoang Sinh, (1) Nguyen Ngoc Trung, (1) Bui Thanh Son, (1) Seunghan Shin, (2) Dinh Tan Thanh, (3) Jin-Young Bae (1)

(1) Department of Polymer Science and Engineering, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, South Korea

(2) Korea Institute of Industrial Technology (KITECH), Cheonan 330-825, South Korea

(3) Tienbo Co. Ltd., Le Van Sy Street 351/60, Ho Chi Minh, Vietnam

Correspondence to: L.H. Sinh; e-mail:

DOI 10.1002/pen.23585

Published online in Wiley Online Library (

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Author:Sinh, Le Hoang; Trung, Nguyen Ngoc; Son, Bui Thanh; Shin, Seunghan; Thanh, Dinh Tan; Bae, Jin-Young
Publication:Polymer Engineering and Science
Article Type:Report
Date:Mar 1, 2014
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