Effect of the nanoclay types on the rheological response of unsaturated polyester-clay nanocomposites.
Polymeric nanocomposites have been of interest to researchers because of their improved properties at low nanoparticle loading. Many researchers are working in this field and there are several reports on the role of nanofillers in improving polymer properties, such as strength, and modulus, lire resistancy, and permeability (1) Today, improving the efficiency of preparation procedures and developing techniques of mare robust characterization methods are very essential.
Among all the nanocomposites, those based on layered silicates and nanoclay have been widely considered, probably due to the easy accessibility of initial nanoclay and well-known chemistry of their modification mechanisms (2). Although the incorporation of layered silicates into polymer matrices has been studied for 50 years, the origin of more modern activities goes back to Toyota research works by starting detailed tests on polymer-layered silicate nanocomposites (1), (3-7). Further works were reported by Pavlidou and Papaspyrides who have shown the intercalation of polymer chains into the galleries of nanoclays by heating mixtures of polymer and nanoclay powder above the polymer glass transition or melting temperatures of the polymer matrices (1).
At present time, various thermoplastics and thermosets with different polarities are being used for making nanocomposites which include: polyethylene, polypropylene, polystyrene, polycarbonate, unsaturated polyesters (UPs), epoxies, etc. With these polymers and layered silicates, two main types of intercalated and exfoliated nanocomposites can be obtained. There are four principal methods for producing polymer-layered silicate nanocomposites: (1) in situ template synthesis, (2) intercalation of polymer or prepolymer formation from solution, (3) in situ inteicalative polymerization, and (4) melt intercalation (1). Regardless of preparation methods, nanocomposites based on layers distances are divided into two groups: intercalated and exfoliated types. The term. "intercalated" refers to a condition of interlayer distance of 1.5 nm, whereas the term "exfoliated" is used when the interlayer distance reaches 8 nm (8). However, the interlayer distance in intercalated situation can be more, depending on the type of polymer and nanoclay. Although each preparation method has its own characteristics, the melt intercalation has drawn more attention for its nonsolvent, environment friendly, cheaper, and convenient process (9), (10).
There are useful tests to prove nanoscale layer formation. Although X-ray diffraction (XRD) and transmission electron microscopy (TEM) are effective tools, they are limited in that they probe a small amount of sample and can be costly for routine characterization of nanocomposites. Moreover, sample preparation for TEM analysis is difficult and time consuming (5), (6), (11). Therefore, researchers have been seeking other methods such as rheology as an indirect method to evaluate nanocomposites preparation methods, because the theological properties are very sensitive to changes in the particulate microstructure (12-14). Rheology offers a means to assess the state of dispersion of nanocomposites directly in the melt state (11). Melt rheology is capable to quantify the degree of intercalation/exfoliation/dispersion across the whole nanocomposite test specimen.
So far, many workers have used rheological properties to assess the quality of the dispersion (11), (13-16) and have compared rheology test results and TEM/XRD analysis of polymer nanocomposites (17), however there are only a few published results on styrene-free UP resin/nanoclay nanocomposites.
Dolgovskij et al. (18) investigated the dispersion of organoclay in polystyrene melt with five different molecular weights and at varied mixing temperatures. They concluded that in polymer nanocomposites both diffusion and stress play active and important roles in the exfoliation of silicate layers. For low molecular weight species, it is suggested that the diffusion is the primary mechanism; however, with increasing shear stress (via mixing at lower temperatures) the dispersion efficiency is improved.
Fornes et al. (19) prepared organoclay nanocomposites based on nylon 6 with three different molecular weights by a melt mixing process. The shear stress which has been exerted was between 0.035 and 225 MPa. They proposed two mechanisms for exfoliation of nanoclays: (1) shearing of
platelets stacks to form smaller tactoids; (2) diffusion of polymer chains into clay galleries and facilitation of platelets peeling apart at low shear intensities.
Borse and Kamal (20) proposed a model for mechanism of size reduction in organoclay polymeric nanocomposites prepared by melt mixing. They reported that the breaking of nanoclay particles into smaller units by dispersion requires shear stresses which are higher than those available in extrusion processing with viscosity of polymers between 1000 and 2000 Pa*s. However, their model showed tactoids that can be peeled from the surface of nanoclays. In organically modified nanoclays, the compatibility established between organoclay and polymer matrix leads to a better diffusion of polymer chains into silicate galleries and thus pushing the ends of the platelets apart. This would help the peeling of the stacks to occur at lower shear stresses.
Kasaliwal et al. (21) investigated the effect of mixing shear stress and polymer diffusion on the dispersion of multiwall carbon nanotube agglomerates in polycarbonate matrix. They achieved a better dispersion of nanotubes with increasing mixing shear up to a certain level; however, further increase in shear stress did not lead to a better result. In addition, they reported that at a given shear stress, better dispersion is achieved with lower molecular weight polymer. This is attributed to a primary diffusion of polymer chains into agglomerates followed by dispersing via shear mechanism.
From the literatures, it can be concluded that the shear stress plays the key role in the dispersion of nanoclays, whereas the diffusion facilitates this by lowering the shear level. To establish a better contrast between high shear condition and the situation in which the diffusion process becomes important, the candidate polymer matrix should has a high viscosity at low temperature, whereas show a dramatic drop in viscosity when is heated at elevated temperatures. Considering these, styrene-free UP resins seem a suitable candidate as it has a glass transition temperature below room temperature and a viscosity in the range 1-2 x [10.sup.6] Pass, depending on the temperature. Moreover, in the production plant of UP resin, styrene is added after resin synthesis, and thus the method can be used in the production stage to make the related nanocomposites.
In this article, the effects of nanoclay type and content on the theological behavior of styrene-free UP resin/nanoclay systems prepared by melt mixing method are investigated. To evaluate the effect of shear force and diffusion on the nanocomposite preparation, samples were prepared at two temperature levels of 40[degrees]C and 150[degrees]C. These are henceforth named as cold-mixed and hot-mixed samples, respectively. In cold mixing, no external heating source is employed and the temperature rise, due to the applied shear forces, is ultimately set at 40LC by air blowing. The rheological tests were performed at temperature range 40-120[degrees]C, and XRD and TEM analyses were used as the complementary techniques.
The styrene-free UP resin was alkyd base of Farapol 101, an orthophthalic type, supplied by Farapayeh Co., Iran. The nanoclays were Cloisite 15A and 30B, obtained from Southern Clay Co.
Nanoclay was dried for 24h at 40[degrees]C in a vacuum oven (0.15 atm) before mixing. Nanoclay was mixed with the resin in a Brabender internal mixer (60 ml nominal volume, blade-type roller) at rotor speed of 40 rpm. The mixing temperature was set at 40 and 150[degrees]C, to prepare two set of samples. Temperature selection was based on the resin viscosity at the corresponding temperature. The resin viscosity at 40[degrees]C is about 3 x [10.sup.6] cp, high enough to provide high shear force during mixing. In contrast, at 150[degrees]C, resin viscosity is less than 100 cp which facilitate diffusion of polymer chain into the nanoclay layers. Mixing was continued until reaching a constant torque. Accordingly, 60 min mixing time was used. Two types of nanoclay (Cloisite 15A and 30B) were used at 1, 2.5, 5, and 7 phr. The main specifications of nanoclays and the corresponding nanocomposites are shown in Tables 1 and 2, respectively.
Test Methods and Equipments
Small angle X-ray scattering (SAXS) studies were carried out using a S3-MICRO diffractometer (Hecus, Austria) equipped with Cu[KAPPA][alpha] radiation source ([lambda] = 0.154 nm) operating at 40 kV and 49 mA. The data were recorded in the reflection mode over a 2[theta] range of ~0-10[degrees] at a rate of 0.04[degrees]/min.
Rheological measurements were done on a Physica MCR 300 rheometer (Anton Paar) with parallel plate geometry (diameter of 25 mm) and a gap of 1 mm. For this purpose, the samples were first degassed in a vacuum oven at 40[degrees]C, then placed between the parallel plates and let to rest for 3 min. The tests were performed in a temperature range 40-120[degrees]C, and higher temperatures were avoided due to data variations. Dynamic strain sweeps were done from 0.1% to 100% at an angular frequency of 1 rad/s to determine the linear regime of the samples. Then the rheological measurements were made at constant shear amplitude of 0.1%, and an angular frequency of 1 rad/s. TEM images were taken from cryogenically mircotomed ultra thin sections using a Zeiss Libra200 TEM.
Shear Stress Measurement
To know if the mixing conditions are properly chosen (from the applied shear stress point of view), one needs to calculate the applied shear stress at the specified temperature, quantitatively. The applied shear stress at the two temperature levels used was calculated using the following equations:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)
v = 2[pi]Rn (2)
where [??] is applied shear rate, v is linear velocity, Y is gap size, R is rotor radius, and n is rotor speed.
In Fig. 1, the cross section of the mixer and its geometrical parameters are shown. The details of calculation are given below.
* The highest shear rate [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
* The lowest shear rate [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
* Rotor speed (rpm) n = 40 [min.sup.-1] = 0.67 [s.sup.-1]
* [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
From the theological measurements: [eta](40[degrees]C) = 29,500 Pa s and [eta] (150[degrees]C) = 2.2 Pa s.
By using equation [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], where [tau] is shear stress, we have
* The highest shear stress at 40[degrees]C is [[tau].sub.1] = 37.9 x 29,500 = 1,118,050 Pa [approximately equal to] 1.1 MPa
* The lowest shear stress at 150[degrees]C is [[tau].sub.2] =37.9 x 2.2 = 83.38 Pa [approximately equal to] 8 x [10.sup.-5] MPa
This means that the applied shear force at 40[degrees]C is 1.3 x [10.sup.4] times greater than that of 150[degrees]C. Therefore, the selected temperature conditions are believed to be appropriate for evaluating the effects of shear force and chain mobility in nanocomposite preparation.
TABLE 2. Specifications of UP resin/nanoclay samples. Sample code Nanoclay Nanoclay Mixing type concentration temperature (phr) ([degrees] C) 15A/1/40 Cloisite 15A 1 40 15A/2.5/40 Cloisite 15A 2.5 40 15A/5/40 Cloisite 15A 5 40 15A/7/40 Cloisite 15A 7 40 15A/1/150 Cloisite 15A 1 150 15A/2.5/150 Cloisite 15A 2.5 150 15A/5/150 Cloisite 15A 5 150 15A/7/150 Cloisite 15A 7 150 30B/1/40 Cloisite 30B 1 40 30B/2.5/40 Cloisite 30B 2.5 40 30B/5/40 Cloisite 30B 5 40 30B/7/40 Cloisite 30B 7 40 30B/1/150 Cloisite 30B 1 150 30B/2.5/150 Cloisite 30B 2.5 150 30B/5/150 Cloisite 30B 5 150 30B/7/150 Cloisite 30B 7 150
RESULTS AND DISCUSSION
Figure 2 shows the variations of complex viscosity at three different temperatures for nanocomposites containing various percentages of nanoclay. A summary of the results are presented in Table 3. According to the table, an increase of almost 10% in viscosities of different nanocomposites prepared by cold mixing and measured at 40[degrees]C is observed, when compared with their hot-mixed counterparts. However, when the viscosity measurements are conducted at 120[degrees]C, huge differences in the results are observed. In 15A series (cold and hot mixed), no significant difference in the viscosities measured at 40[degrees]C is observed, whereas at 120[degrees]C, the viscosity of cold-mixed samples is higher than that of hot mixed.
TABLE 3. Viscosity values of cold- and hot-mixed compounds at 40 and 120[degrees]C. Mixing Viscosity temperature (Pa s) ([degrees]C) at 40 at 120 [degrees]C [degrees]C Neat resin 29,500 3.7 15A/5/([d.sub.001] = 40 (cold mixed) 58.800 1370 31.5[Angstrom]) 150 (cold mixed) 49.900 97 30B/5/([d.sub.001] = 40 (cold mixed) 46,000 44.9 18.5[Angstrom]) 150 (cold mixed) 38.600 9.0
As it can be seen, regardless of mixing temperature, addition of nanoclay increases the viscosity and this becomes more evident at higher concentrations of nanoclay. In addition, all cold-mixed nanocomposites have a higher viscosity than their hot-mixed counterparts. This is in such a way that for the samples tested at 40[degrees]C. the difference in viscosities is about 11%, whereas for those tested at 120[degrees]C cold-mixed samples show viscosities 5 to 140 times greater than the hot-mixed ones. For the latter samples, the difference in viscosities for cold- and hot-mixed samples depends on the initial d-spacing of the nanoclay used in the formulation. The more the d-spacing the higher the viscosity difference. In follow, it has been tried to understand the origin of this behavior.
For better illustration, the complex viscosity variations with temperature was measured for nanocomposite samples containing various types/amounts of nanoclay, mixed at hot and cold conditions. Figure 3 shows the results for hot-mixed samples containing Cloisite 15A and 30B.
As it is evident, with increasing nanoclay content the complex viscosity has increased, but similar to the neat resin its decreasing trend with temperature is also observable. For a given nanoclay content, sample containing Cloisite I5A shows a higher viscosity than Cloisite 30B. This increase is in such a way that the viscosity for the neat resin at 40[degrees]C increases from 29,500 Pa*s to 41,000, 43,200, 54,300, and 65,700 Pa*s for Cloisite 15A at loadings of 1, 2.5, 5, and 7 phr, respectively. For Cloisite 30B and at the same loadings, these are 40,200, 42,500, 42,800, and 47,000 Pa s, respectively.
The same trend, as seen at the temperature 40[degrees]C, can also be observed in viscosity values of hot-mixed samples measured at 100eC. In fact, the slope of the curves, complex viscosity vs. nanoclay content, is almost similar in both temperatures (Fig. 4). This behavior is true for two types of nanoclays, i.e. Cloisite 15A and 30B.
In contrast, cold-mixed samples show different viscosity behavior with temperature, when compared with hotmixed samples. Here, a decrease in viscosity is observed which reaches to a plateau at higher temperatures. The onset of the plateau depends on the nanoclay type and content (Fig. 5). For better illustration, the variations of plateau onset temperature with nanoclay content for nanoclays Cloisite 15A and 3013 are shown in Fig. 6. For samples containing Cloisite 15A, the plateau is observed at all nanoclay loading used (he. 1, 2.5, 5, and 7 phr), whereas for Cloisite 30B this only happens at 5 and 7phr. For either nanoclay types, with increasing nanoclay content the plateau begins at lower temperatures. In addition, the viscosity at plateau for Cloisite 15A is more than one order of magnitude (in logarithmic scale), compared with Cloisite 30B and at the same content. The appearance of a plateau indicates that beyond a certain temperature level, some factors interfere with the decreasing trend of the viscosity and prevent its further reduction as the result of the temperature increase.
In Fig. 7, the variations of complex viscosity with nanoclay content at two selected temperatures. i.e., 40 and 100[degrees]C for cold-mixed samples are shown. As can be seen, the effect of nanoclay on the viscosity increase is much more significant at l00[degrees]C than 40[degrees]C. According to this, the increase in viscosity at 100[degrees]C and at 5 phr of nanoclay is more than 100 and I0 times of the neat resin for the Cloisite 15A and 30B, respectively. This is in contrast with what is observed for the hot-mixed samples (Fig. 4). Whatever the reason, it is obvious that the factors responsible for this phenomenon are much more effective at high temperatures so that even the higher mobility expected for the polymer chains at these temperatures does not lead to a further reduction of viscosity. This means that for the cold-mixed samples the chains slippage and thus the flow is someway restricted, whereas this is not the case for the hot-mixed samples.
Although this behavior (independency of viscosity from temperature) is characteristic of cross-linked structures, however, chemical cross linking cannot be the case here, as this would be more probable for the hot-mixed samples. What is more likely to happen is the formation of a physical network which can be verified by the study of viscoelastic behavior of the samples. Figure 8 shows the variations of the storage (G') and loss modulus (G") of the cold-mixed samples containing 7 phr of either Cloisite 15A or 30B. As it can be seen, loss modulus shows a descending trend with temperature at the whole temperature range studied, whereas the storage modulus curve reaches a plateau at a certain temperature and crosses the G" curve. It is worth mentioning that this crossover point cannot be observed for the hot-mixed samples (Fig. 9).
The initial decrease in the storage modulus is due to the facilitation of the chain slippage as the result of the temperature increase, however, for the cold-mixed samples beyond a certain temperature level further decrease is prevented by the presence of nanoclay particles. It is suggested that a kind of a physical network is Formed in which the polymer chains motion in the nanoclay-rich areas is restricted and hence counteracting the decrease of the storage modulus with temperature so that the net effect is the formation of a plateau. This can also be the reason for the formation of plateau in viscosity-temperature curves. The appearance of the plateau cannot be related to percolation threshold--in which the particles come in contact with each other above a certain concentration--as the viscosity plateau is observed even at low concentrations, e.g. 1 phr (17), (22-25).
A number of studies (25-28) have suggested that the gel point of a thermoset resin occurs at the crossover point of the loss and storage modulus. In addition, the complex viscosity exhibits a trend consistent with the crossover point of the storage and loss modulus. Based on time-temperature super-positioning, however, there is a direct balance between time (the testing frequency) and temperature. Thus, the crossover point of the storage and loss modulus can be regarded as a confirmation of physical network formation.
The formation of physical network for cold-mixed samples and not for the hot-mixed ones can be an indication of more availability of nanoclay particles to polymer chains at the former samples. This is achieved by better dispersion of the particles as well as breaking of the nanoclay stacks through thickness. TEM and SAXS analysis can be used as the complementary techniques to investigate this matter. Figure 10 compares SAXS diagrams of different nanoclays and the related hot and cold-mixed samples containing 5 phr of nanoclay. As it can be seen, for Cloisite 15A in both cases the peak has shifted from 2.7[degrees] for the neat nanoclay to about 2.3[degrees] for the nanocomposite (for Cloisite 30B the shift is from 4.7[degrees] to 2.15[degrees]). In addition, the peak height of the cold-mixed sample is slightly smaller than that of hot mixed. Although the peak intensity is affected by parameters such as nonhomogeneity originated either from mixing process and/or sample preparation, however, it might be related to the number of species having a specific d-spacing. This means that in the cold-mixed samples some of the nanoclay stacks have a lower thickness than the hot-mixed samples; thus the peak intensity is reduced.
Because SAXS itself may not provide enough evidences, further evaluation of the results was carried out using TEM. In Fig. 11, TEM images of the hot and cold-mixed samples containing 5 phr of either Cloisite 15A or 30B are shown. As can be seen, in the cold-mixed samples Cloisite 15A has better dispersion than Cloisite 30B (Fig. 11a and b). Moreover, in the former sample (Fig. 11a) the distance between two set of stacks is below 100 nm, which is less than gyration radius of polymers such as UP. Thus, in the proposed physical network several nanoclay stacks are stuck inside a polymer chain which restricts its flow. In addition, the limited diffusion of polymer chain between the intercalated silicate layers as well as their enhanced entanglements imposes more constraint on the chain motion. Moreover, during dispersion shear forces push the ends of the layers apart (like what is observed in Fig. 11d) and this would provide some spaces where the polymer chains can enter and this increases polymer-clay involvement. In Fig. 12, the schematic of this network and also the suggested polymer chain/nanoclay arrangement for the hot-mixed samples are shown.
In the cold-mixed samples, stack breakage happens through thickness, as the result of high shear generated during mixing process (Fig. 11c and d). For Cloisite 30B, the presence of stacks with high thickness (Fig. 11b) and also their incomplete separation (Fig. 11d) indicate that the applied shear has been less efficient in full separation of the stacks. This can be related to the original d-spacing in Cloisite 30B which is smaller than Cloisite 15A (Table 1).
TABLE 1. Main specifications of the used nanoclays. Nanoclay type Modifier (a) [d.sub.001] ([Angstrom]) Cloisite 30B [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE 18.5 IN ASCII] Cloisite 15A [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE 31.5 IN ASCII] (a) T: tallow (~65% C18; ~3O% C16; ~5% C14).
In the hot-mixed condition, the resin viscosity is low, so the shear force is not high enough to achieve good dispersion of nanoclay particles (Fig. 11e and f). In contrast, the diffusion of polymer chains inside the nanoclay layers and the separation of a few of them are more likely to happen. This mainly happens in outer layers of the stacks which are more accessible to polymer chains (Fig. 11g and h).
From the above discussions, it can be said that for the cold-mixed samples the shear mechanism is dominant and leads to a good dispersion of the nanoclay particles and reduction of the stacks thickness. In these conditions, the chain mobility is low and only limited intercalations of the layers occur, as the results of improved accessibility. This is in agreement with the results obtained from SAXS experiments. In the hot-mixed samples, diffusion mechanism plays the major role, however due to less availability of nanoclay stacks--when compared with the cold-mixed conditions--only exfoliation of the outer layers happens which is not detectable in SAXS (Fig. 11g and h).
Having the formation of a physical network in mind, the theological behavior of the samples can now be better interpreted. For the cold-mixed samples, two factors control the viscosity; polymer chains mobility and the physical network formed by nanoclay-polymer chains. Generally, at low temperatures chain with low mobility resists the flow and this determines the viscosity of the system. This trend continues until the physical network, as an interfering factor, is appeared beyond a certain temperature. At this point, the higher mobility of polymer chains is counteracted by the chain entanglements and also the physical network, thus the viscosity remains constant (Fig. 5).
In contrast, in the hot-mixed samples, there are not enough shear forces to break the stack associations. This leads to the formation of nanoclay-starved areas which mainly determine the viscosity behavior of the system. Thus, it is expected to see a continuous reduction of viscosity with temperature with no plateau region for these samples (Fig. 3).
Melt mixing method was used to prepare thermoset nanocomposites based on styrene-free UP resin and nanoclay. Mixing temperature controls the nanocomposite morphology as well as its theological behavior. Complex viscosity of the nanocomposite system was increased with increasing nanoclay content; however, the trend of increase is much dependant on the mixing temperature. For the hot-mixed samples, a continuous decrease in viscosity-temperature curve was observed with temperature, whereas for their cold-mixed counterparts a plateau was reached beyond a certain temperature which the onset depends on the nanoclay content.
The difference in the theological behavior of the coldand hot-mixed samples was related to the formation of a kind of a physical network in the former. In the cold-mixed samples, high shear forces break the stacks through thickness thus more particles are available for the polymer chains to get involve with them. On the basis of TEM results, it was suggested that in this network the chain slippage and thus polymer flow is restricted by (i).sticking several nanoclay stacks inside a polymer chain, (ii) diffusion of polymer chain between the intercalated silicate layers, and (iii) increasing polymer-clay involvement as the result of entering the polymer chains inside spaces provided by shear forces during dispersion. This counteracts the decrease of the viscosity, as the result of temperature increase, so that the net effect is the formation of a plateau.
The formation of a physical network was further confirmed by the crossover point observed in C'-G" curve. The temperature in which the crossover occurred depends on nanoclay content and decreases with increasing clay content.
Cold-mixed samples containing Cloisite 30B showed a lower viscosity at elevated temperatures, than their Cloisite 15A counterparts. This was attributed to the original d-spacing in Cloisite 30B which is smaller than Cloisite 15A. Regardless of nanoclay type, SAXS results confirmed the intercalation of the silicate layers in both mixing temperatures.
The kind help of Regine Botch from Leibiliz-Institut fur Polymerforschung Dresden e.V. for sample preparation and obtaining the TEM images is acknowledged.
Correspondence to: Masoud EsCaudell: e-mail: email@example.com
Contract grant sponsor: Iranian Nano Technology Initiative Council.
Published online in Wiley Online Library (wileyonlinelibrary.com).
[c] 2012 Society of Plastics Engineers
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Amir Masoud Rezadoust,(1) Masoud Esfandeh,(1) Mohammad Hosain Beheshty, (1) Gert Heinrich (2),(3)
(1) Composites Department, Iran Polymer and Petrochemical Institute, Tehran, Iran
(2) Leibniz-lnstitut fur Polymerforschung Dresden e. V, Hohe Str. 6, D-01069 Dresden, Germany
(3) Technische Universitat Dresden, Institut fut. Werkstoffwissenschaft, D-01069 Dresden, Germany
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|Author:||Rezadoust, Amir Masoud; Esfandeh, Masoud; Beheshty, Mohammad Hosain; Heinrich, Gert|
|Publication:||Polymer Engineering and Science|
|Date:||Apr 1, 2013|
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