Preparation of blends of polymethyl methacrylate copolymers with high glass transition temperatures and low hydrophilicity.
Poly(methyl methacrylate) (PMMA) is a nonpoisonous, environmental polymeric material possessing many admirable advantages, such as high light transmittance, ease of processing, excellent dimensional stability, colorlessness, resistance to weathering corrosion, and so forth. Whereas its application in the optical electronic field, for materials such as compact discs, optical glasses, and optical fibers, was limited because its lower glass transition temperature ([T.sub.g]) was about 100[degrees]C (1), (2).
To raise [T.sub.g] of PMMA, copolymers incorporating monomers with rigid or bulky structures have been reported widely (3-8). For example, copolymerization of methyl methacrylate (MMA) with maleimide (MI) which has a five-member ring was discussed by Kita et al. (9). The copolymers have not only high [T.sub.g], but also excellent toughness (10). Whereas the residue of MI monomer makes the light transmittance of copolymers decrease sharply (11), (12).
Copolymerization with monomers that can form hydrogen bonds with the carbonyl groups of PMMA such as acrylamide (AM), methacrylamide (MAAM), acrylic acid (AA), or methacrylic acid (MAA) was another efficient method to raise the [T.sub.g] of PMMA (13-15). However, water absorbability of the copolymers increased because of the existence of polarity groups of--CONH, or--COOH. To decrease the absorption of moisture, the copolymerization of N-alkyl methacrylamide, MAAM with one hydrogen atom substituted by alkyl groups, with MMA has been investigated by Zhang and coworkers (16). Although the water absorption decreased, [T.sub.g] of the copolymer decreased markedly compared with MMA-MAAM copolymer. Chang and coworkers incorporated styrene (St) as the third component to MMA-MAAM copolymer to reduce the moisture absorption (17), but the weather resistance of the copolymer declined due to the appearance of benzene ring in the chain. Tricyclodecyl methacrylate (TCM) was selected as the third component (18), but [T.sub.g] of the obtained MMA-MAAM-TCM terpolymer decreased markedly compared with the MMA-MAAM copolymer. Techniques that could increase the [T.sub.g] and decrease moisture adsorption of PMMA at the same time have not been acquired.
Recently, Kuo and Tsai prepared blends of poly(methyl methacrylate-co-vinylbenzylthymine) (MMA-VBT) with poly(methyl methacrylate-co-2-viny1-4,6-diamino-1,3,5-triazine) (MMA-VDAT) (19). When MMA-VBT (12 mol% VBT) was blended with MMA-VDAT (20 mol% VDAT), Ts increased about 20[degrees]C relative to the additive value of the two copolymers and near 50[degrees]C to pure PMMA. Under the guideline of the above findings, we prepared MMA-MAAM/MMA-MAA blends and hope to raise Ts of PMMA and improve the moisture resistance of the copolymers at the same time by taking advantage of the intermolecular hydrogen bond interaction.
MMA was purified by washing with 5% NaOH aqueous solution, then with distilled water until neutral. The washed monomer was dried over anhydrous [Na.sub.2]S[O.sub.4], then distilled under reduced pressure. MAAM was recrystallized from toluene. MAA were distilled under reduced pressure directly. MMA, MAAM, and MAA were stored in the refrigerator prior to polymerization. The radical initiator of azobisisobutyronitrile (AIBN) was recrystallized from methyl alcohol prior to use. 1,4-Dioxane was distilled under reduced pressure and then used as the solvent in the solution copolymerization. Dimethylformamide (DMF) was distilled prior to use. All other chemicals were of analytical grade and used as received without further purification.
Syntheses of MMA-MAAM and MMA-MAA Copolymers
The solution copolymerization of MMA with MAAM or MAA was carried out in 1,4-dioxane at 80[degrees]C in a glass flask under nitrogen atmosphere. AIBN (0.5 wt% relative to monomers) was used as an initiator. The mixture was stirred for about 24 h before being poured into excess isopropyl alcohol with vigorous agitation to precipitate the product. The crude copolymer product was purified by redissolving it in 1,4-dioxane and then adding the solution dropwisely into a large excess of isopropyl alcohol. This procedure was repeated several times and then the residual solvent and precipitant in the final product were removed under vacuum at 70[degrees]C for 24 h and white powders of MMA-MAAM and MMA-MAA copolymers were obtained. For convenience, we use the descriptor M-MAAM-1, M-MAAM-2, M-MAAM-3, and M-MAAM-4 that represents a copolymer by copolymerization with 5 wt%, 10 wt%, 15 wt%, and 20 wt% of MAAM, respectively. And it is same for the case of MMA-MAA copolymers. Scheme 1 outlines the synthetic procedures and the structures of the various components.
Blends of MMA-MAAM with MMA-MAA copolymers were prepared through solution blending. Both of weighted copolymers were redissolved in 1,4-dioxane and clear and transparent solution was obtained. Then it was added dropwisely into a large excess of isopropyl alcohol with vigorous agitation to precipitate the product. The ratio of the blend solution to the precipitant was 1:20 to make the copolymers chains precipitate as absolutely as possible. This procedure was repeated several times, make sure that 1,4-dioxane dissolved in isopropyl alcohol completely, then precipitant was removed by suction filtration. The blends were stored in vacuum oven at 70[degrees]C for 24 h to remove the residual solvent and precipitant, finally pure powders of blends were obtained.
[.sup.1]H NMR (400 MHz) spectra of copolymers were recorded on a Brucker AVANCE 400 spectrometer using CD[C1.sub.3] as the solvent. Molecular weights and molecular weight distributions were determined by a Waters 510 HPLC gel permeation chromatography using tetrahydrofuran (THF) as eluent at a flow rate of 0.4 mL/min. The molecular weight calibration curve was obtained using polystyrene standards. [T.sub.g] of the copolymers and blends were determined using a DSC-2910 from TA Instruments. The samples were scanned within the temperature range 25-200[degrees]C at a heating rate of 20[degrees]C/min under nitrogen atmosphere. [T.sub.g] values were obtained from the temperature at the half change of the heat capacity in the second differential scanning calorimetry (DSC) traces. Fourier transform infrared (FTIR) spectra of the copolymers and blends were determined by a Vector 22 FTIR spectrophotometer using the conventional KBr disk method. Thirty-two scans were collected at a spectral resolution of 1 [cm.sup.-1]. The copolymers and blends were dissolved in DMF and then coated on glass slides to evaporate the solvent slowly at room temperature for 48 h and then in a vacuum oven at 50[degrees]C for 48 h. The obtained smooth membranes are clear and transparent. The static contact angles with water were measured in a contact angle meter (SL 200B, Shanghai Solon Information Technology, China) at room temperature. The droplet of water is in a size of 3-5 [micro]L.
RESULTS AND DISCUSSION
Figures 1 and 2 show the [.sup.1]H NMR spectra of MMA-MAAM and MMA-MAA copolymers as examples to illuminate the attributions of diverse characteristic absorption peaks of different functional groups. The compositions of the MMA-MAAM and MMA-MAA copolymers were obtained by reckoning the ratios of peak areas of hydrogen in methyl, amino, or carboxyl, respectively. Tables 1 and 2 tabulate all the monomer feed ratios, copolymer compositions, molecular weights, and polydispersity indexes of MMA-MAAM and MMA-MAA copolymers, respectively.
TABLE 1. Monomer feed mol fractions, copolymer compositions ([F.sub.MAAM] is mol composition and [w.sup.MAAM] is weight composition), and molecular weights of MMA-MAA copolymers. Monomer Polymer feed composition Polymer [f.sub. [F.sub. [.sup.w] [M.sub. [M.sub. [M.sub. MAAM] MAAM] MAAM n] (g/ w] (g/ w]/[M mol) mol) .sub.n] PMMA 0 0 0 18,538 40,502 2.19 M-MAAM-1 0.062 0.044 0.038 20,947 39,268 1.88 M-MAAM-2 0.131 0.092 0.079 16,783 33,300 1.98 M-MAAM-3 0.208 0.137 0.119 12,777 22.206 1.74 M-MAAM-4 0.294 0.179 0.156 11,269 20.691 1.84 TABLE 2. Monomer feed mol fractions, copolymer compositions ([F.sub.MAA] is mol composition and [.sup.w]MAA is weight composition), and molecular weights of MMA-MAA copolymers. Monomer Polymer feed composition Polymer [f.sub. [F.sub. [.sup.w] [M.sub. [M.sub. [M.sub. MAA] MAA] MAA n] (g/ w] (g/ w]/[M mol) mol) .sub.n] PMMA 0 0 0 18,538 40,502 2.19 M-MAA-I 0.061 0.052 0.045 19,987 39,894 2.00 M-MAA-2 0.129 0.091 0.079 19,454 38,675 1.99 M-MAA-3 0.205 0.121 0.112 20,086 40,012 1.99 M-MAA-4 0.291 0.168 0.148 19,990 39,681 1.99
It could be found that [F.sub.MAAM], the copolymer composition, is some lower than [f.sub.MAAM], the feed composition. The reactivity ratios of MMA with MAAM were reported to be [r.sub.MAAM] = 1.38 and [r.sub.MAAM] = 0.24 (15). This indicates that MAAM has much lower activity than MMA in the copolymerization. When [r.sub.1] > 1, the copolymer is richer in monomer 1 than the monomer feed and the opposite holds for [r.sub.1] < 1. Then [f.sub.MAAM] is some lower than [F.sub.MAAM] unless the conversation reaches 100%. Meanwhile the probabilities of formation of diad or triad MAAM sequence are perfectly low, which indicates that the MAAM monomer usually ends up in an isolated single MAAM sequence in the MMA-MAAM copolymer. Taking into account the microstructures of these copolymers, the intermolecular hydrogen bond between the carbonyl group of MMA and the amide group of MAAM has a larger probability of occurring than the self-association through hydrogen bonding of pure PMAAM (15). Similar case was found for MMA-MAA copolymers.
Figure 3a and b displays the DSC curves of pure PMMA, MMA-MAAM, and MMA-MAA copolymers, respectively. All these MMA-MAAM and MMA-MAA copolymers showed one single glass transition which indicates that the incorporation of MAAM or MAA monomer into the PMMA main chain can be considered as statistical. Experimental [T.sub.g] of copolymers versus composition and the theoretical curves base on Fox equation (20) are shown in Figure 4a and b. It was found that [T.sub.g] of the copolymers increased upon increasing MAAM or MAA compositions and a marked positive deviation from the Fox equation was demonstrated which indicates the increase of intermolecular interactions.
FTIR spectroscopy is an efficient tool for identifying and investigating the specific interactions between polymer segments both qualitatively and quantitatively. Figure 5 displays FTIR spectra of MMA-MAAM copolymers with different MAAM contents. The free carbonyl stretching for pure PMMA was observed at 1731 [cm.sup.-1] and PMAAM exhibits two peaks at 1656 and 1601 [cm.sup.-1], corresponding to the amide I band (CO stretching) and amide II band (N--H bending), respectively (15). It could be found that the carbonyl band of PMMA broadens with the increasing MAAM content. Meanwhile, the peak of amide I group shifts to higher wavenumber clearly and its intensity decreases upon increasing MMA content in the MMA-MAAM copolymers. The results may indicate that carbonyl groups of MMA unit forms interassociated hydrogen bonds with amide groups of MAAM.
For convenience, the second-derivative spectrum was used to identify the absorption peaks in amide region of MMA-MAAM copolymers. Figure 6 shows second-derivative infrared spectra of the M-MAAM-4 copolymer in the range of 1550-1750 [cm.sup.-1]. There are five main minima under zero point line (dash line) for (1) the free carbonyl of MMA units at 1730 [cm.sup.-1] (the adsorption of MMA units hydrogen-bonded to amide groups is shown as a shoulder at 1720 [cm.sup.-1]), (2) the free amide I groups of MAAM units at 1681 [cm.sup.-1], (3) the amide I groups of MAAM hydrogen bonding intermolecularly to the carbonyl groups of MMA (HB amide I in Fig. 5) at 1668 [cm.sup.-1], (4) the self-associated, hydrogen bonding amide I groups of MAAM units at 1657 [cm.sup.-1], and (5) the amide II band at 1604 [cm.sup.-1] (15). The adsorption of HB amide I (marked by an arrow) was emphasized here and was found to move to higher frequency and become more clear when the MMA contents increase (1676 [cm.sup.-1] in M-MAAM-2). This indicates that some of the self-associated hydrogen-bonded amide groups of MAAM have become interassociated hydrogen bonds with carbonyl groups of MMA units. Consequently, the [T.sub.g] of copolymers becomes higher than that predicted based on Fox equation.
Figure 7 displays FTIR spectra of MMA-MAA copolymers with different MAA contents. A peak at 2587 [cm.sup.-1] was observed in FTIR spectra of PMAA, which attributed the adsorption of carboxylic dimers (21). But little peak in this region was found in MMA-MAA copolymers, even for M-MAA-4 which has the highest MAA content. This indicates that the self-associate H-bonds between MAA units almost entirely break down in the MMA-MAA copolymers.
The intensity of the absorption of free O--H groups of pure PMAA was located at 3525 [cm.sup.-1], but it broadens and shifts to lower wavenumber clearly with the incorporation of MMA in the MMA-MAA copolymers, which indicates that hydrogen bonds are formed between O--H groups with carbonyl groups of MMA. For the broad peaks, the method of second-derivative could not identify the absorption peaks efficiently. Gauss fitting method was used instead and the fitting result of M-MAA-4 is shown in Fig. 8. It could be noticed that three peaks are observed for (1) the free O--H of MAA units at 3578 [cm.sup.-1], (2) the peaks at 3485 [cm.sup.-1] correspond to the overtone of the carboxyl stretching vibration (21), and the new peaks appeared at 3285 [cm.sup.-1] may be attributed to the hydrogen bonding--OH (HB--OH, marked with an arrow in Fig. 7) of MAA units with carboxyl of MMA units because that it does not appear in the IR spectra of PMMA and PMAA homopolymers. It could be found that the peak moves to lower wavenumber with the corporation of MMA unit, which indicates the influence of the MMA contents. The formation of interassociation hydrogen bond is the main reason for the positive deviation of [T.sub.g], from that predicted by Fox equation.
Figure 9 displays contact angles of MMA-MAAM and MMA-MAA copolymers with various MAAM or MAA contents. The contact angle of pure PMMA is 95[degrees] and decreased rapidly with increasing MAAM or MAA content. This indicates that hydrophilicity of copolymers increases, which is due to that the strong hydrophilic groups of--CON[H.sub.2] of MAAM and--COOH of MAA could form hydrogen bonds with water. The moisture absorption of copolymers is higher than PMMA homopolymer, which will limit their application.
[T.sub.g] of Blends of MMA-MAAM/MMA-MAA
Figure 10 displays DSC curves of MMA-MAAM/MMA-MAA (50/50) blends, where the MMA-MAAM and MMA-MAA copolymers contain different contents of MAAM and MAA, respectively. Each of these binary blends exhibited a single glass transition, indicating that the blends were miscible on the range 20-40 nm or even smaller (19). Based on polarity similar principle, the more interaction among the polymers, the better miscibility should be obtained. A negative Huggins interaction parameter and excellent miscibility are expected because that MMA-MAAM copolymer is a Lewis base while MMA-MAA copolymer is a Lewis acid. A mass of experiments were also carried out to assess the phase behavior of these blends. Mix the solutions of the two kinds of copolymers by different fraction thoroughly and let it stand for about I day. At last, we find that the mix solutions remain uniform transparent. This result may show that MMA-MAAM and MMA-MAA copolymers have excellent miscibility.
Figure 11 illustrates the dependence of [T.sub.g] on the composition of the MMA-MAAM/MMA-MAA blends. Although the values of [T.sub.g] of blends are somewhat lower when MMA-MAA copolymer contents are higher than MMA-MAAM copolymer, all the achieved [T.sub.g]s of blends are quite higher than that predicted by Fox equation and maximum deviations were found when the proportion of MMA-MAAM/MMA-MAA is 50/50. This is similar to the case of MMA-VDAT/MMA-VBT blend that researched by Kuo and Tsai (19). Meanwhile, [T.sub.g]s of the blends increase further with increasing MAAM or MAA content in these two PMMA-based copolymers. It is notable that [T.sub.g] increased by 65[degrees]C relative to that of pure PMMA when M-MAAM-4 was blended with M-MAA-4.
This may due to the presence of intermolecular hydrogen-bonded specific interactions between the copolymers. Over the years, many empirical equations have been proposed to predict the variations in [T.sub.g] of miscible blends and diblock copolymers as a function of composition, such as those suggested by Gordon--Taylor (22), Karasz (23), and Couchman (24). But the most popular and adequate equation for systems displaying intermolecular hydrogen-bonded specific interactions is the Kwei equation (25):
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)
where [w.sub.1] and [w.sub.2] are the weight fractions of the components, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] are the corresponding glass transition temperatures, k is model parameter, and q is fining parameter corresponding to the strength of hydrogen bonding in the blend which reflects a balance between the breaking of the self-association interactions and the forming of the interassociation interactions. In the assumption of the Simha--Boyer rule (26), the Gordon--Taylor equation can be reformulated as the well-known Fox relation (20). Then Kwei equation could be rearranged as:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)
q of M-MAAM-2/M-MAA-2, M-MAAM-3/M-MAA-3, and M-MAAM-4/M-MAA-4 blends were obtained to be 40, 51, and 66, respectively, by fitting the [T.sub.g] data of the bends using Eq. 2. The large positive q indicates that strong intermolecular interaction exists among the components in the blends and is stronger than the self-association interaction in one copolymer chain with increasing of MAAM and MAA contents. This is because that MAAM, a Lewis base unit, could form hydrogen bond with Lewis acid of MAA unit and the formed hydrogen bond has much higher bond energy than that formed between MAAM and MMA or MAA and MMA. The increase of q with increasing MAAM and MAA contents indicates that the interassociation interaction becomes stronger with increasing MAAM and MAA contents. Largest q was obtained to be 66 for the M-MAAM-4/M-MAA-4 blend system. This implies that this blend system featured strongest multiple hydrogen bonds than did the other two blend systems. This is because that this system contains more MAAM and MAA units which increased the possibility of forming the interassociation hydrogen bond.
Figure 12 displays comparison of FTIR spectra of M-MAAM-4/M-MAA-4 (50/50) blend with its corresponding copolymers recorded at room temperature. Unfortunately, it is hard to distribute the different species using the second-derivative spectrum or Gauss fitting method because the complexity of the intermolecular and intramolecular interactions in the blend and the broadening of the absorption peaks. Both M-MAAM-4/M-MAA-4 blend and the copolymers show adsorption peaks in the broad range of 3050-3750 [cm.sup.-1]. The carbonyl and amide adsorption bands of blend and corresponding copolymers shown in Fig. 12 were tentatively used to indicate the structure change after the blending. It could be found that the peak of amide I band (CO stretching) is broader than that of M-MAAM-4 and stronger than half of it. Meanwhile, a broadening and a shift in the carboxyl bond could also be found. This may indicate the formation of intermolecular hydrogen bond between the two copolymers which is responsible for large positive q observed in the blends.
MMA-MAAM and MMA-MAA copolymers are highly hydrophilic because hydrogen bond could be formed between their polar groups with water. It is very important to reduce the hydrophilic property of the copolymers. Figure 13 displays the contact angles of MMA-MAAM/MMA-MAA blends. It is interesting to notice that all the contact angles of blends are larger than the corresponding copolymers. For the M-MAAM-3/M-MAA-3 blends, largest contact angle of 91[degrees], which is close to that of pure PMMA (950), was obtained. This result indicates that the blends exhibited lower hydrophilicity than the corresponding copolymers. This is because that the probability of forming hydrogen bonds between basic functional groups of--CON[H.sub.2] or acid functional groups of--COOH with water decreases due to that they are more likely to form hydrogen bonds with each other. So the hydrophilicity resistance of blends was improved greatly. These blends may have the potential for application.
A positive deviation of the composition dependence of [T.sub.g] from Fox equation was found in not only the copolymers of MMA-MAAM and MMA-MAA but also their blends owing to strong multiple hydrogen-bonding interactions. Both Kwei equation and FTIR analyses provide positive evidence for forming strong hydrogen bond interactions in these blends. The obtained contact angles of blends are much larger than the corresponding copolymers, implying that water resistance of PMMA-based copolymers was improved greatly due to the screening effect of strong hydrogen bond formed between Lewis base of MAAM and Lewis acid of MAA. Therefore, PMMA-based material with significant high [T.sub.g] and low hydrophilicity could be achieved by blending of MMA-MAAM with MMA-MAA copolymers.
Correspondence to: Guo-Dong Liu; e-mail: firstname.lastname@example.org
Contract grant sponsor: Natural Science Foundation of Hebei Province; contract grant number: B2012202148.
Published online in Wiley Online Library (wileyonlinelibrary.com). [c] 2013 Society of Plastics Engineers
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Pan-Pan Wu, Dong-Mei Zhao, Li-Xia Li, Hai-Su Wang, Guo-Dong Liu
School of Chemical Engineering, Hebei University of Technology, Tianjin 300130, People's Republic of China
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|Author:||Wu, Pan-Pan; Zhao, Dong-Mei; Li, Li-Xia; Wang, Hai-Su; Liu, Guo-Dong|
|Publication:||Polymer Engineering and Science|
|Date:||Nov 1, 2013|
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