Thermoreversible crosslinking of maleic anhydride grafted butyl rubber with multiple hydrogen bonding arrays.
Traditional macromolecules, especially the rubber, are crosslinked by slightly connecting each chain together with irreversible, stable, covalent bonds. The main purpose is to enhance the properties, such as enormous extensibility, good elasticity, high toughness, and solvent resistance. However, for rubber technology, most of the crosslinking systems (e.g., sulfur vulcanization) form the permanent network, which prohibits the re-usage of waste vulcanized rubber, thus rising an environmental issue. Then recently supramolecular polymers containing directional and reversible chemical-physical linkages have opened a new avenue of research into thermoreversible crosslinking polymers. Since it weakens significantly at elevated temperatures, hydrogen-bonding has actively been used for formation of thermoreversible crosslinking materials via a self-assembly process [1-7].
Modifying polymer chains is the usual method for preparing hydrogen-bonding supramolecular networks. Sijbesma et al.  found that to significantly increase strength of polymer by crosslinking via hydrogen bonding, a single hydrogen bonding donor and acceptor pair is inadequate, but it is relatively easy to improve the strength by combining hydrogen bonds in arrays. Several hydrogen bonding arrays have been used to obtain reasonably reversible polymer networks. Stadler et al.  and Miiller et al.  reported on the modification of poly(butadiene) with grafted phenylurazole groups, which are capable of forming double hydrogen bonding dimers. The properties were improved in comparison to the non-modified poly(butadiene). However, the improvement is not to a large level. De Lucca Freitas et al.  explained this is due to the relatively weak association constant of phenylurazole groups. Hilger and Stadler [12, 13] showed that two double hydrogen bonding arrays per functional group obtained via the addition of a carboxylic acid group to the phenylurazole, using 4-carboxyphenylurazole units give impressive mechanical properties, due to the formation of large assemblies of associated 4-carboxyphenylurazole groups. Lange and Meijer  and Loontjens et al.  obtained soluble and, hence, reversible polymer networks using imide-diaminopyridine (DAD-ADA) triple hydrogen bonding arrays between melamine and three imide groups of styrene-maleimide copolymers. The materials displayed a rheological behavior similar to covalent networks, although the weak association constant limited the properties.
Sijbesma et al.  and Beijer et al.  introduced 2-ureido-4 [1H]-pyrimidone which can form self-complementary multiple hydrogen-bonding groups with high dimerization constants to the chain-ends of polymers, such as polyethylene oxide-co-propylene oxide)s, polycaprolactone, and telechelic poly(elhylene/butylene). Etter et al.  have reported that thermoreversible elastomers could be obtained by grafting poly(dimethylsiloxanes) with bisurea groups and by preparing segmented block copoly(ether ureas) with small bisurea blocks. The bisurea groups self-assemble into phase-separated stacks via strong, bifurcated hydrogen bonds. Both types of thermoreversible crosslinking elastomers showed excellent mechanical properties, while melting of the stacks at elevated temperatures resulted in processable materials. Recently, reversible polymer networks using low molecular-weight polymers functionalized with three imidazolidone groups and amine to form double hydrogen bonding dimers were illustrated by Toumilhac et al. , Schmidt et al. , and Hu and Lindt .
Chino et al.  introduced triazole groups into polyisoprene via the reaction of maleic anhydride (MAn) grafted polyisoprene with 3-amino-1,2,4-triazole (ATA). The reaction led to the formation of amide triazolecarboxylic acid group at which each group was capable of interacting with other groups via a six-point hydrogen bonding as shown in Fig. 1. These multiple hydrogen bonding arrays should produce a strong crosslinking moiety. Compared to the original MAn grafted polyisoprene, the obtained product showed much higher mechanical properties with thermal reprocessability at least 10 times without affecting the properties.
From above, it can be seen that even though there are many researches focusing on thermoreversible crosslinking polymer, none of the above researches work on butyl rubber (IIR). HR exhibits very unique low gas permeability rate and high dumping. However, it shows very low sulfur curing rate, greatly inhibiting co-curing with general purpose rubbers. The development of thermoreversible crosslinking IIR will extend the usefulness of IIR.
In this work, we synthesize thermoreversible crosslinking IIR. First, the IIR was grafted with MAn. Then the MAn-g-IIR was modified with ATA to form thermoreversible crosslinking MAn-g-IIR via multiple hydrogen bonding arrays same as Fig. 1. The suitable amounts of ATA and tetrabutyl ammonium bromide (TBAB) acting as a catalyst for the formation of thermoreversible cross-linking of MAn-g-IIR via hydrogen bonding geometry was first determined. The designed experiments are shown in Table 1. To determine the occurrence and the conversions of the ATA modification reactions, the chemical structures of obtained products were studied using Fourier transform infrared (FTIR) spectroscopy. The effects of hydrogen bonds on the morphology studied with X-ray powder diffraction (XRD), and the rubber properties, that is, tensile properties were investigated.
IIR with Mn and Mw of 60 and 90 kg/mol, respectively, was supported by NEXEN Tire, Republic of South Korea. The following chemicals, ATA, MAn, TBAB, dimethylbenzene, pyridine, potassium hydroxide (KOH), phenolphthalein, toluene, methanol, ethanol, acetone, and n-heptane were purchased from Aldrich and used as received.
To prepare thermoreversible crosslinked MAn-g-IIR with multiple hydrogen bonds, the sample preparation was divided into three steps as described below.
Step1: Purifying IIR. A proper amount of IIR was dissolved in n-heptane at 50[degrees]C and constantly stirred for 6 h. Then the IIR solution was poured into acetone to precipitate the purified IIR. The precipitated HR was washed with fresh acetone several times and then dried at room temperature under vacuum.
Step2: Grafting IIR With MAn. The purified HR was cut into small pieces and then dissolved in toluene. The mixture was constantly stirred at room temperature until the homogeneous solution was obtained. Then the solution was heated to 92[degrees]C and followed by the addition of solutions of MAn and dibenzoyl peroxide in toluene. The grafting reaction was carried out for certain time at this temperature. Then, the MAn-g-IIR was precipitated in methanol and washed several times with acetone. The grafting product was maintained at 40[degrees]C under vacuum for 48 h.
Step3: Modifying the MAn-g-IIR With ATA. To prepare the thermoreversible crosslinked MAn-g-IIR with multiple hydrogen bonding arrays via reaction with ATA, 10 g of the MAn-g-IIR was first dissolved in 150 ml of toluene at 60[degrees]C. The required amounts of TBAB acting as a phase transfer catalyst was added to the MAn-g-IIR solution while constantly stirring for a certain time. After TBAB completely dissolved, an aqueous solution of ATA was added and the two-phase mixture was stirred for 4 h at 80[degrees]C. The solution was subsequently precipitated by adding an excess of methanol (2-3 times by volume). The product was separated by centrifugation. To ensure that all remaining agents were completely removed, the obtained precipitate was washed with ethanol/water (9/1) and sequentially followed by centrifugation for several times and finally washed with acetone one more time. The obtained precipitate was then dried at 40[degrees]C in a vacuum oven. The resulting product was compression molded between Teflon sheets at 75[degrees]C for 10 min to prepare the sample film for characterization and testing.
The MAn-g-IIR that reacted with ATA to form the thermoreversible crosslinking MAn-g-IIR via multiple hydrogen bonds was named here as TRC-IIR. Table 1 shows the exact amount of MAn and TBAB used in each experiment and sample was named from sample number 1 to 9.
Characterization and Testing Techniques
Degree of MAn Grafting. The absolute degree of MAn (G) grafted on the IIR chains was determined through titration procedure. A small amount (about 1 g) of purified IIR and MAn-g-IIR samples was first dissolved in 50 ml of dimethylbenzene in a conical flask. To transform the grafted MAn groups into their acidic forms, 0.05 [micro]d of pyridine and 0.05 [micro]l of water were added. After heating for 1 h, the carboxylic acid concentration was measured by the titrating the sample solution while it was still hot with 0.05 mol/l of KOH in ethanol. A solution of 1% phenolphthalein in ethanol (about 3 drops) was used as an indicator. The absolute degree of MAn grafting (G) was calculated as follows :
G = C(V - [V.sub.0])M x [10.sup.-3]/2m x 100% (1)
where C is the molar concentration (mol/1) of KOH in ethanol. [V.sub.0] (ml) and V (ml) are the amount of KOH consumed by HR and MAn-g-IIR sample, respectively, m (g) is the weight of the sample. M is the molecular weight of MAn.
FTIR spectra of the purified IIR, MAn-g-IIR, and TRC-IIR were recorded on Nicolet iS5 FT-IR spectrometer by casting films of sample solution on a KBr disk. Each spectrum was obtained from 16 repeated scans in the range of 400-4000 [cm.sup.-1] at a resolution of 4 [cm.sup.-1]. The conversion of the anhydride groups ([X.sub.Anh]) is calculated from the absorbance of the anti-symmetric carbonyl stretching (C=O) vibration band at 1781 [cm.sup.-1] of the anhydride groups before (t = 0) and after (f = t) reaction with ATA using the absorbance of the band at 1470 [cm.sup.-1], which originates from the overlapping anti-symmetric C[H.sub.3] bending and C[H.sub.2] scissoring vibrations of the IIR backbone as an internal reference.
[X.sub.Anh] = (1 - ([([A.sub.1784]/[A.sub.1470]).sub.t=t]/[([A.sub.1784]/[A.sub.1470]).sub.t=0])) x 100% (2)
Data of XRD were collected on a D8 Advance diffractometer (Bruker) with CuK[alpha] ray (1.54051 [Angstrom]), radiation (40 kV, 20 mA), Ni lisp filtering wave, [lambda] = 0. 154 nm, scanning scope: 2[theta] = 5~80[degrees], stepping scanning: [DELTA]2[theta] = 0.1[degrees]/3 s.
Differential Scanning Calorimeter
Thermal properties of samples were investigated with differential scanning calorimetry on a TA Instruments Q2000 differential scanning calorimeter (DSC) with an RCS cooling unit under a nitrogen atmosphere with heating and cooling rates of 10[degrees]C/min (samples of 5-10 mg were measured). The temperature range was from -100 to +200[degrees]C.
Thermoreversibility I Reprocessability by Heating-Cooling Cycle Treatment
The thermoreversibility of hydrogen bond crosslinking MAn-g-IIR with ATA was surveyed by heating-cooling cycle treatment. Samples were first exposed to 186[degrees]C using a hot press for 30 min followed by cooling down until dropping to room temperature. Heating-cooling treatment was done consecutively up to three cycles. Reprocessability of the sample was tested by remolding sample several times at 186[degrees]C. The ability to form a homogenous film of sample indicated the reprocessability.
Dumbbell-shaped tensile bars with dimensions of 75 x 5 x 2 [mm.sup.3] were punched from compression-molded films. Tensile tests were performed at room temperature with a constant speed of 500 mm/min on a UTM (Instron series IX Automated Materials Testing System 7.25). All samples were tested at least in five-fold.
RESULTS AND DISCUSSION
Characterization of MAn-g-IIR
The absolute grafting degree of MAn (G) determined through titration procedure using Eq. I is 1.82%. The chemical structure of MAn-g-IIR was analyzed through FTIR spectroscopy. The result is given in Fig. 2. Compared the FTIR spectrum of MAn-g-IIR with that of the pure HR, two new peaks are found at 1781 and 1858 [cm.sup.-1]. These two peaks are contributed to the functionalities of MAn. The broad and intense peak at 1781 [cm.sup.-1] and the weak absorption peak at 1858 [cm.sup.-1] are caused by symmetric (strong) and asymmetric (weak) C=O stretching vibrations of saturated MAn rings, respectively. The shift of FTIR spectrum of the MAn-g-IIR toward higher wavenumber also confirms the grafting of MAn onto the IIR chain.
Determination of the Optimal Reaction Conditions
As known, the MAn-g-IIR is unable to form hydrogen bond, since the grafted MAn groups do not have a hydrogen bonding donor. Therefore, in this work, the MAn-g-IIR is modified with ATA to form the amide-acid modified HR which can form multiple hydrogen bonding crosslinks (as shown in Fig. 1). As shown in Fig. 3, the amide-acid formed upon the reaction of cyclic anhydrides with ATA gives absorption bands at 1710 [cm.sup.-1], originating from the carboxylic acid group, and at 1645 and 1560 [cm.sup.-1], which are assigned to the C=O (amide-I) and N--H (amide-II) stretching vibrations of the amide, respectively. Besides forming the amide-acids, reaction between cyclic anhydride groups of MAn and amine groups of ATA can also undergo ring closure to form imide, resulting in a loss of hydrogen bonding , However, from this study, there is no band at 1770 [cm.sup.-1] which is typical for the C=O vibration of imide. Therefore, it is clearly confirmed that the thermoreversible multiple hydrogen bond crosslinked MAn-g-IIR is obtained.
Effects of molar ratio of ATA and TBAB to MAn grafting on the IIR chain on the final (equilibrium) conversions ([X.sub.Anh], eq) calculated from FTIR spectra using Eq. 2 are shown in Fig. 4. It is found that the [X.sub.Anh] does not increase monotonically with the increasing of molar ratio of ATA or TBAB, but passes through a maximum. The condition that gives the highest [X.sub.Anh] which is 48.10% is when using 3eq ATA and catalyzed with leq TBAB. Then the optimum molar ratio of ATA and TBAB to MAn is found to be 3eq and leq, respectively. These results show that the anhydride/ATA reaction and, consequently, the level of crosslinking are highly dependent on the molar ratio of ATA and TBAB to MAn grafted on the IIR chain. The product prepared by this optimal condition is used later to determine thermal and mechanical properties.
Morphology of the TRC-IIR via Multiple Hydrogen Bonding Arrays
The morphology of MAn-g-IIR and TRC-IIRs with different crosslinking degrees was investigated using XRD and is shown in Fig. 5. A peak at 2[theta] of 15[degrees] indicates the occurrence of microphase separated aggregates. Originally, for the MAn-g-IIR, the driving force for the formation of these aggregates is due to the large polarity difference between the polar anhydride groups and the apolar IIR chains. Therefore, it can be envisioned as spherical clusters enriched with anhydride groups, surrounded by a layer of IIR chain segments with restricted
mobility. Compared to the XRD peak of the MAn-g-IIR precursor in Fig. 5, a diffraction peak is still present after crosslinking the MAn-g-IIR with ATA. The XRD peaks of all TRC-IIR samples appear nearly the same position as that of the virgin MAn-g-IIR, but much higher diffraction intensity. The increase in diffraction intensity is due to the replacing the relatively weak polar interactions between the original anhydride groups with the multiple hydrogen bonds between the amide-acid groups, resulting in tightening of the aggregates and also forming a larger restricted mobility layer surrounding the aggregates. It may also be contributed to the higher polarity of the TRC-IIR compared to the MAn-g-IIR as well. Figure 5 also shows that the intensity of the diffraction peak increases with increasing [X.sup.Anh]. This seems reasonable to assume that the higher [X.sup.Anh], higher hydrogen bonding network density.
Thermal, Thermoreversible, and Reprocessable Properties of the TRC-IIR via Multiple Hydrogen Bonding Arrays
The DSC curves of the MAn-g-IIR and TRC-IIR are given in Fig. 6. For the MAn-g-IIR, the DSC result clearly shows just a single step transition that is indicative of a glass transition without any evidence of crystallization or other secondary phase transitions. From Fig. 6, it is found that the glass transitions temperature of MAn-g-IIR is around -65[degrees]C. Flowever, the glass transition temperature of TRC-IIR significantly shifts to -60[degrees]C. This is presumably due to the perfect ordering of multiple hydrogen bonds, dramatically resulting in the restriction of HR chains. Moreover, for the TRC-IIR, a sharp endothermic peak is observed when heating up to 186[degrees]C. It has been reported earlier  that for non-crystallized materials, measurable endothermic effects upon heating can be seen at the crosslinked bonding temperature. Therefore, the endothermic peak found at 186[degrees]C is due to multiple-hydrogen bonding dissociation. This result confirms that at higher temperature, above 186[degrees]C, the TRC-IIR flows like a viscoelastic liquid.
Thermoreversibility of the TRC-IIR is surveyed by heating-cooling treatment and its corresponding mechanism for thermoreversibility is elucidated by FTIR spectroscopy. From DSC result, it shows that the dissociation of hydrogen bonding takes place at 186[degrees]C. At first, it is found that when heating to 186[degrees]C, the TRC-IIR can flow and reprocess. Figure 7 compares FTIR spectra of TRCIIR before heating-cooling treatment (cycle 0) and after heating-cooling treatment (cycle 1-3). The TRC-IIR sample is heated from 25[degrees]C to hydrogen bonding breakage temperature which is 186[degrees]C obtained from DSC analysis, and then cooled down to 25[degrees]C. Figure 7 shows that the bands at 1645 [cm.sup.-1] and 1560 [cm.sup.-1] corresponding to the C=O (amide-I) and NH (amide-II) stretching vibrations of the amide are hardly changed after heating-cooling treatment up to three cycles. This result indicates that the thermoreversibility only takes place due to the breakage of hydrogen bond at high temperature, while during cooling hydrogen bonding can reform slowly. Its reversibility is apparent and the process of breaking and reforming of hydrogen bonding can be repeated many times. Reprocessability of the TRC-IIR was tested by remolding at 186[degrees]C. After the first molding, the sample was folded and recompression molded again. It was observed that each time the homogenous film was obtained. These results prove that the re-usage of this material is satisfactory.
Mechanical Property of the Hydrogen-Bonded Materials
Figure 8 shows the stress-strain curves of MAn-g-IIR and TRC-IIR before and after heating-cooling treatment. All samples show the strain at break larger than 500%. It is clearly found that the tensile strength and modulus at 300% of TRC-IIR are significantly improved compared to the MAn-g-IIR. This phenomenon can be described by the introduction of the multiple hydrogen bonds, resulting in forming crosslinking between the ATA modified MAng-IIR. After heating-cooling treatment up to three cycles, the tensile strength of the TRC-IIR was gradually decreased but it was still much higher than that of the MAn-g-IIR. This result proves that when broken or destroyed at high temperature, hydrogen bonding system can be simply recovered. The strain-stress curve of the TRC-IIR with 1 heating-cooling cycle does not reveal any particular difference from that of the TRC-IIR without heating-cooling treatment. However, compared to the TRC-IIR with one heating-cooling cycle, the TRC-IIRs with 2 and 3 heating-cooling cycles show lower stress value but higher strain. These results indicate that during cooling process, as temperature goes down, that broken hydrogen bonds due to heating treatment at high temperature reform. However, intramolecular short hydrogen bond is prone to associate firstly when temperature is still high during cooling process. For further cooling down to comparable lower temperatures, chains become stiffer and more outstretched. Then they start a transition from weak intramolecular short hydrogen association to strong intermolecular long hydrogen association. This transition can be achieved completely for TRC-IIR with heating-cooling cycle 1, so no particular difference in stress can be observed, while partial intramolecular short hydrogen bonding still exist in those curly chains of TRC-IIR with heating-cooling cycle 2 or 3 which do not extend timely as temperature jumps down, it results in reducing the stress and modulus. The strain-stress curve also indicates the discrepancy in toughness among the four samples. Here toughness is defined by the area of strain-stress curve which represents the absorbing energy without fracturing. Crosslinking via hydrogen bonding arrays in TRC-IIR leads to improved toughness compared to MAn-g-IIR precursor. Moreover, after completion of heating-cooling cycles, it exhibits an increase in toughness. This is attributed to the existence of curly chains at which a greater overall strain must be reached before fracture occurs. This leads to a higher energy absorption and high toughness. The increase of toughness also reflects the slight easiness of long range configuration rearrangement in the heating-cooling treated samples.
From this study, a series of novel thermoreversible crosslinking HR (TRC-IIR) with multiple hydrogen bonds is successfully developed. First, the reaction was taken place by grafting MAn onto the HR chain. Then the MAng-IIR was modified with ATA in a secondary step to form the amide-acid groups which can form multiple hydrogen bonding. The degree of conversion of MAn to the amideacid group that directly effects on degree of crosslinking is dependent of the molar ratio of ATA and TBAB to MAn grating onto the HR chain. The highest conversion can be obtained when using molar ratio of ATA and TBAB to MAn at 3eq and leq, respectively. The XRD measurement demonstrates that the microphase-separated aggregates of MAn-g-IIR are significantly enhanced after the introduction of hydrogen bond crosslinking. The TRC-IIR shows outstanding mechanical properties. DSC result shows that the dissociation of hydrogen bonding takes place at 186[degrees]C. The reprocessability of this material can be confirmed by forming of homogenous film after repeatedly compression molding at 186[degrees]C. The product can flow and reshape. After heating-cooling treatment up to three cycles, no significant chemical change is observed from FTIR spectra and the tensile strength of the TRC-HRs is still much higher than that of the MAn-g-IIR. This indicates that reprocessability observed at high temperature is due to the hydrogen bond dissociation, while the higher tensile properties of the TRC-IIRs after heating-cooling treatment compared to the MAn-g-IIR is due to the recovery of hydrogen bonding during cooling process. This indicates that hydrogen bonds are of thermoreversible nature. Due to their unique self-repairing properties and the simplicity of their synthesis, this material bodes well for future application.
The authors are grateful for the financial support of the project on structural control technology of high barrier elastomeric materials (2012-0759). Korea Ministry of Trade, industry & Energy.
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Lin Li, (1) Jin Zhang, (1) Qi Chen, (1) Kanoktip Boonkerd, (2) Jin Kuk Kim (1)
(1) School of Nano & Advanced Materials Engineering, Gyeongsang National University, Gyeongnam, Jinju 660-701, South Korea
(2) Department of Materials Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
Correspondence to: Jin Kuk Kim; e-mail: firstname.lastname@example.org
Published online in Wiley Online Library (wileyonlinelibrary.com).
TABLE 1. Experimental design. Sample The molar ratio The molar ratio number of ATA to MAn of TBAB to MAn 1# 1eq 0.5eq 2# 3eq 0.5eq 3# 5eq 0.5eq 4# 1eq 1eq 5# 3eq 1eq 6# 5eq 1eq 7# 1eq 2eq 8# 3eq 2eq 9# 5eq 2eq
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|Author:||Li, Lin; Zhang, Jin; Chen, Qi; Boonkerd, Kanoktip; Kim, Jin Kuk|
|Publication:||Polymer Engineering and Science|
|Date:||Aug 1, 2014|
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