J. Phys. Chem. B 2008, 112, 6603–6608
6603
Sorption Properties of Functionalized Liquid Crystalline Networks G. Palaprat,† A.-F. Mingotaud,† D. Langevin,‡ J.-D. Marty,*,† and M. Mauzac*,† Laboratoire Interactions Moléculaires et RéactiVité Chimique et Photochimique, UniVersité de Toulouse (Paul Sabatier), IMRCP UMR CNRS 5623, 31062 Toulouse cedex 9, France, and Laboratoire Polymères, Biopolymères, Membranes, UMR CNRS 6522, UniVersité de Rouen, 76821 Mont-Saint-Aignan cedex, France ReceiVed: October 31, 2007; ReVised Manuscript ReceiVed: March 6, 2008
Functionalized polydomain chiral elastomers were obtained by cross-linking side-chain liquid crystalline polysiloxanes bearing acid functions. Sorption experiments were performed by the use of an electronic microbalance, in the presence of one enantiomer of a chiral amine molecule, able to interact with the acid groups. The results showed that Fick’s diffusion law is not valid anymore as soon as an interaction between the material and the molecule is present. Moreover, it was demonstrated that the grafting of interacting groups on a chiral elastomer enhanced both the capacity and selectivity toward one enantiomer. Introduction Separation processes by nonporous polymer membranes have induced a tremendous amount of works for gas separation processes due to the high selectivity obtained.1 Hence, polymer membranes are used commercially to prepare oxygen-enriched air (for medical purpose), to remove carbon dioxide and water from natural gas, to recycle hydrogen from crude oil streams, and so forth.1 For these nonporous polymer membranes, the process could be described as a “solution-diffusion mechanism” characterized by the permeation coefficient P. P can be viewed as the product of the solubility coefficient S of the mobile gas molecules that are participating in the transport process and the diffusion coefficient D, which represents the transport velocity constant of gas flow through the membrane, with P ) S*D. When the gas mixture is introduced to the membrane, a difference of permeativity of the different compounds induced a selectivity of this membrane toward one of the components. For a pair of gases A and B, the ideal selectivity is defined as the ratio of the permeability coefficients for A and B (determined from separate measurements): RA/B ) PA/PB. Many studies elucidate relationships between polymer structures, such as the free volume, polymer chain packing, helical structure and intersegmental distance, and their gas transport properties. Among them, the use of a thermotropic liquidcrystalline polymer (TLCP) as the membrane2–10 or a component of a membrane11 for gas permeation experiments seems, due to their specific properties, to be of special interest.11 For these materials, permeabilities mainly depend on the state of the membrane (below or above the glass temperature, in a liquidcrystalline state or not, and so forth) and the structure of the mesogenic groups (spacer length, chemical structure, and so forth). Recently, we have studied the diffusion of a chiral amine molecule into a TLCP network exhibiting a cholesteric phase.12 The sorption experiments were performed by the use of an electronic microbalance, and the kinetics obtained corresponded to a pure Fickian diffusion behavior. This membrane has shown a pronounced selectivity toward one enantiomer due to a * To whom correspondence should be addressed. Tel: +33 561 55 86 96. Fax: +33 561 55 81 55. E-mail:
[email protected] (M.M.); E-mail:
[email protected] (J.-D.M.) † Université de Toulouse (Paul Sabatier). ‡ Laboratoire Polymères, Biopolymères, Membranes.
difference in solubility coefficients. Up to now, the effect of the introduction of a functional group that interacts strongly with the diffusing gas on the permeation process is still poorly known. One important issue is to know if this addition could improve the efficiency of the membranes. To answer this question, using the same penetrant molecule and similar nonoriented mesomorphic network, we introduced functional acid groups on the polymer backbone that can interact with the amine penetrant (LCFN). For comparison, another liquidcrystalline sample was prepared without functional groups (LCN) as well as reference materials without mesogenic groups or acid functions (RN1 and RN2). The sorption kinetics were investigated by the use of the electronic microbalance. 2. Experimental Section 2.1. Reagents and Apparatus. All reagents were purchased from Aldrich and used without further purification. All solvents (HPLC grade) were from SDS (Peypin, France) and used as received. A xenon lamp (25 mW/m2) equipped with a high pass filter of 335 nm was used to cross-link the polymer. The nature of the mesophases and the temperature at which they occur were determined by polarized-light optical microscopy (Olympus microscope equipped with a Mettler FP82HT hot stage), differential scanning calorimetry (DSC) using a Perkin-Elmer PYRIS 1 calorimeter, and X-ray experiments. For X-ray experiments, the sample was put in a Lindemann glass capillary of 1.5 mm diameter, and its temperature was kept at room temperature or 50 °C within 1 °C. The X-ray (Cu KR, l ) 0.154 nm) monochromatic beam was reflected by a double bent pyrolitic graphite. The scattered X-rays were collected on a cylindrical imaging plate at 60 mm from the sample. The sample and imaging plate were placed in an evacuated camera in order to avoid scattering by air.13 The transition temperatures recorded between the isotropic phase and the mesophase corresponded to those determined from the position of the tops of DSC peaks as the temperature decreased at 2 °C/min; the glass transition temperatures were obtained as the temperature increased at 10 °C/min. 1H NMR analyses were conducted with a Bruker ARX 400 MHz spectrometer using the HRMAS accessory. A Hewlett-Packard UV diode array spectrophotometer (HP 8452A) was used to
10.1021/jp7105082 CCC: $40.75 2008 American Chemical Society Published on Web 05/02/2008
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Palaprat et al. TABLE 1: Characteristics of the Samples compositiona
thermal propertiesb
network x (% mol) y (% mol) z (% mol) Tg (°C) I fN* (°C) LCN LCFN RN1 RN2c
95 87 0 0
0 8 0 0
5 5 50 50
6 6 -10 -10
71 70
a Mol % of substituents x, y, or z linked to the polysiloxane chain; x, y, and z were as defined in Figure 1. b Tg: glass transition temperature; N*: cholesteric mesophase; I: isotropic phase. c In that case, 1,21-docosadiene was used as a cross-linking agent; data extracted from ref 12.
Figure 1. Synthesis of the elastomers.
determine the concentration of the molecule in the extraction experiments. Circular dichroism spectra were obtained from a circular dichroism spectrometer J-815 from Jasco equipped with a 150 W Xe arc lamp light source, using a bandwidth of 2 nm, with medium sensitivity with a maximum of 200 mdeg. Data acquisition was done in a step scan mode. 2.2. Synthesis of the Elastomers. In order to determine the influence of a functional group on the sorption properties of a liquid-crystalline network, four networks were studied. First, two liquid-crystalline networks with (LCFN) or without the (LCN) functional group were synthesized. To compare with, a reference network (RN1) without mesogenic substituents and functional groups was realized. These networks were obtained according to the procedure described in Figure 1. Another reference network, RN2, cross-linked with 1,21-docosadiene instead of benzophenone was also studied to compare. It was synthesized as previously described.12 4-(Butenyloxy)phenyl-4-methoxybenzoate, 4-(3-butenyloxy)benzoic acid, and 4-(10-undecenyloxy)benzophenone were used as substituents on the polymethylhydrogenosiloxane chain (average number degree of polymerization of 80 units). 4-(Butenyloxy)phenyl-4-methoxybenzoate mesogenic substituent (M41) was synthesized as previously described.14,15 It exhibits a monotropic nematic phase at 52.2 °C, determined by polarized-
light optical microscopy. 4-(10-Undecenyloxy)benzophenone (4(10-UO)BP) was synthesized by a Mitsunobu’s reaction between 4-hydroxybenzophenone and 10-undecenylalcohol in the presence of triphenylphosphine, as previously described.16 The analysis of this molecule by polarimetry revealed a close to zero but still present optical rotation, whereas initial reactants present none. The specific rotation, [R]D25, obtained from the value of the rotation angle, measured at 25 °C on different solutions of 70 mg/mL was around +0.45 deg · dm-1 · g-1 · cm3. An analysis of the product was performed after purification through a silica column. A decrease of the specific rotation to 0.042 deg · dm-1 · g-1 · cm3 demonstrated the presence of a chiral impurity that we were unable to identify by analytical methods due to the very low content. 4-(3-Butenyloxy)benzoic acid (4(3-BO)BA) was synthesized by a nucleophilic substitution between 4-bromobut-1-ene and 4-hydroxybenzoic acid, as previously described.14 It exhibits a nematic phase, from the melting point at 110.5 °C and the isotropic state up to 140.1 °C, as a result of dimer formation via homointermolecular hydrogen bonds. Networks LCN, LCFN, and RN1 were then prepared as follows (composition detailed in Table 1): 90 mg of polymethylhydrogenosiloxane backbone (1.5 mmol of silane functions) was previously mixed with all of the vinyl components in 4 mL of toluene. Grafting groups were 10% in excess to favor the Si-H hydrosylilation reaction. [(C2H5)2S]2PtCl2 (245 µL of a 25 mg/mL toluene solution) was added, and the reaction mixture was put into a test tube first enveloped with aluminum paper. The assembly was sealed and immersed inside of a thermoregulated oil bath, and the solution was heated at 60 °C for 72 h under magnetic stirring. A hydrosilylation reaction occurred between the silane functions and the different vinyl end groups, leading to the formation of a functionalized linear polymer. A viscous oil was recovered and dried under vacuum for 10 h in order to eliminate the solvent. The polymer was heated to 100 °C (isotropic state) and then spread on a glass strip that was treated beforehand with a polyvinylalcohol solution (2 wt %), bordered by spacers of 200 µm thickness and placed at 1 cm of the UV lamp.16 After cooling, the sample was exposed to UV light above 335 nm for 2 h at room temperature, which allowed the cross-linking step via benzophenone functions. Benzophenone shows absorption bands at 298 and 340 nm, different from the absorption band of M41 and 4-(3-BO)BA. The liquid crystalline samples were so cross-linked in their mesomorphic phase. Due to the synthesis process, the samples were polydomain. The device was then placed in water to dissolve PVA and to recover the network. In order to remove all of the ungrafted groups, the sample film was washed with a large volume of acetonitrile (swelling ratio of the network, ca. 1.314), leading to diffusion of the low molecular weight molecules from the network in response to a concentration
Sorption in Functionalized LC Networks
Figure 2. Evolution of amine content ((R)-R-methylbenzylamine) as a function of time, during sorption measurements, for the different membranes (]: LCN, ∆: LCFN, O: RN1). Experimental points were fitted using eq 3.
gradient. This step was repeated until the UV absorbance due to each extracted component completely disappeared in the supernatant solution. At the end of the washing process, the solvent was slowly evaporated at room temperature over 4 days to avoid damage such as holes or cracks inside of the materials. 2.3. Sorption Measurements. Sorption measurements have been carried out by using an electronic microbalance (IGA 001 Gas Sorption System from Hiden Analytical, Warrington, England), which enabled a circulation of amine with a controlled vapor pressure around the polymer sample, as described in a previous paper.12 (R)- or (S)-R-methylbenzylamine was used as a penetrant due to its low vapor pressure and to the presence of a phenyl group that allows good interactions with the network. Amine activity can be estimated to aamine ) 0.18 ( 0.002.12 Measurements were performed at 60 °C to ensure that the LCN and LCFN samples were kept in the mesophase. The thickness was around d ) 200 µm for all samples. 3. Results and Discussion 3.1. Phase Behavior of the Mesogenic Networks. Thermal properties of the different networks are presented in Table 1. The two isotropic reference networks (RN1 and RN2) only exhibited a glass transition temperature at -10 °C as determined by DSC measurements. LCN and LCFN presented a glass transition temperature of Tg ) 6 °C and a clearing temperature of TLC-I ) 70 and 71 °C, respectively. X-ray experiments only presented a diffuse ring at 4.5 Å, typical of the nematic (N) or cholesteric phase (N*) without any lamellar organization (see Supporting Information). Circular dichroism (CD) measurements were performed to assess the chiral nature of the materials. As described in the literature, a cholesteric mesophase presents a reflection band due to the selective reflective properties of such materials and another one due to the absorbing groups (for benzophenone, the maximum wavelength absorption was around 335 nm, and it was around 260 nm for liquid-crystalline moieties). In the absence of a planar texture, the first one is broadened, indicating the random distribution of the chiral domains.17 Such a broad band was observed for all of the networks formed in the presence of 4-(10-UO)BP (see Supporting Information), proving their chiral nature. Similar elastomers as LCN and LCFN but synthesized with a 1,21docosadiene cross-linker instead of benzophenone were determined to present a nematic phase.12,14 The cholesteric phase obtained for LCN and LCFN is the result of the chiral impurity presents in the cross-linker. This impurity, even with very low concentra-
J. Phys. Chem. B, Vol. 112, No. 21, 2008 6605
Figure 3. Evolution of Qs ) m(t)/m∞ as a function of t1/2 in the sorption kinetics of (S)-methylbenzylamine and (R)-methylbenzylamine for LCFN (dotted line: linear fit for short time).
tion, acts as a chiral dopant and, as described in the literature,12,18–22 induces a cholesteric behavior of the mesophase for LCN and LCFN (see section 3.3) as well as a chiral structure in the case of RN1. Methylbenzylamine used during sorption measurements could disturb the mesomorphic organization of the network. DSC and X-ray measurements, performed on membranes with different amounts of amine, demonstrated that mesomorphic properties were maintained up to 20% of the amine uptake in mass with a clearing temperature higher than 60 °C.12 Here, the amine adsorbed mass was below 20% of the total mass (see Figure 2 and 3), and measurements were performed at 60 °C; therefore, all of the sorption measurements were carried out in the mesophase of the sample. 3.2. Sorption Properties. The sorption properties toward the (R)-R-methylbenzylamine enantiomer were studied at 60 °C for the samples LCFN, LCN, and RNI. The sorption kinetics should be represented by the evolution, as a function of time, of the sorbed penetrant concentration m(t) ) (M(t) - M0)/M0, M(t) being the sample weight at time t and M0 the dry sample weight (Figure 2). 3.2.1. Sorption Kinetics. Through a conventional rubbery membrane, gas permeation is controlled by the penetrant diffusion, the solution equilibrium being achieved in times very much shorter than the characteristic times involved in the diffusion of the penetrant molecules in the polymer matrix. The sorption curves have a characteristic profile; after a linear initial slope, they bend to the equilibrium mass uptake concentration, m∞. The diffusion process generally satisfies Fick’s first and second laws, giving the rate of penetrant diffusion J and the gradient of the penetrant concentration dC/dx in the thickness of the membrane. Assuming an isotropic and homogeneous membrane and a low concentration of penetrant, these laws are described by the relations
J ) -D dC/dx dC/dt ) -dJ/dx
(1) (2)
with J as the flux density, D as the diffusion coefficient, x as the position coordinate in the thickness d of the membrane, and C as the molar concentration. Fick’s second law of diffusion was solved for a constant diffusion coefficient in the case of a free-standing film which was exposed to a sudden increase of the surface concentration on both sides by the relation in eq 323 ∞
{
8 m(t) D(2n + 1)2π2t )1exp 2 2 m∞ d2 n)0 (2n + 1) π
∑
}
(3)
Assuming that the film thickness d is constant, experimental points for the LCN and RN1 are satisfyingly fitted by using eq
6606 J. Phys. Chem. B, Vol. 112, No. 21, 2008
Palaprat et al.
TABLE 2: Diffusion Coefficient (D), Maximum Sorbed Vapor (m(∞)) and Solubility Coefficient (S) Deduced from Sorption Measurements for R and S Enantiomers of r-Methylbenzylamine diffusion coefficient (×109 cm2/s)a
maximum sorbed vapor content (in percent of initial polymer mass)c
solubility coefficient (g/g of polymer)
network
DR
DS
m(∞)R
m(∞)S
SR
SS
ideal selectivity
LCN LCFN RN1 RN2e
1.3 2.0b 9.6 4.1
1.5 2.3b 8.6 3.8
4.7 21.6d 7.8 5.8
3.2 14.4d 6.8 5.8
0.27 1.19d 0.43 0.32
0.18 0.81d 0.37 0.32
1.39 1.95 1.29 1.07
a Experimental error: 2 × 10-10 cm2/s. b Estimated from linearization of kinetics at short times. c Experimental error: 0.01%. d Value obtained after 3000 min. e Values extracted from ref 12.
3 (correlation coefficients above 0.98 and 0.96, respectively). As previously observed,12 the sorption isotherms exhibit a classical Fickian diffusion behavior for the conventional reference network and for the mesomorphic network LCN. Consequently, the mechanism involved was mainly controlled by the penetrant diffusion. A diffusion coefficient can be determined only at short times. The sorption properties of LCN and RN1 cannot be quantitatively compared because neither the porosity of the materials nor the chemical structure are the same. The values of the corresponding diffusion coefficient are reported in Table 2. In the case of the liquid-crystalline-functionalized network (LCFN), it appears from Figure 2 that experimental points differ significantly from pure Fickian behavior. The diffusion of the amine could no be longer described by a simple dissolution in a homogeneous host phase. In that case, the host membrane had sites that interacted with the guest molecule. Hence, behavior should be better described by a dual-mode sorption model. The sorption curve was then plotted as Qs(t) ) m(t)/ m∞ over t1/2 (with m∞ ) m(∞); sorbant penetrant concentration taken at 3000 min). As depicted in Figure 3, after a short time of induction, a linear evolution can be observed (for times below 40 min). This behavior is characteristic of a permeation process mainly explained by a Fickian-type diffusion. For those short times, an apparent diffusion coefficient could be calculated from the initial slope of the curve by using the relation Qs(t) ) 4 · D1/2 · t1/2/(d · π1/2).12 This value is reported in Table 2. For longer times, the absorption phenomena of the amine on the acid sites occurs, and their contributions strongly modify the diffusion process. The diffusion coefficients for all of the samples were found in the range of 10-9 cm2/s. This is in good agreement with those measured by other authors through mesogenic materials.4,8,12 3.2.2. Capacity of the Samples. LCN and RN1 samples reached a plateau of amine uptake. The corresponding m∞ sorbed vapor concentration is linked to aamine by a solubility coefficient S, which is representative of the level of interaction between the polymer network and amine enantiomer. For dilute penetrant solutions, S is independent of penetrant concentration. It is given by Henry’s law: m∞ ) S · aamine. From each sorption kinetics, the maximum quantity of amine that can be sorbed by the polymer, m∞, was determined from the fit, and S was calculated for the vapor activity a ) 0.18 in order to compare the affinity of each material toward each enantiomer (Table 2). The experimental error on m∞ was estimated from the repetition of the same experiment and was found equal to 0.01%. Without acid groups, the capacity of the liquid crystalline sample LCN (4.7% of initial polymer mass) is slightly lower than those of the references (7.8% of initial polymer mass). This could be explained by the effect of mesogens, which make the sample denser. The capacity of the LCN sample is on the same order of magnitude as those we reported on a similar chirally
doped network12 and those described by S. Courty and coworkers on other liquid-crystalline networks.20–22 For LCFN, no plateau was observed on the kinetics curve, but it is undeniable that the addition of functional groups is accompanied by a significant increase of maximum quantities of amine sorbed. The content of (R)-R-methylbenzylamine goes to 21.6 wt % at 3000 min after the beginning of the sorption process. The capacity of the acidic sample is very high, corresponding to around 1.3 mmol/g of polymer. This could be explained by the additional specific molecular interactions between amine and acid groups. This great increase of solubility (by a factor of 4.6) compared to that of LCN was accompanied by a small increase (by a factor of 1.6) in the value of diffusion coefficients and a higher increase of the permeation coefficient (by a factor of 6.8). 3.3. Enantioselectivity. Chiral networks show significant enantioselective absorption toward chiral molecules.12,20–22 These materials can be obtained either from mesogens generating chiral phases or from nematogenic mesogens by the use of a chiral dopant removed after their synthesis. In the last case, for example, S. Courty and co-workers demonstrated, using a macroscopically oriented cholesteric elastomer, the ability of the material to preferentially absorb and retain the “correct” chirality isomer from a racemic solvent.20–22 In a previous paper,12 we observed similar effects from a chirally doped polydomain liquid-crystalline material, proving that a macroscopic orientation is not essential for the enantiomeric separation in which the phenomena occur at the molecular scale. In order to evaluate if any synergistic effect could be obtained by the combination of the carboxy group and the cholesteric supramolecular structure of the elastomer on the separation properties of two enantiomers, we realized the same sorption experiments as those described in section 3.2 but using the other enantiomer of the R-methylbenzylamine. Evolution of the amine content ((S)-R-methylbenzylamine or (R)-R-methylbenzylamine) as a function of time during sorption measurements is depicted in Figure 4. It appears that reference RN1 has a preference for the (R)-R-methylbenzylamine, whereas another reference (RN2) synthesized using an aliphatic 1,21docosadiene cross-linker has no preference toward one enantiomer or another.12 Therefore, this effect should be the consequence of the chiral impurity introduced during the crosslinking step by 4-(10-undecenyloxy)benzophenone. The chiral impurity can interact either specifically with one enantiomer or via the induced chiral structure. This latter effect can be related to the phenomenon of chiral amplification described by Yashima.24 Indeed, he showed that an enantiomeric excess as low as 5% of an interacting molecule was sufficient to induce the formation of a 100% single-handed helix of polyphenylacetylenes. When compared to RN1, a slightly higher preference toward the (R)-R-methylbenzylamine is found for LCN that presents
Sorption in Functionalized LC Networks
J. Phys. Chem. B, Vol. 112, No. 21, 2008 6607 to be equal to 22%. Such a variation shows the positive effect on enantioselectivity of adding functionalized groups in a cholesteric mesophase that can interact with the penetrant. Moreover, it has to be noted that for each network, similar diffusion coefficients were obtained for (R)- and (S)-methylbenzylamine. These results are consistent with theoretical predictive models developed by Frezzato and al.25,26 These models showed that in a cholesteric liquid crystal, the enantiomer penetrant with shape chirality opposite to that of the LC phase was slowed down more that the other but that the difference was quite small (1%) and occurred only when the pitch length of the phase was on same order of magnitude as the molecule size. Here, as demonstrated in a previous article,17 the pitch of the helical structure was much higher, typically in the µm range. Consequently, the differences observed at equilibrium in the sorption of the two enantiomers could not result from a difference in the diffusion coefficient but mainly from a difference in solubility. 4. Conclusion Sorption experiments were performed by the use of an electronic microbalance on functionalized polydomain liquidcrystalline elastomers. The results showed that Fick’s diffusion law is not valid any more as soon as an interaction between the material and the molecule penetrant is present. It was also demonstrated that the grafting of interacting groups enhanced the capacity of the material to more than 20%. Moreover, the introduction of a small concentration of chiral molecules in the cross-linker was sufficient to obtain a significant enantioselectivity of the materials. Compared to chirally doped materials previously studied,12 the grafting of acid groups allows one to improve the separation of enantiomers with a higher capacity of the materials synthesized and demonstrates quite clearly a synergistic effect between the chiral mesophase and acid groups in separation properties. Acknowledgment. The authors thank P. Davidson, Laboratoire de Physique des Solides Université Paris-Sud, for X-ray scattering measurements and fruitful discussions. Supporting Information Available: X-ray scattering data and circular dichroism spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 4. Evolution of amine content ((S)-R-methylbenzylamine or (R)-R-methylbenzylamine) as a function of time during sorption measurements for a) reference sample RN1, b) liquid-crystalline network LCN, and c) liquid-crystalline-functionalized network LCFN.
additional mesogens (Figure 4 and Table 2). The ideal selectivity is found to be equal to 1.29 and 1.39, respectively. This corresponds to an enantiomeric preference, determined at equilibrium as (m∞R - m∞S)/(m∞R + m∞S), of 6 and 10%, respectively. These values are similar to the values obtained with the chirally doped polydomain liquid-crystalline materials (8.6%)12 and may suggest the formation of a cholesteric mesophase due to the chiral impurity, enhancing the selectivity properties. If acid groups are incorporated into the cholesteric material (LCFN), the difference of the sorbed penetrant concentrations for the R or S enantiomer greatly increases. Taking the value of D obtained from the Fick equation at short times and the value of S determined at 3000 min, we estimated the ideal selectivity to be equal to 1.95 and the enantiomeric preference
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