J. Phys. Chem. B 2007, 111, 9239-9243
9239
Enantioselective Absorption of Chirally Doped Liquid Crystalline Networks Studied by the Use of an Electronic Microbalance G. Palaprat,† J.-D. Marty,*,† D. Langevin,‡ H. Finkelmann,§ and M. Mauzac† Laboratoire Interactions Mole´ culaires et Re´ actiVite´ Chimique et Photochimique, UMR CNRS 5623, UniVersite´ Paul Sabatier, 31062 Toulouse cedex 9, France, Laboratoire Polyme` res, Biopolyme` res et Membranes, UMR CNRS 6522, UniVersite´ de Haute Normandie, BouleVard Maurice de Broglie, 76821 Mont Saint Aignan, cedex, France, and Institut fu¨r Makromolekulare Chemie, Albert-Ludwigs-UniVersita¨t, Stefan-Meier-Strasse 31, D-79104 Freiburg, Germany ReceiVed: March 2, 2007; In Final Form: May 24, 2007
A polydomain cholesteric elastomer was obtained by cross-linking a nematic side-chain polysiloxane in the presence of a chiral dopant. After extraction of the chiral dopant, sorption experiments were performed, by the use of an electronic microbalance, in the presence of each enantiomer of a chiral amine molecule. The sorption kinetics corresponds to a Fickian diffusion behavior. They allowed us to determine the diffusion coefficients and to show that the doped polymer has a more pronounced affinity toward one of the enantiomers.
1. Introduction In recent years, design and synthesis of optically active macroand supramolecules has promoted great interest in their relationship with biological phenomena and in their potential application in materials science, including as chiral selectors for separation, as catalysts, as adsorbents, and, especially, as chiroptical materials.1 In the case of polymers, three-dimensional networks can exhibit a memory function which relates to the configuration of the polymer chains at the points of cross-linking. So, de Gennes first suggested that a helical macroscopic symmetry should be introduced by simply generating the polymer network in an oriented chiral solvent. The originally achiral polymer would remember the induced chirality after removal of the solvent.2 Moreover, in designing new functional materials, the control of chirality in response to an external stimulus, such as temperature, pH, or solvent, is of special interest.3 Cholesteric liquid crystalline polymers are temperature-dependent systems. They could be prepared by a one-step polymerization/crosslinking reaction of nematic monomers in a chiral solvent.4,5 Likewise, Hasson and co-workers showed that a chiral structure could be imprinted by cross-linking an achiral liquid crystalline polymer in the presence of a low molecular weight chiral mesogen.6,7 They showed that low levels of cross-linking (1 mol % of the monomer units) are sufficient to imprint the memory of a chiral mesophase into the elastomer. Even after heating to the isotropic state, the samples exhibited the same chiral structure in the liquid crystalline phase. The liquid crystalline materials retained a helical structure after extracting the dopant solvent, owing to the orientation imposed on the mesogenic side-groups by the chiral mesophase and the coupling between the mesogenic side-groups and the polymer backbone. A theoretical model of chiral imprinting from a nematic network was presented by Mao and Warner8 in which the authors discuss * Corresponding author. Phone: 33 5 61 55 61 35. Fax: 33 5 61 55 81 55. E-mail:
[email protected]. † Universite ´ Paul Sabatier. ‡ Universite ´ de Haute Normandie. § Albert-Ludwigs-Universita ¨ t.
the “robustness” of the imprinted phase chirality as a function the elastic properties of the polymer and the twisting power of the dopant. Using similar macroscopically oriented cholesteric elastomer, Courty and co-workers9-11 presented a study relative to the separation of chiral isomers. They demonstrated the capacity of the material to preferentially absorb and retain the “correct” chirality molecules from a racemic solvent. In these studies, the monodomain texture was needed for the technical analysis, based on the study of the optical rotation in the medium. However, as discussed by Courty and co-workers, this seems not necessarily relevant for the enantiomeric separation for which the phenomena occur at the molecular scale. In this work, the gravimetric sorption measurement was chosen as a new technique to be used to investigate the preferred absorbed molecules on a polydomain chiral network. For this, we synthesized nonoriented liquid crystalline materials in which a chiral molecule was introduced during the synthesis. This dopant induced a helical structure inside the initial nematic liquid crystalline network which was locked by the cross-linking step. The characterizations of the transport kinetics, the capacity, and the stereoselectivity of the material obtained after extraction of the chiral dopant were investigated and reported here. 2. Experimental Section 2.1. Film Preparation. Cholesteric elastomer was synthesized according to the procedure described in Figure 1. 2.1.1. Reagents and Materials. 4-(Butenyloxy) phenyl 4-methoxybenzoate mesogenic substituent (M41) was synthesized as previously described.12,13 It exhibits a monotropic nematic phase at 52.2 °C determined by polarized-light optical microscopy (Olympus microscope equipped with a Mettler FP82HT hot stage). Polymethylhydrogenosiloxane (average number degree of polymerization of 80 units), 1,21-docosadiene (1,21-DD), cholesteryl acetate (CholOAc), and (S)- or (R)-R-methylbenzylamine were purchased from Aldrich Fine Chemicals (Saint Quentin Fallavier, France). CholOAc presents a monotropic cholesteric phase at 94.5 °C. Dichloro(dicyclopentadienyl)platinum (II) (DCPPtCl2) was purchased from Strem Chemicals
10.1021/jp071711n CCC: $37.00 © 2007 American Chemical Society Published on Web 07/17/2007
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Figure 1. Synthesis of the chirally doped liquid crystalline network.
(Newsburyport, U.S.A.). All solvents (HPLC grade) were from SDS (Peypin, France) and used as received. 2.1.2. Synthesis of Polydomain Cholesteric Elastomer (Doped Network). The mesogenic group M41 (477 mg, 1.6 mmol), the cross-linker 1,21-DD (87 mg, 0.2 mmol), and a fixed concentration of CholOAc (172 mg, 0.4 mmol, 20% of total weight) were mixed with polysiloxane chains (120 mg, 2 mmol of silane functions) in 1.5 mL of toluene. The concentration of the polymer chains in the solvent was set at 37 vol % in order to perform the reaction above the gel point.14 The catalyst of the hydrosilylation reaction DCPPtCl2 (15 µL of a 1 mg/mL toluene solution) was added. The reaction mixture was then filtered through a PTFE membrane filter (0.45 µm) and poured into a cylindrical Teflon mold, with a diameter of 5 cm and a height of 2 cm. The mold was sealed and fitted onto the top of a specially adapted centrifuge, heated at 60 °C and spun at 4000 rpm for 48 h as already described in the literature.15 A hydrosilylation reaction occurred between the silane functions and the different vinyl end-groups of M41 and 1,21-DD leading to the formation of a mesomorphic network (Figure 1). The network was slightly swollen by toluene; nevertheless, at the concentration used, the preservation of the mesomorphic behavior at 60 °C was checked by X-ray experiments. This result confirmed previous studies made on similar samples.13,14 So all networks were cross-linked in their thermotropic cholesteric phase. The swollen elastomer obtained was then removed from the cell and was let floating on water. As a result, the solvent slowly evaporated during 2 days. In order to remove the chiral dopant (Chol) the material was placed in a large volume of
nonchiral polar solvent (acetonitrile, swelling ratio of the network ca. 1.113) allowing a diffusion of the dopant from the network in response to a concentration gradient. This step was repeated until the UV absorbance at 250 nm, due to the CholOAc, completely disappeared in the supernatant solution. At the same time, the FT-IR spectrum of the polymer exhibited no remaining characteristic peaks of CholOAc. 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 the materials. Films of 200 µm thickness were obtained. 2.1.3. Synthesis of Nonmesogenic Elastomer (Reference Network). Polysiloxane backbone (120 mg, 2 mmol of silane functions) and 1,21-DD (328 mg, 1.1 mmol), without CholOAc, were previously mixed in toluene with DCPPtCl2. The network was then prepared under similar conditions as above. 2.2. Sorption Measurements. Amine sorption measurements have been carried out by the use of an electronic microbalance (IGA 001 gas sorption system from Hiden Analytical, Warrington, England) equipped with a humidifier module which enabled a circulation of amine with a controlled vapor pressure around the polymer sample (Figure 2). The humidifier module was connected to the analyzer (1) containing the microbalance (resolution: 0.2 µg). Its electronics were maintained at 55 °C by a thermoregulator. In the reactor (2), thermostated at 60 °C by a thermoregulated water bath, the sample was positioned in a grid pan, hung to the balance head by a long gold chain and a tungsten wire. A gas flow controller supplied a nitrogen stream (U grade from Air Liquide) which was divided into two parts, A and B (controlled flow rates fA and fB, respectively). After
Enantioselectivity of Chirally Doped Networks
J. Phys. Chem. B, Vol. 111, No. 31, 2007 9241
Figure 2. Scheme of the electronic microbalance.
bubbling in a tank (3) containing the liquid amine ((S)- and (R)-R-methylbenzylamine) at temperature θ (in these experiments, θ ) 60 °C), stream B rejoined stream A (total flow rate: fA + fB ) 200 cm3‚min-1) and the gas mixture circulated along the sample at atmospheric pressure. Assuming that gas B is totally saturated with amine vapor (so that vapor pressure pB ) saturated vapor pressure psat and activity aB ) pB/psat ) 1), the vapor activity in the mixture would be equal to the ratio r ) fB/(fA + fB) whatever the temperature may be. However this hypothesis needed to be verified since the level of saturation of the B fluid probably depends on the bubbling conditions. The calibration of the system was thus carried out using water as the vaporized liquid in the same flowing conditions (total flow ) 200 cm3‚min-1). A humidity sensor placed in the reactor not far from the sample allows the measurement of water activity in the A/B mixture as a function of r varying from 0 to 1. It was concluded that the vapor activity in the mixture could be linked to r with a good accuracy by the relation aamine ) (0.90 ( 0.01)r. In the amine sorption measurements, a single value of fB was used (fB ) 40 cm3‚min-1) so that the amine activity aamine can be estimated to aamine ) 0.18 ( 0.002 for all experiments. Before sorption measurements the sample was dried by circulating pure nitrogen (r ) 0) to a constant weight (see the Supporting Information). By this way, all volatile molecules (solvent, residual low molecular weight substituents, or dopant, ...) were expected to be extracted from the material. Then the amine vapor was admitted inside the reactor and diffused in the polymer network. The microbalance recorded the weight variation as a function of time until an equilibrium was reached. 3. Results and Discussion 3.1. Phase Behavior of the Mesogenic Network. An elastomer previously synthesized without CholOAc exhibited a nematic behavior (checked by X-ray experiments) and gave a glass transition temperature Tg ) 5 °C and a clearing temperature TN-I ) 75 °C.16 The cholesteric network (with no chirality at the molecular level) was obtained as described previously in the literature: the addition of a chiral dopant during the formation of the nematic network was followed by its extraction.6,7,9-11 During synthesis, the temperature was kept below the clearing temperature of the final network swollen
with toluene (see the Experimental Section). As a consequence, even if at the beginning the mixture of reactants presented no mesomorphic properties, cross-linking phenomenon occurred in the mesomorphic state before the end of the network formation. Moreover, the addition in the initial mixture of the chiral dopant induced a cholesteric behavior of the final network, as already described in the literature.9-11 After extraction of the chiral dopant, a cholesteric network was obtained that presents a helical supramolecular structure with no molecular chirality. Due to the synthesis process, the obtained sample was polydomain. Differential scanning calorimetry (DSC) measurements gave a glass transition temperature Tg ) 8 °C and a clearing temperature TN*-I ) 69 °C. In order to obtain a pure Fickian behavior during sorption measurements, an isotropic reference network was chosen. It was also analyzed by DSC and only exhibited a glass transition temperature at -10 °C. (S)- and (R)-R-Methylbenzylamine used during sorption measurements could disturb the mesomorphic organization of the network. DSC and X-ray measurements, performed on the doped network with different amounts of adsorbed amine (up to 50% in mass), demonstrated that mesomorphic properties were maintained up to 20% of amine uptake in mass. Here, the amine adsorbed mass was below 10% of the total mass (see Figures 3 and 4) and measurements were performed at 60 °C, so all the sorption measurements were carried out in the cholesteric mesophase of the sample. 3.2. Sorption Properties. Methylbenzylamine was chosen for sorption measurements, due to its relatively low vapor pressure and to the presence of a phenyl group enabling interactions between the network and the amine. The doped sample and the reference nonmesogenic elastomer (each of 200 µm thickness) were thermostated at 60 °C. Their sorption properties toward each pure (R)- or (S)-R-methylbenzylamine enantiomers were compared. The sorption kinetics could be represented by the evolution, as a function of time, of the sorbed penetrant content m(t) ) (M(t) - M0)/M0, M(t) being the sample weight at time t and M0 the dry sample weight. The sorption experimental data of (R)- or (S)-R-methylbenzylamine in the reference sample or in the doped network are presented in Figures 3 and 4, respectively. 3.2.1. Sorption Kinetics. All the curves (Figures 3 and 4) have the same characteristic profile : after a linear initial slope they
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Figure 3. Evolution of amine content as a function of time, during sorption measurements, for the reference sample (a) (S)-R-methylbenzylamine and (b) (R)-R-methylbenzylamine. Experimental points were fitted by using eq 3.
Figure 4. Evolution of amine content as a function of time, during sorption measurements, for the doped sample (a) (S)-methylbenzylamine and (b) (R)-methylbenzylamine. Experimental points were fitted using eq 3.
bend to the equilibrium mass uptake. 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.17,18 The diffusion process generally satisfies Fick’s first and second laws giving the J rate of penetrant diffusion J and the gradient of 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 following relations:
J ) -D dC/dx
(1)
dC/dt ) -dJ/dx
(2)
with J the flux density, D the diffusion coefficient, x the position coordinate in the thickness d of the membrane, and C the molar concentration. Crank19 solved Fick’s second law of diffusion for a constant diffusion coefficient for a free-standing film which is exposed to sudden increase of the surface concentration on both sides.
m(t) m∞
∞
)1-
∑
n)0
8 (2n + 1)2π2
{
exp -
}
D(2n + 1)2π2t d2
(3)
Assuming the film thickness d is constant, experimental points for the two samples and the two enantiomers are satisfactorily
TABLE 1: Diffusion Coefficient (D), Maximum Sorbed Vapor (m(∞)), and Solubility Coefficient (S) Deduced from Sorption Measurements for (R)- and (S)-Enantiomers of r-Methylbenzylamine diffusion max sorbed vapor content solubility coefficient (in percent of initial coefficient (×109 cm2/s)a polymer mass)b (g/g of polymer) doped network reference network
DR
DS
m(∞)R
m(∞)S
SR
SS
7.1
5.7
8.8
7.4
0.49
0.41
4.1
3.8
5.8
5.8
0.32
0.32
a Experimental error 2 × 10-10 cm2/s. An alternative analysis of sorption curves to determine D was proposed in the Supporting Information and leads to similar results. b Experimental error 0.01%.
fitted by using eq 3 (all correlation coefficients above 0.99). As expected, the sorption isotherms exhibit a classical Fickian diffusion behavior for the conventional reference network but also, as shown in Figure 4b, for the mesomorphic one. Consequently, the mechanism involved was mainly controlled by the penetrant diffusion. Corresponding values of the diffusion coefficients values obtained are reported in Table 1. These values, all in the range of 10-9 cm2/s, are in good agreement with those measured by other authors through conventional or mesogenic materials.20 Moreover, for the cholesteric network, similar diffusion coefficients were obtained for (R)- and (S)-methylbenzylamine. Consequently, the differences observed at equilibrium in Figure
Enantioselectivity of Chirally Doped Networks 4 could not result from a difference in diffusion coefficient but mainly from a difference in solubility. 3.2.2. Capacity and EnantioselectiVity of the Samples. From Figures 3 and 4, we can directly evaluate the maximum quantity of amine sorbed at equilibrium, ∆M(∞) ) M(∞) - M0, by the polymer for a given vapor activity aamine, and so the maximum sorbed vapor content m(∞) ) ∆M(∞)/M0. m(∞) is linked to aamine by a solubility coefficient, S, representative of the level of interaction between the polymer network and the amine enantiomer. For dilute penetrant solutions, the solubility coefficient is independent of penetrant concentration and is given by Henry’s law: m(∞) ) Saamine. The values of m(∞) and S deduced from measurements with aamine ) 0.18 are reported in Table 1 for each (R)- or (S)-enantiomer. Experimental error on maximum sorbed vapor m(∞), evaluated from deviation between the two curves in Figure 3, parts a and b, was 0.01%. The doped network retained 8.8 wt % of (R)-R-methylbenzylamine and around 7.4 wt % of the other enantiomer. These values are in the same order of magnitude as those reported by Courty et al.9 who estimated, assuming that only molecules with the appropriate chirality were retained, that 5.7 wt % of 2(S)bromopentane was retained in a similar 20% cross-linked cholesteric network. The capacity of the cholesteric sample used here was around 0.6 mmol/g of polymer toward (R)-R-methylbenzylamine. For comparison, in nonmesogenic molecularly imprinted materials (imprinted by a chiral molecule) this capacity is at best around 1 µmol/g.21 The sorption properties of the reference sample and of the doped sample cannot be quantitatively compared because the stiffness and the porosity of the materials are not exactly the same, as well as the chemical structure. Nevertheless, the greater solubility coefficient for the mesomorphic network could be mainly attributed to strong interactions between phenyl groups of the liquid crystal pending groups of the network and the one of the chiral amine. As expected, the reference sample (Figure 3) has no preference toward one enantiomer or the other (SR ) 0.32 g/g and SS ) 0.32 g/g). On the contrary, the doped sample (Figure 4) absorbs preferentially (R)-R-methylbenzylamine (SR ) 0.48 g/g and SS ) 0.41 g/g). The corresponding enantiomeric preference, determined at equilibrium and defined as (∆M(∞)R - ∆M(∞)S)/ (∆M(∞)R + ∆M(∞)S) is around 8.6%. As CholOAc presents several asymmetric carbons, the preference observed here for the (R)-enantiomer was difficult to predict from a purely structural point of view. So after complete extraction of the chiral dopant, whereas no chirality remained at the molecular level, the doped sample absorbed one of the enantiomer: the network retained a memory of the phase chirality frozen during the cross-linking step, and this supramolecular chirality is sufficient to induce a stereoselection phenomenon. This result, obtained with the polydomain sample, was in the same order of magnitude as the effect shown by Courty and co-workers with an oriented monodomain sample using another technique (optical rotation).9-11 As suggested by these authors, this underlines the importance of the local nematic order on the stereoselective absorption properties of the network. 4. Conclusion By the use of an electronic microbalance, we analyzed the sorption kinetics of a nonoriented polymer network (polydomain) in which cholesteric order has been topologically frozen by cross-linking a nematic polymer in the presence of a cholesteric dopant. The network retained a memory of the chirality even when the dopant was completely removed, leaving
J. Phys. Chem. B, Vol. 111, No. 31, 2007 9243 an internally stored helical twist in the material. We observed the effect of a significant stereoselection between left- and righthanded molecules. The electronic microbalance proved to be successful in analyzing accurately the sorption kinetics of the penetrant. It will be used to study monodomain samples in order to compare with the present results. The material described here exhibits a phase chirality but no molecular chirality. In previous papers22,23 a cholesteric molecularly imprinted elastomer was obtained by cross-linking a nematic side-chain polysiloxane around a chiral template linked to the polymer via hydrogen-bond interactions, then removed by washing. In this case, there are two scales of chirality topologically imprinted in these materials, one related to the molecular chirality formed by the template molecules inside the micropores, the other one being the phase chirality of the resulting cholesteric phase. In such materials, enantiomeric separation should occur by recognition phenomenon inside the molecular imprinted cavities as well as by specific adsorption by the polymer matrix. Experiments are in progress to determine, by the use of an electronic microbalance, the respective contribution of these two parts in the enantiospecific separation. Supporting Information Available: Desorption measurements and an alternative approach to determine diffusion coefficients. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Nakano, T.; Okamoto, Y. Chem. ReV. 2001, 101, 4013-4038. (b) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem. ReV. 2001, 101, 3893-4011. (c) Green, M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook, R.; Lifson, S. Science 1995, 268, 1860-1866. (d) Yashima, E.; Maeda, K.; Nishimura, T. Chem. Eur. J. 2004, 10, 42-51. (2) de Gennes, P. G. Phys. Lett. 1969, 28A, 765. (3) (a) Huang, F.; Switek, K. A.; Gibson, H. W. Chem. Commun. 2005, 3655-3657. (b) Stadler, A.-M.; Kyritsakas, N.; Lehn, J.-M. Chem. Commun. 2004, 2024-2025. (c) Sanda, F.; Terada, K.; Masuda, T. Macromolecules 2005, 38, 8149-8154. (d) Lee, J. W.; Kim, K.; Kim, K. Chem. Commun. 2001, 1042-1043. (e) Choi, H. S.; Huh, K. M.; Ooya, T.; Yui, N. J. Am. Chem. Soc. 2003, 125, 6350-6351. (f) Tang, H.-Z.; Boyle, P. D.; Novak, B. M. J. Am. Chem. Soc. 2005, 127, 2136-2142. (4) Kelly, S. M. J. Mater. Chem. 1995, 5, 2047-2061. (5) Broer, D. J.; Hynderickx, I. Macromolecules 1990, 23, 2474-2477. (6) Hasson, C. D.; Davis, F. J.; Mitchell, G. R. Chem. Commun. 1998, 22, 2515-2516. (7) Hasson, C. D.; Davis, F. J.; Mitchell, G. R. Mol. Cryst. Liq. Cryst. 1999, 332, 2665-2672. (8) Mao, Y.; Warner, M. Phys. ReV. Lett. 2000, 84, 5335-5338. (9) Courty, S.; Tajbakhsh, A. R.; Terentjev, E. M. Phys. ReV. Lett. 2003, 91, 085503. (10) Courty, S.; Tajbakhsh, A. R.; Terentjev, E. M. Eur. Phys. J. E 2003, 12, 617-625. (11) Courty, S.; Tajbakhsh, A. R.; Terentjev, E. M. Phys. ReV. E 2006, 73, 011803. (12) Marty, J.-D.; Tizra, M.; Mauzac, M.; Rico-Lattes, I.; Lattes, A. Macromolecules 1999, 32, 8674. (13) Marty, J.-D.; Fournier, C.; Mauzac, M.; Rico-Lattes, I.; Lattes, A. Liq. Cryst. 2002, 29, 529-536. (14) Zanna, J.-J.; Mauzac, M.; Boue, F. Macromolecules 1999, 32, 2962-2966. (15) Kim, S. T.; Finkelmann, H. Macromol. Rapid Commun. 2001, 222, 429-433. (16) Zanna, J.-J.; Stein, P.; Marty, J.-D.; Mauzac, M.; Martinoty, P. Macromolecules 2002, 35, 5459-5465. (17) Stern, S. A.; Frisch, H. L. Annu. ReV. Mater. Sci. 1981, 11, 523550. (18) Fujita, J. Fortschr. Hochpolym. Forsch. 1961, 3, 1. (19) Crank, J. Mathematics of Diffusion; Clarendon Press: London, 1967. (20) Modler, H.; Finkelmann, H. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 836-856. (21) Kempe, M. Anal. Chem. 1996, 68, 1948. (22) Marty, J.-D.; Gornitzka, H.; Mauzac, M. Eur. Phys. J. E 2005, 17, 515-520. (23) Marty, J.-D.; Mauzac, M. AdV. Polym. Sci. 2005, 172, 1-35.
