E7 mixtures

European Polymer Journal 37 (2001) 1079±1082

www.elsevier.nl/locate/europolj

Phase diagrams of poly(dimethylsiloxane)/E7 mixtures Nicolas Gogibus a,b, Ulrich Maschke a,*, Farida Benmouna c, Bernd Ewen b, Xavier Coqueret a, Mustapha Benmouna c a

Laboratoire de Chimie Macromol eculaire, CNRS (UPRESA No 8009), B^ atiment C6, Universit e des Sciences et Technologies de Lille, F-59655 Villeneuve d'Ascq, France b Max-Planck-Institut fur Polymerforschung, Postfach 3148, D-55021 Mainz, Germany c Facult e des Sciences, University Aboubakr Belkaõd, Bel Horizon, BP119, 13000 Tlemcen, Algeria Received 7 September 2000; received in revised form 5 October 2000; accepted 19 October 2000

Abstract The phase behavior of blends of poly(dimethylsiloxane) and the eutectic mixture of liquid crystals E7 is investigated. Experimental phase diagrams are established by polarized optical microscopy for two systems with widely di€erent polymer molecular weights. Surprisingly, we ®nd a very slight loss of miscibility of the mixture as a result of an increase of the polymer molecular weight by almost an order of magnitude. This unexpected phase behavior is in contrast with the results reported so far on other systems. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Polymer; Polymer physics; Polymer composite materials

1. Introduction Blends of polymers and low molecular weight liquid crystals (LMWLCs) are widely used in display devices, light shutters and privacy windows [1±3]. Most applications of such systems are based upon electro-optical properties since under applied electric ®elds, they can be switched from opaque to translucid ®lms. Electro-optical response functions and degree of transparency of these materials depend crucially upon their phase behavior and morphology under operating conditions. Therefore, a precise knowledge of the phase diagram is a necessary step towards a better understanding of electrooptical and light transmission properties. Polymer/E7 mixtures such as NOA 65, poly(n-butylacrylate), poly(styrene) (PS) and poly(methylmethacrylate) (PMMA) were reported before [4±7]. In certain cases, these diagrams show peculiar properties that were attributed to the multicomponent nature of E7 [4,7]. This LC is an eutectic mixture of four LMWLCs but it has a single

nematic±isotropic transition temperature TNI ˆ 60°C [8]. The purpose of this work is manifold. First, it is meant to check whether peculiar properties observed in certain systems involving E7 are also found in the case of poly(dimethylsiloxane) (PDMS)/E7. Second, we would like to explore the e€ects of molecular weight on the miscibility of E7 and PDMS and see whether the relatively high incompatibility of these two molecular species bring about additional information not known before. The phase diagrams are established for two PDMS molecular weight systems in a wide range of temperature using polarized optical microscopy (POM). Third, an attempt is made to analyze the equilibrium phase diagrams theoretically using a model that combines the Flory±Huggins [9] theory of isotropic mixing and the Maier±Saupe theory of nematic order [10,11].

2. Experimental 2.1. Materials

*

Corresponding author. Fax: +33-3-2043-4345. E-mail address: [email protected] (U. Maschke).

PDMS has been prepared by an anionic living polymerization method using n-butyllithium as initiating

0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 0 ) 0 0 2 4 1 - X

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N. Gogibus et al. / European Polymer Journal 37 (2001) 1079±1082

species and trimethylchlorosilane as end capper. The obtained products were puri®ed and characterized by gel permeation chromatography. The molecular weight and the degree of polydispersity …Mw =Mn † are obtained from toluene solutions at 25°C giving Mw ˆ 5500 g/mol with Mw =Mn ˆ 1:3 and Mw ˆ 45 000 g/mol with Mw =Mn ˆ 1:1. These polymers will be referred to as PDMS5500 and PDMS45 000, respectively. The LMWLC is the eutectic mixture commonly known as E7 exhibiting the nematic± isotropic transition temperature TNI ˆ 60°C. It was purchased from Merck Encolab GmbH (Darmstadt, Germany). E7 contains 51 wt.% of 4-cyano-40 -n-pentylbiphenyl (5CB), 25 wt.% of 4-cyano-40 -n-heptyl-biphenyl, 16 wt.% of 4-cyano-40 -n-octyloxy-biphenyl, and 8 wt.% of 4-cyano-400 -n-pentyl-p-terphenyl. 2.2. Sample preparation Sample preparation was made following a combination of solvent induced phase separation and thermal induced phase separation [2]. The polymer and the LC were dissolved in THF which is a common organic solvent at a weight fraction of 55% for the PDMS5500 and 70% for the PDMS45 000 at room temperature. The resulting mixtures were stirred mechanically for 2 h. A small quantity of the mixture was cast on a clean glass slide before drying at room temperature for 24 h. After complete evaporation of THF, another glass slide was put on top of the ®rst one and made ready for POM measurements. 2.3. Polarized optical microscopy The thermo-optical studies were performed on a POM (ZEISS) equipped with a heating/cooling stage (Linkam) temperature control unit. All the samples were treated similarly. They were heated without interruption with a rate of 0.5°C/min from room temperature to above the transition temperature in the isotropic phase. At least two samples were prepared independently with the same composition to check the reproducibility of the results.

3. Results and discussion Fig. 1 displays the experimental phase diagram of the PDMS5500/E7 system as obtained by POM. It is worth noting that duplicate samples (i.e., with the same composition) prepared independently lead to practically the same data points and this gives us con®dence of their reproducibility. The symbols in this ®gure represent averaged values. One can see three distinct regions. In the upper part, the usual single isotropic phase characteristic of a linear polymer/LMWLC is recovered. The

Fig. 1. The experimental phase diagram obtained by POM for the system PDMS5500/E7 of PDMS Mw ˆ 5500 g/mol. Diamonds show the (N ‡ I) to (I ‡ I) transition whereas circles are for the (I ‡ I) to I transition. Empty and ®lled symbols represent onsets and ends of transition, respectively. N: nematic phase, I: isotropic phase.

rest of the (temperature T, LMWLC volume fraction u1 ) frame exhibits two biphasic domains depending essentially on the temperature range. Below 60°C, the data reveal an isotropic polymer rich phase coexisting with a nematic pure LMWLC phase. The fact that the (N ‡ I) to (I ‡ I) transition takes place nearly at constant temperature (i.e., TNI ˆ 60°C) con®rms that the polymer content of the nematic phase is practically zero. Above 60°C, a large isotropic (I ‡ I) miscibility gap characteristic of highly interacting molecular species is found, in spite of the relatively low molecular weight of the polymer. Systems involving PMMA and PS for example with similar molecular weights do not exhibit such a miscibility gap. POM data show a clear distinction between the onsets and the ends of transitions represented by the empty and ®lled symbols, respectively. Diamonds concern the (N ‡ I) to (I ‡ I) transition while circles are used for the (I ‡ I) to I transition. A general trend towards a higher di€erence between the two sets of data is found in the latter transition. This di€erence tends increasingly towards zero as u1 approaches 1. Very little distinction can be made between onset and end of transition in the case of the (N ‡ I) to I data. Some of these features are recovered in Fig. 2 giving the experimental phase diagram of the second system PDMS45 000/E7 investigated in this work. Large di€erences between onsets and ends of transition are observed in both (N ‡ I) to (I ‡ I) and (I ‡ I) to I. The (N ‡ I) to (I ‡ I) transition temperature exceeds in some cases 60°C which is the TNI for the pure E7. This is especially the case for the end of transition data and the highest PDMS molecular weight system. For example, the sample with 10 wt.% E7 exhibits a

N. Gogibus et al. / European Polymer Journal 37 (2001) 1079±1082

Fig. 2. The same as Fig. 1 for the system PDMS45 000/E7 of PDMS Mw ˆ 45 000 g/mol. Triangles represent (N ‡ I) to (I ‡ I) transitions and squares show the (I ‡ I) to I transition.

transition temperature N ! I almost 15°C above TNI of pure E7 which is totally unexpected. However, similar peculiar observations were made for other systems such as poly(n-butylacrylate)/E7 [7] and NOA/E7 [4]. In both cases, transition temperatures N ! I exceeding 60°C were observed for samples with a relatively low E7 content. These peculiarities were analyzed by invoking the multicomponent nature of the eutectic mixture E7 and the possibility of a preferential anity of the polymer towards certain constituents of the LC. From comparison of Figs. 1 and 2, one observes that the phase diagrams of PDMS5500/E7 and PDMS45 000/ E7 exhibit no signi®cant changes besides a slight increase of the (I ‡ I) region in spite of the fact that the polymer molecular weights are di€erent by almost a factor 8. This can be clearly seen in Fig. 3 where POM equilibrium data corresponding to the onset transition ((I ‡ I) to I) of the two diagrams are put together. Solid and dashed curves represent the calculated binodals and spinodal extensions of the unstable regions. Both diagrams admit a critical point. The dotted line gives the limiting LC volume fraction uNI below which the system does not exhibit a nematic order versus temperature. The calculations are made starting from a free energy model f representing a sum of the Flory±Huggins [9] free energy of isotropic mixing f …i† and the Maier±Saupe [10,11] free energy of nematic order f …n† . These free energies are recalled here without further details for shortness …i†

f u u ˆ 1 ln u1 ‡ 2 ln u2 ‡ vu1 u2 kB T N1 N2 and f …n† u1 ˆ kB T N1

 ln Z ‡

 mu1 s2 : 2

…1†

…2†

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Fig. 3. The equilibrium phase diagrams of PDMS5500/E7 and PDMS45 000/E7. The empty symbols are POM data for the onset of transition and the curves are the calculated binodal (±±) and spinodal (±). The dotted line represents uNI against T. To plot the theoretical curves, the following parameters were used v ˆ 11:63 ‡ 4619=T ; N2 ˆ 16 for PDMS5500/E7 and N2 ˆ 80 for PDMS45 000/E7. In all the theoretical curves we use N1 ˆ 1; TNI ˆ 60°C.

The details of the procedure used to establish the theoretical diagram were given elsewhere [5±7]. In Eqs. (1) and (2), ui and Ni are the volume fraction and number of repeat units of the molecular species i ˆ 1; 2. The Flory±Huggins interaction parameter v is assumed to be function of temperature according to the form vˆA‡

B ; T

…3†

where A and B are constant independent of T. Knowing the numbers of repeat units of constituents 1 and 2 in the mixture, one has a mean ®eld estimate of the critical parameter  2 1 1 1 p ‡ p : vc ˆ 2 N1 N2 If the critical temperature Tc is known experimentally, then one can express B in terms of A as B ˆ …vc A†Tc [7] implying that only one adjustable parameter A is needed to ®t the data. In Eq. (2), s represents the nematic order parameter, Z the partition function and m the Maier±Saupe quadrupole interaction parameter m ˆ 4:54

TNI : T

…4†

The other symbols have their usual meaning (see Ref. [7] for more details). This model cannot account for an N ! I transition temperature exceeding 60°C except in the case where the polymer presents a preferential anity towards certain constituents of E7. Surprisingly, the v-parameter is the

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N. Gogibus et al. / European Polymer Journal 37 (2001) 1079±1082

same for both systems and showing a low sensitivity to the polymer molecular weight contrary to analogous PDMS/5CB systems [12]. Here the dependence upon Mw is entirely accounted for via the entropy part of the Flory±Huggins free energy. This fact is probably a direct consequence of the very low miscibility of E7 in PDMS even with a Mw as small as 5500 g/mol. 4. Conclusions The phase behavior of PDMS/E7 with two di€erent molecular weights is investigated using POM technique. The phase diagram exhibits a large (I ‡ I) miscibility gap even for the lower molecular weight system Mw ˆ 5500 g/mol. Surprisingly, the molecular weight has only a little e€ect on the phase behavior con®rming the low intrinsic miscibility of E7 in PDMS. POM data show a clear distinction between the onset and the end of transition, especially for the (I ‡ I) to I transition. This di€erence is attributed to the multicomponent nature of E7 and the possibility of preferential anity of the polymer towards certain components of E7. The discrepancy between onset and end of transition is slightly enhanced for the higher polymer molecular weight and decreases systematically with increasing LC concentration. The equilibrium phase diagram is analyzed with a theoretical model combining the Flory±Huggins theory of isotropic mixing and the Maier±Saupe theory of nematic order. The interaction parameter v is found to be independent of polymer molecular weight for the systems under consideration.

Acknowledgements N. Gogibus thanks Professor H.W. Spiess for his interest in this work and the Max-Planck-Institut f ur Polymerforschung for ®nancial support. M. Benmouna thanks the University of Lille for hospitality as a guest professor while this work has been completed.

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