Spinodal decomposition kinetics of a mixture of liquid crystals and polymers

Transcription

1 Macromol. Chem. Phys. 200, (1999) 943 Spinodal decomposition kinetics of a mixture of liquid crystals and polymers Zhiqun Lin, Hongdong Zhang, Yuliang Yang* Department of Macromolecular Science, Lab of Macromolecular Engineering, SEDC, Fudan University, Shanghai , China (Received: July 23, 1998; revised: October 28, 1998) SUMMARY: The spinodal decomposition (SD) kinetics of the mixture of polystyrene (PS) and a low molar mass liquid crystal (LC) (E7) has been studied by means of time-resolved small angle light scattering (SALS) and polarized optical microscopy (POM). It is found that, in the isotropic spinodal region, the mixture first undergoes an isotropic concentration fluctuation, and then the ordering process that occurs in pure LC will appear in the LC-rich phase after phase separation. In contrast to that, however, the concentration and orientation fluctuations take place simultaneously in an anisotropic spinodal region. A rapid phase separation via SD was observed in the anisotropic spinodal region due to the fact that the ordered LC phase excludes the polymer coils more strongly than the isotropic region. I. Introduction As an important technique for preparing inhomogeneous composite materials, the spinodal decomposition (SD) of binary polymer mixtures has attracted theoretical and experimental attention due to academic interest and their potential technological importance 1 4). In recent years, the composites composed of microdroplets of low molar mass liquid crystals in a solid polymer matrix (PDLC) are promising materials for electro-optical applications 5 12). The simplest way of preparing such materials might be the thermally induced phase separation by cooling the homogeneous mixture of LCs and thermoplastic polymers 13, 14). The electro-optical performance of the PDLC materials depends strongly on the morphology of the phase-separated structure, and this morphology is mainly determined by both the thermodynamics and the kinetics of phase separation during the preparation process. Thus, the understanding of the phase equilibrium and phase separation dynamics of a mixture of LCs and polymers is of central importance for optimizing the performance of PDLC materials. On the other hand, the dynamics of spinodal decomposition in a mixture containing anisotropic moieties is also an interesting theme for the study of complex fluids or soft matters. However, there have been only a few reports 15 21) on this kind of research for mixtures with LC as one component. Kim and Palffy-Muhoray 15, 16) have experimentally investigated an epoxy-based PDLC system that is formed via polymerization-induced phase separation of the mixture of LCs and polymers. However, it seems to us that these studies are limited to the case of SD in a liquidliquid (L-L) spinodal region. Our previous linear theoretical analysis of LC/polymer mixtures has shown that, except the L-L region, there may exist two other spinodal regions, i. e., an isotropic region and an anisotropic region 17 19). The theory has also predicted that the SD kinetics in these two regions is quite different 19, 20). In this paper, we present our experimental results on SD kinetics of a binary mixture of E7/polystyrene (PS) driven by a thermal quench into these two regions. This paper is organized as follows. In section II, detailed descriptions of the material used in the experiment and the experimental techniques are given. In section III, we firstly present the measured phase diagram of E7/PS mixture together with an anisotropic-isotropic transition line, theoretically fitted binodal and spinodal curves. To verify the various spinodal regions, SALS and POM techniques are used to monitor SD kinetics of the E7/PS mixture at several LC compositions. Time-evolution SALS patterns and POM images under Vv and Hv polarization conditions are presented together with the discussions. The main conclusions drawn from this work are summarized in section IV. II. Experimental part The materials used in this study are a nearly monodispersed PS (M n = 5100 and M w/m n a 1.05, E. Merck Inc.) and a eutectic nematic LC (commercially known as E7, BDH Ltd.). The nematic liquid crystal, E7, is a mixture of 51 wt.-% 5CB (49-pentyl-4-cyanobiphenyl), 25 wt.-% 7CB (49- heptyl-4-cyanobiphenyl), 16 wt.-% 8OCB (49-octoxy-4-cyanobiphenyl) and 8 wt.-% 5CT (499-pentyl-4-cyanoterphenyl). Although the composition of E7 is quite complicated, it just shows a single nematic-isotropic or anisotropic-isotropic (NI) transition temperature, i.e., T NI =608C. The PS/E7 samples at various compositions were prepared by adding a minimum amount of the solvent 1,2-dichloroethane (its volatility is C) to the PS/E7 mixtures to Macromol. Chem. Phys. 200, No. 5 i WILEY-VCH Verlag GmbH, D Weinheim /99/ $ /0

2 944 Z. Lin, H. Zhang, Y. Yang form a homogeneous solution under ambient stirring. Then the solution was cast on a glass slide and heated to 908C on a hot stage to evaporate the solvent. This sample was removed from the heating stage and put into a vacuum oven at room temperature for 48 h to completely remove the residual solvent. After solvent evaporation, the sample was heated to 908C again and a cover slide was put on top to make a sandwich sample. In this way, the PS/E7 mixture was filled between the two glasses with a gap distance of L20 lm controlled by a spacer. The final sample was in a turbid state because of the refractive index mismatch between E7 and PS domains. The phase diagram of E7/PS mixture was determined by POM (Leitz, Orthoplan) equipped with a heating stage and a programmable temperature controller (the accuracy of the order is about l0.18c). The E7/PS sample was first heated to 808C to form a homogeneous transparent state and kept for 20 min, and then was cooled down slowly at a rate of 0.58C/ min to room temperature. Under the Vv (vertical polarizer with vertical analyzer) polarization condition, there was no liquid-liquid (L-L) coexistence region above the LC transition temperature observed because of the low molecular weight of PS 17 20). Therefore, the Hv (horizontal polarizer with vertical analyzer) condition was adopted to determine the phase equilibrium point at various LC compositions. The same heating procedures were repeated 3 l 4 times for each sample to minimize the experimental uncertainties. By fitting the experimental phase diagram with the phase equilibrium theory 18 20), the binodal and spinodal curves can then be obtained. Although the spinodal curve obtained in this way is not very accurate, we believe that it can give a reasonable estimation for the SD regions. POM and a home-built time-resolved SALS apparatus with 7.5 mw He-Ne laser (632.8 nm) as a light source 22) were used to investigate the kinetics of spinodal decomposition. The Vv and Hv scattering patterns and their corresponding POM images are monitored and recorded with a chargecoupled device (CCD) camera. III. Results and discussion Phase diagram of E7/PS mixtures The phase equilibrium of the mixture of low molecular LCs and polymers can be described by the statistical thermodynamics theory developed in our previous papers 17 20). In this theory, we combine the Flory-Huggins model for the flexible polymer solutions and the Lebwohl- Lasher model for the nematogens to derive the free energy of mixtures of LCs and polymers 17 20) : F u; S ˆ ulnu 1 u x ulnz u 1 2 v LLu 2 S 2 ln 1 u v LP u 1 u where u is volume fraction of LC, x is the polymeric chain length, v LP is the Flory-Huggins interaction parameter between polymer chain segments and LC molecules, and v LL is Maier-Saupe s anisotropic interaction parameter between LC molecules. Z(u) and S are orientational partition function and order parameter of LC, respectively, and can be calculated by the self-consistent relations from Maier-Saupe s theory. Thus, only three adjustable parameters, i.e., x, v LP, v LL are needed to calculate the phase diagram. v LL can be directly calculated from T NI of LC, and v LP can be simply expressed by the relation v LP = A/T(K)-B. It has been shown that this theory agrees with the experimental results quite well 17, 18). The experimentally measured phase diagram of E7/PS mixture, together with the calculated anisotropic-isotropic (N-I) line, theoretical binodal curves, and two theoretical spinodal curves, are shown in Fig. 1. It can be seen that the N-I transition line divides the spinodal region into two parts, i. e., the isotropic and anisotropic spinodal regions. Through the fitting procedure, we obtain the Flory-Huggins interaction parameter v LP = A/T(K)-B with A = 410 and B = For the details of the fitting of the phase diagram, one can refer to our pervious papers 17 20). This phase diagram exhibits an upper critical solution temperature (UCST) character. However, the liquid-liquid biphasic coexistent region is not existing. This can be attributed to the reason that, based on theoretical grounds, both the molecular weight of PS and the Flory-Huggins interaction parameter v LP are not high enough. SD kinetics in the isotropic spinodal region The SD kinetics of E7/PS mixture in the two spinodal regions was studied by SALS and POM techniques. For the mixture containing LC as one component, two order parameters, i. e., concentration and orientation order parameters, should be considered to describe the SD kinetics 20, 23, 24). According to the light scattering theory of anisotropic systems 25, 26), the Vv scattering pattern is relevant to the concentration and orientation fluctuations, however the Hv scattering pattern just attributes to the orientation fluctuation. Therefore, in order to obtain information on the concentration and orientation fluctuations, both Vv and Hv scattering patterns should be measured. In order to study the SD kinetics in the isotropic spinodal region, the sample containing 70.1 wt.-% of E7 was investigated by a thermal quench from 708C to 258C. It means that this quench trajectory is only located in the left side of the N-I transition line, i. e., isotropic spinodal region. We should note that the calculated critical LC concentration at N-I transition at 258C is L89 wt.-%. In Fig. 1, it is denoted as u c. Time-evolution SALS patterns and the POM images of the isotropic spinodal region under Vv and Hv conditions are shown in Fig. 2 a and 2 b. It can be seen that a typical isotropic spinodal ring scat-

3 Spinodal decomposition kinetics of a mixture of liquid crystals and polymers 945 Fig. 1. Phase diagram of the mixture of E7/PS with T NI = K, v LP = 410/T The molecular weight of PS is Quench trajectories are given by the arrows. Solid line ( ): calculated phase equilibrium curve. Dashed line (- - -): calculated isotropicanisotropic transition line. Dotted line ( N N N ): calculated spinodal curve. Black solid circles (0): experimental points. Open triangle (H): critical LC concentration u c (89 wt.-%) where the N-I transition takes place at 258C tering pattern is observed in the Vv scattering when the evolution time is less than 5 seconds (t a 5s). This scattering pattern is independent of the azimuthal angle l and can be attributed to the concentration fluctuation. Within this period, the corresponding POM images show a bicontinuous phase structure (see Fig. 2 a). It is interesting to note that no Hv scattering patterns and no corresponding POM images are available in this time period (t a 5s) (see Fig. 2b). That is to say, only the isotropic concentration fluctuations, but almost no orientation fluctuations, can be observed in the beginning of SD. This character of SD mechanism qualitatively agrees with our previous theoretical prediction about the SD mechanism in isotropic spinodal region 20). After 5 s, the Vv scattering intensity increases, the spinodal ring gradually shrinks and turns out to be an anisotropic two-fold symmetry pattern 24). In the mean time (t F 5 s), the Hv scattering pattern appears as a four-fold symmetry pattern 24) with maximum intensity in the direction of the odd multiple of the azimuthal angle l = 458. The corresponding POM images under Hv condition show that nematic LC droplets emerge from the dark background. It indicates that the LC-rich phase undergoes isotropic-anisotropic transition because the LC concentration in the LC-rich phase increases and crosses over the N-I transition line. SD kinetics in the anisotropic spinodal region To investigate the SD kinetics in the anisotropic spinodal region, the sample containing 91.8 wt.-% E7 was quenched from 708C to 258C, i.e., quenching into the anisotropic spinodal region, which is located in the right side of the N-I transition line. The time-dependent Vv and Hv scattering patterns are shown in Fig. 3a and 3b. A two-fold symmetry Vv scattering pattern appears when the evolution time is less than 3 s (t L 3s). It reveals that the anisotropic structure has already formed in this period due to high LC concentration (91.8 wt.-%). The formation of anisotropic structure can also be confirmed by the birefringence pattern of POM and the appearance of the four-fold symmetry Hv scattering pattern. Both the Vv scattering pattern and the Hv scattering pattern collapse rapidly to a small diameter, and their intensities increase continuously with time evolution. It seems that orientation ordering and phase separation may take place simultaneously and a coupling between concentration and orientation fluctuations exists. As predicted by our previous theory 20), the phase separation progresses spontaneously faster in the anisotropic spinodal region than in the isotropic spinodal region. The reason could be that LCs in the highly ordered state might exclude the polymer coils more strongly in the anisotropic spinodal region. Therefore, it results in an increase of the thermodynamic driving force for phase segregation and hence the growth rate of the domains. In order to further compare the kinetic behavior between isotropic and anisotropic spinodal regions, the relationships of lni m Vv (t) vs. lnt and lni m Hv (t) vs. lnt for the SD behavior in the isotropic and anisotropic spinodal regions are shown in Fig. 4 and Fig. 5, respectively. We

4 946 Z. Lin, H. Zhang, Y. Yang (a) (b) Fig. 2. Time-evolution of the SALS patterns and POM images of sample containing 70.1 wt.-% of E7 in the isotropic spinodal region. (a) Vv condition; (b) Hv condition. The evolution time and the direction of the polarizer (P) and analyzer (A) are indicated

5 Spinodal decomposition kinetics of a mixture of liquid crystals and polymers 947 (a) (b) Fig. 3. Time-evolution of the SALS patterns and POM images of sample containing 91.8 wt.-% of E7 in the anisotropic spinodal region. (a) Vv condition; (b) Hv condition. The evolution time and the directions of the polarizer (P) and analyzer (A) are indicated

6 948 Z. Lin, H. Zhang, Y. Yang regions are quite different. The SD kinetics of the mixture of E7/PS in the anisotropic spinodal region progressed much faster than that in the isotropic spinodal region. These results agree with theoretical grounds qualitatively. In addition to that, in the isotropic spinodal region, the isotropic concentration fluctuation develops first and then follows the LC ordering process in the LC-rich domains. In contrast to that, in the anisotropic spinodal region, isotropic concentration fluctuation and LC ordering process may take place simultaneously and a coupling between concentration and orientation fluctuations exists. Fig. 4. Time-evolution of the scattering intensity of E7/PS mixture containing 70.1 wt.-% of E7 under Vv (-0-) and Hv (-9-) conditions in the isotropic spinodal region Acknowledgement: The financial support by NSF of China, State Key Project-Macromolecular Condensed State Physics MSTC, Qiu Shi Foundation of Hong Kong and The Commission of Science and Technology of Shanghai Municipality are gratefully acknowledged. Fig. 5. Time-evolution of the scattering intensity of E7/PS mixture containing 91.8wt.-% of E7 under Vv (-0-) and Hv (9) conditions in the anisotropic spinodal region obtain the I m Vv by measuring the maximum intensity in the azimuthal angle l = 08 and 1808, i.e., the maximum intensity of the two-fold symmetry Vv scattering patterns. The intensities of I m Hv are obtained by measuring the maximum intensity in the direction of the odd multiple of the azimuthal angle l = 458, i.e., the maximum intensities of the four-fold symmetry Hv scattering patterns. It can be seen from Fig. 4 that the Hv scattering intensity is much lower than the Vv scattering intensity in the isotropic spinodal region. However, in the anisotropic spinodal region (Fig. 5), the initial intensity of the Hv scattering is higher than that of the Vv scattering. IV. Conclusions We have demonstrated the SD kinetics of the mixture of E7/PS in the isotropic and anisotropic spinodal regions by time-resolved SALS and POM techniques. The experimental results show that the SD behaviors in two spinodal 1) G. H. Fredrickson, K. Binder, J. Chem. Phys. 91, 7265 (1989) 2) T. Hashimoto, M. Takenaka, T. Izumitani, J. Chem. Phys. 97, 679 (1992) 3) M. Takenaka, T. Hashimoto, J. Chem. Phys. 96, 6177 (1992) 4) T. Hashimoto, Current Topics in Polymer Science, R. Ottenbrite, L. A. Utracki, S. Inoue, Eds., MacMillian, New York ) J. W. Doane, N. A.Vaz, B. G. Wu et al., Appl. Phys. Lett. 48, 269 (1986) 6) P. S. Drazaic, J. Appl. Phys. 60, 2142 (1986) 7) G. Chidichimo, G. Arabia, A. Golemme, Liq. Cryst. 5, 1443 (1989) 8) C. Cipparrone, C. Umeton, F. Simoni et al., Mol. Cryst. Liq. Cryst. 179, 269 (1990) 9) Y. K. Fung, D. K. Yang, J. W. Doane, SPIE 41, 1664 (1992) 10) G. P. Montgomery, Jr., N. A. Vaz, Appl. Opt. 26, 738 (1987) 11) N. A. Vaz, G. Smith, G. P. Montgomery, Jr., Mol. Cryst. Liq. Cryst. 146, 1 (1987) 12) J. L. West, Mol. Cryst. Liq. Cryst. 157, 427(1988) 13) T. Kajiyama, A. Miyamoto, H. Kikuchi, Y. Morimura, Chem. Lett. 813 (1989) 14) J. W. Doane, Liquid Crystals, Application and Uses, B. Bahadur, Ed., World Scientific, Chap. 14, ) J. Y. Kim, P. Palffy Muhoray, Mol. Cryst. Liq. Cryst. 203, 93 (1991) 16) J. Y. Kim, C. H. Cho, P. Palffy-Muhoray et al., Phys. Rev. Lett. 71, 2232 (1993) 17) Y. Yang, J. Lu, H. Zhang, T. Yu, Polym. J. (Tokyo) 26, 880 (1994) 18) H. Zhang, F. Li, Y. Yang, Sci. China, Ser. B 38, 412 (1995) 19) H. Zhang, Z. Lin, D. Yan, Y. Yang, Sci. China, Ser. B 40, 128 (1997) 20) Z. Lin, H. Zhang, Y. Yang, Macromol. Theory. Simul. 6, 1153 (1997) 21) F. Benmouna, L. Bedjaoui, U. Maschke et al., Macromol. Theory. Simul. 7, 599 (1998) 22) J. Zhang, H. Zhang, Y. Yang, Sci. China, Ser. B 40, 15 (1997) 23) J. R. Dorgan, J. Chem. Phys. 98, 9094 (1993) 24) A. Nakai, T. Shiwaku, T. Hashimoto, Polymer 37, 2259 (1996) 25) R. S. Stein, J. J. Keane, J. Polym. Sci. 17, 21 (1955) 26) R. J. Samules, J. Polym. Sci., Part A-2 9, 2165 (1971)