submitted papers Characterization of Polymer-Dispersed Liquid Crystal Systems by FT-IR Microspectroscopy SUDARSANA R. CHALLA, SHI-QING WANG, and JACK L. KOENIG* Department of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio 44106 Infrared microspectroscopy was used to generate functional group images of liquid crystal (ET) droplets dispersed in poly(n-butyl methacrylate) (PBMA). The spatial concentration fluctuations that occur within the system were studied as a function of time. This approach is possible because spectral information can be obtained by focusing on regions on the order of tens of micrometers. The peak intensities were used as a measure of concentration of the components. The amount of liquid crystal dissolved in the polymer matrix determines the extent to which the polymer is plasticized, which in turn affects the shape and size of the droplets. The growth of the domains at any temperature is also deter- mined by whether the system is maintained above or below the glass transition temperature of the matrix. It is observed that the growth of the droplets follows temporal power laws. The spatially resolved spec- troscopic images provide valuable insight into the phase separation pro- eess and the formation of mierodroplets of E7 in PBMA. Index Headings: FT-IR microspectroscopy; E7; Poly(n-butyl methac- rylate); PDLC; Ostwald ripening. INTRODUCTION Polymer-dispersed liquid crystals (PDLCs) are prom- ising new materials for electro-optic applications such as projection displays and privacy windows. 1-3 The PDLC films consist of micrometer-sized droplets of nematic liq- uid crystal dispersed in a polymer matrix. They can be switched between an opaque "off" state and a transparent "on" state because of mismatching or matching of the refractive indices between the droplets and the matrix. This phenomenon is achieved by the application of elec- tric or magnetic fields across the films. The electro-optic properties such as the driving voltage and switching times depend on several factors such as the shape and size of the droplets and the amount of liquid crystal dissolved in the polymer matrix. 4-6 The dispersion of the LC within the matrix is achieved by phase sepa- ration processes which are brought about by polymeriza- tion, solvent evaporation, or a thermal quench depending upon the materials used. I-6 Understanding and control of the kinetics of phase separation are essential for producing quality displays. FT-IR microspectroscopy is a powerful tool for char- acterizing domains of size greater than 10 ~tm.7,8 This Received 21 July 1994; accepted 23 November 1994. * Author to whom correspondence should be sent. capability permits the probing of the two separated phases to yield information about the composition of each phase. This technique is used to obtain spatially resolved func- tional group images. Any changes that occur within a system as a function of time, temperature, or method of preparation can be studied by spatial imaging. This work mainly deals with the thermoplastic PDLC system comprised of liquid crystal E7 dispersed in a poly(n-butyl methacrylate) matrix. If the system is main- tained at a temperature above the Tg of the polymer ma- trix, the droplets have a tendency to grow through the processes of coalescence and Ostwald ripening. This phe- nomenon is examined with the use of IR microspectros- copy. EXPERIMENTAL The low-molecular-weight liquid crystal used in the studies was E7, an eutectic liquid crystal mixture con- sisting of four types of cyano-biphenyls. E7 is manufac- tured by BDH Chemical Co. Ltd., Poole, England and distributed by EM Industries Inc., Hawthorne, New York. Poly(n-butyl methacrylate) 300,000 g/mole in molecular weight was purchased from Aldrich Chemical Co. The materials were used as received. A combination of solvent-induced phase separation and thermally induced phase separation techniques was used to make the samples. A mixture of E7, PBMA, and chlo- roform in the ratio of 7:3:90 was prepared, and a few drops were transferred onto a KBr disk. The solvent was allowed to evaporate in air for an hour to leave a thin film consisting of E7/PBMA in the ratio of 70:30 and then placed in a vacuum oven maintained at 70°C for an hour to remove any solvent that may have been remain- ing in the system. This film was sandwiched by using another KBr disk. Glass fibers 5 #m in thickness were used as spacers. The system was quenched to room tem- perature, and the experiments were run. Spectra were recorded on a Spectra-Tech IR#S infrared microscope system equipped with a 250-tzm x 250-#m narrow-band MCT detector. A 15 x objective coupled with a 10 x condenser was used to obtain transmission spectra with 50 scans at a resolution of 4 cm -t . A region of 240 ~tm was imaged with a 24-tzm x 24-#m aperture with a step size of 12 #m. Volume 49, Number 3, 1995 0003.7028/95/4903.026752.00/0 APPLIED SPECTROSCOPY 267 @ 1995Society for AppliedSpectroscopy