Polymer Journal, Vol. 31, No.2, pp 160-166 (1999) Polymer Surface-Induced Order of Liquid Crystalline Molecular Alignment Based on Nematic-Isotropic Phase Transition Behavior Sung-Kyu HoNG, Hirotsugu KIKUCHI, and Tisato KAJIYAMA t Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashiku, Fukuoka 812--8581, Japan (Received August 5, 1998) ABSTRACT: Alignment order of liquid crystal (LC) molecules at the (polymer/LC) interface was evaluated by comparing nematic-isotropic phase transition temperatures (TN1) at the interface with TN1 in the bulk. The thickness dependence of TN1 was measured using the LC cell in which an LC thin layer was sandwiched between polymer surfaces without surface treatment. The order of LC molecular alignment at the (polymer/LC) interface increased with surface free energy of the polymer substrate. The surface of crystalline polymer provided higher order of LC molecular alignment. Hydrophobic intermolecular interactions at the (polymer/LC) interface may be more responsible for increase in the interfacial molecular alignment order of LC in comparison with the hydrophilic one. KEY WORDS Liquid Crystal/ Order Parameter J (Polymer/Liquid Crystal) Interface/ Nematic-Isotropic Phase Transition Temperature I Surface Free Energy I Substantial research has been devoted to under­ According to the Landau-de Gennes theory, a nematic­ standing the anchoring mechanisms at the interface isotropic phase transition temperature (TN1) at a bulk between liquid crystalline (LC) molecule and solid nematic LC should be dependent on the order para­ substrate. The macroscopic three-dimensional alignment meter of LC molecular alignment. Substrate surface of LC molecules in an LC cell is strongly dependent on effects on the nematic-isotropic transition of LC have two-dimensional LC molecular alignment at the LC-solid also attracted much attention. Sheng studied the substrate interface. LC-polymer substrate interfacial nematic-isotropic transition in a thin layer of nematic characteristics are inevitably important for making LC LC being held between substrates inducing higher display devices, for example, TN (twisted nematic), STN orientational order of LC on the basis of Landau-de (super twisted nematic) type and so on, because LC Gennes theory. 8 •9 He found that the TN1 of LC layer molecules fill space between polymer substrates coated increases with the thickness of LC layer, in the case of on two glass plates. In the case of the (polymer/LC) that the solid surface induces the higher order of LC. composite films, the anchoring effect at the (polymer/LC) Poniewierski and Sluckin applied Sheng's theory to the interface has much influence on the electro-optical substrate surface with the lower orientational order of switching characteristics, such as the rise or decay LC. 10· 11 The experimental results above apparently response time and contrast between light scattering and indicate that TN1 of LC layer shifts either upward transparent states, because LC molecules are bicontin­ or downward according to whether the substrate is uously embedded in a three-dimensional polymer net­ orientationally ordering or disordering to LC molecules, work. 1 - 3 Despite the practical importance for the respectively, following the thermodynamic Kelvin design and construction of LC display devices, the equation. If the substrate is neutral to the alignment mechanism of the surface-induced alignment of LC order of LC layer, the magnitude of TN1 of LC at the molecules is not well understood. (polymer/LC) interface remains the same as that in the When LC molecules contact a solid polymer substrate, bulk nematic LC, regardless of the thickness of LC the order parameter of the LC molecular alignment at layer. Though it is a valuable method for evaluating the (polymer/LC) interface is not always the same as the interfacial order of LC to measure the shift of TN1 that in the bulk region, and is strongly dependent on in the vicinity of the interface, results for polymer surfaces intermolecular interactions with the polymer surface. without any surface treatment have not been reported yet. The interfacial order parameter of LCs on the rubbed In this study, the molecular alignment order of LC poly( vinyl alcohol) (PV A) surface is higher than that in induced at the (polymer/LC) interface was investigated the bulk region. The situation is opposite in the case of based on the thickness dependence of the nematic-iso­ the SiO evaporated surface. Nevertheless, both substrate tropic phase transition temperature (TN1) using the LC surfaces give the same homogeneous bulk molecular cell in which the LC thin layer was sandwiched between alignment to LC. 4 polymer surfaces without surface treatment, to clarify Since no direct method is available to measure the the relationship between the interfacial order of LC and order parameter of LC molecular alignment at the polymer surface characteristics. interface, an indirect method on the basis of the Landau-de Gennes theory has been proposed. 5 -lz t To whom correspondence should be addressed. 160 Polymer Surface-Induced Order of LC Molecular Alignment Polymer I Poly(diisopropyl fumarate) (Pdi-iPF) 6 Polyimide (PI) 2 Poly(l-cyano-1-ethylisopropyl fumarate) 6-1 Solvent soluble PI (S-PI) t;::t(51co, / CH3 ± " N-Q;-CH, 0 N........... H CH co fa I CH2 R-O-c=o n n DA-1, DA-2, DA-3 n 3 Poly( vinyl alcohol) (PVA) -tCH2-crH+ (DA-1) (DA-3) OH n 4 Poly( vinyl chloride) (PVC) ' r&cc, (DA-2) 5 Poly(vinylidene floride) (PVDF) -fcH2-cF2T 6-2 Thermally polymerized PI (T-PI) Liquid Crystal 4-cyan,J-4'-n-pentylbiphenyl (5CB) TKN =297K TN! =308K C5H11--g-g-c N E J. =6.9 €11 =17.9 =11.0 Figure 1. Chemical structures and physical properties of polymers and liquid crystal in this study. EXPERIMENTAL with both surfaces coated with polymer thin films. The thickness of the LC layer was minimum at the center of Materials the cell and increased continuously along the circum­ Figure 1 shows the chemical structures and physical ference from the center. Maximum thickness of LC layer properties of polymers and liquid crystal used in this at both ends of the optical window was 60 f1m. The cell study. Poly(diisopropyl fumarate) (Pdi-iPF, Nippon Oil was surrounded by brass materials to prevent inhomo­ & Fats Co., Ltd.), poly(l-cyano-1-ethyl-isopropyl fu­ genous heat transfer at the entire cell as shown in marate) (PCNEt-iPF, Nippon Oil & Fats Co., Ltd.), Figure 2. The polymers were dissolved in good solvents, poly( vinyl alcohol) (PV A, Kuraray Co., Ltd.), poly( vinyl and the both convex lens and fiat substrate were chloride) (PVC), poly(vinylidene fioride) (PVDF), four spin-coated with their solutions of 1 wt% under types of solvent soluble polyimides (S-PI, JSR Co., Ltd.) conditions of 4000 rpm and 293 K. y-Butyrolactone for with different side chain groups and thermally four types of S-Pis, pure water for PV A, toluene for polymerized polyimide (T -PI, Chisso Co., Ltd.) were used Pdi-iPF, chloroform for PCNEt-iPF, cyclohexanone for as polymers. 4-Cyano-4' -pentylbiphenyl (5CB, Merck PVC and N,N-dimethylacetamide for PVDF were used Co., Ltd.) was used as LC. 5CB presents a nematic phase as the solvents. After spin-coating, the substrates were at room temperature. annealed at 423 K for S-Pis, PVA, PVC, PVDF and at 373 K for Pdi-iPF, PCNEt-iPF for 2 h, respectively. In Measurement of Thickness-Dependent TN1 of LC cell order to prepare the T-PI film, a solution of poly(amic Figure 2 shows a cross-sectional view of the LC cell acid) as a polyimide precursor was spincoated on both designed to measure the thickness dependence of TN1 of convex lens and the fiat substrate and then, thermally the LC layer. 5CB was sandwiched between optical-fiat polymerized in vacuum at 573 K for 2 h. The thickness glass plate and half-convex lens with a 5 m focal length of the resulting polymer films on the substrates ranged Polym. J., Vol. 31, No.2, 1999 161 S.-K. HoNG, H. KIKUCHI, and T. KAJIYAMA Figure 2. Cross-sectional view of the LC cell designed to measure thickness dependence of TN, of the LC layer. L p B Len A C L: light P: polarizer B: constant-temperature bath Len: lens A: analyzer C: video camera Figure 3. Schematic representation of experimental set up to measure thickness dependence of TN, of LC layer. from sub-micrometer to a few micrometers. polymer surface was carried out in an isotropic phase at Figure 3 shows a schematic representation of the 313K. The aggregation structure of each polymer thin experimental set up to measure the thickness dependence film was investigated based on wide angle X-ray of TN 1 of the LC layer. The thickness dependence of TN 1 diffraction (W AXD) study. W AXD patterns were taken of the LC layer was measured based on optical-texture on an imaging plate by using Ni filtered Cu-Ka changes under crossed nicols upon heating or cooling of (A.=0.15405nm) radiation from 40kV, 300mA X-ray the LC cell placed in a constant-temperature bath in source of X-ray generator (Mac Science Ml8XHF). which temperature was uniformly controlled over the entire sample. LC layer thickness was estimated from a RESULTS AND DISCUSSION curvature ratio of lens and the distance from the lens center. White light was used as the light source. Evaluation of the Order of LC Molecular Alignment at Temperature in the thermostat was monitored during (PolymerjLC) Interface Based on Phase Transition observation of change of the optical texture. The preci­ Behavior sion of temperature measurement should be controlled Figure 4 shows the W AXD photographs of each within ± 10- 3 K by a quartz thermometer, because shift polymer.