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Polymer Journal, Vol. 31, No.2, pp 160-166 (1999)

Polymer Surface-Induced Order of Crystalline Molecular Alignment Based on Nematic-Isotropic Transition Behavior

Sung-Kyu HoNG, Hirotsugu KIKUCHI, and Tisato KAJIYAMA t

Department of Physics and , Graduate School of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashiku, Fukuoka 812--8581, Japan

(Received August 5, 1998)

ABSTRACT: Alignment order of liquid (LC) at the (polymer/LC) interface was evaluated by comparing nematic-isotropic 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 / 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) and 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 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 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 (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( ) (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 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 of 1 wt% under types of solvent soluble (S-PI, JSR Co., Ltd.) conditions of 4000 rpm and 293 K. y-Butyrolactone for with different 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-, 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 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. PVA, PVDF, and T-PI showed strong Debye of TN 1 is in the range of 10- 2-10- 3 K. Heating and rings at the Bragg spacings of 0.41-0.48 nm, and thus cooling rates were carefully controlled at 0.01 K min - 1 . were crystalline polymers. PVC and DA-1, DA-2, DA-3, and ALI 051 were amorphous, because diffuse amor­ Surface Free Energy and Aggregation Structures of phous halos at Bragg spacings of 0.52-0.58 nm were Polymers and Contact Angles of 5CB to Each Polymer observed. For Pdi-iPF and PCNEt-iPF, strong inner Surface De bye rings at the Bragg spacing of 1.08 nm and 1.15 nm The surface free energy and aggregation structures of and diffuse rings at the Bragg spacings of 0.46 nm and polymers and of 5CB to each polymer 0.44 nm were observed. Strong inner Debye rings may surface were measured to investigate their dependence be due to regular packing of semi-rigid chains in a similar on the order of LC molecular alignment at the (poly­ fashion to LC molecules, owing to the repulsive force merjLC) interface. The surface free energy of each between bulky side groups in Pdi-iPF and PCNEt-iPF polymer was evaluated from measurement of static molecules. 14 Pdi-iPF and PCNEt-iPF may thus be in contact angle with two solutions such as water and an amorphous state with quasi-hexagonal packing of methylene iodide using a Kyowa contact angle meter. rod-like molecules. Surface free energy was calculated on the basis of Owens Interfacial interactions affecting the shift of TN1 may equation13 and yd, yh, and y, corresponding to the dis­ extend over 100 nm from the LC-substrate interface, and persion component, the hydrogen bonding component continuously decrease with increase in distance from the and the sum of the two components, respectively, were interface. 8 Therefore the degree of interfacial interaction

obtained. Contact angle measurement of 5CB with each can be evaluated from thickness dependence of TN 1 of

162 Polym. J., Vol. 31, No.2, 1999 Polymer Surface-Induced Order of LC Molecular Alignment

Pdi-iPF PCNEt-iPF PVC S-PI(DA-1) S-PI(DA-2)

S-PI(DA-3) S-PI(AL1051) PYA T-PI PVDF Figure 4. WAXD photographs of polymers obtained by WAXD.

Nematic phase__.,. Isotropic phase (Upon heating )

a) (Pdi-iPF/SCB) b) (PCNEt-iPF/SCB) c) (PVC/5CB), (PVA/5CB) (PVDF/5CB), (Pis/5CB) Figure 5. Photographs of the LC cell perpendicular to the substrate surface under crossed nicols in the process from the nematic phase to isotropic one upon the heating process. the LC layer using LC cell as shown in Figure 2, since 5 during phase transition with both TN1 of LC should be dependent on the order parameter heating and cooling. Figures 5 and 6 show that TN1 in of LC molecular alignment in a nematic phase. the cell is apparently higher at the edge part than the Figure 5 shows photographs of the LC cell along the center in the (Pdi-iPF /5CB) system, and T NI is lower at direction perpendicular to the substrate surface under the edge than center in the (PVA/5CB), (PVC/5CB), crossed nicols in the process from the nematic phase to (PVDF/SCB), and (Pis/5CB) systems. For (PCNEt­ the isotropic one upon the heating process. The dark iPF/SCB) system, TN1 is nearly same throughout the cell. and bright regions correspond to isotropic and nematic The difference in TN 1 between the center and edge of the phases, respectively, because the LC molecular align­ observation field roughly corresponds to the difference ments in the nematic state are almost planar to the between TN1 at the (polymer/LC) interface and TN1 of the substrate surface in all cases studied here. In the LC cell, bulk LC, because the surface of polymer substrates the thickness of LC layer sandwiched between polymer coated on the glass plate and convex lens are in contact substrates continuously changed from 0 at the center to at the center of the cell (d=: 0) and d at the edge of the 60 ,urn at the edge due to the curvature of half-convex cell is quite large, 60,um. The experimentally obtained lens. It is clear from Figure 5 that magnitude defined by eq I are listed in Table I. phase transition upon heating process occurred from the l'lTN TN1(center)- TN1(edge) (1) center, or the thinnest part of the 5CB layer sandwiched 1= between Pdi-iPF films, while from the edge, from the TNI at the (Pdi-iPF /5CB) interface should be lower than thickest part, in the cases of the (PVA/5CB), (PVC/ that of the bulk, because l'lTN1 was negative for the LC 5CB), (PVDF /5CB), and five (Pis/5CB) systems. For cell of (Pdi-iPF /5CB). T NI at the interface should be (PCNEt-iPF/5CB) system, the phase transition occurred higher than that of the bulk for (PVC/5CB), (PVDF/ throughout an observation field. 5CB), (PV A/5CB), and (Pls/5CB) systems because 11 TN1s Figure 6 shows a schematic representation of the were positive. From the Sheng, Poniewierski and Sluckin optical texture changes for three LC cells shown in Figure theory, TN1 increases with decrease of thickness of the Polym. J .• Vol. 31, No.2, 1999 163 S.-K. HoNG, H. KIKUCHI, and T. KAJIYAMA

1) (Pdi-iPF/SCB) system

Front View (N--+I--+N) a) Nematic _.,. Isotropic b) Isotropic__.. Nematic

Cross Sectional 1 View (N--+I) 'z..r___ <.... • '...7 __ 9(---JJ .,.'&.t___ <-31 ... 'z..r___ <-31 ... a;.F'__ "1---..31

2) (PCNEt-iPF/SCB) system Front View 0· 0-0 (N--+I--+N) a) Nematic __.,. Isotropic b) Isotropic ____.. Nematic

Cross Sectional 'p }( .,. ___i-31 .,. '.._p___ •'.._p ___ .,. 'p..__ __.,---..3/ View (N--+I) ,._____ ---JJ ------

3) (PVC/SCB), (PVA/SCB), (PVDF/SCB) and (Pis/SCB) systems

Front View (N--+I--+N) a) Nematic Isotropic b) Isotropic Nematic

'&.T__ '.:.p__ a_lli_.. .. J?a;. __ "i_-31 •'a:.f"__ \4_..31 .,. 'a:.r:__ \4---..31 Figure 6. Schematic representation of optical texture changes for the three LC cells during the nematic-isotropic phase transition during heating and cooling.

Table I. defined as the difference between dispersion component, I'd and hydrogen bonding com­ TN1(center) and TN1(edge) based on ponent, 'l'h of the surface free energy, respectively. For thickness dependence of TN1 at amorphous polymers such as Pdi-iPF, PCNEt-iPF, (each polymer/5CB) system PVC, and S-Pis, it is apparent from Figure 7 that ATN, I'd· Sample Sample linearly increases with No significant relationship between ATN, and 'l'h was detected for amorphous Pdi-iPF -0.003 S-PI(AL1051) 0.015 polymers as shown in Figure 8. The hydrophobic group PCNEt-iPF 0 PVC 0.017 or moiety in a polymer chain is preferentially oriented S-PI(DA2) 0.007 PVA 0.031 S-PI(DA1) 0.010 PVDF 0.035 toward the outside from the surface, while the hydrophilic S-PI(DA3) 0.011 T-PI 0.050 group is so innerwardly to minimize the interfacial free energy between LC molecules and polymer surface, especially in the case of amorphous polymers. In nematic nematic LC layer sandwiched between substrates which LC the magnitude of surface free energy of LC increases enhance the order of the LC molecular alignment. Figures with LC molecular alignment. 17 •18 This indicates that I'd 5, 6, and Table I thus indicate that the Pdi-iPF surface increases with the interfacial order of LC to minimize may reduce the interfacial alignment order of 5CB the difference of dispersion component in surface free molecules, while PVA, PVC, PVDF, and Pis surfaces energy between amorphous polymer and LC. LC molec­ enhance it. PCNEt-iPF is neutral with respect to en­ ular alignment at the (amorphous polymer/LC) inter­ hancement of the interfacial alignment order of 5CB. face, in other words, the intermolecular interaction be­ Table II shows the surface free energy of polymer tween polymer surface and LC molecules may thus be surfaces, I'd• 'l'h• and 'l's which correspond to a disper­ strongly dependent on the magnitude of dispersion sion component of surface free energy, a hydrogen component rather than hydrogen bonding component. bonding component and sum of I'd and 'l'h• respectively, The hydrophobic intermolecular interaction at the as well as the contact angle of 5CB with the polymer (polymerjLC) interface would be more responsible for film surface. Pdi-iPF, PCNEt-iPF, and PVDF had lower increase in the interfacial molecular alignment order of surface free energy in comparison with PVA, PVC, and LC. Pis. The surface free energy of 5CB is 32--40 mJ m- 2 , 15•16 There should thus be correlation between yd of polymer which is close to that of Pdi-iPF, PCNEt-iPF, S­ and contact angle of LC on the polymer surface. Figures PI(DA-2), and less than that of others except PVDF. 9 (a) and (b) show plots of I'd and 'l'h against the contact In the cases of amorphous polymers, ATNI of 5CB angle of 5CB with each polymer surface. From Figure contacted with the polymer surface with higher surface 9(a) the magnitude of contact angle of 5CB with each free energy was larger than for polymer surfaces with polymer surface decreases almost linearly with increase lower surface free energy as shown in Table I. 5CB in the dispersion component of surface free energy. molecules may thus align on the polymeric surface so as However, no systematic correlation between the hy­ to minimize interfacial free energy between LC and drogen bonding component of polymer surface free polymer surface. energy and contact angle was observed as shown in Figures 7 and 8 show plots of ATN, against the Figure 9(b). Interactions at the (crystalline polymerj5CB) 164 Polym. J., Vol. 31, No. 2, 1999 Polymer Surface-Induced Order of LC Molecular Alignment

Table II. Surface free energy, aggregation state of each polymer and contact angle of 5CB with the polymer surface

Surface free energy jmJ m- 2 Contact angle of 5CB Aggregation Sample with each polymer;o state '1/d "lh " Pdi-iPF 26.9 7.7 34.6 38 Amorphous PCNEt-iPF 30.9 8.2 39.1 27 Amorphous S-PI(DA-3) 33.6 18.5 52.1 18 Amorphous S-PI(DA-2) 34.1 3.0 37.1 17 Amorphous S-PI(DA-1) 37.6 11.0 48.6 7 Amorphous PVC 43.9 1.6 45.5 5 Amorphous S-PI(AL I 051) 44.0 7.6 51.6 2 Amorphous PVDF 23.2 7.1 30.3 14 Crystalline PVA 37.1 7.5 44.6 2 Crystalline T-PI 41.2 3.5 44.7 2 Crystalline

0.07 50 0 Crystalline polymer 0 Crystalline polymer 0.06 e Amorphous polymer e Amorphous polymer " 40 Pdi-iPF 0.05 0 Oil • PVDF T-PI ""'" 0.04 r:o 30 PCNEt-iPF 0 u z PO' V') E- 0.03 ..... •

Figure 7. Plots of L'lTNt against rd· (a)

0.06 50 .------,.------,.,,------., T-PI 0 Crystalline polymer 0 Crystalline polymer 0.05 0 e Amorphous polymet e Amorphous polymer 40 Pdi-iPF 0.04 PVDF Oil • ""'" 0.03 PCNEt-iPF z 0.02 PVC • E- S-Pl(ALI051)