Dielectric relaxation of liquid crystalline side-chain poly(vinyl ether)s U. W. Gedde*, F. Liut, A. Hult, F. Sahl6n and R. H. Boydt Department of Polymer Technology, Royal Institute of Technology, S- 100 44 Stockholm, Sweden tDepartment of Materials Science and Engineering, University of Utah, Salt Lake City, UT 84112, USA (Received 16 April 1993) Dielectric permittivity and loss have been measured over the frequency range 10-2 Hz-10 kHz between 100 K and 350 K for samples of two mesomorphic side-chain poly(vinyl ether)s, poly(4-(11-(vinyloxy)undecyloxy)- 4-ethoxyphenylbenzoate)(P-EtO) and poly(4-(11-(vinyloxy)undecyloxy)-4-cyanophenylbenzoate) (P-CN) of different degree of mesogen group orientation. X-ray scattering showed that P-EtO was in a semicrystalline state below 345 K, whereas P-CN displayed a tilted smectic structure at temperatures lower than 315 K. Four relaxation transitions were found in P-EtO: ~, the glass-rubber transition occurring at 290-300 K, and three subglass processes referred to as/~, 7 and 6. P-CN exhibited only three dielectric processes, ~,/~ and 7. The low temperature process, 6, was absent in P-CN and could be assigned to torsion about the pendent phenyl-carbon-ether-oxygen bond. Subglass processes/~ and 7 exhibited an Arrhenius temperature dependence with activation energies of 60-105 and 32+ 2 kJmol-1, respectively. The activation energy of the 7 process was insensitive to morphology and it was assigned to local motions in the spacer group. The activation energy of the/~ process varied considerably among the studied polymers with higher values for the highly ordered P-EtO than for P-CN. It is suggested that the/~ process leads to reorientation of the carboxylic group in the phenyl benzoate moiety. (Keywords: liquid crystalline polymers; poly(vinyl ether); mesogen group orientation) INTRODUCTION The dielectric relaxation of liquid crystalline side-chain polymers has recently received some attention 1-13. Interest in this group of polymers is motivated by their potential use in optronics and electronics. The focus of this paper is the dielectric relaxation in subglass processes of partially crystalline or smectic polymers showing smectic (SA) mesomorphism at higher temperatures. Data obtained by n.m.r. 14, small-angle neutron scattering 15 and thermal analysis 16 provide evidence in favour of a two-phase morphology. The smectic layers with the mesogens, which at lower temperatures may crystallize, organize in stacks together with the more disordered domains essentially consisting of the backbone chains sandwiched between the crystalline lamellae. The spacer group decouples the main chain from the mesogen and allows it to attain an anisotropic 'random' chain conformation 15. Evidence for limited interpenetrating flexible-chain and mesogenic components has been presented15,17. The molar mass dependence ofisotropiza- tion of different side-chain oligomers and polymers was found to depend on the chain structure 17. It was shown that the order within the smectic layers and the penetration by the flexible backbone chains of the smectic layers was not appreciably affected by changes in molar mass in polysiloxanes or in polymethacrylates whereas in poly(vinyl ether)s a reduction in order of the smectic layers was recorded for the high molar mass systems 17. * To whom correspondenceshould be addressed 0032-3861/94/10/2056~07 © 1994 Butterworth-HeinemannLtd 2056 POLYMER Volume 35 Number 10 1994 Five dielectric relaxation processes were found in a series of isotropic side-chain phenylbenzoate-containing polyacrylates and polymethacrylates, oligomethylene spacers with two to six carbons, and methoxy, n-butoxy and cyano end groups4: a high temperature relaxation (6) with an activation energy (AE) of 150kJmo1-1, a glass-rubber transition (~) exhibiting Williams-Landel- Ferry (WLF) temperature dependence and three sub- glass processes referred to as fl (AE=50kJmol-X), 71 (AE=35kJmo1-1) and 72 (AE=24kJmol-1) • It was proposed 4 that the subglass processes were due to reorientation of the ester group of the mesogen (fl), motions within the oligomethylene group causing re- orientation of the surrounding ether dipoles (71) and torsion about the bond between the outer phenylene carbon and the end group (72). These findings were further substantiated in later work 1° on combined main-chain and side-chain polymers. Pranoto et al. 5 studied isotropic and oriented samples of polysiloxanes with phenylbenzoate as mesogen, a six carbon methylene spacer, and methoxy, cyano or chlorine end groups and observed that the fl process (AE=51 kJmol-t), ascribed by the authors to reorien- tation of the ester dipoles, was strongest for the samples in which the mesogens were oriented perpendicular to the direction of the applied electric field. The fl process was broad and followed the symmetric Cole-Cole equation. The glass-rubber transition (~) and the relaxation appearing at high temperatures was also reported 5. The extensive work of Williams et al. TM 1,12 has shed more light on the ~ and 6 processes.