L iquid C rystals, 1997 , V ol. 22 , N o. 6 , 669±677 Eect of a lateral substituent on the mesomorphic properties of ferroelectric side chain liquid crystalline polysiloxanes by CHAIN-SHU HSU* and CHIA-HSUN TSAI Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30050, Republic of China ( Received 26 August 1996; in ®nal form 23 January 1997; accepted 31 January 1997 ) The synthesis of side chain liquid crystalline polysiloxanes containing oligooxyethylene spacers and (S )-2-methylbutyl 4-[(4-oxybiphenyl-4¾-yl)carbonyloxy]-3-¯uorobenzoate mesogenic side groups is presented. Dierential scanning calorimetry, optical polarizing microscopy and X-ray diraction measurements reveal liquid crystalline properties for all synthesized mon- omers and polymers. All three precursor ole®nic monomers reveal cholesteric and smectic A phases. The ole®nic monomer which contains two oligooxyethylene units in the spacer is the only one which reveals a twist grain boundary A phase and a blue phase, besides the cholesteric and smectic A phases. All three polysiloxanes present enantiotropic smectic A and chiral smectic C phases. The mesomorphic behaviours of the monomers and polymers are compared with those of the corresponding monomers and polymers without the lateral ¯uoro substituent. The results seem to demonstrate that incorporating a lateral ¯uoro substituent in the mesogenic cores of the monomers aects not only the mesophase thermal stability, but also the nature of the mesophases formed. However, incorporating a lateral ¯uoro substituent in the mesogenic cores of the polymers aects only the thermal stability of the mesophases formed. The lateral ¯uoro substituent has a more profound eect on the mesomorphic behaviour for the monomers than that for the polymers. 1. Introduction ethylene spacer can dramatically increase the thermal The potential application of ferroelectric liquid crys- stability of the chiral smectic C phase. tals (FLCs) in fast switching, high resolution electro- Investigations of the eect of lateral substituents on optical devices is well documented [1±3]. Besides low liquid crystalline behaviour may have several objectives, molar mass FLCs, a number of ferroelectric liquid such as the desire to determine to what extent clearing crystalline side chain polymers have been successfully points and the relative stabilities of dierent mesophases prepared during the past few years [4±27]. These poly- are aected by the size and type of lateral substituents mers are interesting because of their ferroelectric proper- or to ®nd out how physical properties, e.g. dielectric ties and this ®eld has been reviewed by LeBarney and anisotropy, elastic constants and viscosity are modi®ed. Dubois [28]. However the detailed structure±property The eect of molecular structure on the occurrence of relationships of ferroelectric liquid crystalline polymers tilted smectic C phases is very dierent from that on (LCPs) has not been clear until now. It seems necessary nematic or orthogonal smectic phases. The molecules to introduce additional functional groups into the meso- should possess some degree of steric asymmetry, so that genic units, spacers and polymer backbones of side chain they can arrange themselves in a tilted layer when liquid crystalline polymers to create some new property packed together. The in¯uence of a lateral substituent combinations in order to achieve these goals. on the formation of a chiral smectic C phase for low In our previous reports [29±31], the eects of polymer molar mass liquid crystals has been studied by Coates backbone and spacer on the formation and thermal et al . [32]. Their experimental results demonstrated that stability of the chiral smectic C phase for ferroelectric incorporation of a lateral substituent in the mesogenic side chain LCPs have been extensively studied. The core of a molecule reduces the occurrence of higher results demonstrate that the tendency towards a chiral ordered smectic phases, and also decreases their melting smectic C phase increases with increasing spacer length point in most cases. According to some other reports and ¯exibility of the polymer backbone, and the oligoox- on low molar mass liquid crystals in the literature [33, 34], introducing a suitable lateral substituent, e.g. ¯uoro, into the mesogenic cores can enhance the formation of a tilted smectic C phase. *Author for correspondence. 0267±8292/97 $12´00 Ñ 1997 Taylor & Francis Ltd.