Effect of Spacer Length on the Thermal Properties of Side-Chain Liquid Crystal Polymethacrylates. 2.^† Synthesis and Characterization of the Poly[ω-(4′-cyanobiphenyl-4-yloxy)alkyl methacrylatejs

A new homologous series of side-chain liquid crystal polymers, the poly[ω-(4′-cyanobiphenyl-4-yloxy)alkyl methacrylate]s, has been synthesized in which the spacer length is varied from 3 to 12 methylene units. The thermal behavior of the polymers has been characterized using differential scanning calorimetry and polarized light microscopy. The propyl homologue is amorphous while the remaining members of the series all exhibit liquid crystallinity, specifically smectic behavior. The glass transition temperatures decrease with a small odd-even effect before reaching a limiting value as the length of the spacer is increased. The clearing temperatures exhibit a more pronounced odd-even effect on increasing the spacer length. In both alternations the odd members exhibit the higher values. The molecular significance of the odd-even effect exhibited by the glass transition temperatures was unclear while that shown by the clearing temperatures was rationalized in terms of the change in the average shape of the side chains as the parity of the spacer is varied. The properties of this cyanobiphenyl-based series are compared to those of the analogous materials containing methoxybiphenyl, and differences are accounted for in terms of the local side-chain packing within the mesophase. The influence of backbone flexibility on liquid crystal properties of side-chain polymers is also considered, and this polymer series tends to support the view that increasing backbone flexibility increases the clearing temperature while decreasing the entropy change associated with the transition.