Effect of spacer length on the thermal properties of side-chain liquid-crystal poly(methacrylate)s

Abstract

A series of side-chain liquid crystal polymers, the poly[ω-(4′-methoxybiphenyl-4-yloxy)alkyl methacrylate]s, have been synthesized in which the spacer length is varied from 3 to 12 methylene units. This is the first example of a poly(methacrylate)-based side-chain liquid crystal polymer series for which as many as 10 homologues have been prepared. The thermal properties have been characterised using differential scanning calorimetry and polarised light microscopy. All 10 homologues exhibit smectic behaviour; in addition, the butyl homologue is nematogenic. The clearing temperatures and associated entropy changes exhibit a distinct odd–even effect as the length and parity of the spacer is varied with the odd members exhibiting the higher values. This behaviour is rationalised in terms of the change in the average shape of the side chain on varying the parity of the spacer. The properties of the polymers are compared to those containing the same mesogenic group but different polymer backbones in order to consider the effect of flexibility on thermal behaviour. In general, these comparisons support the view that increasing backbone flexibility enhances the clearing transition while tending to decrease the entropy change associated with the transition. Exceptions to this rule are rationalised either in terms of molecular weight dependent effects or in the case of poly(norbornene)-based materials, are thought to arise from the complex microstructure of the polymer chains. Replacing the alkyl spacer by an oligo(ethylene oxide) chain reduces the clearing transition of the polymer. This is rationalised in terms of the flexibility and preferred conformations of an oligo(ethylene oxide) spacer.