Effect of backbone flexibility on the thermal properties of side-group liquid-crystal polymers

A new series of side-group liquid-crystal polymers, the poly[omega-(4-methoxybiphenyl-4'-yloxy)-alkyl acrylate]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. All 10 homologues exhibit thermotropic liquid crystalline behavior. The glass transition temperatures decrease initially before reaching a limiting value as the spacer length is increased. This is attributed to a plasticisation of the backbone by the side groups. A small odd-even effect in the clearing temperatures is observed on increasing the spacer length in which the odd members display the higher values. The properties of these polymers are compared to the analogous polymethacrylate-based materials and also to the corresponding polymers containing 4-cyanobiphenyl as the mesogenic unit. This reveals that the structure of the liquid-crystal phase influences the glass transition temperature. Thus, the 4-methoxybiphenyl-containing polymers exhibit the higher glass transition temperatures reflecting the higher packing density observed in the smectic phases exhibited by these polymers when compared to the interdigitated phases exhibited by 4-cyanobiphenyl-containing polymers. A comparison of the clearing temperatures reveals that the expected trend in which the more flexible backbones exhibit the higher clearing temperatures is not found. It is suggested that for the 4-methoxybiphenyl-based polyacrylates the clearing transitions may be entropically driven whereas for interdigitated phase structures the clearing transitions are driven by the interaction between the 4-cyanobiphenyl units.