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-metho xybiphenyl-4'-yloxy)-alkyl acrylate]s, has been synthesized in which the sp acer length is varied from 3 to 12 methylene units. The thermal behavior of the polymers has been characterized using differential scanning calorimetr y and polarized light microscopy. All 10 homologues exhibit thermotropic li quid crystalline behavior. The glass transition temperatures decrease initi ally before reaching a limiting value as the spacer length is increased. Th is is attributed to a plasticisation of the backbone by the side groups. A small odd-even effect in the clearing temperatures is observed on increasin g the spacer length in which the odd members display the higher values. The properties of these polymers are compared to the analogous polymethacrylat e-based materials and also to the corresponding polymers containing 4-cyano biphenyl as the mesogenic unit. This reveals that the structure of the liqu id-crystal phase influences the glass transition temperature. Thus, the 4-m ethoxybiphenyl-containing polymers exhibit the higher glass transition temp eratures reflecting the higher packing density observed in the smectic phas es exhibited by these polymers when compared to the interdigitated phases e xhibited by 4-cyanobiphenyl-containing polymers. A comparison of the cleari ng temperatures reveals that the expected trend in which the more flexible backbones exhibit the higher clearing temperatures is not found. It is sugg ested that for the 4-methoxybiphenyl-based polyacrylates the clearing trans itions may be entropically driven whereas for interdigitated phase structur es the clearing transitions are driven by the interaction between the 4-cya nobiphenyl units.