Phase Behaviors of Side-Chain Liquid Crystalline Polyacetyleneswith Different Length of Spacer: Where Will the Decoupling EffectAppear?

Phase structures and transitionsof a series of biphenyl containingside-chain liquid crystalline polyacetylenes (SCLCPAs) with differentlength of spacer and alkyl tail, poly((m + 2)-{[(4′-alkyloxy-4-biphenylyl)carbonyl]oxy}-1-yne)[P-m,7 (m= 4, 5, 8), P-m,5 (m= 9), P-m,5L(m= 3, 9), mis the numberof methylene units in the spacer], were studied by differential scanningcalorimeter, polarized light microscopy, one- and two-dimensionalwide-angle X-ray diffraction. It is revealed that phase structuresof the samples are strongly dependent on the lengths of spacer andalkyl tail. The samples with short spacer (m≤4) render the molecular shape of sheet-like due to the strong couplingbetween the backbone and the biphenyl groups on side-chains. Theyform highly ordered smectic phase with an additional ordering on thesubnanometer scale (SmXo) at low temperatures. For m> 5, P-m,7 exhibits the low-temperaturephase of smectic B (SmB), while P-9,5 and P-9,5Lform smecticE (SmE). As the long spacers adopt more gaucheconformationsto satisfy the ordered packing of biphenyl groups, the coupling ofmain- and side-chain is relieved. Among the samples studied, P-5,7is unique, which can retain the sheet-like shape but form the SmBstructure based on a fully interdigitated packing manner. Therefore,the spacer length of m= 5 sets the boundary betweencoupling and decoupling of the backbone and side-chains. Phase transitionsequences of the SCLCPAs studied are also found to be influenced clearlyby varying the spacer and tail lengths. [ABSTRACT FROM AUTHOR]