Phase behaviors of side-chain liquid crystalline polyacetylenes with different length of spacer: Where will the decoupling effect appear?

Phase structures and transitions of a series of biphenyl containing side-chain liquid crystalline polyacetylenes (SCLCPAs) with different length 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,5^L (m = 3, 9), m is the number of methylene units in the spacer], were studied by differential scanning calorimeter, polarized light microscopy, one- and two-dimensional wide-angle X-ray diffraction. It is revealed that phase structures of the samples are strongly dependent on the lengths of spacer and alkyl tail. The samples with short spacer (m ≤ 4) render the molecular shape of sheet-like due to the strong coupling between the backbone and the biphenyl groups on side-chains. They form highly ordered smectic phase with an additional ordering on the subnanometer scale (SmX^o) at low temperatures. For m > 5, P-m,7 exhibits the low-temperature phase of smectic B (SmB), while P-9,5 and P-9,5^L form smectic E (SmE). As the long spacers adopt more gauche conformations to satisfy the ordered packing of biphenyl groups, the coupling of main- and side-chain is relieved. Among the samples studied, P-5,7 is unique, which can retain the sheet-like shape but form the SmB structure based on a fully interdigitated packing manner. Therefore, the spacer length of m = 5 sets the boundary between coupling and decoupling of the backbone and side-chains. Phase transition sequences of the SCLCPAs studied are also found to be influenced clearly by varying the spacer and tail lengths.