Features of critical behavior of bend elastic constants of polymer liquid crystal near nematic-smectic A phase transition point

摘要

The temperature dependences of the elastic splay K$-1$/ and bend K$-3$/ constants and birefringence $Delta@n of a comblike combined side- and main-chain polymer near nematic (N) - smectic A (S$-A$/) phase transition were investigated using the method of the Freedericksz threshold transitions. Experiments were performed on a series of fractions in the range of molecular weight from 900 to 10200. The splay constant K$-1$/ and birefringence $Delta@n have practically normal `nematic' temperature dependence in the investigated temperature range. For some fractions near the N$ARLR@S$-A$/ transition, the bend constant K$-3$/ exhibits a rapid increase with decreasing temperature, which is explained by the fluctuations of smectic order in the nematic phase. For high molecular weight fractions, the fluctuation-dependent part of K$-3$/ varies as (T -T$-N-SmA$/)$+$MIN$rho$/ according to theoretical predictions. Hence, the values of critical exponent $rho are very large (1.1 - 1.4). For low molecular weight fractions, no agreement is observed between the results obtained and theoretical predictions. The features of critical behavior of bend elastic constants of liquid crystal investigated may be explained by the molecular weight polydispersity of polymer fractions.