A critical behavior of orientational elastic modulus of polymeric liquid crystal in the vicinity of the nematic-smectic phase transition

For the comb-shaped polymer containing mesogenic groups in the backbone and side chains, orientational elastic deformations in the vicinity of a nematic (N)-smectic A (S A ) phase transition were studied using the Freedericksz threshold method under the action of the magnetic field. On approaching the N-S A transition, the bend elastic constant K 3 dramatically increases, and this increase may be explained by the existence of the smectic-order fluctuations in the N-phase. In this case, the critical behavior is controlled by the molecular mass of the polymer. For the high-molecular-mass fractions, an increase in modulus K 3 qualitatively agrees with the theoretical predictions; however, the experimental values of critical index p are abnormally high. As molecular mass is decreased, the critical change in K 3 does not agree with the theory, whereas, for the lowest molecular mass fraction, all pretransition phenomena completely disappear. The effect of the polymer molecular mass and polydispersity on the character of the temperature dependence of K 3 in the vicinity of the N-S A transition is analyzed.