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Liquid Crystals
Year: 2008  |  Volume: 35  |  Issue: 9  |  Page No.: 1061 - 1071

Calorimetric study of the isotropic to nematic phase transition in an aligned liquid crystal nano-colloidal gel

F. Crucenau, D. Liang, R. L. Leheny and G. S. Iannacchione    

Abstract: A high-resolution calorimetric study of the specific heat (Cp ) has been carried out for the isotropic to nematic phase transition in an aligned liquid crystal (octylcyanobiphenyl - 8CB) and aerosil nano-colloid gel. A stable alignment was achieved by repeated thermal cycling of the samples in the presence of a strong uniform magnetic field, which introduces anisotropy to the quenched random disorder of the silica gel. In general, the specific heat features of the I-N transition in aligned (anisotropic) gel samples are consistent with those seen in random (isotropic) gel samples, namely the observance of two Cp peaks and non-monotonic transition temperature shifts with increasing silica concentration. However, larger transition temperature shifts with silica density, modification of the phase conversion process in the two-phase coexistence region, and a larger effective transition enthalpy are observed for the aligned samples. The lower-temperature aligned Cp peak is larger and broader while exhibiting less dispersion than the equivalent peak for the random gel. This may be a consequence of the alignment altering the evolution from random-dilution-dominated to random-field-dominated effects. The exact origin of the larger transition temperature shifts is uncertain but the larger enthalpy suggests that the nematic state is different in the aligned system than in random gels. The general non-monotonic behaviour of the transition temperature is interpreted using dimensional analysis as a combination of an effective elastic stiffening of the liquid crystal combined with a liquid crystal and aerosil surface interaction energy.

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