Volume transition of nematic gels in nematogenic solvents

Abstract

Equilibrium swelling and phase behavior of liquid crystalline (LC) networks swollen in miscible nematogenic solvents has been investigated by polarizing microscopy as a function of temperature. Four systems, i.e., each of two different LC networks comprising dissimilar mesogens in two different nematic solvents, exhibit essentially the same swelling and phase characteristics. The swelling characteristics strongly correlate with the phases of the LC molecules inside and outside the gel. The two independent nematic-isotropic transition temperatures for the gel (TNIG) and the surrounding pure solvent (TNIS; TNIG>TNIS for all the systems examined) yield three characteristic temperature regions. In the totally isotropic and nematic phases (T>TNIG and T<TNIS, respectively), the degree of equilibrium swelling (Q) is almost independent of T, and the magnitudes of Q in these phases are comparable. Meanwhile, Q strongly depends on T in the region TNIS<T<TNIG where the LC phases inside and outside the gel are different, i.e., nematic and isotropic, respectively. Upon cooling down to TNIG from the totally isotropic phase, the nematic network and the miscible nematic solvent inside the gel form a single nematic phase, which causes a discontinuous decrease in gel volume: The swollen isotropic gel is discontinuously transformed into the shrunken nematic gel. As temperature decreases further, Q of the shrunken nematic gel surrounded by the isotropic LC solvent increases again, and Q at T≈TNIS reaches almost the same magnitude as that in the totally isotropic phase. The nematic ordering of the surrounding pure solvent takes place at TNIS, which yields an inflection of the Q-T curve without discontinuity. A mean field theory for nematic gel successfully describes the swelling and phase behavior observed. The theory also demonstrates that the nematic-isotropic transition of gel drives a volume transition; an increase in nematic order inside gel induces a further swelling of nematic gel surrounded by isotropic LC solvent.