Dynamic Viscoelasticity of Thermotropic Liquid Crystalline Cellulose Derivatives

Abstract

Dynamic measurements were made on two thermotropic liquid crystalline cellulose derivatives abbreviated as HEC-C5 and TBC. HEC-C5 is a hydroxyethyl cellulose (HEC) fully acylated with pentanoic acid. Since the chemical structure of the HEC is disordered (DS=1.5 and MS=3.0), that of the tripentanoate also becomes disordered. TBC is a tri-O-β-butoxyethyl cellulose, fully etherified cellulose (DS=MS=3.0) having an ordered structure. In both samples, the temperature dependence as well as the frequency dependence of the dynamic viscoelastic functions clearly and unambiguously reflected the three states of the polymers, i.e., an anisotropic liquid, isotropic liquid, and anisotropic/isotropic co-existing state. TBC was found to undergo an anisotropic-to-isotropic transition in a very narrow temperature span (less than 10°C), the dynamic viscosity η′ increasing by a factor about 10 in this temperature span. Moreover, the temperature dependence of η′ of TBC measured at a constant frequency showed two characteristic temperatures, other than the anisotropic-to-isotropic transition temperature, which indicated certain structural changes in this polymer in the liquid crystalline state. No such detailed information was obtained from the η′-temperature diagram of HEC-C5, which showed only a very broad anisotropic/isotropic co-existing region extending from about 90°C to 145°C. These marked differences in the transition and other behavior between the two polymers are believed to originate from their difference in chemical structural order.