Isotropization of nematic liquid crystals by TMDSC

Temperature-modulated differential scanning calorimetry (TMDSC) and traditional DSC are used to study the transition between the nematic liquid crystalline state and the isotropic liquid for two small molecules [4,4'-azoxyanisole and N,N'-bis(4-n-octyloxybenzal)-1,4-phenylenediamine] and one macromolecule (4,4'-dihydroxy-alpha-methylstilbene copolymerized with a 1 : 1 molar mixture of 1,7-dibromoheptane and 1,9-dibromononane). The DSC measurements with 4,4'-azoxyanisole were used for temperature calibration with varying heating and cooling rates. The breadths of the transitions were analyzed with quasi-isothermal TMDSC, using a small temperature-modulation amplitude and standard TMDSC with underlying heating and cooling rates. It could be verified that the isotropization transition of a nematic liquid crystal is, indeed, reversible for all three molecules. The nature of the transition changes, however, from relatively sharp for small and rigid molecules, to about 3 K wide for the small molecule with flexible ends, and to as broad as 20 K for the macromolecule. It was also demonstrated that quantitative heats of fusion of sharp transitions can be extracted from TMDSC, but only from the heat-flow signal in the time domain. (C) 1998 Elsevier Science B.V. All rights reserved.