Investigation of mesophase transitions in liquid crystals using inverse gas chromatography

Abstract

Inverse Gas Chromatography, IGC, has been used to study phase transitions in three low molar mass liquid crystals and one side-chain liquid crystalline polymer. This extension of previous work has shown that transitions between all of the various mesophases can be detected and their temperatures measured with an accuracy and precision comparable to that of more conventional techniques. For one of the compounds, 4,4'-(n-hexyloxy)cyanobiphenyl, the kinetics of solidification from the supercooled nematic phase have been measured. The process took place in two stages which could be interpreted in terms of a modified version of the Avrami model of polymer crystallization. An explanation of the results is that the solid support is providing a large number of nucleation sites for the initial part of the solidification, which is completed in a slow, diffusionlimited process. However, further work is required to confirm this mechanism.Key words: inverse gas chromatography (IGC), liquid crystals, mesophase transitions, activity coefficients, retention volumes.Resume : On a utilisC la chromatographie en phase gazeuse inverse (IGC) pour Ctudier les transitions de phase de trois cristaux liquides de faibles masses molaires et d'un polymkre cristal-liquide avec chai'ne latCrale. Cette extension du travail prCcCdent a permis de dCmontrer que l'on peut dCtecter toutes les transitions entre les diverses mCsophases et de mesurer leurs tempkratures avec une exactitude et une prkcision comparables i celles des techniques plus conventionnelles. Pour l'un des composCs, le 4,4'-(n-hexyloxy)cyanobiphCnyle, on a mesurC la cinCtique de la solidification B partir de la phase nCmatique surrefroidie. Le processus se produit en deux Ctapes que l'on peut interpreter en fonction d'une version modifiCe du modkle d'Avrami de la cristallisation d'un polymkre. On peut expliquer les rksultats en supposant que le support solide fournit un grand nombre de sites de nuclCation pour la partie initiale de la solidification que est complCtCe par un processus lent, B diffusion limitCe. Toutefois, des travaux ultCrieurs seront nicessaires pour confirmer ce micanisme.

Applications of inverse gas chromatography in the study of liquid crystalline stationary phases

“…Phase transition behaviour One of the most common methods for determining the phase transition is differential scanning calorimetry, DSC. The DSC measurements of the transition temperatures for HCB, OCB and PDCBBS have been described previously 30 . Those for MBHPT and PMMBTPS showed similar properties.…”

“…The probe-to-probe reproducibility was good. The mass fraction crystallinity and its time variation can be used to investigate the mechanism of crystallisation and the method has been published previously 30 .…”

Chromatographic investigation of the effect of dissolved carbon dioxide on the glass transition temperature of a polymer and the solubility of a third component (additive)

“…This model of absorption at high temperatures and adsorption at low temperatures was first postulated by Guillet, 21 and has proven to be adequate for many polymeric systems. 22,23 However, this model has sometimes been questioned in the literature; a few investigators have suggested that the characteristic S-shaped van't Hoff plots were a At a pressure of 75 atm, a small change in temperature at temperatures less than 30°C causes a large change in the solubility of both CO 2 and isooctane in PMMA. In a chromatographic experiment, this leads to long retention times of the probes; thus, experiments were not carried out at these lower temperatures.…”

Inverse gas chromatographic measurement of solubility parameters in liquid crystalline systems

“…The methods and techniques used were entirely standard for thermodynamic measurements at infinite dilution by IGC. Full details have been published previously [22,23].…”