The detection of conformational disorder by thermal analysis

Conformational disorder in crystals is found in many molecules that possess a plurality of conformational isomers. Typical examples are linear macromolecules such as polyethylene, polytetrafluoroethylene and trans-1,4-polybutadiene; and small molecules such as paraffins, cycloparaffins, soaps, lipids and many liquid-crystal forming molecules. Conformational motion is often coupled with the cooperative creation of disorder. In this case a heat and entropy of transition is observed by thermal analysis. Levels of transition entropies can be estimated, assuming most of the disorder can be traced to conformational isomerism. In case there is conformational disorder frozen-in at low temperature, thermal analysis can be used to find the glass transition of a condis crystal. An Advanced Thermal Analysis System has been developed, and will be described that permits a detailed interpretation of the thermal analysis traces. It rests with the establishment of high quality heat capacity for the rigid solid state (vibration only) and the mobile liquid state (vibrations and large amplitude cooperative motion). THE TRANSITIONS BETWEEN MESOPHASES, CRYSTALS AND GLASSES In a recent review of thermotropic mesophases and their transitions (ref. 1) it was suggested that crystals with dynamic conformational disorder do not fit into the standard classification of mesophases as either plastic crystals (ref. 2) or liquid crystals (ref. 3). At that time it was proposed that a distinct third type of mesophase exists, that of the condis crystal (with dynamic -formational -order and long-range positional and orientational order). Figure 1 illustrates schematically the various condensed phases and R A S T l C CRYSTAL U D LIQUID CRYSTAL S Fig. 1. Schematic digram of the relationship between the three limiting phases (in double outline) and the six mesophases. their interrelations. The three classical condensed phases: glass, crystal and melt, are outlined doubly. Glasses and crystals are solids, the melt is a liquid. Condis crystal, plastic crystal, and liquid crystal are increasingly less solid (more liquid). The arrows on the right-hand side indicate the possible transitions between the phases that are often first order transitions, i.e. transitions that involve a discontinuity in the first derivative of the Gibbs energy function, the entropy (ref. 4).