DIELECTRIC AND CALORIMETRIC STUDY OF PHASE-TRANSITIONS IN MIXED DIACETYLENE SINGLE-CRYSTALS POLYMERIZING IN THE SOLID-STATE

Dielectric and calorimetric experiments were performed on a series of pTS-pFBS mixed diacetylene single crystals. The electric permittivity measurements were carried out in the direction of growing polymer chains in the temperature range 80-333 K, as a function of the degree of polymerization of the samples. The specific heat measurements were performed on monomeric and fully polymerized samples in the temperature range 120-220 K. Phase transitions, already described in the literature for pure pTS crystals, were found to shift toward lower temperatures upon increasing the pFBS content in mixed crystals. In pFBS-rich and pure pFBS crystals, no evidence of any phase transition was found. A qualitative model employing an approach based on the Ising model was put forward to explain the observed shifts of the transition temperatures. A difference in the magnitude of the dipole moments associated with the polar side groups of pTS and pFBS diacetylene molecules is thought to be the main factor responsible for this phenomenon.