Twist phase transition in poly(p-phenylene) oligomers: Heat capacity of p-quinquephenyl

The heat capacity of crystalline p-quinquephenyl was measured by adiabatic calorimetry below room temperature. A broad hump was detected around 264 K and attributed to a phase transition. through which an average molecular co nformation changes from the planar to the twisted one on cooling. A similar anomaly was also detected for crystalline p-sexiphenyl around 295 K by cho pped-light ac calorimetry. The broad shape of the thermal anomalies arising from the twist transitions are discussed in relation to the number of poss ible intramolecular twisting modes. Properties of the twist transition in p olymer are discussed on the basis of those of oligomers. Standard thermodyn amic functions are tabulated for p-quinquephenyl.