Study on the thermal behavior of a solution-cast liquid-crystalline polymer film by positron-annihilation lifetime spectroscopy

The thermal behavior of a solution-cast liquid-crystalline polymer (LCP) film was extensively studied by positron-annihilation lifetime spectroscopy (PALS). From the positronium (Ps) lifetimes of the first heating process from 40 to 250 °C at a heating rate of 2.5 °C/h, four characteristic temperatures (140, 170, 200, 235 °C) were observed. From a combined investigation with conventional differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA), the first three characteristic temperatures were found to correspond to reorientation, glass-transition, and softening temperatures, respectively. The fourth temperature was related to the commencement of crystallization, which was observed above about 235 °C from a decrease in the Ps lifetime. A low-temperature PALS experiment exhibited the γ-transition due to rotation of the phenyl moiety at about −53 °C.