Glass transition and crystal melting in semicrystalline polyimides

Abstract

AbstractQuantitative thermal analysis of a family of semicrystalline polyimides containing from one to three ethylene glycol spacer units has been conducted. Special emphasis has been placed upon the changes in the thermodynamic properties resulting from the successive additions of ethylene glycol units. The solid and liquid heat capacities were measured in the temperature range 230–640 K. The solid heat capacities were also calculated from the vibration spectra of the polymers. An in‐depth description of the glass transitions, the heat capacity increases at the glass transition temperatures, and the widths of the glass transitions is provided. Three parts of the heat of fusion have been identified: Wc (H), that contributed from the high melting peak; wc (L), that contributed from the low melting peak; and wc (C), that developed during cooling after isothermal crystallization. The metastability, sequence of crystallization, and rate of reorganization of the low and high melting peaks have been investigated by stepwise crystallization and by analysis using different heating rates after isothermal crystallization. A rigid amorphous fraction is needed to explain the failure of the two‐phase (crystallinity) model.