Structure and thermal history dependent enthalpy relaxation at the glass transition of semicrystalline polyimides

Abstract

Quantitative differential scanning calorimetry (d.s.c.) measurements were conducted to assess the enthalpy relaxation behaviour of a series of semicrystalline polyimides with different chain flexibilities. The polymers consisted of a rigid group (the imide and aromatic rings, 4,4′-oxydiphthalic anhydride, (ODPA)) and a flexible linkage (ethylene glycol sequence). The polymers are designated as ODPA( n = x) where x is the number of flexible ethylene glycol repeating units ( n = 1, 2 and 3). Non-isothermal experiments at different cooling and heating rates lead to hysteresis effects. Isothermal experiments in which the ODPAs were held for various times at three sub- T g annealing temperatures (undercoolings) show physical ageing behaviour. The results are analysed based on the multi-order parameter model, which assumes thermorheological simplicity, a distribution of relaxation times, and structure and temperature dependent relaxation times. The apparent activation energies ( Δh) of these ODPA polyimides were obtained through non-isothermal experiments, and they are assumed to be independent of thermal history. The parameter that characterizes the distribution of relaxation times (β) is, however, dependent on annealing temperature in the isothermal experiments. The parameter ( x) used to fractionize the temperature and structure dependency of the relaxation time shows for each polymer only very minor changes with annealing temperature. The variation of these parameters with chain flexibility is also discussed.