Vitrification/devitrification phenomena during isothermal and nonisothermal crystallization of poly(aryl-ether-ether-ketone) (PEEK) and PEEK/poly(ether-imide) blends

Abstract

We have established time-temperature transformation and continuou-sheating transformation diagrams for poly( ether-ether-ketone) (PEEK) and PEEK/poly(ether-imide) (PEI) blends, in order to analyze the effects of relaxation control on crystallization. Similar diagrams are widely used in the field of thermosetting resins. Upon crystallization, the glass transition temperature (T-g) of PEEK and PEEK/PEI blends is found to increase significantly. In the case of PEEK, the shift of the alpha-relaxation is due to the progressive constraining of amorphous regions by nearby crystals. This phenomenon results in the isothermal vitrification of PEEK during its latest crystallization stages for crystallization temperatures near the initial T-g of PEEK. However, vitrification/devitrification effects are found to be of minor importance for anisothermal crystallization, above 0.1 degrees C/min heating rate. In the case of PEEK/PEI blends, amorphous regions are progressively enriched in PEI upon PEEK crystallization. This promotes a shift of the alpha-relaxation of these regions to higher temperatures, with a consequent vitrification of the material when crystallized below the T-g of PEI. The data obtained for the blends in anisothermal regimes allow one to detect a region in the (temperature/heating rate) plane where crystallization proceeds in the continuously close proximity of the glass transition (dynamic vitrification). These experimental findings are in agreement with simple simulations based on a modified Avrami model coupled with the Fox equation. (C) 1998 John Wiley & Sons, Inc.