Irradiation and the glass transition in PEEK

The influence of electron irradiation on the glass transition temperature, Tg, of poly(ether ether ketone) (PEEK) has been investigated by differentia l scanning calorimetry, up to a dose of 100 MGy. For amorphous PEEK, the ob served Tg increases linearly with absorbed dose at a rate of 0.18 degreesC MGy(-1). This indicates the formation of crosslinks, as deduced elsewhere. Above similar to 50 MGy, these crosslinks prevent crystallization on heatin g to above Tg, whereas at lower doses, the polymer is able to crystallize t o some degree. Qualitatively, the variations in Tg seen in these partially crystalline samples can be explained in terms of a number of factors. The c rystals serve to constrain the amorphous fraction, resulting in a direct el evation in Tg. This is reinforced through the rejection of crosslinks from crystalline regions, so resulting in an increase in the local amorphous cro sslink density. Conversely, crosslinking can also serve to inhibit crystall ization. Quantitatively, models based solely upon the percentage crystallin ity are, however, unable to account fully for the variations in Tg that are seen. In contrast, a model based on both overall crystallinity and lamella r thickness gives good agreement with experiment over the complete crystall inity range. (C) 2001 Elsevier Science Ltd. All rights reserved.