Glass transition characteristics of poly(aryl ether ketone ketone) and its copolymers

The glass transition characteristics of poly(aryl ether ketone ketone) (PEKK) have been investigated as a function of backbone structure and crystallization history; PEKK 100/0 homopolymer and PEKK 70/30 and 60/40 copolymers were examined, where the numbers represent the terephthalic/isophthalic (T/I) ratio. For the all-para-connected homopolymer, the presence of crystallinity had a significant influence on the calorimetric glass transition properties of the amorphous phase: T_g was offset by as much as 20°C in the crystallized amples as compared to the wholly amorphous material, and a sizeable (W_RAP ∼ 0.30) rigid-amorphous-phase fraction was observed. In the copolymers, crystallinity had only a very minor effect on T_g. The rigid-amorphous-phase fraction in the copolymer samples was smaller as compared to the homopolymer, and was negligible (W_RAP → 0) for samples prepared under less-restrictive crystallization conditions. Dielectric studies indicated progressive mobilization of the rigid-amorphous-phase fraction above T_g or the copolymer samples, with full mobilization of the on-crystalline fraction observed for those samples crystallized at the highest temperatures. These results show that the introduction of 1,3-connected isophthalate moieties in the PEKK copolymers has a disrupting influence on the persistence of crystalline constraint into the amorphous phase. Similar observations have been reported for poly(phenylene sulfide) copolymers and thermoplastic polyimides that incorporate meta-phenylene linkages in the chain backbone.