TENSILE DRAWING OF POLY(ARYL ETHER ETHER KETONE) .1. BIREFRINGENCE, INFRARED DICHROISM AND SHRINKAGE-STRESS MEASUREMENTS

Birefringence, infra-red dichroism and shrinkage-stress data of poly(a ryl ether ether ketone) (PEEK) uniaxially drawn above the glass transi tion have been analysed in the framework of the theory of rubber elast icity. It is shown that PEEK chains having a molecular weight M(n) alm ost-equal-to 13 000 only contain three entanglements per chain on aver age. The very low level of shrinkage stress of the samples having a dr aw ratio below the threshold of strain-induced crystallization is perf ectly consistent with a weakly entangled network. The birefringence ve rsus stress curve discloses an orientation-dominated regime at low str ains and a deformation-dominated regime starting from the onset of the strain-induced crystallization. The length of the random link, L = 1. 8 nm, is in very good agreement with the one that can be theoretically predicted for the stiff-chain polymer relatives, poly(ethylene tereph thalate), polycarbonate and poly(phenylene oxide). A value for the int rinsic birefringence of the amorphous phase, DELTAn(a)degrees = 0.38 /- 0.04, is also reported.