A transient mesophase on drawing polymers based on polyethylene terephthalate (PET) and polyethylene naphthoate (PEN)

This paper is centred around the identification of a transient mesophase during the drawing of PET and PEN and their associated random copolymers. While the observation of the mesophase has previously been announced as a note (Macromolecules, 31, 1998, p. 7562), this paper sets out the work in appropriate detail. Samples across the full random copolymer composition range are hot drawn, and, either immediately quenched or examined in-situ using synchrotron radiation. In each case a mesophase has been observed under drawing conditions which produced little or no crystallinity. That the phase was not the product of the quench was confirmed by the high temperature dynamic measurements. The signature of the mesophase is a sharp, meridional peak in the WAXS fibre diagram which shows no other sharp peaks suggesting crystallinity; the equatorial maxima, while equiaxial and indicating a high level of chain orientation, are diffuse. The mesophase is thus classified as smectic A. SAXS measurements on quenched samples, give no indication that the mesophase is associated with a particular microstructure. The mesophase is seen across the PET/PEN composition range, its observation being more straightforward in the random copolymers where the rate of crystallisation is significantly reduced. Subsequent crystallisation of the fibres by suitable annealing, replaces the meridional mesophase peak on the first layer line with off-meridional crystal reflections characteristic of the triclinic unit cell. The layer line is at a slightly lower angle after crystallisation indicating a longer axial repeat in the crystal than in the mesophase. The development of crystallinity gives rise to a distinct SAXS pattern indicative of a two phase microstructure. The transition from smectic A mesophase to triclinic crystal provides a rationale for the occurrence of oblique lamellae in PET, and associated random copolymers with more than 70% ethylene terephthalate units, as indicated by the four point SAXS patterns, and also for the c axis tilt observed in the WAXS patterns. Similarly, for the PEN rich copolymers, the absence of obliquity and consequent c-axis tilt, coupled with the marked layer line streaking of the triclinic reflections, can be rationalised in terms of the c-shear of the transition being compensated by faulting in the crystal.