Shrinkage behaviour of uniaxially drawn poly(ethylene 2,6-naphthalate) films

摘要

Amorphous films of poly(ethylene 2,6-naphthalate) (PEN) were drawn isothermally at 145 deg C up to the desired draw ratios mainly to study structure formation during uniaxial drawing by differential scanning calorimetry and to characterise the shrinkage behaviour of the drawn films with or without heat treatment. During drawing, a rigid phase structure is induced and the amount of induced rigid phase structure is linearly related to th square root of the extra first strain invariant. The stress-strain curves are characterised by a necking behaviour and the end of the yielding or necking is reached when the amount of induced rigid phase is attaining 50%. The stretching behaviour of PEN is characterised more by the strain induced rigid phase formation (SIRP) than by the stress or strain induced crystallisation. The shrinkage behaviour is characterised by two regimes. A first one for draw ratios below the necking behaviour where the films shrink back to their original length for temperatures between 100 and 140 deg C with a mid-value of 120 deg C, corresponding to the glass transition temperature of the amorphous phase. The second regime, for draw rations above the necking behaviour is characterised by a shrinkage behaviour for temperatures between 120 and 160 deg C, with a mid-value of 140 deg C, corresponding to the transition temperature of the induced rigid phase structure. In this regime, the films never shrink back below the draw ratio after necking and a linear relation between the initial draw ratio and the final draw ratio after shrinkage is obtained. A heat treatment of the oriented films with fixed ends stabilises the induced structures and the shrinkage of these heat-set films is zero for temperatures up to the heat-setting temperature.