Optimising process conditions for solid state drawing of high modulus poly(vinyl alcohol) fibres

The optimum process conditions for production of high modulus poly(vinyl alcohol) (PVA) fibres using a solid state drawing process have been considered. The high degree of molecular mobility required to allow conformational changes in the polymer chains during the drawing process may be achieved by either drawing at elevated temperatures or by the addition of water, which acts as a plasticiser. Although it has been determined previously that temperature and water content have a similar effect on enhancing molecular mobility in PVA, it is contended that the latter method is detrimental to the achievement of a high degree of molecular orientation. The addition of water was found to destroy crystallinity in PVA and to have a lasting effect on molecular mobility, with the result that any gains in orientation are offset by the tendency of the chains to relax towards a random coil configuration upon removal of the drawing stress. Increasing the temperature, in contrast, enhances the ability of the chains to unravel and orientate in the direction of the applied drawing stress without promoting molecular relaxation, since the mobilising effect of increased drawing temperature is negated once the fibre has been removed from the elevated temperature environment of the drawing apparatus.