INFLUENCE OF COPOLYMER COMPOSITION ON THE CRYSTALLIZATION IN PCL SAN BLENDS

Blends of poly(epsilon-caprolactone) (PCL) and poly(styrene-co-acrylonitrile) (SAN) show a miscibility window, i.e, for a fixed blend ratio phi, they are able to form a one-phase mixture in the liquid state, which is dependent on the copolymer composition beta, and the temperature T For miscible blends the Flory interaction parameter chi is smaller than chi(s), the chi parameter at the spinodal. The minimum of chi, for a fixed T and phi, is at approximately 20 wt% acrylonitrile in SAN. Below the melting point the homogeneous liquid undergoes a liquid-solid phase transition (i.e. crystallization). We have studied the crystallization kinetics of miscible PCL/SAN blends as a function of beta, phi and the crystallization temperature T(c). The rate of crystallization G, which is equivalent to the radial spherulite growth rate, also exhibits a minimum, at a fixed T(c) and phi, for blends containing SAN with 20 wt% acrylonitrile. This behaviour is described in terms of a modified Hoffman-Lauritzen (HL) theory. From secondary nucleation plots it was shown, in addition, that the surface free energy (sigmasigma(e))1/2 has a minimum at a copolymer composition with 20 wt% acrylonitrile, again for fixed values of phi and T(c). In contrast, according to the HL theory the lower surface free energy would lead to a faster crystallization. In order to explain the influence of polymer-polymer interactions on the crystallization kinetics it has to be assumed that the reptation of a crystallizable polymer chain towards the growing front of the crystal is slowed down by favourable interactions. Therefore, an additional term, DELTAU*, was introduced into the mobility expression used in the HL theory. The experimental findings show that the influence of U*, the activation energy for the transport of crystallizable material, on the rate of crystallization predominates over the influence Of (sigmasigma(e))1/2.