Blends of poly(~-caprolactone) (PCL) and poly(styrene-co-acrylonitrile) (SAN) show a miscibility window, i.e. for a fixed blend ratio ~b, they are able to form a one-phase mixture in the liquid state, which is dependent on the copolymer composition/~, and the temperature T. For miscible blends the Flory interaction parameter Z is smaller than Z,, the Z parameter at the spinodal. The minimum of Z, for a fixed T and q~, 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 fl, q~ and the crystallization temperature T~. The rate of crystallization G, which is equivalent to the radial spherulite growth rate, also exhibits a minimum, at a fixed T c and ~b, 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 (aao) ~/2 has a minimum at a copolymer composition with 20 wt% acrylonitrile, again for fixed values of ~b and To. 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, AU*, 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 (O'O'e) 1/2,
Посилання на статтю:
Influence of copolymer composition on the crystallization in PCL/SAN blends / Jorg Kressler, Petr Svoboda and Takashi Inoue // Polymer. – 1993. – Vol 34. – P. 3225-3233.