Reactive blending: inhomogeneous interface grafting in melts of maleinated polystyrene and polyamides

To be competitive, most blends need compatibilizers, usually copolymers with a blocky architecture, the chains of which cover the interfaces between the blend phases, refining the phase morphology and improving the interface strength. When the blend components are suitably functionalized, such copolymers can be conveniently generated in situ, in processes of reactive blending. Normally, graft copolymers are created. The polymer–polymer coupling proceeds exclusively in the interfaces. This interface grafting is (i) pivotal in the design of modern blend systems and (ii) an interesting route towards novel copolymers. The complex kinetics of interface grafting in blend melts have so far attracted little attention. In a model study, amino terminated polyamide 12 (PA) was grafted in the melt onto heavily maleinated polystyrene (SMA; S: styrene and MA: maleic anhydride). Anhydride and amino functions react at high temperatures fast and irreversibly by imide condensation. A series of SMA/PA blends differing in composition and PA chain lengths was investigated, with the aim of driving the grafting to high conversions so a pure graft copolymer SMAgPA would result, instead of an SMA/PA/SMAgPA blend. However, a pure copolymer was never obtained. The grafting remained incomplete, except with very short-chained PA and only at equal weight fractions of SMA and PA. More importantly, the SMA chains were never grafted evenly. Instead, “overgrafted” and “undergrafted” chains SMAgPA coexisted in one and the same product. It appears that the SMAgPA chains form an auto-inhibitory barrier in the interfaces that prevents random grafting. Grafting proceeds to high conversion only in SMA/PA blends with a co-continuous phase morphology where the interfaces are constantly torn apart and renewed, during melt blending, so the reaction is constantly reactivated. © 1998 John Wiley & Sons, Ltd.