Segmental orientation in ionically cross-linked blends of polystyrene and poly(ethyl acrylate)

Ionically complexed blends are potentially useful model compounds for studying selected aspects of segmental orientation behavior in blends. This paper presents a study of orientation behavior in stoichiometric blends of poly[styrene-co-(styrenesulfonic acid)] and poly[ethyl acrylate-co-(4-vinyl pyridine)] (heteroblends), where ionic cross-links are produced by proton transfer from the acid to the pyridine moieties, thus leading to an effective ionic network whose density is controlled by the counit content of the blend components. Orientation measurements were obtained by infrared dichroism. For blends composed of components with similar counit contents (symmetric blends), the segmental orientation of both the polystyrene and poly(ethyl acrylate) segments increases with ion content when compared at the same relative temperature, a direct result of the effective ionic cross-links. The polystyrene orientation is, however, significantly lower than that in previously-studied ionomer blends based solely on polystyrene (homoblends). This suggests that the orientation of the polystyrene segments in the heteroblends is reduced by a plasticization effect from the poly(ethyl acrylate) segments, attributed to relaxation coupling between the dissimilar chains during stretching. In an asymmetric blend of a sulfonated polystyrene copolymer of low acid content and a poly(ethyl acrylate) copolymer of high pyridine content, the orientation of the styrene segments is much higher than that in the symmetric blend of the same acid content. It is concluded that relaxation coupling is not present in the asymmetric blend,:attributed to the high orientation of the ethyl acrylate segments and to the lower ethyl acrylate content.