Morphology evolution, crystalline orientation, and thermal expansion of PA6/SEBS blends with nanolayer networks

Abstract

A blend of polyamide 6 (PA6) and styrene-ethylene/butylene-styrene (SEBS) with a co-continuous nanolayer network was fabricated by reactive compounding and subsequent injection molding. The nanostructured polymer alloy was found to exhibit an extremely low coefficient of linear thermal expansion (CLTE) in the flow direction, accompanied by a largely suppressed molding shrinkage. To clarify the influence of the microstructure on thermal expansion behavior, a systematic study of morphology evolution, crystalline orientation, and confined crystallization of the PA6/SEBS (60/40) blend was carried out by means of TEM, DMA, DSC and WAXD measurements. It was found that a lower viscosity of SEBS and the capability of in situ compatibility with PA6 enable a morphology evolution from a disordered co-continuous to droplet-continuous and, finally, to a nanolayer network structure. Multi-scale orientations take place during the injection molding process, and the large reduction of CLTE may originate from the high order microstructure in two aspects: (1) the rubber-deformation-induced orientation of PA6 crystalline in which the b-axis with a negative CLTE orients along the flow direction, and (2) the co-continuous orientation of the rubber and plastic nanolayers, of which the thermal expansion favors towards the normal direction.