Anisotropic thermal expansion in polypropylene/poly(ethylene-co-octene) binary blends: influence of arrays of elastomer domains

In injection molded specimens consisting of isotactic polypropylene (iPP)/poly(ethylene-co-octene) (EOR) blends with different viscosity ratio of η(EOR)/η(iPP), the coefficient of linear thermal expansion (CLTE) was investigated by thermal mechanical analysis (TMA). It was found that the blend with a smaller viscosity ratio showed the larger anisotropy of CLTE depending upon the directions. TEM observations revealed that the shape of rubber domains varied from slabs, cylinders to ellipsoids in shape, by increasing η(EOR)/η(iPP). The crystal orientation analysis by WAXD have revealed that the blend with 'slab' EOR domains showed the orientation of the c-axis of iPP crystals was preferably oriented to FD (flow direction) and TD (transverse to FD), and that the b-axis was exclusively oriented to ND (thickness direction). The CLTE of each FD and TD was in good agreement with the rules-of-mixing for CLTE by introducing the effect of the arrays of the elastomer domains and the PP crystal orientation. On the other hand, the CLTE in ND showed massive discrepancy between the calculation and observation. It was found that the incorporation of the retraction effect could explain the discrepancy to some extent.