Morphology and Properties of Polymer Blends -Polypropylene and a Thermoplastic Elastomer -Polypropylene/Polyamide (1/1) and a Modified Thermoplastic Elastomer
Abstract: The search for polymeric materials with new property profiles has stimulated a great interest into polymer blends and reactive processing. During recent years many new blends have reached the market. Although some of these are homogeneous mixtures of polymers, most commercial blends are composed of immiscible polymers, and contain two or more polymeric phases. The properties of such blends depend on composition as well as blend morphology. Most combinations of immiscible polymers are incompatible, i. e. they give blends of low tensile and impact strengths. This is due to weak polymer - polymer interactions across phase boundaries. The properties of such combinations of polymers can often be dramatically improved by adding a suitable compatibilizer. The present work concentrates on two blend systems, one intrinsically compatible, one which demonstrates the role of an efficient compatibilizer. The compatible blend system studied is based on polypropylene (PP) and polystyrene-block-poly(ethylene-stat-butylene)-block-polystyrene (SEBS), extended by a processing oil (90 parts per 100 parts of SEBS). Within a broad composition range, this system gave blends with a bicontinuous network structure of the type commonly referred to as a physically cross-linked interpenetrating polymer network (IPN). The presence of a pronounced melting point depression of PP indicated partial miscibility in the melt. This suggested that the formation of an IPN structure is due to nucleation of polypropylene across the phase boundaries in the cooling two-phase melt. In injection moulding, this mechanism of structure formation leads to weld lines free from weaknesses. The low viscosity of the two phase blends simplifies processing and form-filling. The presence of an IPN structure in the solid state contributes to interesting mechanical properties, properties which may be varied within wide limits. Polyamide (PA) and PP are incompatible, but may give useful blends if combined with a suitable compatibilizer. The present study is concerned with blends prepared by melt mixing of PP, PA and either SEBS modified to contain an EB mid-block with succinic anhydride groups (SEBS-MA), or unmodified SEBS. In blends prepared using SEBS-MA, the actual compatibilizer is the graft copolymer formed as the succinic anhydride groups react with polyamide end groups. This graft copolymer was found to give blends with much finer PA domains and better mechanical properties than unmodified SEBS. The effect of compatibilizer concentration and mixing strategy on blend morphology, as well as the effect of blend morphology on mechanical properties, the crystallization behavior of PP and PA in the blends, and the rate of water uptake are discussed.
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