Crystallization of PP in PP/SEBS blends and its correlation with tensile properties

Abstract

SynopsisCrystallization of polypropylene (PP) in the blends of PP with styrene+thylene butylene-styrene triblock copolymer (SEBS) is studied through differential thermal analysis (DTA) and X-ray diffraction measurements. Analysis of crystallization exotherm peaks in terms of crystallization nucleation and growth rates, crystallite size distribution, and crystallinity revealed differences in the morphology of PP component in the blend in the different regions of blend composition. Crystallinity determined by X-ray diffraction and DTA showed identical variations with blend composition. Variations in tensile properties of these blends with blend composition are also reported. Correlations of the various tensile properties with the crystallization parameters, viz., the crystallinity and crystallite size distribution, are presented, which confirm the influence of crystallization of PP component on the tensile properties of these blends. INTRODUCTIONIt has been shown1 that blending of polypropylene (PP) with high density polyetheylene (HDPE) or reinforcement with short glass fiber affects the crystallization and morphology of PP. An understanding of the crystallization behavior of PP is of great importance to explain the mechanical properties of polyblends and composites involving PP as the major component.Blends of PP with various other polymers have been r e p~r t e d~-~ to show improvements in mechanical and rheological properties suited to many applications. SEBS (styrene-ethylene butylene-styrene) triblock copolymer, obtained by hydrogenation of the butadiene sequence, is a recently developed thermoplastic elastomer. Blends of SEBS with HDPE and polystyrene (PS) and the development of thermoplastic interpenetrating polymer networks (IPN) with SEBS were recently reported.6 Lindsey, Paul, and Barlow7 found that addition of SEBS to the binary HDPE-PS blends improved the ductility, toughness, and various mechanical properties but lowered the strength and modulus. In the case of thermoplastic IPN's, Siegfried, Thomas, and SperlingG developed the two types of thermoplastic IPN's (chemically blended and mechanically blended) with styrene, methacrylic acid, and isoprene monomers. TheyG found a dual phase continuity after ionomer formation in both types of these IPNs, while the melt viscosity was lower in the chemically blended IPN than the mechanically blended one. Blends of SEBS with PP, reported8 recently by us, have shown considerable improvement in the melt rheological properties of PP, favorable from the point of view of processing.In this paper we present a study of crystallization of PP in the blend of PP and SEBS, with SEBS content varying from 5 to 25 wt %. The crystallization be- havior and the degree of crystallinity are determined through the differential thermal analysis (DTA) and X-ray diffraction measurements. Use of thermal analysis techniques (DTA or DSC) for the study of crystallization kinetics of PP is illustrated in the literaturel~~ DTA or DSC thermogram of PP shows welldeveloped crystallization exot...

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