THERMOPLASTIC ELASTOMERS FROM IPP EPR BLENDS - CRYSTALLIZATION AND PHASE-STRUCTURE DEVELOPMENT

The phase morphology and structure of thermoplastic elastomers obtaine d from isotactic polypropylene (iPP) and ethylene-propylene random cop olymer (EPR) blends by means of the dynamic curing of EPR rubbery comp onent carried out during its melt mixing with iPP in a Banbury mixer a t 180-degrees-C were investigated. Samples obtained by compression mol ding and by using isothermal crystallization conditions of the iPP pha se were analyzed by means of differential scanning calorimetry, of opt ical, scanning, and transmission electron microscopy, and of wide-angl e and small-angle X-ray diffraction. The influence of cooling below th e melting point and of EPR molecular structure on the kinetic and ther modynamic parameters related to crystallization process of the iPP pha se was also studied. It was found that the process of dynamic curing o f the EPR component dramatically affects the development of the phase morphology and structure in the material. As a matter of fact, the ble nd containing the uncured EPR is characterized by the presence of iPP domains randomly distributed in the EPR rubbery matrix, whereas in the blend containing the cured EPR the iPP phase becomes the continuous p hase crystallizing in a structure that resembles a cobweb tending to s urround the EPR cured particles; moreover such an iPP cobweb appears t o be constituted by row structures of stacked lamellae. It was found t hat the addition of EPR phase interferes dramatically with the crystal lization process of the iPP, thus inducing drastic modification in its intrinsic morphology (size, neatness, regularity of spherulites, inne r structure of spherulites, etc.). Such interference was found to be c omparatively stronger when the iPP phase crystallizes in presence of c ured EPR. The elastic behavior of the thermoplastic elastomer material was accounted for by applying the ''leaf spring model'' to the morpho logy and structure of the iPP phase crystallized in presence of cured EPR. (C) 1994 John Wiley & Sons, Inc.