Isotactic polypropylene/polyisobutylene blends: morphology-structure-mechanical properties relationships

Abstract

Morphological observations by optical and scanning electron microscopy, wide (WAXS) and small (SAXS) angle X-ray scattering, differential scanning calorimetry (DSC) and mechanical tests have been performed on sheet specimens of isotactic polypropylene (iPP)/polyisobutylene (PIB) blends obtained under different crystallization conditions. Two kinds of morphologies have been observed, particularly at high crystallization temperatureT _c, on thin sections of the same sheets: a spherulitic one in the centre and a row-like structure on the edges. The size of the spherulites, as well as the thickness of the row-like regions, decreases with diminishingT _c, and seemsto be independent of the amount of rubber. The adhesion among the spherulites and between the spherulites and the row-like regions seems to become poorer with higherT _c. The rubber particles seem to be evenly dispersed into the iPP matrix for samples quenched at low temperatures, whereas for samples isothermally crystallized (at highT _c) their concentration seems to be slightly higher at the border of the spherulites than in the centre. The overall crystallinity measured by DSC and by WAXS is an increasing function ofT _c and decreases with increasing amount of PIB. Theβ index of iPP phase, quite low indeed (max ≃ 3%), drops with loweringT _c and with enhancing PIB percentage. The long spacingL for a given quenching temperatureT _q is independent of PIB content, whereas for isothermally crystallized samples at low undercooling varies differently according toT _c. The lamellar thicknessL _c is always a decreasing function of rubber content. Stress-strain analysis shows a more and more brittle behaviour both with increasingT _c (beyondT _c=122° C all the specimens are very brittle irrespective of PIB amount) and PIB amount in accordance with the morphological observations. Some tentative hypotheses have been made to explain the observed behaviour.