iPP Phase Research Articles

Investigation of the structure of isotactic polypropylene—thermoplastic elastomer blends by means of X-ray scattering

Blends of semicrystalline isotactic polypropylene (iPP) with two types of elastomer, butadiene/styrene (BS) and hydrogenated isoprene/styrene (HIS), block copolymers were investigated by means of wide-angle and small-angle X-ray scattering. Except for the sample with a low concentration of BS (5 wt%), where the copolymer macromolecules are probably dispersed in the amorphous phase of iPP, the blends can be described as two-component systems (copolymer-iPP), in which the crystalline phase of iPP is only slightly influenced by the copolymer. X-ray results in the structural investigation of the blends correlate well with the mechanical properties of the blends.

Morphology, phase structure and thermal behaviour of films of isotactic polypropylene/hydrogenated oligocyclopentadiene blends: 1. Extruded isotropic films

The effect of hydrogenated oligocyclopentadiene (HOCP) on the structure and morphology of isotropic isotactic polypropylene (iPP) films obtained by melt extrusion has been studied using wide-angle X-ray scattering, thermal analysis and electron microscopy. It was found that the addition of HOCP causes the formation of the smectic phase of iPP at temperatures where bulk iPP crystallizes only in the monoclinic form. The amount of smectic phase present in the blend is dependent on the blend composition. The spherulitic morphology of iPP is completely modified by the presence of HOCP. For blends containing up to 10% HOCP, the iPP spherulite dimensions are drastically reduced. In the case of blends of higher HOCP content, the film crystallinity is reduced, no spherulites are visible and the film surface appears smoother.

Examination of the deformation of isotactic polypropylene and isotactic polypropylene/ethylene-propylene rubber blends by wide-angle X-ray scattering

Wide-angle X-ray investigations have been carried out on isotactic polypropylene (iPP) and on isotactic polypropylene/ethylene-propylene rubber (iPP/EPR) blend fibres. The initial material has been crystallized and drawn at various temperatures using an Instron machine. The index of anisotropy I a and the degree of orientation have been calculated from the X-ray photographs. It can be shown that drawing at T d = 150°C creates a bimodal crystal texture if the iPP/EPR blends contain 20wt% EPR or more. The EPR prevents complete orientation of the iPP matrix crystals. This can be shown by the decreases in the index of anisotropy and degree of orientation with increasing EPR content. A bimodal crystal texture was also observed for iPP and the iPP/EPR 80 20 blend drawn at T d = 120°C immediately after neck formation. This bimodal texture disappeared during further drawing, in contrast to fibres drawn at 150°C. An oriented ‘smectic’ phase of iPP was observed for samples drawn at T d = 20°C, whilst iPP and the blend fibres consist of oriented monoclinic α-phase crystals in all other cases. The study shows that two main deformation processes occur: deformation of the initial spherulites to a fibre structure and then plastic deformation of the oriented fibre structure.

Changes in interface shape during crystallization in two-component polymer systems

The interfaces between isotactic polypropylene (iPP) and polyethylene (PE) or polyoxyethylene (POE) were investigated in model sandwich-like systems by means of optical microscopy and scanning electron microscopy. It was found that during crystallization of iPP the shape of the interface undergoes significant changes. The interface surface changes from initially flat to highly developed with many deep and branched influxes of the second polymer flowing into the iPP phase. The formation of influxes is caused by the volume defect that appears during conversion of the iPP melt to crystal. The volume defect in regions of iPP melt that are closed by a continuous front of growing spherulites on one side and the interface on the other induces a deformation of the interface and flow of the melt of the second polymer into these regions. It was shown that the driving force for influx formation is the adhesion between melts of both polymers being in contact. The presence of influxes increases the interface strength mainly by increasing its area. Deformation of the interface in polymer blends can result in the deformation of the dispersed particles during crystallization of the matrix, and as a consequence in the improvement of mechanical properties of the blend.

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

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.

Morphology, crystallization and melting behaviour of films of isotactic polypropylene blended with ethylene-propylene copolymers and polyisobutylene

The crystallization, the morphology and the thermal behaviour of thin films of isotactic polypropylene (iPP) blended with elastomers such as random ethylene-propylene copolymers (EPM) with different ethylene content and polyisobutylene (PiB) were investigated by means of optical microscopy, differential scanning calorimetry and wide angle X-ray diffractometry. During crystallization EPM copolymers are ejected on the surface of the film forming droplet-like domains. A different morphology is observed in iPP/PiB blends. For these mixtures the elastomers separate from the iPP phase forming spherical domains that are incorporated in the iPP intraspherulitic regions. Both EPM and PiB elastomers act as nucleant agents for iPP spherulites. This nucleation efficiency is strongly dependent on the chemical structure and molecular mass of the elastomers. The addition of EPM causes an elevation of the observed and equilibrium melting temperature of iPP. This unusual effect may be accounted for by assuming that the elastomers are able to extract selectively the more defective molecules of iPP. The depression of the growth rate of spherulites and the observed and equilibrium melting temperature of iPP, noted in iPP/PiB blends, suggests that these two polymers have a certain degree of compatibility in the melt.