Z. Bartczak

Spherulite nucleation in blends of isotactic polypropylene with high-density polyethylene

Abstract Primary nucleation of spherulites in blends of isotactic polypropylene (iPP) with high-density polyethylene (HDPE) has been investigated by means of differential scanning calorimetry and optical microscopy. The number of iPP nuclei in blends crystallized isothermally at temperatures greater than 127°C decreases with increasing HDPE concentration to a much greater extent than follows from the decreasing amount of iPP, whereas in blends crystallized below 127°C this number increases strongly. The shapes of the spherulite size distributions indicate that athermal (heterogeneous) nucleation is disturbed. Experiments with nucleating agents demonstrate that heterogeneous nuclei migrate acrossinterphase boundaries from iPP melt to HDPE melt during mixing due to the difference in interfacial energy between the nuclei and the molten components of the blend. At temperatures below 127°C the HDPE crystals growing in the blend cause additional heterogeneous nucleation of iPP spherulites. This nucleating activity of HDPE crystals is responsible for the increase in nucleation density of the blend in spite of migration.

On the plastic deformation of the amorphous component in semicrystalline polymers

The molecular orientation of the amorphous component of semicrystalline high-density polyethylene (HDPE) induced by plane strain compression was studied by wide-angle X-ray scattering measurements utilizing the pole figure technique and separation of the scattering produced by the crystalline and amorphous components. It was found that the oriented amorphous component produced by large strain plastic deformation consists of domains of extended chain segments, closely packed in a two-dimensional pseudo-hexagonal aggregation, which are separated by less ordered regions. The deformation leads to the formation of a texture of the amorphous component which is common to the whole sample. In this texture, the direction of the chains coincides with the direction of flow, and one of the (100) pseudo-planes of the pseudo-hexagonal structure in every domain is perpendicular to the loading direction. It was suggested that the most important deformation mechanisms in the ordered amorphous component were the glide of the chain segments along their axes and the slip of the pseudo-planes of the ordered chains in the direction perpendicular to the chain axis, with resembling the crystallographic slip processes. Such a specific deformation of the amorphous layers most probably resulted from the strong constraint imposed by the slip deformation in the crystalline component to which the amorphous component is intimately connected.

Changes in the morphology and orientation of bulk spherulitic polypropylene due to plane-strain compression

Abstract Studies on the morphology and the development of texture in isotactic polypropylene (iPP) subjected to plane-strain compression are reported. The iPP samples were compressed in a channel-die at 110°C up to the true strain of 1.89 (compression ratio, CR=6.6). The structure of deformed specimens was investigated by means of light microscopy, differential scanning calorimetry, density measurements, small- and wide-angle X-ray diffraction techniques and dynamic mechanical analysis. A scheme of morphology changes on all structural levels was proposed. It was found that initial spherulitic morphology was destroyed and was transformed into stacks of crystalline lamellae with their normals rotating towards loading direction, while chain axis tending towards the flow direction at the true strain near 1.1 (CR≈3). The main active deformation mechanisms found were the crystallographic slips along the chain direction: (010)[001], (110)[001] and (100)[001] slip systems, supported by the deformation of the amorphous component by interlamellar shear. No evidence of the twinning modes was found. The intense chain slip caused the fragmentation of the lamellae into smaller crystalline blocks due to slip instabilities. That transformation occurred above true strain of 1.39 (CR=4). Further slips in these fragmented crystallites led to formation of a sharp orientation of the chains along the flow direction. The final texture of the compressed iPP found at the true strain of 1.89 (CR=6.6) was the multi-component texture with two main components of (010)[001] and (110)[001]. Mechanical properties of deformed samples follow the evolution of their structure through successive increase of storage modulus and a decrease of mechanical loss, ascribed to the glass–rubber transition, with increasing strain. The behavior of mechanical loss evidences substantial stiffening of the amorphous component with increasing strain.

Primary nucleation and spherulite growth rate in isotactic polypropylene-polystyrene blends

Abstract The morphology, primary nucleation of spherulites and their growth rate in immiscible blends of isotactic polypropylene (iPP) and atactic polystyrene (aPS) were studied. It was found that PS inclusions are dispersed in an iPP matrix and their size depends on the mixing conditions. During crystallization of iPP aPS inclusions are engulfed by growing spherulites without any pushing or deformation before engulfing. The spherulite growth rate in the blends does not depend on either concentration of aPS in the blend or time of mixing. The spherulite number per iPP volume unit in the blend increases with increasing concentration of aPS and also with increasing mixing time. It was found that additional nuclei which appear in iPP/aPS blends are of two types: heterogeneous and self-seeded primary nuclei. It was demonstrated that the reasons for the increase in primary nucleation are the migration of impurities from aPS melt to iPP melt during mixing and the nucleation of a fraction of spherulites by the iPP-aPS interface. It is suggested that the driving force for the migration process is the interfacial free energy of an impurity with respect to the iPP melt and the analogous energy with respect to the aPS melt.

Spherulite nucleation in polypropylene blends with low density polyethylene

Abstract Primary nucleation of spherulites in blends of isotactic polypropylene (iPP) with low density polyethylene (LDPE) was investigated by means of differential scanning calorimetry and optical microscopy. The number of iPP spherulites in the blend decreases with increasing LDPE concentration to a much greater extent than follows from the decreasing amount of iPP. The shapes of spherulite size distributions indicate that athermal (heterogeneous) primary nucleation is inhibited. The density of primary nucleation in the blends decreases strongly with increasing mixing time. The same effect was observed in the blends with the nucleating agent which was added to iPP or LDPE. These experiments demonstrate that heterogenoeus nuclei migrate across interphase boundaries from the iPP melt to the LDPE melt during the mixing process. It is suggested that the interfacial energy difference between the nuclei and the molten components of the blend is responsible for the migration of nuclei.

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

Abstract 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.

Homogeneous nucleation in polypropylene and its blends by small-angle light scattering

Abstract The homogeneous primary nucleation of spherulitic crystallization in isotactic polypropylene (iPP) and its blends with atactic polypropylene (aPP) was studied. Bulk samples of iPP and iPP/aPP blends were crystallized isothermally under high undercoolings (∼ 100°C) using a specially designed crystallization cell. In crystallized samples the fifth-order average spherulite radius was determined on the basis of small-angle light scattering measurements. The parameters for homogeneous primary nucleation were obtained from the fitting of curves calculated on the basis of theoretical predictions for regime III to the experimental data. For that purpose the theoretical background for homogeneous nucleation in polymer blends and for small-angle light scattering by an assembly of impinged spherulites was developed. The results obtained for homogeneous nucleation in plain iPP are in very good agreement with theoretical predictions. For iPP/aPP blends it was found that the main reason for depression of nucleation in blends is the additional energy barrier connected with the separation of components of a homogeneous blend during crystallization of one of the components (iPP).

Primary nucleation behaviour in isotactic polypropylene/ethylene-propylene random copolymer blends

Abstract Primary nucleation of spherulites in blends of isotactic polypropylene (iPP) with ethylene-propylene random copolymer (EPM) has been investigated using optical microscopy. The number of spherulites generally increases with increasing EPM content. It is shown that this increase is caused by migration of heterogeneous nuclei across interface boundaries from EPM to iPP during melt-mixing. The migration was observed in the blends with the nucleating agent initially added to EPM before blending with iPP. It is suggested that the interfacial free energy difference between nuclei and the molten components of the blend is responsible for the migration of nuclei. It is also shown that self-seeded nucleation becomes damped in the blends due to partial solubility of the components and that the degradation of the blends during melt-annealing depresses the primary nucleation.

Orientation and properties of sequentially drawn films of an isotactic polypropylene/ hydrogenated oligocyclopentadiene blend

Films of isotactic polypropylene (iPP) and a blend of iPP and hydrogenated oligocyclopentadiene (HOCP) oriented by one-way (uniaxial) and two-way (sequential biaxial) drawing were studied. The mechanical and thermal properties of the oriented iPP and iPP/HOCP blends as well as orientation behaviour were investigated by means of mechanical tests, d.s.c. and small- and wide-angle X-ray scattering including the pole figures technique. It was found that drawing of iPP/HOCP blends to a high strain leads to the formation of highly oriented films having mechanical properties greatly improved compared to plain iPP oriented under the same conditions. The studies revealed in one-way drawn samples the multicomponent texture with chain axis, c, oriented in the machine direction, MD, for all texture components. The lamellae are oriented with normals parallel to the c direction. Due to differences in the amorphous phase content the texture in the blend samples is less developed than in deformed plain iPP. The subsequent drawing along the transverse direction, TD, leads to the reorientation of the c axis towards the TD, the sharpening of texture and the destruction of the lamellar structure. It was found that the major deformation mechanisms active in iPP and iPP/HOCP blends are: the crystallographic slips systems of (010)[001], (100)[001], (110)[001] and {110} twinning modes. In addition the interlamellar sliding is active as the main deformation mechanism in the amorphous phase. The (010)[001] system is the easiest of the slip systems active and dominates the other mechanisms.

Ternary blends of high‐density polyethylene–polystyrene–poly(ethylene/butylene‐b‐styrene) copolymers: Properties and orientation behavior in plane–strain compression

Ternary blends of high-density polyethylene (HDPE) with atactic polystyrene (PS) and styrene–ethylene/butylene–styrene block copolymer (SEBS) were deformed by plane–strain compression in a channel die. The samples were deformed up to the true strain of 1.8 (compression ratio of 6) at 100°C. Thermal and mechanical properties of the deformed blends were studied in addition to the study of the deformation process. The basic mechanism of plastic deformation is crystallographic slip, the same as that active in deformation of plain HDPE and binary blends of HDPE and PS. This slip is supplemented by the plastic deformation of an amorphous component. In blends of high SEBS content, the role of deformation of an amorphous component by shear and flow increases markedly due to reduced overall crystallinity of these blends. In such blends an amorphous component includes a semicontinuous embedding of crystallites, and therefore, the deformation process is dominated by deformation mechanisms active in a more compliant amorphous phase. Consequently, with increasing the content of SEBS in the blend, the texture of the oriented blends changes from a single-component (100)[001] texture to a texture with a strong fiber component in addition to a (100)[001] component. In blends with high content of SEBS, the crystalline lamellae of polyethylene do not undergo fragmentation up to the compression ratio of 6, while in blends with low and moderate content of SEBS, such lamellar fragmentation was detected.