Andrzej Galeski

Confined Crystallization of Polyethylene Oxide in Nanolayer Assemblies

The design and fabrication of ultrathin polymer layers are of increasing importance because of the rapid development of nanoscience and nanotechnology. Confined, two-dimensional crystallization of polymers presents challenges and opportunities due to the long-chain, covalently bonded nature of the macromolecule. Using an innovative layer-multiplying coextrusion process to obtain assemblies with thousands of polymer nanolayers, we discovered a morphology that emerges as confined polyethylene oxide (PEO) layers are made progressively thinner. When the thickness is confined to 20 nanometers, the PEO crystallizes as single, high-aspect-ratio lamellae that resemble single crystals. Unexpectedly, the crystallization habit imparts two orders of magnitude reduction in the gas permeability.

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.

Compatibilization and properties of poly(ethylene terephthalate)/polyethylene blends based on recycled materials

The morphology and physical/mechanical properties of non-compatibilized and compatibilized blends of poly(ethylene terephthalate) (PET) and polyethylene (HDPE) obtained from virgin and post-consumer packaging materials were investigated. The blend compatibilization was carried out by melt-mixing in the presence of various polyolefins functionalized with reactive groups (HDPE-g-MA, EPR-g- MA, E-AA, E-GMA, SEBS-g-MA). The effect of the type and concentration of compatibilizer, as well as the mixing conditions on the phase morphology, crystallization behavior, chemical interactions and melt rheology of the blends was then examined by means of electron microscopy (SEM), scanning calorimetry (DSC), FTIR and NMR spectroscopy, and capillary rheometry. The results pointed out that ethy- lene-co-glycidyl methacrylate copolymer (E-GMA) displayed a higher compatibilizing efficiency giving rise to a neat improvement of phase dispersion and interfacial adhesion in the blends, as compared to the other compatibilizers examined. This was accounted for by a high reactivity of epoxy groups with the chain end-groups of PET during meltmixing and the effect of in situ formed graft copolymer on the interfacial properties (reduction of coalescence, interpenetration with homopolymer phases). Tensile mechanical tests showed that elongation at break of recycled PET/HDPE (75/25) blends was markedly enhanced upon addition of E- GMA amount lower than 5 wt.-%.

Characterization of scrap poly(ethylene terephthalate)

Abstract The goals of the investigation were: to indicate the methods of characterization of recycled polymers, to show general tendencies in properties deterioration and characterize recyclates on Central Europe and European Community markets. The properties and composition of scrap poly(ethylene terephthalate) from several sources were characterized by: TGA, DSC, FTIR, tensile properties, dynamic viscosity, intrinsic viscosity and thermo-oxidative stability. We found that all PET regrinds contained admixture of other polymers (0.1–5 wt%). The presence of more than 50 ppm PVC makes PET worthless for advanced application as film forming, because it catalyzes the hydrolysis and reduces the strength of material. Although the individual flakes of recycled PET show almost unchanged molecular characteristics and properties, the processed regrinds always exhibit worse properties. Partial restoration of recycled PET properties can be achieved by careful working, removing the dust fraction and by proper drying. The difference between studied PETs results from different applied recycling procedure. Admixtures of polymers without compatibilizer always deteriorate tensile properties. Various levels of stability of polymer viscous flow during film and tape extrusion were observed, depending on composition of recycled PET from various sources. Microgels were observed in all samples during film extrusion.

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.

Crystal phase and crystallinity of polyamide 6/functionalized polyolefin blends

Abstract Blends of Nylon 6 (Ny6) and polyolefins functionalized with acrylic acid (polyethylene—PE–AA, polypropylene—PP–AA) were investigated in terms of crystallization behavior and resulting Ny6 crystalline structure. Thermal analysis showed that in the case of blends with functionalized polyolefin as a matrix: (a) Ny6 crystallization is spread and dramatically shifted toward lower temperatures, approaching that of the polyolefin component 125–132°C; (b) Ny6 γ crystal polymorph is the major phase present; confirmed and quantitatively evaluated by use of deconvolution computations performed on WAXS spectra of the blends. When Ny6 is dispersed in functionalized polyolefin matrix, the weight content of Ny6 γ crystals increases up to three times with respect to analogous, non-compatibilized blends and up to ≈16 times with respect to Ny6 homopolymer. These phenomena are explained by the reduction of size of Ny6 dispersed particles, caused by the interactions between the functional groups of polyolefin and the polar groups in polyamide chain. The nucleation mechanism is changed due to the lack of heterogeneous nuclei in most small Ny6 droplets, which results in the enhanced γ crystal formation.