Miroslaw Pluta

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.

Metallocene Catalyzed Polymerization of Ethylene in the Presence of Graphite, 1. Synthesis and Characterization of the Composites

Polyethylene/graphite composite have been prepared by two different methods. In a first approach, the ethylene polymerization has been catalyzed by metallocene in the presence of neat graphite particles (NGC composite). A second series of composites (TGC) has been prepared by the polymerization-filling technique, which requires that the mettallocene/methylamoxane catalyst is fixed onto the graphite surface prior to the ethylene polymerization. The two series of composite exhibit significantly different morphology and thermal propreties. The filler distribution is very heterogeneous in the NGC series. The morphology changes from an intimate mixture of PE and filler filler particles at low graphite content to graphite covered by patches of PE at high filler loading. The graphite distribution is mich more homogeneous in the TGC samples, and the morphology consists of particles covered by a layer of PE in the whole composition range. Differences in the thermal properties are discussed in relation to the morphology.

Metallocene-catalyzed polymerization of ethylene in the presence of graphite. II. Structure and electrical properties of the composites

Structure and electrical properties of conducting polyethylene/graphite composites have been studied in relation to the preparation method, i.e. (i) the polymerization-filling technique (PFT), in which the polyolefinic chains are growing from the graphite surface, (ii) the slurry polymerization of ethylene in the presence of untreated graphite, and (iii) the mechanical blending of preformed polyethylene and graphite. The extent of the filler dispersion depends on the method used for the composite preparation. Moreover, the graphite particles can be oriented by the molding of the samples used for the measurement of the electrical properties. This orientation is as pronounced as the melt viscosity of polyethylene is low, this characteristic feature changing with the preparation method. These structural details have consequences on the electrophysical properties and the percolation threshold. Finally, the thermal dependence of the electrical resistivity has been studied.

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.

Aggregation of polylactide with carboxyl groups at one chain end in the presence of metal cations

Polylactides with one or more carboxyl groups at one chain end were synthesized by cationic polymerization according to activated monomer mechanism and by application of “thiol-yne” click chemistry for subsequent functionalization. End groups of such obtained polylactides were converted into ionic groups by neutralization of polymer solutions with metal oxides, mainly calcium oxide, and the aggregation of individual stereoisomers as well as that of the mixture of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) was investigated. The extent and progress of the aggregation was followed by viscosity measurements, and aggregated polymers in the solid state were examined by SEM and DSC. Solution viscosity increase was observed upon the aggregation of individual PLA stereoisomers, whereas PLA stereocomplex precipitation occurred in the case of the aggregation of PLLA/PDLA/metal oxide mixture.

Mechanical and rheo-optical studies of the relaxation processes in low-density polyethylene

Abstract Dynamic birefringence and dynamic light scattering techniques have been used to characterize the morphological changes in low-density polyethylene during sinusoidal deformation with various frequencies at constant temperature. Measurements have been conducted on samples with various morphologies. Two separate relaxation processes in the frequency range from 0.003 to 20 cps have been found to be strongly dependent on the morphology (i.e. spherulite perfection, degree of orientation correlation and sizes of crystalline elements) and less dependent on crystallinity of samples. By comparison of the results with calculated stress distribution inside spherulite mechanical model, the locations of distinguished relaxation processes in particular morphology elements have been discussed.

Phase structure and viscoelastic properties

The phase structure and dynamic mechanical properties of three polypropylene/polystyrene (PP/PS) systems of similar composition but various dispersion of the minor PS component have been examined. Two different PP/PS systems were prepared by polymerization of styrene (ST) molecularly dispersed in PP matrices (with the same initial structure) under the conditions leading to a linear or crosslinked PS component. The third PP/PS system has been prepared blending the homopolymers in the molten state. Studies of materials containingin situ polymerized PS revealed nanoscale phase separation of PS (atomic force microscopy) and pointed to the presence of physical entanglements between PS and non-crystalline phase of PP (DSC, dynamic mechanical analysis). The PS component in material prepared by melt mixing appeared to be completely phase-separated into micron-sized domains. Dynamic mechanical analysis revealed also the dependence of viscoelastic behavior of the PP/PS systems on dispersion of the PS inclusions and on the nature of the interface.

Swelling Induced Surface Layer Reorganization of the Crystalline Elements in Isotactic Polypropylene

The influence of styrene treatment on the interface and interphase layer has been investigated in isotactic polypropylene. The structure of polypropylene films was controlled by the crystallization conditions from the molten state and the interfacial regions with the various order were obtained. The modification effects have been shown to be dependent on the initial order and perfection of the crystalline phase of polymer. The role of the interphase layer modification in the total structural changes in the modified films was identified.