Robert A. Shanks

Kinetics of polymer crystallisation

Abstract The methods, models, theoretical developments and techniques for the study of kinetics of polymer crystallisation are described and discussed. Some of the pertinent fundamental issues and recent results, especially of polymer crystallisation under non-isothermal conditions and crystallisation of polymer blends, are surveyed. It is demonstrated that non-isothermal crystallisation under varying cooling rates can be described and predicted through an integral method. Hotstage microscopy, improved by the technique of digital image processing and analysis, is useful for the study of the kinetics and mechanisms of polymer crystallisation through establishing a relationship between nucleation, spherulite growth and overall crystallisation rate. The effect of polymer blending on crystallisation is discussed on the basis of two fundamental factors: nucleation and spherulite growth.

PP–elastomer–filler hybrids. I. Processing, microstructure, and mechanical properties

Three-component systems with a polypropylene (PP) matrix consisting of polar elastomer (ethylene–propylene rubber and styrene–ethylene–butylene–styrene grafted with maleic anhydride) or of polar PP (PP grafted with maleic anhydride) and filler were investigated. Three microstructures of PP–elastomer–filler hybrids were obtained by processing control and elastomer or PP modification with the maleic anhydride: fillers and rubber particles were separated in the PP matrix, rubber particles with filler core were distributed in the PP matrix, and mixed microstructures of the first and second. A study of mechanical properties showed that the elastic modulus increased in the first microstructure and impact strength increased in the second microstructure. Mechanisms for the relationships between microstructure, processing, and mechanical properties are discussed.

Properties of Poly(3-hydroxybutyric acid) Composites with Flax Fibres Modified by Plasticiser Absorption

Natural fibre-biopolymer composites have been preparred from flax and polyhydroxybutyrate (PHB). The flax was modified by drying, followed by plasticiser absorption to replace the water lost to prevent embrittlement. This protects the fibres from problems associated with their water content and changes in water content due to equilibration with the environment. Flax and PHB showed good interfacial adhesion, which was decreased when plasticisers were present. Some plasticiser migrated from the flax to PHB and caused complex changes in the glass transition, crystallisation and crystallinity of the PHB. Morphology of the composites was examined by scanning electron microscopy (SEM) and optical microscopy (OM), SEM provided information on the interfacial adhesion through fractography. OM showed extensive transcrystallinity along the fibre surfaces. Dynamic mechanical analysis was used to measure elastic and damping characteristics and their relation to composition and morphology.

Miscibility and isothermal crystallisation of polypropylene in polyethylene melts

Three blends were made, consisting of 20% polypropylene (PP) homopolymer and 80% high density polyethylene (HDPE), low density polyethylene (LDPE) or linear low density polyethylene (LLDPE). Isothermal crystallisation of PP, at temperatures where the PE remained molten was studied by differential scanning calorimetry (DSC) and hot-stage optical microscopy (HSOM) with polarised light. The resulting semi-crystalline morphology was studied by transmission electron microscopy (TEM). It was observed by HSOM that the PP crystallised as open armed, diffuse spherulites in the PP-LLDPE blend, similar to the crystal morphology observed in miscible blends, while in the PP-LDPE and PP-HDPE blends, PP crystallised in phase separated droplets. The crystallisation rate of PP decreased significantly in the PP-LLDPE blend, but in PP-LDPE, PP-HDPE blends, it was similar to that of the pure PP. The difference in crystallisation rates indicated that PP was much more dispersed in the LLDPE melt, either by dissolution, or greater dispersion of droplets without nuclei, or both. The DSC and HSOM results suggested that the PP was miscible with the LLDPE at elevated temperatures at a PP composition of 20%, while the PP was immiscible with the HDPE and LDPE at these temperatures. However, TEM showed that there was phase separation in the PP-LLDPE (20:80) blend as well, though the droplets were much finer. Nevertheless, the form of spherulitic growth implies a substantial amount of PP dissolved in the molten LLDPE.

PP/elastomer/filler hybrids. II. Morphologies and fracture

Three microstructures of polypropylene (PP)/elastomer/filler hybrids were obtained by processing control and elastomer or PP surface modification: (a) filler and elastomer particles are dispersed in a PP matrix to form separate particles; (b) elastomer particles with a filler core are distributed in a PP matrix; or (c) there are mixed microstructures of (a) and (b). Morphologies and fracture of different components and microstructures were studied by SEM. When the lower-temperature cut samples were carefully etched, the differences between the various microstructures were clearly observed under SEM. The core-shell microstructure provided an elastomer interlayer between the filler and the PP matrix, which resulted in changing the fracture mechanism from microcrack to shear yield. The SEM micrographs were digitized and analyzed by IMAGE 1.52. Rubber particle size and distribution were studied. The relationship between the morphologies and mechanical properties, especially the brittle–toughness transition, was discussed. DSC was used to confirm the difference of microstructures, crystallization behavior, and compatibility.

Developing gelatin-starch blends for use as capsule materials

Blends of gelatin with up to 50% hydroxypropylated high amylose (80%) corn starch were developed as capsule materials. Poly(ethylene glycol) (PEG) was used as both a plasticizer and a compatibilizer in the blends. In order to prepare hard capsules for pharmaceutical applications using the well-established method of dipping stainless steel mold pins into solution, solutions with higher solids concentrations (up to 30%) were developed. The solutions, films and capsules of the different gelatin-starch blends were characterized by viscosity, transparency, tensile testing, water contact angle and SEM. The linear microstructure of the high amylose starch, and the flexible and more hydrophilic hydroxylpropylene groups grafted onto the starch improved the compatibility between the gelatin and starch. SEM revealed a continuous phase of gelatin on the surface of films from all blends. The water contact angle of pure gelatin and the different blends were similar, indicating a continuous phase of gelatin. By optimizing temperature and incubation time to control viscosity, capsules of various blends were successfully developed. PEG increased the transparency and toughness of the various blends.

Crystallisation of single-site polyethylene blends investigated by thermal fractionation techniques

Abstract Branched polyethylenes, low density polyethylenes (LDPE) or long-chain branched very low density polyethylenes (VLDPE), were blended with VLDPEs containing short branches. The melting behaviour of pure copolymers and their blends were investigated using differential scanning calorimetry (DSC) after applying stepwise isothermal crystallisation (‘thermal fractionation’). Thermal fractionation separates polymers according to their branching densities and fractionated curves used to determine the short-chain branching distribution (SCB), crystallisation and miscibility of blends. When both polymers have similar unbranched segments, they may co-crystallise if they are miscible in the melt. The co-crystallisation is observed to occur in all sets of blends, however, the extent of co-crystallisation varies from blend to blend. The blends of metallocene-catalysed VLDPE1 and LDPEs show significant deviation from the additivity rule indicating the greater co-crystallisation and hence melt miscibility at all compositions. The extent of co-crystallisation decreases for the VLDPE1 blends containing long-chain branched VLDPE2 and increases for Ziegler–Natta-catalysed VLDPE3–VLDPE2 blends, as the VLDPE2 content increases. DSC fractionated curves allow detailed examination of co-crystallisation and miscibility of blends that is also comparable to the results gained by temperature rising elution fractionation.

Mechanical properties and morphology of polyethylene–polypropylene blends with controlled thermal history

The effect of time–temperature treatment on the mechanical properties and morphology of polyethylene–polypropylene (PE–PP) blends was studied to establish a relationship among the thermal treatment, morphology, and mechanical properties. The experimental techniques used were polarized optical microscopy with hot-stage, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and tensile testing. A PP homopolymer was used to blend with various PEs, including high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and very low density polyethylene (VLDPE). All the blends were made at a ratio of PE:PP = 80:20. Thermal treatment was carried out at temperatures between the crystallization temperatures of PP and PEs to allow PP to crystallize first from the blends. A very diffuse PP spherulite morphology in the PE matrix was formed in partially miscible blends of LLDPE–PP even though PP was present at only 20% by mass. Droplet-matrix structures were developed in other blends with PP as dispersed domains in a continuous PE matrix. The SEM images displayed a fibrillar structure of PP spherulite in the LLDPE–PP blends and large droplets of PP in the HDPE–PP blend. The DSC results showed that the crystallinity of PP was increased in thermally treated samples. This special time–temperature treatment improved tensile properties for all PE–PP blends by improving the adhesion between PP and PE and increasing the overall crystallinity. In particular, in the LLDPE–PP blends, tensile properties were improved enormously because of a greater increase in the interfacial adhesion induced by the diffuse spherulite and fibrillar structure.

Miscibility and crystallisation of polypropylene-linear low density polyethylene blends

Abstract Crystallisation, morphology and miscibility of polypropylene (PP) and linear low density polyethylene (LLDPE) blends were studied by polarised optical microscopy connected to a computer with digital image processing and analysis. In particular the effects of LLDPE and its melt flow index (MFI) on the kinetics of PP crystallisation was investigated through establishing a relationship between nucleation density, spherulitic growth rate and overall crystallisation growth rate. All the blends contained 20% by mass of PP and the LLDPEs used were of the similar grades. The crystallisation of PP was controlled to occur isothermally at temperatures where LLDPEs were in molten state. It was found that, the PP crystallised as open-armed diffuse spherulites, similar to those observed in the miscible blends, suggesting that the PP and the LLDPE may be miscible at some temperatures. The nuclei density, spherulite growth rate and overall crystallisation rate of PP decreased significantly in the blends, indicating that the LLDPE retarded crystallisation of PP, possibly due to various reasons such as the dilution of PP by LLDPE (LLDPE as a solvent in molten state), hindrance of viscous LLDPE to the PP crystallisation front, and decreased supercooling degree because of the miscibility between the PP and LLDPE. This provided further evidence that the PP and the LLDPE could be miscible at crystallisation temperatures selected. In addition, the spherulite growth rate of PP decreased with a decrease in MFI of LLDPE while the MFI of LLDPE had negligible effect on the nuclei density, showing that the diffusion process controlled overall crystallisation rate when the nucleation density were similar for blends with various MFI. This further confirmed that PP and LLDPE were miscible at elevated temperatures since the more viscous LLDPE (lower MFI) reduced the crystallisation rate of PP at a greater degree.

Comonomer Distribution in Polyethylenes Analysed by DSC After Thermal Fractionation

Ethylene copolymers exhibit a broad range of comonomer distributions. Thermal fractionation was performed on different grades of copolymers in a differential scanning calorimeter (DSC). Subsequent melting scans of fractionated polyethylenes provided a series of endothermic peaks each corresponding to a particular branch density. The DSC melting peak temperature and the area under each fraction were used to determine the branch density for each melting peak in the thermal fractionated polyethylenes. High-density polyethylene (HDPE) showed no branches whereas linear low-density polyethylenes (LLDPE) exhibited a broad range of comonomer distributions. The distributions depended on the catalyst and comonomer type and whether the polymerisation was performed in the liquid or gas phase. The DSC curves contrast the very broad range of branching in Ziegler—Natta polymers, particularly those formed in the liquid phase, with those formed by single-site catalysts. The metallocene or single-site catalysed polymers showed, as expected, a narrower distribution of branching, but broader than sometimes described. The ultra low-density polyethylenes (ULDPE) can be regarded as partially melted at room temperature thus fractionation of ULDPE should continue to sub-ambient temperatures. The thermal fractionation is shown to be useful for determining the crystallisation behaviour of polyethylene blends.