Fractionation and Crystallization of Isotactic Polypropylenes Prepared Using Homogenous Metallocene Catalyst

A series of polypropylenes were prepared using rac-ethylene-bis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride (Et(H4Ind)2ZrCl2 (EI(4H)) as a homogeneous metallocene catalyst. The molecular weight of the polypropylene polymers was controlled using molecular hydrogen which was used as a transfer agent. To obtain materials with differing molecular weight and tacticities, polymers were fractionated with prep-TREF. Polypropylene polymers and fractions were fully characterized using various analytical techniques: differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), 13C nuclear magnetic resonance spectroscopy (13C-NMR) and high temperature gel permeation chromatography (HT-GPC). It was found that the addition of molecular hydrogen to propylene polymerizations catalyzed using EI(4H) catalyst system resulted in a reduction in polymer molecular weight and isotacticity. Furthermore, the molecular weight, the isotacticity and the degree of crystallinity of the TREF fractions increased significantly with increasing elution temperature for all samples. Also, only the crystal form existed in all TREF fractions of different samples.Civil and Chemical Engineerin

A better mousetrap? a new look at some old issues

Albert van Reenen was born in Stellenbosch in 1957. He obtained both his MSc (1985) and PhD (1988) in polymer science at Stellenbosch University. He joined Stellenbosch University as a researcher at the Institute for Polymer Science in 1989 and was subsequently appointed as senior lecturer in the Department of Chemistry. From his early research, which focused on polymeric membranes for waste water treatment, an interest in understanding the fundamental relationship between polymer microstructure and macroscopic material properties developed. Coupled with a long-standing collaboration with local industry, Prof van Reenen’s research has been primarily focused on the structure-property relationship in commercially important thermoplastics, in particular the polyolefins. Related to this are research projects that encompass wood-polymer composites, polyolefin nanocomposites, and the study of solution crystallisation of polymers. A new focus area that is being developed at present is the use of bioactive polymers or polymer systems in packaging applications. The collaboration with industry has also allowed him to leverage substantial funds for the purchase of analytical equipment that is essential for successful research. Professor Van Reenen has a keen interest in furthering polymer science education in Southern Africa and through a collaborative effort with colleagues regularly presents short courses and workshops in both academia and for local industry

Poly(2-propenoic acid) co- and terpolymers for dynamic membranes

Thesis (Ph. D.) -- University of Stellenbosch, 1988.Full text to be digitised and attached to bibliographic record

The synthesis, characterization and properties of zirconium-chelating polymers

Thesis (M. Sc.) -- University of Stellenbosch, 1985.Full text to be digitised and attached to bibliographic record

Molecular weight and tacticity effect on morphological and mechanical properties of Ziegler–Natta catalyzed isotactic polypropylenes

Abstract The morphological and mechanical properties of Ziegler–Natta catalyzed isotactic polypropylenes as influenced by the molecular weight and tacticity was investigated. Polypropylene samples were injection moulded into standard disks with a HAAKE MiniJet II injection moulder at 190 °C and 200 bar for morphological and mechanical tests. The morphological and mechanical properties of specimens were investigated by means of optical microscope (OM), scanning electron microscopy (SEM), microhardness (MH) and dynamic mechanical analysis (DMA). The samples exhibited a typical α-modification spherulite structure of isotactic polypropylenes crystallized from the melt. It was found that the most important factor affecting the structure and properties of these polymers is the isotacticity content. A clear molecular weight effect was also found for samples with low molecular weights. The microhardness and storage modulus values increased as crystallinity did. Accordingly, isotacticity degree is considered as the main parameter affecting the crystallinity of samples

Polymerization of Olefins With Bulky Substituents. 1. Homo- and Copolymerization of 3-(1-adamantyl)Propene

The use of bulky substituents like adamantane has been shown in the past to influence the glass transition temperatures of polymers significantly. In this paper the synthesis and homopolymerization of the monomer 3-(1-adamantyl) propene is described, as well as the copolymerization of this monomer with ethene, propene, 1-pentene, 4-methyl-1-pentene, 1-hexene and 1-octene. The resultant copolymers proved to be largely insoluble in organic solvents. It was also demonstrated that the presence of the bulky methyladamantyl side group influenced the glass transition temperatures of the copolymers in comparison with the corresponding homopolymers. (C) 2003 Elsevier Ltd. All rights reserved

CHEMICAL FORCE MICROSCOPY ANALYSIS OF WOOD-PLASTIC COMPOSITES PRODUCED FROM DIFFERENT WOOD SPECIES AND COMPATIBILIZERS

Alien invasive species are posing a serious and direct threat to biodiversity, water security, and productive use of land in South Africa. Most of these species need to be cleared and are therefore regarded as waste material, which could become raw material for wood-plastic composites (WPCs). WPCs containing wood from Pinus radiata, Eucalyptus grandis, Acacia mearnsii, Acacia longifolia, Acacia saligna, and Casuarina cunninghamiana trees, low-density polyethylene (LDPE) and three different compatibilizers: namely the commercially available ethylene vinyl alcohol (EVOH), polyethylene graft-maleic anhydride (PE-g-MA), and thermally degraded LDPE (dPE)—were studied. The determined properties included MC, density, tensile strength, and adhesive forces between the wood and compatibilizer components. The adhesive forces were determined using chemical force microscopy with functionalized, coated tips. WPC samples were compounded and injection molded. EVOH as compatibilizer proved to be very sensitive to the wood species incorporated into the WPC blend. Composites containing PE-g-MA and dPE as compatibilizer had a higher tensile strength for all the wood species. Composites containing dPE as compatibilizer showed less variation in all samples for tensile strength and adhesive force measurements. The densities and tensile strengths of the samples compares well with some commercial WPCs. The study shows that the inexpensive dPE outperforms commercially available compatibilizers and effectively promotes adhesion in WPCs. It was also shown that the studied invasive wood species can be incorporated into WPCs, if the correct compatibilizer is chosen. The differences in the results of the study seem difficult to relate due to the many factors such as the wood species, MC, density, compatibilizers, and processing method. However, the micro properties can give enough information regarding the macro properties of WPCs.

Larix during the Mid-Pleniglacial (Greenland Interstadial 8) on Kotelny Island, northern Siberia

Needles, wood and pollen of Larix, recorded in a peat deposit from Kotelny Island, northern Siberia, indicate the local occurrence of larch around 38 000 cal. a BP, which is during the Middle Pleniglacial (Greenland Interstadial 8) of the Weichselian last glacial period. The pollen record, dominated by Cyperaceae, Poaceae, Betula, Alnus, Salix and Artemisia, indicates steppe‐tundra conditions with some shrubs and trees. The distribution of Larix species plays an important role in the reconstruction of climatic conditions during the Weichselian in Siberia. Nowadays Larix does not occur on Kotelny Island and in the present situation the temperature may not be the limiting factor for Larix, but instead the long, snowy winters and the moist tundra vegetation. Our data may contribute to a better understanding of Larix refugia and Weichselian climatic conditions in northern Siberia

Solution Crystallization and Dissolution of Polyolefins as Monitored by a Unique Analytical Tool: Solution Crystallization Analysis by Laser Light Scattering

For the first time, the solution crystallization and dissolution behavior of polyolefins in a variety of solvents was investigated by using a recently developed crystallization based analysis technique, solution crystallization analysis by laser light scattering (SCALLS). SCALLS results provide clear evidence that crystallization and dissolution of linear polyethylene (PE) and isotactic polypropylene (iPP) are greatly influenced by the type of solvent used. It was demonstrated for a blend of PE and iPP that cocrystallization effects are minimal in solvents such as TCB and <i>o</i>-DCB and are significantly more pronounced in xylene and decalin. Surprisingly, in xylene, individual dissolution curves (bimodal SCALLS profile) for both PE and iPP with minimal codissolution effects were observed while in TCB, <i>o</i>-DCB, and decalin both components dissolve simultaneously. These findings provide a novel and facile approach to understand the effect of solvents on cocrystallization and codissolution of chemically dissimilar components in preparative fractionations such as prep TREF (which normally uses xylene), by using TCB as the crystallization solvent and xylene as the eluent