The fractionation of dextran polymer by ultrafiltration to yield clinical products
A review of ultrafiltration (UF) theory and equipment has been made. Dextran is fractionated industrially by ethanol precipitation, which is a high energy intensive process. The aims of this work were to investigate the fractionation of dextran using UF and to compare the efficiency and costs of UF fractionation with ethanol fractionation. This work is the continuation of research conducted at Aston, which was concerned with the fractionation of dextran using gel permeation chromatography (GPC) and hollow fibre UF membranes supplied by Amicon Ltd. Initial laboratory work centred on determining the most efficient make and configuration of membrane. UF membranes of the Millipore cassette configuration, and the DDS flat-sheet configuration, were examined for the fracationation of low molecular weight (MW) dextran. When compared to Amicon membranes, these membranes were found to be inferior. DDS membranes of 25 000 and 50 000 MW cut-offs were shown to be capable of fractionating high MW dextran with the same efficiency as GPC. The Amicon membranes had an efficiency comparable to that of ethanol fractionation. To increase this efficiency a theoretical UF membrane cascade was adopted to utilize favourable characteristics encountered in batch mode membrane experiments. The four stage cascade used recycled permeates in a counter- current direction to retentate flow, and was operated 24 hours per day controlled by a computer. Using 5 000 MW cut-off membranes the cascade improved the batch efficiency by at least 10% for a fractionation at 6 000 MW. Economic comparisons of ethanol fractionation, combined GPC and UF fractionation, and UF fractionation of dextran were undertaken. On an economic basis GPC was the best method for high MW dextran fractionation. When compared with a plant producing 100 tonnes pa of clinical dextran, by ethanol fractionation, a combined GPC and UF cascade fractionation could produce savings on operating costs and an increased dextran yield of 5%. 37-5379 Photoexcitation and transport of charge carriers in Durham-route polyacetyiene Townsend, P.D. Cambridge Ph.D. 1987 D1h. In recent years there has been a rapid growth of interest in conjugated polymers such as polyacetyiene, which form a new class of organic semiconductors. The interesting electronic properties of polyacetyiene arise from the presence of delocalised v- electron states which are formed by the overlap of carbon p» orbitals along the polymer chain. In contrast to an inorganic semiconductor, the addition of charge to a polyacetyiene chain via photoexcitation or charge-transfer doping is not expected to result in the population of extended band states. Instead, since the carbon backbone has only two-fold coordination it is energetically favourable for the carriers to spatially localise by distorting the polymer chain. These defects may take the form of 'solitons' or 'polarons', and their motion is expected to be highly anisotropic with strong confinement to the chain through the structural relaxation. This anisotropy strongly affects both the nature of charge transport, and the probability of charge separation following photoexcitation. The conventional synthetic route to polyacetyiene involves the catalytic polymerisation of acetylene gas, and results in the formation of open-structured films consisting of randomly oriented fibrils. Since polacetylene is both insoluble and infusible, little structural modification is possible, and until recently these material problems had meant that it was not possible to experimentally investigate the proposed anisotropic electronic properties. In contrast, the Durham synthetic route makes use of a soluble and therefore processible precursor polymer, which can be cast in film form and thermally transformed to give a fully-dense form of polyacetyiene. Stretch-orientation of the precursor during the thermal transformation step results in the production of well-aligned and crystalline samples which are ideally suited for measurements of polyacetyiene's anisotropic physical properties. This Ph.D. dissertation describes the method which has been developed to produce these highly-oriented films, and shows how they have been used along with unoriented samples to gain an understanding of the charge transport and photoexcitation mechanisms in polyacetyiene. The characterisation of the samples used for these studies has been carried out by other workers using resonance Raman scattering, infra-red spec-troscopy and X-ray diffraction techniques. The results of these measurements are discussed, and used to interpret the effect of factors such as the degree of orientation, and the length of uninterrupted conjugation sequences on the observed electronic properties. Oriented films show evidence for longer straight chain sequences than in unoriented films, and this is considered to arise from the removal of conformational defects from the polymer chains during the stretch-orientation process. Information on the nature of the conformational defects is obtained from an analysis of the dispersion of the resonant Raman modes with excitation wavelength. As-grown Durham polyacetyiene is a semiconductor with transport properties characteristic of hopping conduction between localised states. Measurement of AC and DC conductivity and thermoelectric power have shown that the charge carriers present are extrinsic in origin, are positively charged, and exhibit an activated mobility with a hopping energy of 0.4 eV. It is argued that hops of this energy correspond to rate- determining inter-chain events which limit the DC conductivity. The possible identification of the carriers as like-charged soliton pairs is discussed, and comparison made with the results of photoconductivity measurements which indicate a similar transport mechanism for both photocarriers and dark carriers. Resistance network simulations of oriented films suggest that 'free' motion of carriers along chains takes place over distances no greater than 100A. Intra-chain transport is probably confined to such small distances by the presence of structural defects on the chains which tend to impede carrier motion. Experimental investigations of the optical properties of oriented Durham polyacetyiene has been carried out using polarisation-dependent measurements of optical absorption and reflectivity, photoinduced absorption (PA) and photoconductivity (PC). These measurements have shown that the generation of long- lived (msec) charged soliton-like defects in polyacetyiene requires the inter-chain separation of electron-hole pairs. This finding is in disagreement with existing theories which predict that soliton generation is an intra-chain process. Although sub- picosecond PA measurements have shown that intra-chain electron- hole excitations do relax to form charged solitons, these pairs undergo non-radiative geminate recombination on a timescale of less than 2 picoseconds. It is thought that this intra-chain decay route may involve a neutral intermediate state which gives rise to a PA feature at 1.4eV. Experiments in the millisecond time-range indicate that the small proportion of these neutral excitations which survive the short-time recombination are rendered metastable by existing structural defects in the polymer. Photoexcitation measurements on unoriented samples are also described and discussed.