Copolymerisation of ethyl cellulose (EC) with methyl methacrylate (MMA) and vinyl acetate was carried out in solution, using a number of free radical initiators, to form graft copolymers with grafting yields up to 63% and weight average molecular weight about 9 x 104 g mol-1 Graft copolymerisation of MMA and EC appears to involve participation of comonomer charge transfer complexes at the initial stage of reaction, Interaction of EC with the carbonyl group of MMA enhances the electron-accepting ability of EC and promotes the formation of a donor-acceptor complex as matrix, with the resultant generation of a covalent bonded monomer radical, which then propagates in the usual manner to produce EC-g-PMMA. The influence of a number of experimental variables on the grafting efficiency of the EC-g-PMMA reaction, and the properties of the resulting copolymers, was investigated. The initiator has a significant effect on the grafting efficiency, which decreases in the order azobisisobutyronitril>ammonium persulphate>potassium persulphate>Ce(IV) ion. Benzoyl peroxide was found to be unsuitable because it promoted degradation of ethyl cellulose chains. The order of solvent influence on the grafting yield is chloroform>toluene>benzene. The grafting yield is also reaction time and temperature dependent. A2-3 hour reaction time and 40-50°C reaction temperature gives a substantial increase in both reaction rate and the molecular weight of the graft copolymer product. The introduction of PMMA graft side chains onto EC main chains leads to a significant decrease in the radius of gyration in solution, as determined by size exclusion chromatography, compared to linear EC. This finding is explained by the theory of Stockmayer and Fixman in terms of the number and type of long chain graft points in the molecule, Calculation of viscosity of EC-g-PMMA in solution using the Kurata equation gave values in good agreement with the experimentally determined viscosity. A significant result is that grafting PMMA branches onto EC reduces the hydrodynamic volume, and differential scanning calorimetry showed conclusively that increasing PMMA sidechain length reduces the chain flexibility, by reduction of free volume. The glass transition temperature of EC-g-PMMA, produced by using a large excess of MMA monomer and reasonably long reaction time, was shifted to higher temperature with increase of grafting yield and molecular weight. A single glass transition is observed for copolymers made using low initiator concentrations, and two glass transitions are found for higher initiator concentrations. In addition, EC-g-PMMA samples with higher grafting yield are more brittle than those with lower grafting yield. However, both the bending modulus (determined by dynamic mechanical thermal analysis) and elongation at break increase with grafting yield. Our experiments proved that the crystallinity of EC-g-PMMA is less than that of pure EC and greater than that of pure amorphous PMMA. The specific oxygen gas-permeability coefficients of EC-g-PMMA films decrease with increase of grafting yield. The reason is that grafting decreases of hydrodynamic volume. Interestingly, X-ray diffraction showed that the higher the grafting yields of EC-g-PMMA, the higher its crystallinity. Films of EC-blend-PMMA, made by solution casting and examined in cross section using scanning electron microscopy, showed an essentially multiple layer morphology with non-uniform dispersion of PMMA in the EC matrix. Scanning electron microscopy confirmed, as expected, that EC-g-PMMA cast films were homogeneous with no evidence of phase separation. The tensile strength of EC-g-PMMA films was typically about 20% higher than that of EC-blend-PMMA films, and the graft copolymer films exhibited greater oxygen permeability. A new model for graft copolymerisation to EC is proposed. The model provides for the possibility that grafting to EC with 48% ethoxy content can occur at the five OC2H5 groups and one OH group per unit. It was established using NMR spectrometry of EC-g-PMMA and EC-g-Poly (vinyl acetate) that under some conditions the ethoxy groups are the favoured graft sites. The liquid crystalline properties of EC and EC-g-PMMA were extensively investigated. Optical microscopy with crossed polarisers revealed that the relatively short mesomorphic temperature range of ethyl cellulose is extended downwards almost to ambient temperature in the graft copolymer. Low frequency Raman scattering showed that EC-g-PMMA has higher light scattering intensity than EC at ambient temperature. But it was lower than that of EC at 191°C. Moreover, the Jaynes-Cummings model cannot predict the light scattering intensity of EC-g-PMMA as a function of frequency very well, whereas the EIT-Kerr scheme is able to successfully account for the experimental observations. It was found that EC and EC-g-PMMA are pseudoplastic, lyotropic and thermotropic materials whose viscosity in solution is dependent on many factors, particularly on temperature and solution concentration. Both materials show unusual rheological behaviour, including maxima in the isothermal concentration dependence of viscosity. Unmodified EC exhibits thixotropic flow behaviour, whereas grafting PMMA side chains onto EC changes the flow behaviour to rheopectic, which is also typical of PMMA itself. As the temperature and concentration increase, the viscosity increases in response to the transition from an isotropic to an anisotropic state of EC-g-PMMA. Furthermore, experiments on the distinguishing characteristics of EC-g-PMMA indicate that different solvents affect its anisotropic flow behaviour to a greater or lesser extent. In particular, chloroform shows a stronger flow orientation effect than tetrachloroethane on EC-g-PMMA solutions. The rheological behaviour of EC and EC-g-PMMA in the nematic phase is much more complex than that of ordinary polymers or low molecular weight liquid crystals. The Doi theory is usually applicable to ordinary polymers or low molecular weight liquid crystals, but cannot predict shear stress as a function of shear rate; particularly at very low shear rates for EC and EC-g-PMMA. Whereas, the polydomain model is better to account for the observations. Measurements of thermally stimulated depolarisation current showed that EC-g-PMMA has a longer dipolar relaxation time in an electrical field. Molecular dipole motions of EC give a maximum depolarisation current at 138 °C in the glass transition region and another maximum near 200 °C, corresponding to dipolar relaxation in the liquid crystalline phase. EC-g-PMMA shows a broader dipolar relaxation temperature range in the liquid crystalline phase and the depolarisation current maximum shifts to a lower temperature. We have observed that field induced polarisation of EC-g-PMMA is affected by the length of the PMMA side chain, and obeys an interfacial model of the Maxwell-Wagner-Sillars type. It was confirmed by X-ray photoelectron spectroscopy that the length of the PMMA side chain could affect the surface properties. EC-g-PMMA has higher photoemission intensity than EC, and the photoemission intensity and depolarisation current increase with grafting yield.
| Identifer | oai:union.ndltd.org:ADTP/276089 |
| Date | January 2003 |
| Creators | Zheng, Betty Qilan |
| Publisher | ResearchSpace@Auckland |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated., http://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm, Copyright: The author |
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