P.D. Calvert

Synthetic and biological composites formed byin situ precipitation

Filler particles are frequently put into polymers to improve the stiffness or strength. There are a number of other applications such as magnetic coatings and piezoelectric transducers where second phase particles are introduced to give the composite special properties. Many biological materials, including bone, are also composite with particles reinforcing a polymer matrix. In the preparation of the synthetic materials the filler is normally blended into the polymer. In the biological materials the particles are grownin situ within the polymeric matrix, and under the control of the matrix. Here we review the growth of particles when constrained by a polymeric matrix, both in natural and synthetic materials. We discuss the principles governing such processes and the potential for the production of highly structured composite materials by this route.

Localization of oxidation in polypropylene

Abstract The distribution of oxidative degradation in isotactic polypropylene has been monitored using ultra-violet microscopy. Reaction of carbonyl groups in the oxidized polymer with 2,4-dinitrophenylhydrazine allows the oxidation to be seen with illumination at 350 nm. There is much extraction of oxidized material during the staining process. It is shown that oxidation initially favours the atactic fraction of the polymer. With increasing oxidation prior to crystallization rejection of oxidized material to the spherulite boundaries leads to increasing weakness in these regions although there is no evidence that their subsequent oxidation rate is enhanced. Localized oxidation in diluent-phase polymer is shown to correlate strongly with the distribution of catalyst residues in the polymer. Similar localization can be observed in polymer formed on a high efficiency, gas phase, catalyst although catalyst residues are not directly visible in this polymer.

Durham poly acetylene: preparation and properties of the unoriented material

Abstract Polyacetylene prepared by the Shirakawa route is a mat of crystalline fibrills, which are capable of being oxidized or reduced to a highly conducting state. Polyacetylene can also be produced from a soluble precursor polymer by a thermally driven transformation reaction (the ‘Durham’ route) as a solid, coherent film. This transformation consists of the elimination of a substituted benzene unit followed by the isomerization of the largely cis polyacetylene produced to a disordered trans isomer. We have studied the effect of different transformation conditions and from these studies it is possible to gain an insight into these processes. Techniques such as caloremetry, infrared, resonance Raman and optical spectroscopy, e.s.r., X-ray diffraction and electrical conductivity have all been used to probe the transformation and the product. The resultant information gives a consistent picture of the overall transformation processes detailed above but also shows an ordering associated with higher transformation temperatures. A thorough knowledge of these processes enables the orientation of the precursor to produce highly crystalline material.

Abrasive wear of particle-filled polymers

The abrasive wear rates of quartz and glass particle-reinforced PMMA have been measured as a function of filler volume fraction for silicon carbide, quartz and calcite abrasives. The wear rates were found to exceed those predicted by a simple series model. Abraded surfaces were studied by surface profile measurement and SEM. The excess wear is attributed to rapid wear of the filler at the matrix interface and to particle pulls-outs.

Stability and degradation of some electrically conducting polymers

Abstract The oxidative degradation reactions of polyacetylene, prepared from a soluble precursor polymer, are described and compared with those of more common polymers and of polyacetylene prepared by the conventional method. Because the band structure of the π-electron system of the polymer allows the formation of charge-transfer complexes with oxygen, the initial process in the pristine polymer is oxygen doping, with increasing conductivity. This is followed by irreversible degradation which is much faster than that of polyolefins or polydienes and faster than that of the crystalline polymer. Doping to low levels with electron acceptors removes the electrons involved in oxygen doping and the polymer becomes much more stable in air. Doping to high levels leads to new instabilities as the polymer reacts slowly with its counter-ions. Studies of polypyrrole and polythiophene show that these polymers are much more stable than polyacetylene but still undergo degradation reactions. The general features of their degradation mechanisms are discussed.

Thermal stability of electrochemically prepared polythiophene and polypyrrole

The degradation behaviour of electrochemically prepared polythiophene and polypyrrole has been studied by thermal gravimetric analysis technique. Studies on both the polymers show that they are more stable than polyacetylene but still undergo degradation reactions which involve two steps, viz. loss of dopant and then degradation of polymer backbone. The general features of degradation mechanisms are discussed.

THE SOLUBILITY OF STABILIZING ADDITIVES IN POLYPROPYLENE

Abstract The solubilities of several groups of common stabilizing additives have been measured in polypropylene over a range of temperatures. Regular solution theory is shown to give a good basis for extrapolation of data but cannot predict solubilities. Structural effects on solubility are discussed. The solubility of an additive is dependent upon intrinsic properties (melting point and heat of fusion) and upon interactions with the polymer and both may be equally important.

Studies of diffusion in polymers by ultraviolet microscopy: 2. Diffusion of atactic fractions in isotactic polypropylene

Abstract Ultraviolet (u.v.) absorbing and fluorescent labels have been attached to isotactic and atactic polypropylene by reaction with appropriate sulphonyl azides. The distribution is random and substitution does not affect the crystallization. The labelled polymer can be visualized by u.v. absorption or fluorescence microscopy. By monitoring penetration of isotactic polymer by labelled atactic, the diffusion coefficient of the atactic polymer has been determined over a range of temperatures. The redistribution of atactic polymer during growth of spherulites of isotactic polymer has been studied. Atactic polymer behaves quite differently to small additive molecules. The fibrosity of polypropylene spherulites is associated with rejection of atactic polymer from crystallizing regions. There is significant non-uniformity of atactic content between the spherulite centres and the boundaries.

Effects of water on glass-filled methacrylate resins

Composite materials have been prepared with up to 50 vol % glass beads in two methacrylate resins: polymethyl methacrylate and 64% bisphenol A di(glycidyl methacrylate) — 36% triethyleneglycoldimethacrylate. These composites were immersed in water at 60° C for up to six months and the changes in weight, elastic modulus and compressive strength were followed. In the polymethyl methacrylate composites the water uptake levels off after 5 days. The strength and modulus decrease over this period due to plasticization of the resin, but then increase again. The cross-linked methacrylate composites show the same decrease but no subsequent increase. Silane treatment of the glass beads reduces the water uptake by the resin and the decrease in modulus and strength.

FT-IR studies on thermal degradation of electrically conducting polymers

Abstract The degradation behaviour of electrochemically prepared polythiophene has been studied by the FT-IR spectroscopic technique. Studies show that it is more stable than polyacetylene but still undergoes degradation reactions which involve two steps, namely, loss of dopant and then degradation of polymer backbone. The general features of degradation mechanisms are discussed.