David Neville Batchelder

The study of model polydiacetylene/epoxy composites: Part 1 The axial strain in the fibre

A mode composite system consisting of one polydiacetylene single crystal fibre in an epoxy resin matrix has been subjected to tensile strain parallel to the fibre direction. The strain at all points along the length of the fibre was determined by resonance Raman spectroscopy while that of the matrix was measured by conventional techniques. Comparison of the fibre and matrix strain showed two distinct regions. Below about 0.5% matrix strain the composite followed Reuss-type behaviour with equal stress in the fibre and the matrix. At higher matrix strain the composite followed Voigt-type behaviour with any increase in matrix strain matched by an equal increase in fibre strain. In this region the strain distribution along the length of the fibre could be approximately described by the shear-lag model of Cox. The critical length of the fibre was found to increase linearly with fibre diameter as predicted by that model. Good qualitative agreement was found with the predictions of a calculation based on finite element analysis over the full range of applied stress.

Study of model polydiacetylene/epoxy composites: Part 2 Effect of resin shrinkage

A model composite has been prepared consisting of a polydiacetylene single-crystal fibre in an epoxy resin matrix. The strain at points along the length of the fibre has been measured, using Raman spectroscopy, as a function of matrix strain and the post-cure temperature of the matrix. Good quantitative agreement has been found between the behaviour of the cold-cured system and the shear-lag model of Cox. The critical length of the fibres was measured as a function of fibre diameter and compared with recent calculations using finite difference methods. Higher post-cure temperatures have been shown to lead to compressive deformation of the fibres due to matrix shrinkage on cooling. The deformation is manifest as twinning, which disappears reversibly during tensile deformation. The amount of matrix shrinkage was determined by measuring the level of strain required to remove the twins. This was found to be in good agreement with the shrinkage determined from the linear thermal expansivity of the matrix.

The study of model polydiacetylene/epoxy composites

Raman spectroscopy and conventional mechanical testing have been employed to study a range of polydiacetylene fibre composite specimens with varying volume fractions cured at either room temperature for a week or at 100 ‡C for 24 h. The high-temperature cured composites were found to contain thermal stresses sufficient to cause twinning in the polydiacetylene fibres which affected the mechanical response of the bulk composite. A simple modification to discontinuous fibre reinforcement theory which accounts for the effect of thermal stresses in the material has been presented, giving good agreement with the experimental data. Small cracks were found to be produced at the end of the fibres by thermal stresses.

Raman spectroscopy of conductive polypyrroles

Recent studies of conductive polymers have focused on the need to produce air-stable processable materials. Since the discovery that conductive polypyrrole films could be prepared electrochemically [1], this polymer has attracted interest because of its much greater stability in air compared with polyacetylene [2]. Recently it has been shown that film morphology and mechanical properties are strongly influenced by the choice of electrolysis conditions. Films with thicknesses of 1 mm or more with properties ranging from rubbery to tough elastomeric can now be obtained with conductivities in the range 20 100 S/cm [3 5]. Factors that influence film properties are solvent, counter-ion and preparation temperature. It is clear that the differences observed at the macroscopic level reflect differences in polymer structure at the microscopic level. The ideal polypyrrole chain structure is a planar array of monomer units joined by ~ (2 5) bonds. This is not the only possible bonding sequence; other links also occur, as shown by the greater crystallinity that results when these links are blocked by the use of fi fl substituted monomers [6]. The elastomeric nature of the polymer films also suggests that they contain crosslinks. It has not yet proved possible to determine directly the concentration of these different defect structures in polypyrrole films by a non-destructive technique. In principle, Raman spectroscopy provides a method for characterization of polypyrrole films, since it has proved to be a powerful method for the study of other conjugated polymers [7]. To date there have been very few reports of Raman spectra for polypyrrole, particularly in the oxidized, conducting state [5]. The broad absorption bands and the rough sample surfaces result in dense black samples, which give weak Raman spectra. We have studied a number of samples prepared using different electrode materials and obtained well-resolved Raman spectra from the surface adjacent to the electrode. The spectra show well-resolved bands in the range 800 to 1600 cm 1. Differences in band positions and frequencies show a strong influence of the electrode material and growth temperature on polymer structure. As yet, a complete assignment of the observed spectra has not been possible. Model calculations and comparison of other polymer properties are being used to aid interpretation.

Deformation mechanisms in polymer crystals Part 2 Twinning in macroscopic single crystals of bis(p-toluene su/phonate ) diacet ylene polymer

Polymeric sulphur nitride (SN)x has been recently found to undergo a stress-induced martensitic transformation from the normal monoclinic phase to a new orthorhombic phase. The theories of shear-induced transformations in crystalline solids developed by Bevis, Crocker and co-workers have been used to predict the shear modes between the two forms of (SN)x. A comparison has been made between the theoretically predicted and experimentally measured modes for (SN)x and the transformation has been compared with a similar one that is observed in polyethylene.

Residual strain mapping in carbon fibre/PEEK composites

Abstract The strain dependency of the Raman frequencies of carbon fibres has been used in order to map the residual fibre strains in a HMS4/PEEK composite tape. The fibres have been found to be subjected to residual compressive strains of the order of 0.2%. Such compressive strains are thought to be developed principally during manufacture.

Surface enhanced Raman spectroscopy of molecular oxygen adsorbed on polydiacetylene single crystals

Abstract Surface enhanced Raman scattering has been observed for O2 molecules adsorbed on polydiacetylene single crystals. The spectral dependence of the Raman cross section suggests that the enhancement is due to a well-defined electronic transition of energy 2.39 eV which involves a significant degree of charge transfer from the conjugated polydiacetylene backbone to the O2 molecule.

The Study of Electronic Excitations of Polydiacetylenes and Resonance Raman Spectroscopy

Much of what is known about the electronic excitations of PDAs has been learned from optical and resonance Raman (RR) spectroscopy (1, 2). In addition to measurements on single crystals the polymer has been studied in solutions, precipitates and thin films. This paper will attempt to review the experimental evidence for the nature of the electronic excited states in PDAs which are accessible by these spectroscopic techniques. Starting from the optical spectra for single crystals the effect of changing environment and conformation on the dominant electronic transition will be explored. Additional information, particularly about vibrational coupling, which can be brought out by RR spectroscopy will be included. Finally, electronic transitions which have been detected by RR spectroscopy when small molecules such as oxygen are adsorbed by the PDA backbone will be discussed. Selection of the spectroscopic data has been made with the aim of giving a representative rather than a complete picture of the experimental situation.

Electron microscopy of photo-oxidized polydiacetylene crystals

The photo-oxidation of toluene sulphonate polydiacetylene single crystals has been studied by scanning and transmission electron microscopy. Submicron particles were observed as reaction products on the surfaces of crystals which had been exposed to 514 nm light from an argon ion laser. Electron diffraction showed that these particles were single crystals ofparatoluene sulphonic acid. The appearance of the particles was correlated with the decrease in intensity of a Raman line associated with the presence of adsorbed oxygen on the crystal surface.

Pressure dependence of phase transitions in two diacetylene crystals

Abstract The phase transitions which occur in a diacetylene polymer crystal (PTS) at 195 K and a diacetylene monomer crystal (DCH) at 142 K have been studied as a function of pressure. For the second-order phase transition in PTS the pressure dependence of the transition temperature was found to be less than 0.05 K MPa −1 ; for the first-order transition in DCH it was 0.63 ± 0.08 K MPa −1 .