Geoffrey R. Mitchell

The effect of dopant molecules on the molecular order of electrically-conducting films of polypyrrole

X-ray scattering curves have been measured for a range of electrochemically-prepared conducting polypyrrole films employing a variety of counterions in aqueous solutions. Films containing counterions based on aromatic rings exhibit an anisotropic molecular organization. The degree of anisotropy is enhanced through the use of highly planar counterions. The electrical conductivity of such films is also improved if the charge/volume ratio of the counterion is maintained at a high level. Polypyrrole films prepared using ‘spherically’ shaped counterions such as SO42− do not display such anisotropic molecular organizations, and exhibit lower electrical conductivities. The competing structural roles of the counterions within these molecular composites are discussed.

Structure of polystyrene glasses

Wide-angle X-ray scattering from both unoriented and axially deformed glassy polystyrenes (atactic and quenched isotactic) is compared with that calculated for isolated molecules in different conformations. No satisfactory fit is obtained. It is apparent that the scattering beyond s=1.0 A−1 is very similar to that from benzene and styrene, having a large contribution from contacts between phenyl groups attached to neighbouring molecules which are not represented in a single chain model. The only significant difference between the scattering of the polymer and the two low molecular weight liquids is that there is a small peak at s=0.75 A−1 (0.62 A−1 for the isotactic glass) which forms on polymerization and was first reported by Katz in 1927. For drawn samples the peak intensifies on the equator and apparently represents interchain correlations. However, in comparison with those of other non-crystalline polymers this interchain peak is weak, at a surprisingly low scattering angle in relation to the expected spacing of the chains and shows a very significant increase in intensity with increasing temperature. A model is proposed in which the phenyl groups segregate on a molecular scale to form stacks; there are fewer stacks than chain backbones and they have a low electron density core which expands considerably in relation to its small diameter as the sample temperature increases. The model accounts for the position and temperature sensitivity of the interchain ‘polymerization’ peak. It also shows some similarities to the organization of isotactic molecules in the crystalline state. The stacks of phenyl groups contribute to the X-ray pattern as if they were flexible superchains. The cylindrical distribution function derived from the scattering pattern of the oriented polymer indicates that the phenyl groups are in register in all directions over distances of the order of several chain diameters.

A wide-angle X-ray study of the development of molecular orientation in crosslinked natural rubber

Molecular orientation parameters have been measured for the non-crystalline component of crosslinked natural rubber samples deformed in uniaxial tension as a function of the extension ratio and of temperature. The orientation parapeters 〈P2(cosα)〉 and 〈P4(cosα)〉 were obtained by an analysis of the anisotropy of the wide-angle X-ray scattering functions. For the measurements made at high temperatures the level of crystallinity detected was negligible and the orientation-strain behaviour could be compared directly with the predictions of molecular models of rubber elasticity. The molecular orientation behaviour with strain was found to be at variance with the estimates of the affine model particularly at low and moderate strains. Extension of the crosslinked rubber at room temperature led to strain-crystallization and measurements of both the molecular orientation of the non-crystalline chains and the degree of crystallinity during extension and relaxation enabled the role of the crystallites in the deformation process to be considered in detail. The intrinsic birefringence of the non-crystalline component was estimated, through the use of the 〈P2(cosα)〉 values obtained from X-ray scattering measurements, to be 0.20±0.02.

Molecular organisation of electrochemically prepared conducting polypyrrole films

Molecular structure-preparation condition relationships are reported for electrochemically prepared polypyrrole-toluene sulphonate electrically conducting films. Polypyrrole films have been prepared from aqueous solutions employing a range of anodic potentials (0.5-1.6 V versus SCE) and at several temperatures. Consistently films grown using higher potentials and at lower temperatures exhibited greater electrical conductivities. The molecular organisation in these series of films was evaluated using quantitative X-ray scattering procedures. The X-ray scattering data show that polypyrrole molecules adopt a planar-type conformation in which the planes of the pyrrole moieties have a preferred orientation with respect to the electrode surface. This anisotropy is enhanced in samples prepared at low temperatures or at high anodic potentials, and such anisotropic films have greater electrical conductivity. Structural mechanisms for these observations are discussed.

Molecular orientation distribution derived from an arbitrary reflection

A straightforward method using Legendre series enables the orientation distribution in a specimen with uniaxial symmetry to be derived from the azimuthal profile of a single arbitrary reflection. Moreover, the moments of the distribution (P2n(cosα) can be obtained directly from the azimuthal profile without needing to calculate the complete distribution.

Structural studies of side-chain liquid crystal polymers and elastomers

Abstract The preparation of a range of acrylate-based side-chain liquid crystal elastomers is described. Materials containing up to 13 mol% of crosslinking units exhibit a nematic-isotropic transition. In the isotropic phase the mechanical properties are similar to those observed in conventional elastomers, and are used to obtain an estimate of the level of effective crosslinking. Wide-angle X-ray scattering procedures are employed to obtain structural information on macroscopically unaligned and aligned samples, the alignment being introduced by mechanical means. Analysis of the scattering from aligned samples shows that although the coupling chain length is short, the mesogenic side-chains are preferentially aligned in the direction of extension. Furthermore, the analysis shows that a considerable number of the coupling chains adopt gauche isomers. A more detailed examination of the alignment process shows that only a few percent extension is necessary to achieve high levels of molecular alignment, when the deformation is performed in the liquid crystal phase. Attention is drawn to the parallel with electro-optic and magneto-optic effects.

Molecular correlation in thermotropic copolyesters

Uniaxially oriented samples of thermotropic random copolyesters of hydroxybenzoic and hydroxynaphthoic acids, and of hydroxybenzoic acid and ethylene terephthalate, have been characterised in detail using wide-angle X-ray diffraction. The technique was used to measure the global chain orientation, and in the case of the first polymer the meridional scattering has been analysed in terms of diffraction from an isolated straight random chain. Optical microstructures were observed between crossed polars, with circularly polarized light and in plane-polarised light with no analyser present. Interpretation of the contrast seen shown that the material is optically biaxial and leads to the conclusion that, in the liquid-crystalline phase, there is long-range correlation of the rotations of the molecules about their chain axes. The polymers are examples of ‘biaxial nematics’. Thermal analysis indicates that the solid phase has a higher level of conformational order than the melt and that the melting range is very broad. Annealing below the melting point leads to the development of localised regions of enhanced order, especially in specimens with a high degree of overall orientation. The combination of electron diffraction and electron microscopy provides evidence that the lateral growth of ordered entities does not necessarily require regular sequences within the otherwise random copolymer molecules. Small crystals based on identical but non-periodic sequences within the molecules are (NPL crystals) proposed as being consistent with the experimental evidence and the implications of a brief statistical analysis.

A comparison of photoinduced poling and thermal poling of azo‐dye‐doped polymer films for second order nonlinear optical applications

Photoinduced poling (PIP) is a new technique which allows the room‐temperature preparation of guest/host polymer films exhibiting significant polar order for nonlinear optical applications. We report a comparison of this novel technique with the conventional electrode poling procedure performed at the glass transition temperature of the polymer using disperse red 1/poly(methylmethacrylate) films. In particular, in situ second harmonic generation measurements show that levels of polar order achieved using these two techniques are similar. In contrast, the stability of the polar order is reduced by up to 20 times in terms of the decay time constant in films prepared using PIP although the stability is very dependent upon the temperature at which the poling was performed.

Stereo‐thermal‐lithography: a new principle for rapid prototyping

A new fabrication process for rapid prototyping is proposed in this paper. Optical and thermal effects are simultaneously used in this process to locally induce a phase change in a liquid resin. This phase change phenomena is used to “write” three‐dimensional shapes or patterns. Such objects or patterns can involve macroscopic engineering prototypes through to nanostructures for exploitation in waveguiding and photonic crystals. Several advantages can be achieved through this new process, in terms of accuracy, cost and time.

Structure property relationships in polyethylene/montmorillonite nanodielectrics

The influence of a montmorillonite (MMT) nanoclay, functionalized with dimethyl-di(hydrogenated tallow) quaternary amine, on structural evolution and electrical characteristics of a designed polyethylene system has been studied. Samples were prepared by mixing a polyethylene/MMT masterbatch into a matrix system containing 10% high density polyethylene and 90% low density polyethylene using an extruder; X-ray diffraction results suggest good dispersion and exfoliation, as no basal peak was observed. The introduction of this MMT system was found to result in little disruption of the polymer crystallization process and analysis of the crystallization kinetics of the matrix polymer suggests that it interacts only weakly with the incorporated MMT. This is in sharp contrast to our previous studies of a differently functionalized MMT system. Electrically, this combination of highly dispersed MMT within a weakly interacting polymer matrix results in a significant enhancement in short-term breakdown strength. However, this is accompanied by a massive increase in dielectric loss.