Fred J. Davis

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.

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.

Liquid-crystalline elastomers

Liquid-crystalline elastomers are produced by the introduction of crosslinking into liquid-crystalline polymer systems. This crosslinking results in materials with a number of unusual properties. Of particular note are the effects of an applied stress coupled with such a system. Theory predicts that these should include stress-induced phase transitions and spontaneous elongation of samples in the liquid-crystalline phase; such properties have now been observed experimentally. The materials exhibit a number of other properties, also induced by the coupling of network elasticity to the liquid-crystalline phase. These include a temperature-induced memory effect; materials with a chiral mesophase exhibit piezoelectric behaviour.

Molecular Switching in Liquid Crystal Elastomers

Abstract The synthesis is described of some liquid crystal polymers containing various degrees of cross linking units. Such materials show properties over and above those observed either from low molecular weight liquid crystals, or from conventional rubbers. Thus, though these compounds show simple rubber like elasticity in the isotropic phase, on cooling to the liquid crystal region, more complex behavior is observed, an example being the high levels of orientation produced at relatively low strain. Such materials also show unusual properties in that, when swollen with a low molecular weight liquid crystal, they undergo macroscopic shape changes in the presence of applied electric fields, in addition to the more usual electro-optic effects.

Coupling between mesogenic units and polymer backbone in side-chain liquid crystal polymers and elastomers

The levels of alignment of the mesogenic units and of the polymer backbone trajectory for polyacrylate based nematic side-chain liquid crystal polymers and elastomers were evaluated by using wide angle X-ray and small angle neutron scattering procedures. The X-ray scattering measurements show that substantial levels of preferred orientation of the mesogenic units may be introduced through magnetic fields for uncrosslinked polymers and through mechanical extension for liquid crystal elastomers. Small angle neutron scattering measurements show that for highly aligned samples an anisotropic polymer backbone trajectory is observed in which the envelope is slightly extended by ∼ 10% in the direction parallel to the axis of alignment of the mesogenic units. The sense of this coupling differs from that recorded for other uncrosslinked side-chain liquid crystal polymers. Possible mechanisms to account for this anisotropy and its relationship to the properties of liquid crystal elastomers are discussed. The observed deformation behaviour of the liquid crystal elastomer is non-affine and this appears to confirm the dominating influence of the liquid crystal order of the side chains on the mechanical properties of these novel networks.

The role of the counter-ion during electropolymerization of polypyrrole-camphor sulfonate films

Abstract The electrochemical preparation of polypyrrole-based films doped with camphor sulfonate which exhibit an anisotropic molecular organization is reported. The role of the dopant in determining the final structure of the film is explored by polymerization from electrolytes containing mixed counter-ion species. Smaller counter-ion units are preferentially included in the growing polymer film at the expense of the bulkier camphor sulfonate units. The mechanism of electropolymerization is discussed, particularly with reference to the interactions between the growing chain and the electrolyte which are dependent upon the hydrophobic/hydrophilic nature of the dopant.

Liquid crystalline elastomers: the relationship between macroscopic behaviour and the level of backbone anisotropy

Nematic monodomain liquid crystalline elastomers have been prepared through in situ cross-linking of an acrylate based side-chain liquid crystalline polymer in a magnetic field. At the nematic–isotropic transition, the sample is found to undergo an anisotropic shape change. There is found to be an increase in dimensions perpendicular — and a decrease parallel — to the director, this is consistent with alignment of the polymer backbone parallel to the direction of mesogen alignment in the nematic state. From a quantitative investigation of this behaviour, we estimate the level of backbone anisotropy for the elastomer. As second measure of the backbone anisotropy, the monodomain sample was physically extended. We have investigated, in particular, the situation where a monodomain sample is deformed with the angle between the director and the extension direction approaching 90°. The behaviour on extension of these acrylate samples is related to alternative theoretical interpretations and the backbone anisotropy determined. Comparison of the chain anisotropy derived from these two approaches and the value obtained from previous small-angle neutron scattering measurements on deuterium labelled mixtures of the same polymer shows that some level of chain anisotropy is retained in the isotropic or more strictly weakly paranematic state of the elastomer. The origin and implications of this behaviour are discussed.

Effect of media polarity on the photoisomerisation of substituted stilbene, azobenzene and imine chromophores

The influence of substituents and media polarity on the photoinducedE→Z geometrical isomerisation of the stilbene, azobenzene and N-benzylideneaniline chromophores has been compared and assessed. The efficiency of the process in all three systems is markedly dependent on the presence and characteristics of electron-donor and electron-acceptor substituents at the 4- and 4′-positions. The results are discussed in terms of relaxation of the E-excited singlet state. In the absence of a nitro substituent, relaxation to the S 1 orthogonal state competes effectively with non-productive intramolecular electron transfer; in the presence of a nitro substituent, the T 1 orthogonal state is formed from inter-system crossing. For systems with a 4-nitro and a 4′-electron-donor substituent, access to the triplet state is inhibited by polar solvents promoting formation of the inactive charge-transfer state from the S 1 state, and no isomerisation is observed. Similar effects are observed in both solution and polymer films. Such variations in behaviour have important implications for the utilisation of the chromophores in nonlinear optical phenomena including photorefractivity.

Highly anisotropic electrically conducting films based on polypyrrole

Abstract Electrically conducting films of polypyrrole prepared using a pulsed electrochemical potential exhibit a marked increase in molecular anisotropy, electrical conductivity and surface smoothness, as compared to equivalent films synthesized under constant potential conditions. These enhancements are only observed when the polypyrrole films are prepared from aqueous electrolytes containing toluene sulphonate as the dopant or counterion. X-ray scattering patterns show that the polypyrrole films are highly anisotropic with the planes of the pyrrole rings lying preferentially parallel to the electrode surface. The pulse lengths and voltages are most important to preparing enhanced films and appear to indicate that these improved films grow as a consequence of an increase in the number of equivalent growth sites.

Liquid crystal elastomers: controlled crosslinking in the liquid crystal phase

A method of chemically crosslinking side-chain liquid crystal polymers at carefully controlled temperatures and with variable crosslinking densities in the bulk phase is described. The nematic-isotropic phase transition temperatures of the resulting liquid crystal elastomers are found to show a marked dependence on the history of the sample. In particular, crosslinking a polymer in the nematic phase results in an elastomer in which the nematic phase shows an enhanced stability. The results are compared with theoretical expectations, and related to the coupling between the mesogenic units and the polymer backbone.