1993 рік

A polycarbosilane is prepared by condensation between dichlorodimethylsilane and dianionic species of thiophene. The structure of the polycarbosilane is characterized by spectroscopic methods, gel permeation chromatography and thermal analysis. This polymer shows strong u.v. absorption at 2 .... as found for thiophene itself, which indicates that the a-Tt interaction through the polymer backbone is not significant. The polymer is also found to be photodegradable upon illumination with u.v. light.

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Monoethylphenyl itaconate was synthesized and polymerized. Polymer fractions were characterized by viscometry and light scattering. Thermodynamic and dimensional parameters were determined and calculated. The results are compared with those of poly(benzyl itaconate)s previously reported

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Molecular dynamics (MD) calculations have been used to study the behaviour of isolated rigid rod molecules of poly (p-phenylene), poly (p-phenylene benzobisthiazole) and poly (p-phenylene benzobisoxazole). The molecular mechanics force field was initially modified to improve agreement between minimized structural geometries and available X-ray data, as well as results from semiempirical molecular orbital calculations. The MD simulations show the molecules to be surprisingly flexible, with changes in end-to-end distances as large as 16%. An examination of the energies (calculated by various methods) associated with out-of-plane bending deformation, suggests that the rigid rod polymers may in fact be even more flexible than the simulations indicate. The results provide rationalizations for the relatively short persistence lengths measured in solution and for bending observed in high-resolution electron micrographs of these materials

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Side-chain liquid crystalline polymers (SCLCPs) with differing spacer groups have been prepared as copolymers or as blends of the individual homopolymers. The resulting materials have been examined by thermal and optical methods. The copolymers showed intermediate properties between those of the homopolymers. In contrast, blends of SCLCPs with either chemically distinct or similar side-chain spacer units showed two clearing points (both of which are lower than those of each homopolymer) and one major melting point. We interpret the latter results in terms of partial contamination of each phase by a small amount of the other phase.

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The kinetic features of vinylidene fluoride (VDF) radical polymerization in solution, initiated by bis(4-t-butylcyclohexyl)peroxydicarbonate, have been investigated in three different solvents (acetone, ethyl acetate and methyl acetate) under homogeneous conditions. Many significant deviations of the dependence of the polymerization rate on the 'ideal' 1.0 and 0.5 exponent values of monomer and initiator concentrations, respectively, are indicative of anomalous kinetic behaviours linked to concurrent reactions, which are operative during such polymerization. The complexity of the kinetic behaviour prevents, inter alia, the correct evaluation of chain transfer constants to the solvents from kinetic data. 19F nuclear magnetic resonance spectroscopy has been used in order to get full microstructural characterization of the low-molar-mass poly(vinylidene fluoride) (PVDF) synthesized under the above conditions. Both chain transfer constants to the solvent and extent of chain inversions in PVDF have been evaluated by this technique.

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The orientation development that takes place during injection moulding of liquid crystalline polymer (LCP) is analysed by FTi.r. specular reflection. The LCP used is the commercial Hoechst-Celanese product Vectra B900 based on terephthalic acid, p-aminophenol and 6-hydroxy 2-naphthoic acid. Dichroic ratios were measured from the 1016 cm 1 absorption band which is associated with a vibration of the 1,4-disubstituted benzene rings. The results, which are in good agreement with previous photoacoustic measurements, show that orientation is maximum along the principal axis of the mould with two significant minima of orientation located approximately at 2 and 3.2 cm from the injection gate. Furthermore, as the sample position shifts along the transverse direction towards the mould edge, there is a marked fall-off in orientation and the orientation minima occur at positions closer to the gate.

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Diphenyl-o-methoxyphenylmethyl methacrylate was polymerized with several organolithium complexes of chiral ligand such as (-)-sparteine (Sp) and (S,S)-(+)-2,3-dimethoxy-l,4-bis(dimethylamino)butane (DDB). (+)-DDB was effective in preparing a polymer of high optical rotation, whereas (-)-Sp only gave oligomers with low optical rotation for the repulsive hindrance between the bulky ester group and the rigid ligand. The optical rotation of the polymer decreased rapidly to a constant value due to the propeller-propeller transition, which has been demonstrated by ~H n.m.r, and circular dichroic spectra.

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Copolymers of 3-hydroxybutyrate (3HB) and 3-hydroxypropionate (3HP) were synthesized in Alcaligenes latus from mixed carbon substrates of sucrose and 3-hydroxypropionic acid. The composition of copolyesters varied from 0 to 26 mol% 3HP, depending on the fraction of carbon substrates. The microbial copolyester was characterized by 1H and 13C nuclear magnetic resonance (n.m.r.) spectroscopy, gel permeation chromatography and differential scanning calorimetry. The copolymers were shown to have a random sequence distribution of 3HB and 3HP units by analysis of the 125 MHz 13C n.m.r, spectra. The biosynthetic pathway of copolymers has been discussed

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Poly(4,4'-p-phenylenebis[2,6-diphenyl]pyridinium-alt-bisparaphenylene ditetrafluoroborate) and a model compound exhibit fluorescence emission from dilute solutions in dimethylformamide and vitrified samples dispersed in glassy poly(methyl methacrylate). The excitation and emission spectra, excitation polarization spectra and quantum yields for fluorescence were characterized in the dilute solution limit in dimethylformamide. Strong depolarization of the fluorescence was observed in dilute solution. The time-dependent decays of the intensity of the fluorescence were analysed to determine the lifetimes of the singlet excitation. In order to assist in the interpretation of the experiments, the conformations and molecular dynamics were investigated for an isolated dimer in the electronic ground state. The minimum energy conformations were found to be similar to those populated by other ortho-phenyl substituted para-phenylene structures in the solid state. The molecule experiences frequent transitions between rotational isomers that involve changes in dihedral angles in the main chain and between the main chain and pendant phenyl groups. The trajectory shows that there are correlations in these conformational transitions. The depolarization is attributed to an intramolecular non-radiative energy transfer mechanism coupled with frequent conformational changes in the main chain and pendant group torsional angles during the lifetime of the singlet excited state.

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The structural response of a molten main-chain liquid crystal polymer to an applied magnetic field has been investigated microscopically, with the microstructure being characterized as a function of field strength, time in the field and molecular weight. The application of the field to a structure in which the director wandered gradually on a scale of about 100 ~tm resulted in the formation of domains delineated by well defined boundaries. The domains were elongated along the field axis to an aspect ratio of about 8 in the case of a 1.1 T field, although for the higher field strength of 5.6 T the domains were no longer visible. Evidence is presented that the domain boundaries correspond to a director reorientation of ~ rad. Etched domain microstructure revealed a uniform distribution of crystallites, representing regions of local ordering in the material, on a scale of a few hundred nanometres. The crystallites had the form of platelets normal to the local director and acted as orientational markers across the domain boundaries.

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