1993 рік

A new class of free radical photoinitiators based on pyrazolone azomethine dyes has been investigated. Results show that the free radical formation occurs via intermolecular electron transfer. Changes in the dye structures are translated into changes in the rate of polymerization. Dyes which have electron withdrawing substituents initiate the free radical polymerization more readily than those with electron releasing groups. The introduction of heavy atoms into the dye molecule decreases the rate of initiation of free radical polymerization.

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Elastic modulus E l of the crystalline regions of poly(ethylene-2,6-naphthalate) (PEN-2,6) in the direction parallel to the chain axis was measured by X-ray diffraction. The El value for the (74 1 1 0) plane of PEN-2,6 was 145 GPa at room temperature, which coincided with the calculated value (144 GPa) obtained by Treloar's method. The E~ value of PEN-2,6 is larger than that of poly(ethylene terephthalate) (observed, 108 GPa; calculated, 122 GPa). This difference could be attributed to the difference of the deformability between naphthalene and phenyl rings.

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A lattice theory is presented for liquid-liquid equilibria in binary systems containing random copolymers. This theory takes into account deviations from random mixing through a non-randomness factor which follows from a generalization of Monte-Carlo calculations for the three-dimensional Ising model. While the lattice remains incompressible, the effect of specific interactions (hydrogen bonding) is introduced by superimposing on the non-random (Ising model) expression for the Helmholtz energy of mixing a correction based on the lattice-gas model by ten Brinke and Karasz. The resulting theory can predict immiscibility caused by lower critical solution temperatures. Several theoretical miscibility maps at fixed temperature were computed; these are compared with those predicted by the random-mixing Flory-Huggins theory. Theoretical miscibility maps are also compared with experiment for a few systems with strong specific interactions.

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Sterically stabilized colloidal dispersions of polypyrrole have been prepared using a novel tailor-made block copolymer stabilizer of narrow molecular weight distribution, synthesized by group-transfer polymerization techniques. These polypyrrole dispersions have a spherical morphology, are slightly flocculated and have a relatively high stabilizer content (54% w/w)

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Polystyrene (PS) microparticles were prepared by freeze-drying a very dilute cyclohexane solution and by spraying a dilute benzene solution into methanol. D.s.c. studies showed that the microparticles have a transition at a temperature lower than the Tg of bulk PS. This low temperature transitiotJ was regarded as the glass transition of the PS microparticles, and it was suggested that the decrease in Tg of the particles was due to their high surface area to volume ratio.

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Coil-helix transition has been studied for poly(L-lysine) salts, with counterion species C1- (PLLHC1), Br- (PLLHBr) and NO3 (PLLHNO3), in aqueous alcohol mixtures (water/MeOH, EtOH, 2PrOH, tBuOH) as a function of the alcohol composition and/or salt concentration. The dependence of the helix content on the solvent species and composition is interpreted in terms of the three solvent properties i.e. dielectric constant, Gutmann-Mayer acceptor number and water activity, which have been found to be critical for contact ion-pair formation in the study of poly(L-glutamic acid). The salt-induced transition is also analysed by taking account of the contact ion-pair formation. Comparisons are made with the results on the poly(L-glutamic acid) systems

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By application of absorptive correction factor and some other improvements, the general procedure for evaluating apparent crystallinity and substantial crystallinity from wide-angle X-ray scattering patterns of binary and ternary copolymers with sufficient thickness was extended to multicomponent polymers with any thickness. The total correction factors of semicrystalline poly(ethylene oxide), amorphous poly(methyl methacrylate), polyacrylate acid, poly(ethyl acrylate), poly(butyl acrylate), poly(dimethylsiloxane), polystyrene, polybutadiene and poly(propylene oxide) were calculated. The results showed that when a diffractometer is used with reflection geometry, the influence of absorption factor can be ignored when la ~>0.5, but it should be taken into account when #t <0.5 (# is linear absorption factor and t is sample thickness).

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Processable precursor polymers to aramids, poly(imino-l,4-phenyleneiminocarbonyl-l,4-phenylenecarbonyl) and poly(imino-l,3-phenyleneiminocarbonyl-l,4-phenylenecarbonyl), were prepared and studied. The precursor polymers were made from the reaction of 2,5-dichlorobicyclo[2.2.2]octane-l,4-dicarbonyl chloride with 1,4-phenylenediamine or 1,3-phenylenediamine. The polymeric precursors were subsequently converted in approximately 97% yields, via dehydrohalogenation and retro Diels-Adler reactions, to the corresponding aramids by refluxing in 1 M solution of potassium hydroxide or sodium hydroxide. Only the p-phenylene precursor polymer exhibited liquid crystal characteristics in solution. The solubility and thermal properties were examined.

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The solubility and association of water-soluble copolymers of N-(2-hydroxypropyl)methacrylamide (H PMA) containing both p-nitroaniline-terminated oligopeptide side-chains and N-2-hydroxyethylpiperazine-N'- ethanesulfonic acid (HEPES) side-chains in aqueous solvents were investigated using light scattering methods. The weight-average molar masses, second virial coefficients, diffusion coefficients and hydrodynamic sizes of the copolymer molecules and their aggregates were studied as a function of the copolymer concentration, temperature and solvent pH. The copolymers with a low content (< 2 mol%) of hydrophobic side-chains terminated in p-nitroaniline were soluble molecularly in aqueous solutions. The water solubility was substantially enhanced by the presence of HEPES side-chains. The copolymers with a high content (~ 7 mol%) of p-nitroaniline-containing side-chains associated in water, generally forming two types of micelles of different sizes. Micelle formation depended on hydrophobic and amphiphilic side-chain content, polymer concentration, temperature and solvent pH. Thus, macromolecules containing a higher amount of p-nitroaniline and a small amount of HEPES side-chains tended to form small, compact micelles with the hydrophobic p-nitroaniline molecules oriented into the core, while macromolecules with a high content of HEPES side-chains tended to form large micelles. The formation of large micelles was strongly dependent on solvent pH, due to the electrolyte behaviour of the HEPES moieties. The solution properties of the copolymers generally reflected the competition between these two association processes.

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The room temperature crystal structures, unit cell dimensions at ll0 K and phase transitions of three poly(p-phenylene) oligomers are reported. The structures of p-quinquephenyl (PQP), C3oH22 , p-sexiphenyl (PSP), C36H26 , and p-septiphenyl (PSeptiP), C42H30, each belonging to space group P21/c, are similar to those of shorter oligomers. The unit cell dimensions are a = 22.056 A, b = 5.581 A, c = 8.070 A and/~ = 97.91 ° for PQP, a=26.241A, b=5.568A, c=8.09lA and /~=98.17 ° for PSP, and a=30.577A, b=5.547A, c=8.034A and fl=100.52 ° for PSeptiP. The a axis increases with molecular length. The molecules are linear and planar in all three structures. The herringbone nature of the packing is similar for PQP and PSeptiP, while a considerably greater tilt occurs in PSP. At 110 K, the unit cell parameters b and c are approximately doubled while a remains nearly the same as in the room temperature cell. A time-dependent solid state transition is observed for PQP, PSP and PSeptiP when crystals are cooled to 110 K. At elevated temperatures, thermal measurements indicate the oligomers to be thermotropic liquid crystals. The crystal smectic transition temperatures are reported for PQP, PSP, PSeptiP and p-octiphenyl (POP), C4sH34. The results of a molecular mechanics study on the conformation and packing of PSP are also presented. The competition between intramolecular forces (such as ortho hydrogen repulsions) and intermolecular crystal packing forces was examined in particular. Molecular mechanics calculations predict non-planar conformations in isolated polyphenyls, implying that conjugation between phenyl rings is insufficient to overcome ortho hydrogen repulsions. In a crystalline environment, however, intermolecular forces tend to force a planar conformation. Calculations on arrays of PSP molecules show that changing the phenyl-phenyl torsion angles from the coplanar value increases the total energy of the structure. The most favourable intermolecular interactions between oligomers are achieved for conformations having the phenyl rings coplanar.

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