1992 рік

A method of structural analysis based on analogies and differences in the X-ray diffraction patterns of unoriented samples of chemically related polymers is proposed. By this method we study a family of new basic multifunctional polymers based on the repeat unit -S-CH2-CH 2-X-CH2-CH 2-S-CH2-CH2-Y-CH2-CH 2 - where the functional groups X and Y can be considered as independent variables. Eleven patterns are compared here and used in the discussion.

Детальніше...  

Solid-state polymerizations of N-carboxy anhydrides (NCAs) of ~-amino acids (L-leucine, L-alanine, 7-benzyl-L-glutamate and glycine) were carried out using butylamine as initiator in hexane at 20 50°C. The results were compared with those for polymerization in acetonitrile solution. L-Leucine NCA was the most reactive among the NCAs examined, and formed high-molecular-weight polypeptide in the solid state. In contrast, L-alanine NCA polymerized faster than any other NCAs examined and formed high-molecular-weight polypeptide in acetonitrile compared to the solid-state polymerization. X-ray analysis, electron microscopy and infra-red spectroscopy revealed that L-leucine NCA polymerized to form a-helical polypeptide predominantly along the c axis in the crystal, while the a-helical polymer grows in random directions in the crystal of L-alanine NCA. Although 7-benzyl-L-glutamate NCA polymerized a little slower in the solid state than in acetonitrile, its polymerization reached 100% conversion and formed polypeptide with higher molecular weight than the value expected from the molar ratio of the monomer to the initiator. Glycine NCA polymerized very slowly in the solid state. These differences could be explained by their crystal structures. Favourable features for solid-state polymerization were found in the crystal of L-leucine NCA : five-membered rings of the NCA make a layer sandwiched by two hydrophobic side-chain layers and the NCA molecules polymerize to form the a-helical polypeptide in a layer without any interference from the other layers. On the other hand, such a layer structure was not seen in the crystal structure of L-alanine NCA. In the crystal of 7-benzyl-L-glutamate NCA, the layer structure similar to L-leucine NCA was observed and this seemed to activate the polymerization of the NCA, while the long side chains were considered to make molecular rotation difficult to some extent in the crystal. Glycine NCA seemed to be stable because of the dimer structure in the crystal

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The molecular structure of ultradrawn films of polyethylene has been studied with a combination of different techniques of vibrational spectroscopy, namely attenuated total reflection at different angles of incidence (thus doing optical microtoming), infra-red absorption and Raman scattering in various geometries and polarizations. The experimental results are interpreted on the basis of the theoretical concepts of the lattice dynamics of ordered and disordered polymethylene systems previously worked out. A gradient of molecular and crystalline structures is found from the surface into the core. The skin is highly orthorhombic; moving inside the film, both monoclinic and orthorhombic lattices coexist. Moreover, in the core a new phase with 'isolated' trans-planar sequences is observed. Various kinds of collective and oriented or isolated conformational disorder are identified. From the experimental data collected, a molecular model is proposed for the process of orientation during drawing. A problem of general relevance in Raman scattering of polymers is presented by the fact that the scattering observed arises only from the upper skin of the sample.

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3C n.m.r, spin-lattice relaxation times have been determined for carbons in a number of solid alkanes and in two linear polyethylenes. ~3C spin-lattice relaxation times, Tls, for the internal methylene carbons of the linear alkanes depend upon structure and chain length. There is almost a 100-fold increase in T1 in going from orthorhombic C2~H44 to C6oH~22. The central methylene carbons in the two polyethylenes give rise to two exponential decays. Analysis of the methyl resonances in one of these polyethylenes suggests that 60% of the methyl ends are in a crystalline environment. Low molecular weight orthorhombic alkanes are more mobile than expected from the 1 / T~ versus a dimension of the unit cell relationship observed for linear and branched polyethylenes

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The morphologies of poly(ethylene terephthalate) (PET) films annealed at various temperatures after drawing to various extents were studied by measurements of density and birefringence, wide-angle X-ray analysis, electron microscopy and measurement of the contraction on annealing. It is proposed that a new type of texture with a-axis orientation is formed when the resultant films have negative birefringence. Namely, thin lamellae, in which polymer molecules are folded, are aligned preferentially along the original drawing direction, which means that polymer molecules are oriented preferentially perpendicular to the drawing direction. It is suggested that, on annealing a drawn film with a slight positive birefringence and very low crystallinity, crystallites or a similar conformation of molecules (nuclei) with c-axis orientation may rotate, the extent depending on that of the frozen elongation of the amorphous tie-chains between the crystallites or nuclei, so as to make the tie-chains relax, giving rise to the texture in question.

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Preparation of a good hydrogel supported on a fluorinated polymer film has been achieved by means of simultaneous radiation-induced grafting of N-vinyl-2-pyrrolidone (NVP) onto (tetrafluoroethyleneperfluorovinyl ether) copolymer (PFA). The conditions at which the graft polymerization proceeded with a suitable yield were determined. Ammonium ferrous sulphate (Mohr's salt), as an inhibitor, failed to inhibit the homopolymerization of NVP during the radiation grafting process. In contrast, the addition of Cu2C12 (0.5 wt%) effectively inhibited the homopolymerization process and smooth grafted films with suitable grafting yield were obtained. The grafting onto PFA films, which scarcely swell in any solvent or monomer, was critically controlled by the diffusivity of reactants through the grafted layers initially formed near the film surface. The addition of ZnCI2 (3 wt %) to the monomer solution slightly enhanced the grafting and much smoother grafted films were obtained. The dependence of initial rate of grafting on NVP concentration was of the order 0.54. The graft copolymer possessed good mechanical properties and high thermal and chemical stability. Such non-ionic supported hydrogel on fluorinated polymer may be of interest for some practical use in which high electrical conductivity is not required, as in biomaterials

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High-resolution XH and 13C nuclear magnetic resonance spectra are used to investigate the intermolecular interaction of polyester-polyurethanes with different contents of hard segments and different degrees of ionization. Because there are several kinds of hydrogen bonds, the hydrogen bonds play an important role in phase separation and physical properties. The higher the percentage of intermolecular hydrogen bonds, the greater are the tensile strength and strength at definite elongation. The interaction between 4,4'-methylene diphenyl diisocyanate and N-methyldiethanolamine is weak for the ES-33.4 series. Ionization does not change the percentage of intermolecular hydrogen bonds for the ES-33.4 series and ES-41 series, and ionization does not improve the physical properties and phase separation

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The effects of pre-drawn morphology of melt-spun poly(ethylene terephthalate) (PET) fibres, induced by treatment at room temperature with mixtures of dimethylformamide (DMF) and water, on the two-stage drawing behaviour have been studied. The maximum achievable draw ratio (DRma x) in the first-stage cold drawing was dependent on the treatment time and the fraction of water in the mixtures. The fibres treated by ~<91/9 (vol) DMF/water mixtures showed depression of the glass transition temperature. However, the DRm,x of the treated fibres was not improved appreciably. When the fibres were treated by a 95/5 mixture for 3 rain, the fibres could be cold drawn up to a significantly higher draw ratio of 7. The value was about 30% higher than that for untreated fibres. The improved cold drawability of the treated fibres was explained by the presence of small and/or less perfect crystals induced by the mixture. With decreasing fraction of water and increasing treatment time, the treated fibres became brittle, which reduced the deformability at room temperature. This was primarily due to increased crystal perfection and surface roughening appeared on the fibre surface. By the second-stage hot drawing at 230°C, the initially cold-drawn fibres with the highest DRma x of 7 could be drawn up to a total draw ratio of 11.5. These highly drawn fibres showed tensile moduli and strengths of ~<225 and ~< 10 g/d, respectively.

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The dielectric properties of polyamide-4,6 were examined as functions of frequency, temperature and moisture content. The dielectric spectra were compared with the results of dynamic mechanical measurements in torsion at 0.2153 Hz, as a function of temperature, and with the dynamic dielectric properties of commercial polyamide-6,6. Three relaxation processes are observed in polyamide-4,6, which are related to either local or collective molecular mechanisms of motion, similar to the relaxation processes observed in other polyamides. At temperatures above the glass transition temperature, ionic conductivity is observed, which, due to the semicrystalline structure of the material, gives rise to a strong Maxwell-Wagner-Sillars (MWS) polarization. With increasing temperature, the conductivity increases sharply, eventually causing electrode polarization, and thus obscuring the dielectric effects in the material. All loss processes shift to lower temperatures with increasing water content. The activation energies of the secondary relaxations decrease with increasing water uptake, while the fl and ct relaxations are enhanced in magnitude.

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The molecular motions of two spin nitroxide probes of vastly different geometries dissolved in poly(vinyl acetate) have been studied over a large temperature range. The smaller spherical probe's motion follows the main glass relaxation at higher temperatures and the 7 transition at lower temperatures. Its small size allows it to occupy multiple sites as indicated by the observed spectra. The larger probe appears to occupy a single type of site. Its motion follows the main relaxation at higher temperatures and appears to abruptly jump to the fl relaxation at ~330 K.

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