1993 рік

The variation of ~/o with amylose fraction is analysed, separating the effect of the polymer concentration from the effect of temperature. This gives a critical amylose volume fraction ~0"=0.14 for the onset of entanglements. The corresponding critical molecular weight, Mc=29400 fulfils correlations recently presented in the literature, in particular the correlation between critical end-to-end distance and molecular chain diameter.

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The polymerization of unsaturated polyester resins was studied using electron spin resonance (e.s.r.) spectroscopy, differential scanning calorimetry (d.s.c.), and Rheometrics dynamic analyser (RDA). The effects of crosslinking and thermoplastic additive on radical concentration and reaction kinetics were investigated. Variables studied included initiator concentration, resin composition and curing temperature. It was found that the radicals formed were very stable and their concentration was much higher than that for the linear polymerizations. From the e.s.r, spectrum, the liquid-solid transition could be identified, which agreed well with the gel time measured by RDA

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In the reaction of phenyl glycidyl ether with aliphatic alcohols in the presence of benzyl dimethylamine, four different oligomers are found at temperatures above 100°C. In the temperature range of 100-180°C, the length of oligomer chains decreases with increasing temperature. In bifunctional epoxide-alcohol systems the number of branched products increases simultaneously. These small branched oligomers are terminated by non-reactive end groups, preventing network formation above 140°C. At lower temperatures (< 60°C), the proportion of the addition mechanism exceeds 90%, alternating with a combined mechanism including the homopolymerization of glycidyl ether at 100°C. This mechanism yields the fastest gel times in corresponding bifunctional epoxide-alcohol systems. Above 100°C, gel times decrease again owing to hindered crosslinking in an oligomeric system with short, branched and non-reactive products

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We report the effect of phenol on the phase behaviour and physical properties of polydiacetylene gels. Specifically, they are o-dichlorobenzene (o-DCB) solution of poly[4,6-decadiyne-l,10-diol-bis(nbutoxycarbonylmethylurethane)], abbreviated as P(3BCMU), and toluene (Tol) solution of poly[5,7- dodecadiyne-l,12-diol-bis(n-butoxycarbonylmethylurethane)], abbreviated as P(4BCMU). The polymer concentration was 0.1 to 2 wt%, and the molar ratio z of phenol to the monomer unit of the polydiacetylenes was regulated within 20. The gel-to-sol transition temperatures T t were determined by measurements of shear modulus for the P(3BCMU)/o-DCB system and by a falling ball method for the P(4BCMU)/Tol systems. Tt decreased with increasing z and the systems with polymer concentration of 1 wt% turned into a sol at room temperature in the range of z > 10. The temperature dependence of alternating-current (a.c.) conductivity a,c at 1 kHz exhibited a maximum at a temperature ca. 20 K below the transition temperature. The value of ~rac decreased steeply at the electrical transition temperature T~, which also decreased with increasing z. Ultra-violet and visible-light spectra for the P(3BCMU)/o-DCB and P(4BCMU)/Tol systems shifted to the short-wavelength side with increasing z. The infra-red spectra indicate that the intramolecular hydrogen bonds between the N H and C=O groups of the side groups are cleaved by phenol. These results indicate that the conjugated rod-like conformation is made unstable by phenol. To clarify the mechanism of crosslinks of the gel network, the crystallinity of concentrated P(3BCMU) gels was determined by wide-angle X-ray diffraction. No crystallinity was observed in the gels of C less than 30 wt%.

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13C nuclear magnetic resonance spectra of poly(methyl acrylate) (PMA), poly(ethyl acrylate) (PEA) and poly(butyl acrylate) (PBA), obtained from graft copolymers onto amylose or amylopectin, have been studied. The spectra of the polymers were recorded with a 75 MHz spectrometer and were analysed by triad, tetrad, pentad or hexad placements. All of them follow Bernoullian statistics, with an isotactic parameter of 0.51, 0.44 and 0.43 for PMA, PEA and PBA respectively. By comparison with other values found in the literature it can be asserted that the presence of the carbohydrate has no effect on the tacticity of the grafted polymer.

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In this work, FTi.r. spectroscopy has been used in order to verify the proposed influence of the hydroxyl group, present in each monomeric unit of the phenoxy (PH) resin, in the observed miscibility of this polymer with a variety of other polymer families such as polyoxides or polyesters. Two different approaches have been used for this purpose. First, FTi.r. studies at different temperatures have been carried out on mixtures of PH with polyoxides, in order to demonstrate the higher strength of the hydrogen bonds in the mixture with respect to those existing between pure PH molecules. Second, chemical modification of the PH resin, with a controlled elimination of the repetitive hydroxyl, has been carried out intending to relate the percentage of hydroxyl substitution to the observed miscibility of PH. In the second approach, mixtures of PH and modified PH with poly(N-vinyl-2-pyrrolidone) have been studied.

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The non-linear mechanical behaviour exhibited by oriented thermotropic liquid crystalline polymers has been attributed to an increase in molecular orientation caused by the applied tensile stress. In this work we have detected the postulated change in orientation by polarized infra-red spectroscopy. It is shown that there is reasonable quantitative agreement between the infra-red measurement of change in molecular orientation and the change predicted by modelling the change in tensile modulus with stress.

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An alternative idea of decay of free radicals in solid polymers based upon a concept of voids existing in each polymer with the mobility induced by the motion of respective polymer segments and vice versa has been put forward taking into account formal reaction kinetics. It has been shown that this approach enables the qualitative interpretation of second-order decay, stepwise decay of free radicals initiated by a stepwise increase of temperature as well as the effect of high pressure on the overall rate constant of the decay process.

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Methyl acrylate and methyl methacrylate have been copolymerized using the initiator 2,2'- azobisisobutyronitrile over a wide composition and conversion range. Free-radical copolymerization has been carried out in benzene solution using 3 and 5 mol 1 1 as the overall concentrations of monomers. Reactivity ratios do not, apparently, change by dilution. Cumulative copolymer composition was independent of total monomer concentration, being described satisfactorily by the integrated Mayo-Lewis equation. The overall copolymerization coefficients are not affected by total monomer concentration. They were analysed on the basis of a penultimate effect on the propagation reaction.

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A normal vibrational spectroscopic analysis has been carried out for polybenzamide for both cis-trans and all-trans structures. The valence force field suitable for non-redundant coordinates was taken from several earlier studies. The calculated values coincide well with the experimental data including the polarization observations. Several vibrations above 1000cm-1 were found to be sensitive to changes in intermolecular interactions. The changes in these specific vibrations have been interpreted as originating from the electronic redistribution of the cross-conjugated backbone, directly related to changes in the hydrogen bond interactions. The potential energy terms associated with the backbone stretching vibrations are found to be perturbed significantly. The theoretical moduli of two molecular crystals involving both the cis-trans and the all-trans conformations have been calculated.

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