Search

The spin-probed copolymers of styrene and acrylonitrile differing in monomer content and chain structure were examined by the electron spin resonance (e.s.r.) methods in a wide temperature range. The measurements of spin–lattice relaxation time, T1DM, in slow motional region by the double modulation e.s.r. technique reveal two distinct relaxation regions in copolymers and pure polyacrylonitrile matrices. At very low temperatures (region I) a very distinct difference between T1DM values of pure polyacrylonitrile, alternating and statistical copolymers and polystyrene-rich copolymer is ascribed to the effect of packing density of polymer chains. The density modulates the electron spin–lattice relaxation rates and other vibrational modes contributing to the total relaxation rate. At higher temperatures (region II), where the skeleton of a spin probe begins to move, alternating and statistical copolymers of a similar composition show different relaxation rate. The data are reconciled on the basis of the sequence structure of copolymer chains. The difference in glass transition temperature between alternating and statistical copolymers as determined by the e.s.r. measurements reflects local motional heterogeneity as a consequence of chain microstructure. The presence of local motional heterogeneity of a probe near and above the glass transition results from the styrene-rich sequences contributing to the structural heterogeneity in statistical copolymer.

The electron spin resonance (e.s.r.) spin-probe method has been applied to investigate the matrix heterogeneity of natural rubbers differing in molecular weight, molecular weight distribution and gel content. The composite temperature-dependent e.s.r. spectra above the glass transition temperature (Tg) were evaluated in order to obtain the population of spin probes contributing to restricted and fast motion. From the ratio of spin probes distributed in the two motionally different environments, microstructural heterogeneities of the rubber matrix are observed. It is shown that the restricted spin-probe motion above Tg is predominantly determined by the amount of gel phase and structure of the gel. Chemical treatment and mastication of natural rubber, which have a complex influence on gel fraction, molecular weight and intermolecular interactions, have been considered in analysis of the temperature dependence of the e.s.r. spectra. A comparison between natural rubber and a synthetic polyisoprene confirms the contribution of the gel phase and its structure to the microstructural characteristics of rubber matrices.

The structure and dynamics of a network formed by radiation induced crosslinking of polyesters based on 1,6-hexane diol and 1,2-propylene glycol and maleic anhydride (HDF and PGF, respectively) with styrene is studied by proton pulsed n.m.r. spectroscopy. The dependence of spin-lattice, T1, and spin-spin, T2, relaxation times on the structure of polyester chain, molar ratios of styrene to polyester unsaturations and the radiation doses are analysed in terms of network formation and structure, and their effect on molecular motion. Above the gel point, at temperatures above the glass transition, the presence of two T2 components reflects the heterogeneity of the network structure in both resins. The glass transition temperature, determined from the T2 relaxation measurements for PGF resins having molar ratio of styrene to polyester unsaturation (RU) up to 1.67:1 continuously decreases with the increasing styrene content. Generally, all the PGF-styrene mixtures have a higher degree of conversion of diol component than the styrene component. The largest shift in the glass transition to higher temperatures for HDF-styrene resins, which indicate the less mobile network, is found for resins with RU about 1:1. Glass transition temperatures also increase with increasing fumarate content as a consequence of better reactivity of styrene with the trans form. The shift in glass transition and the conversion data suggest a formation of different network structure in PGF- and HDF-based networks, which is a consequence of different chain structure. Parallel with the n.m.r. relaxation measurements the crosslink density was determined from the extracted gel phase or double bonds (fumaric and styrene) participating in the crosslinking process. Although the degree of cis-trans isomerization in the HDF series has an influence on the network formation, samples of both series having 50 mass per cent of styrene after irradiation with 3kGy do not reach the gel point.

The reaction steps of the polyesterification of maleic anhydride and 1,2-propylene glycol were followed by 13C and 1H n.m.r. spectroscopy. The number and structure of mono- and diesters of both maleates and fumarates in the reaction mixture are determined by the possible glycol unit configurations. The structure of the growing polyester chain reflects a statistical distribution of approximately equal numbers of symmetric and asymmetric arrangements of the polymer sequences.

The polyesterification and isomerization reaction of 1,6-hexane diol and maleic anydride in a melt without catalyst was studied by 13 C and 1 H n.m.r. spectroscopy. The structure and concentration of oligoester species during the polyesterification and isomerization were determined depending on the reaction temperature and time. According to the number and configuration of repeating units determined from 1 H n.m.r. spectra kinetics of both reactions were also considered. The degree of isomerization is continuously increasing in the investigated reaction range.