Zorica Veksli

ESR spectroscopy for the study of polymer heterogeneity

Abstract In recent years the motional and structural heterogeneity of polymers and their phase separation on a nanoscale length have been approached by many experimental techniques. This review compiles the literature on the application of electron spin resonance (ESR) spectroscopy — spin label/probe methods in heterogeneous polymer systems over the past decade. The achievements and limitations of the ESR method in the study of homopolymer heterogeneity, crosslinked polymer networks, polymer blends, interpenetrating networks, copolymers and block copolymers are overviewed and compared with some other allied methods. Recent analyses of complex ESR spectra and their corresponding motions are presented in the article. Information on molecular dynamics in a wide range of spin label/probe motion, free volume and free-volume distribution, phase separation, phase morphology and small molecule penetration are the main issues of the remaining sections. The last section covers some other ESR techniques such as continuous wave, linear and nonlinear methods and pulse ESR supported by their possible features or those already achieved in the field of polymer research. ESR advanced methods offer new possibilities to expand the limits as well as sensitivities of molecular dynamics detection in gaining information on realistic molecular behavior in complex polymer materials.

Study of matrix inhomogeneity of natural rubber and synthetic polyisoprenes by a spin probe method

Abstract Nitroxide spin probes have been used to investigate the matrix morphology of natural rubber (NR), deproteinized natural rubber (DPNR), synthetic polyisoprene (PI) and modified synthetic polyisoprene (PI-M). Composite electron spin resonance spectra with broad and narrow components appear in NR, DPNR and PI-M, above the glass transition temperature. The slow component is attributed to the spin probes in regions of higher density or gel phase, whereas the narrow component is identified with the nitroxides located in the sol phase. The intensity of the slow component depends both on the amount of gel phase and on the specific gel structure. The restricted spin probe motion above the glass transition is considered to be a measure of the participation of naturally occurring protein components or modifiers in the formation of gel and its structure. Several spin probes differing in size and shape are used in order to assess the free volume distribution.

Motional heterogeneity of segmented polyurethane–polymethacrylate mixtures: an influence of functional groups concentration

Abstract The motional heterogeneity in the polymer mixtures of segmented polyether polyurethane (PU) with carboxylic groups and methacrylic copolymer (PM) with tertiary amine groups was studied by the electron spin resonance (ESR)—spin probe and spin label methods. Pure polymer components containing varying amounts of functional groups and their 1:1 mass ratio mixtures have been analysed. The results of motional heterogeneity and polymer interaction were complemented with the glass transition temperatures, structural and morphological characteristics. Spin probed PU/PM mixtures indicate that the probe motion and the phase separation deduced from the temperature-dependent ESR spectra are sensitive to a free volume determined by the polymer–polymer interactions. The interaction between the two components in PU/PM mixture with the highest functional groups concentration disorganizes hard segment domains with spherulitic character at the microscopic level as compared with the ordered hard phase in the corresponding pure PU sample. The influence of PU hard and soft segments on the motional dynamics of PM chains is analysed from the ESR spectra of spin labelled PM chains in the mixtures with unlabelled PU components. The fractional amount of the PM fast motion depends on the temperature and concentration of functional groups.

A spin-probe study of heterogeneity in the natural rubber matrix: The effect of molecular weight, molecular weight distribution and gel phase

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.

An electron spin resonance study of molecular dynamics and heterogeneity in the styrene-acrylonitrile copolymers

Abstract 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.

Effect of thermal oxidative ageing on the morphology of natural rubber networks as viewed by ESR

The change of network morphology of unfilled sulfur crosslinked natural rubber exposed to thermal oxidative ageing has been studied. Three model networks with different sulfur contents (efficient network, semi-efficient and conventional vulcanizate) have been analysed by applying spin probe ESR, equilibrium swelling measurements and mechanical measurements. By combining the density distribution deduced from the complex ESR spectra and the equilibrium swelling density, it is shown that the major network changes occur in the networks with predominantly polysulfidic bonds (conventional network) after 96 h of ageing at 373 K. The effect of ageing on the network with predominantly monosulfidic bonds is much less pronounced. Both effects (main chain scission and additional local crosslinking) have been analysed in the course of prolonged ageing up to 240 h. A correlation between network structure and its mechanical performance is established. © 2000 Society of Chemical Industry

1H n.m.r. relaxation of radiation induced crosslinking in polyester―styrene systems

Abstract 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.

Polyisoprene matrix inhomogeneity studied by nitroxide spin probe motion

SummaryThe spin probe ESR method was applied to study natural rubber and synthetic polyisoprenes with different content of cis-configuration over a wide temperature range. The ESR spectra of natural rubber near and above the glass transition indicate the existence of two distinctly different mobilities as a consequence of the spin probe distribution in sol and gel phase. The results show that the spin probe method can yield information about the inhomogeneity of the polyisoprene matrix and the character of gel phase.

Study of irradiated polymers using double-modulation electron spin resonance and size exclusion chromatography

Abstract Irradiated polymers such as polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(α-methylstyrene) (PαMS) have been studied using size exclusion chromatography and double-modulation electron spin resonance (d.m.e.s.r.). Size exclusion chromatography coupled on-line with low-angle laser light scattering and viscometry have demonstrated that PS irradiated in the solid state gives only slight increases of molecular weight and viscosity with increasing dose. When irradiated in solution, the results confirm the existence of a highly crosslinked and heterogeneous PS matrix. PMMA and PαMS undergo degradation at the same doses. The d.m.e.s.r. spectra of a spin probe in the slow-motion region have been used to investigate the heterogeneity of the polymer matrix introduced by changes in crosslinking or by degradation. Discrete molecular motions of a spin probe above the internal probe transition (≈ 190 K) are very sensitive to local matrix density and to side-chain relaxation modes. The d.m.e.s.r. spectra indicate clearly a bimodal motion of spin probes located in regions of two considerably different crosslink densities of the PS matrix irradiated in solution.

Slow molecular motion in uniaxially stretched poly(ethyl methacrylate) as observed by double modulation electron spin resonance

SummaryThe double modulation ESR (DMESR) technique has been applied to investigate the change of very slow molecular dynamics ({ei235-1} >10-7 s) of nitroxide spin probe in amorphous poly(ethyl methacrylate) (PEMA) at low drawing ratios. The polymer has been characterized by means of size exclusion chromatography (SEC) apparatus fitted with three detectors in series with continuous viscometer, low angle light scattering photometer and differential refractometer. The side chain group motion and segmental motion well below the glass transition temperature are discussed in terms of local density changes induced by orientational anisotropy of the chain segments.