Srećko Valić

Analysis of deuterium NMR spectra of probe chains diffusing in a stretched polybutadiene network

Abstract Polybutadiene network was swollen with short perdeuterated network-like chains (PBD). Deuterium NMR spectra of these probe chains are measured as the rubber matrix is uniaxially elongated. Due to the presence of two kinds of deuterons (methylene and ethylene) and two possible configurations of monomer units (cis and trans) in PBD chains, composite NMR spectra are observed and analysed. They evidence for different orientational behaviour of cis and trans units. ‘Rod-like’ motional behaviour of the trans units leads to higher degree of orientational order than for the cis units.

Orientational diffusion of homopolymer chains in a lamellar structure of diblock copolymer: a deuterium n.m.r. study

Chain dynamics in lamellar microdomains of polystyrene-polybutadiene (PS-PB) diblock copolymer is investigated by deuterium nuclear magnetic resonance (n.m.r.). The experiments are performed on perdeuterated PB homopolymer chains dissolved in the PB sublayers of a macroscopically oriented PS-PB sample. The n.m.r. spectra show that homopolymer chain segments undergo uniaxial reorientations around the direction normal to the layers. This local uniaxial order implies that the PB chains of the diblock are slightly elongated and that the segments between chains are correlated in orientation.

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.