NMR imaging of solid polymers: Strategies for chemical-shift selectivity

One reason that the solid-state imaging field is exciting is because of its potential to provide spatially resolved chemical information from deep inside a solid sample. To achieve this goal, methods are needed which exploit the many NMR parameters to generate image contrast. We describe two strategies for employing the chemical-shift anisotropy as a contrast mechanism in images of abundant nuclei in solid polymers. We illustrate these approaches with images which map spatial variations of molecular orientation in samples of polytetrafluoroethylene.     

Nondestructive evaluation of polymer materials by solid state NMR imaging

Many polymer products are heterogeneous. Examples of heterogeneities are the chemical composition, distributions of mechanical stress, and variations in flexibility or molecular orientation. Applications of NMR imaging to polymers are summarized and investigations of the aging of rubber are reported as well as novel deuteron NMR techniques to image molecular order and mobility of polymers below the glass transition temperature.     

ESR application to radiation chemistry of polymers

Important results obtained in our group in the field of ESR application to the study of irradiated polymers are summarized. They are the analysis of the decay reaction of the free radicals, spur-like trapping of the free radicals and the related discussions. A diffusion controlled bimolecular reaction scheme was a good way of analyzing the data of the decay reaction. Power saturation phenomenon of ESR spectra of the free radicals showed a circumstance of the spur-like trapping of the free radicals in irradiated polyethylene. The phenomenon of spur-like trapping was quite consistent with the interpretation of the decay reaction of the free radicals.     

New materials for ESR dosimetry

New materials for electron spin resonance (ESR) dosimetry have been investigated with the aim to find systems more sensitive than L-alanine accepted as a standard for high dose determinations. Among the investigated systems ammonium tartrate, 2-methylalanine, salts of formic acids and dithionates have been found to be more sensitive than alanine by a factor 2-10. The lower limit applies to tissue equivalent materials, while much higher sensitivities were obtained with formates and dithionates containing heavier atoms. The increased sensitivity was mainly attributed to suitable ESR properties of the room temperature stable radicals as regards spectral shape (narrow lines, little or no hyperfine structure) and microwave saturation properties (short relaxation times). The radical structures have when necessary been clarified by ENDOR spectroscopy, while the saturation properties have been screened by pulsed ESR measurements.     

ESR and magnetic properties of electrically conducting metallophthalocyanine polymers

Polymeric metallophthalocyanines of Cu, Ni, Co in which benzene rings are shared in common with the macrocyclic phthalocyanine rings containing peripheral carboxylic groups have been synthesized and their electrical conductivities are shown to increase from 5 to 8 orders of magnitude by thermal treatment. The cobalt polymer exhibits greater conductivity than its nickel or copper analogs. The heated polymers show very broad electron spin resonance (ESR) signals and large paramagnetic susceptibilities. The bulk magnetic susceptibility of these polymers shows Curie-like behavior when the samples are heated from room temperature to 473 K. This is explained on the basis of coexistence of fixed mobile and conduction electron spins in the system and the interaction of the soliton type defects present in these polymers with the polymeric network to give dipolar charge carriers. This is also supported by the decrease in ESR intensity with increasing temperature when the samples are heated in the ESR cavity. The effect of the presence of unpaired electrons on the d shell of the central metal atoms on the line width and the line shape of the ESR spectra is also explained. © 1994 John Wiley & Sons, Inc.     

Phonons in solid polymers

On the basis of a qualitative theory the phonon spectrum is obtained. Two- and three phonon interactions are examined. The theoretical results are applied for the analysis of the experimental data: for heat conduction, dielectric relaxation, heat capacity.     

ESR imaging investigations of two-phase systems

The possibilities of electron spin resonance (ESR) and electron spin resonance imaging (ESRI) for investigating the properties of the spin probes TEMPO and TEMPOL in two-phase systems have been examined in the systems water/n-octanol, Miglyol/Miglyol, and Precirol/Miglyol. Phases and regions of the phase boundary could be mapped successfully by means of the isotropic hyperfine coupling constants, and, moreover, the quantification of rotational and lateral diffusion of the spin probes was possible. For the quantitative treatment of the micropolarity, a simplified empirical model was established on the basis of the Nernst distribution and the experimentally determined isotropic hyperfine coupling constants. The model does not only describe the summarized micropolarities of coexisting phases, but also the region of the phase boundary, where solvent molecules of different polarities and tendencies to form hydrogen bonds compete to interact with the NO group of the spin probe.     

ESR studies of the orientational and reorientational properties of mesomorphic polymers

Abstract

The orientational properties of two mesomorphic azobenzene-containing polyacrylates are investigated by ESR spectroscopy using the spin probe technique. Procedures to achieve a uniform alignment of the director in the presence of a static magnetic field are described in detail. The recovery of the alignment after the creation of controlled non-equilibrium states in the presence of a magnetic field is studied. It is found that the recovery time is proportional to the reciprocal square of the static magnetic field, analogous to that which occurs in low molar mass liquid crystals.     

ESR studies on polymers with particular electronic and magnetic properties

Measurements on insulating and conducting polymers from the polyaniline (PANI) family and investigations on semiconducting poly(tetrathiafulvalenes) (PTTF) have been used for illustration and discussion of some methodical questions of electron spin resonance (ESR). This concerns especially the new possibilities of the recently developed high-resolution ESR in the 2 mm wave band. It was applied for the study of the nature and dynamics of paramagnetic centres (PC) and charge carriers in PANI and PTTF. The rate of the quasi-one-dimensional (1D) intrachain electron motion and of the three-dimensional (3D) interchain electron hopping has been estimated separately. In iodine - doped PTTF the maximum electrical conductivity is 10⁻⁴ S ·cm⁻¹. It is almost identical with the 3D-conductivity estimated by ESR and shows the typical temperature dependence of a semiconductor. The quasi-1D-conductivity is several orders of magnitude higher and shows in its dependence on temperature similarities with a metal. The PANI samples show in the highly doped form a maximum conductivity of about 10 S ·cm⁻¹ and relativly small differences between the 1D- and 3D-conductivity, supporting the model of metallic islands.     

ESR study of the rotation of peroxy radicals in polymers

The nature and the frequencies of rotation of peroxy radicals in polyethylene and polytetrafluorethylene are determined over a wide temperature range theoretically calculated ESR spectra.     

Quasi-one-dimensional materials: polymers and chains

Results of density-functional calculations on materials that can be considered being quasi-1D are reported. The materials include some based on carbon (fullerene nanotubes and a polyyne), chains of metal atoms (Pb, Au, and Ag), and charged chains as building blocks of crystals (PtS₂ chains surrounded by K atoms and NiN chains separated by Ca atoms). In many cases it is found that 2D or 3D behaviors can be recognized, but in some cases the smaller changes due to the change from true 3D or 2D to quasi-1D have profound effects.     

Biodeterioration of polymers and polymer composite materials

The problem of the biodeterioration of polymers and polymer composite materials is considered. Products made from both thermoplastic and thermosetting polymers are found to undergo biodeterioration. An analysis of methods for investigating biocorrosion is performed. It is established that the application of conventional testing methods under laboratory conditions is restricted due to their complexity and work content, as well as because of the low efficiency at the stage of material development. In connection with this, the possibility of applying indirect investigation methods for the qualitative and quantitative evaluation of biodeteriorations of polymers is considered, and testing is carried out on samples of crosslinked polymers in aerotanks. It is shown that biocorrosion is not caused by the microorganisms themselves, but rather by their waste products, e.g., organic acids and ferments. The possibility of evaluating biocorrosion via modeling biodeterioration using solutions of individual substances (various organic acids) is revealed. Urgent problems of investigations on this topic are defined.     

Morphology of adsorbed polymers and solid surface wettability

The adsorption of a polyacrylamide (MW 14600) and two polysaccharides (MW 9260 and 706 x 10(3)) onto model silica surfaces of different hydrophobicities was investigated. In all cases, adsorption adhered to the Freundlich isotherm, reflecting the heterogeneous character of the solid substrates. The latter strongly influenced the character of the adsorbed polymer, with morphologies from chainlike structures to thin films and patches being observed. Surface roughness, polymer type, and molecular weight also play roles in controlling adsorbed polymer morphology. Surface wettability is strongly influenced by the thickness of the adsorbed layer.     

NMR imaging of elastomeric polymers

We have used NMR imaging to determine the dispersion in crosslinking density in complex elastomers both along the chains as well as spatially. Our work has established that the ordinary industrial accelerator systems generate a broad distribution of crosslink densities in rubber articles due to the poor mixing of the solid accelerator/sulfur recipe in the raw rubber. This high inhomogeneity in elastomeric network has an adverse effect on the properties of the elastomer system. In particular, the variation in crosslink density leads to substantial differences in the coefficients of thermal expansion and high internal stresses in the rubber article.     

Atomic mechanism of fracture of solid polymers

Macromolecular chain scission under mechanical stress has been studied by infrared spectroscopy. The dependence of accumulation of chemical bond scissions on temperature T and uniaxial tensile stress σ has been investigated. The rate constant K for bond dissociation under mechanical stress has been found to obey the modified Arrhenius equation: K = K₀ exp{ ? (E_A ? ασ)/RA}. The quantitative connection between the rate constant for bond dissociation and mechanical lifetime τ has been established. Analysis of the experimental data indicates that the strength and mechanical lifetime of polymers is determined by the kinetics of mechanochemical scission of the main chains of polymer molecules.     

Librational Raman scattering from solid polymers

Broad, low frequency Raman bands (below 100 cm^?1 were observed in several solid polymers (polymethylmethacrylate) (PMMA), polystyrene, poly n-butyl methacrylate and poly 4-methyl pentene-1) and attributed to the librational motions of the side groups. A structural model for the librating side group is proposed which exhibits the observed frequency dependence on mass.