V. A. Shevelev

The influence of the chemical crosslinking network on the spin-spin relaxation of crosslinked and swelling polymer systems

Abstract Crosslinked and swollen polymer systems have been found to give rise to a ridge on the plots of spin-spin relaxation times T 2 against temperature; this is explained by an incomplete averaging of the nuclear magnetic dipole-dipole interactions during an isotropic movement of chain segments between network joints. A simple sectional model of a chain with both ends fixed is used to get a theoretical dependence of the T 2 in the range of the ridge on the length and equilibrial rigidity of the chain section. The theoretical results are in good agreement with the experimental as far as it concerns the T 2 dependence on the number of crosslinkages and the absolute value of T 2 in the region of the ridge.

Theory of the Relaxation spectra of polymer networks with included hard rodlike particles manifested in NMR

A theory of nuclear magnetic resonance relaxation of13C nuclei and nuclear Overhauser effect (NOE) of polymer networks with included rodlike particles is developed for the case when the length of each rod is comparable or greater than the average distance between neighboring crosslinks. The long-scale dynamics of the network is described by means of a regular cubic “coarse-grained” model. The effects of entanglements of long rods in the network are described by a quasi-elastic potential acting between rods and network fragments. The frequency dependences of 1/T1C and NOE are calculated for the case when the internuclear vector is directed along each rodlike particle. The frequency dependences of 1/T1C and NOE for rods included into a polymer network are shifted to high frequencies as compared with these dependences for free rods due to quasi-elastic interactions between rods and network fragments. At strongly different viscoelastic parameters of rods and network fragments, the frequency dependences of 1/T1C and NOE may have two maxima. The high-frequency maximum corresponds to localized motions of rods at immobile network domains. The low-frequency maximum is caused by involving rods in long-scale network motions. The intensity of the low-frequency maximum increases when the degree of interactions between rods and the network increases.

Proton magnetic relaxation in concentrated polymethylmethacrylate solutions

Studies were made of the temperature relations of proton magnetic relaxation times T1 and T2 at a resonant frequency of 18·6 Mc/s for solutions of PMMA fractions (M = 3×104−1·7×106) in proton free solvents with polymer concentration of 5–80 mole%. It was shown that the minimum on the curve of the temperature dependence T1 observed in PMMA solutions with polymer concentrations of 5–30% is due to superposition of two coexisting ranges of relaxation caused by the motion of the main chain and the natural rotation of α-methyl groups. The effective activation energy of the relaxation process, of which the correlation times determine T2, increases with a reduction of temperature from 4–5 to 8–10 kcal/mole. Time T1 is independent of M in the range of M values studied. Starting from a polymer concentration of 10% time T2 depends on M at high temperatures; the higher the concentration, the more marked is this dependence and on lowering temperature, dependence on M ceases. An assumption is proposed, according to which an increase in the effective activation energy for T2 and a weakening of the dependence of T2 on M are due to intensification of interaction between the chains as temperature decreases. The nuclear intermolecular effect for high polymer concentrations is ∼20% of the total effect, i.e. relaxation is basically due to intramolecular nuclear interactions. Molecular mobility decreases with an increase of viscosity and a deterioration of the thermodynamic quality of the solvent. The effect of solvent properties on T2 is stronger than on T1.

Proton magnetic relaxation in concentrated polystyrene solutions

Studies were made of temperature relations of proton magnetic relaxation times T1 and T2 of solutions of polystyrene fractions (M = 2 × 103−1·4 × 106) with concentration of 5–40 mole% in proton free solvents of different viscosities and thermodynamic properties. Features of T1 and T2 observed in the temperature range preceding solution separation are due to the formation of long life polymer-polymer contacts (associates, aggregates) affecting correlation times and proton-proton interactions. In the temperature range where ωτ ⪡ 1 (ω is the frequency at which resonance is observed, τ-correlation time) it was found that T1 is sensitive to the molecular weight of polystyrene when M<105 for all concentrations studied. 1T2 is proportional to log M with a concentration of 5% and at higher concentrations it increases; the higher the value of M concentration, the greater the increase. An increase in solvent viscosity or deterioration of thermodynamic properties reduces times T1 (when ωτ⪡1) and T2 and consequently, increases correlation times.

Theory of relaxation spectra for two identical interpenetrating polymer networks

Viscoelastic models for the description of the relaxation characteristics of two identical swollen interpenetrating polymer networks with different topologies moving against the background of an external viscous medium are considered. Two dynamic models that differ in the character of mutual interaction between network junctions are proposed. According to the first model, viscoelastic interaction is assumed to be constant and provided by the entanglements between a junction of one network with eight symmetrically arranged junctions of the other network. The second model involves (i) the predominant interaction between multiple-network junctions most closely located owing to entanglements and (ii) a weaker interaction with more distant junctions of neighboring cells. For the systems composed of two interpenetrating networks, relaxation-time spectra and average inverse relaxation-time spectra are compared with the corresponding spectra and characteristic times for individual noninteracting regular networks. Both models can involve two branches of the relaxation spectrum. One branch is a collective branch corresponding to the motion of the double network, whose parameters are controlled by the constants of elasticity of each of the interacting networks as well as by the effective mutual viscoelastic interactions between networks. This low-frequency branch is characterized by a broad spectrum of relaxation times. The second branch is a high-frequency branch that is primarily provided by mutual local motions of two interacting networks. This branch is characterized by a comparatively narrow relaxation spectrum and depends on quasielastic constants, which describe network entanglements, and on the characteristic elasticity of each network. The second branch does not involve any infinitely long relaxation times for infinitely continuous networks.

Proton magnetic relaxation study of pretransitional phenomena in the isotropic phase of a nematic liquid crystal I. Dynamics of local order fluctuations

Nuclear magnetic relaxation was investigated in a broad temperature region above the clearing point (Tc) of a nematic liquid crystal. Dependence of spin-spin relaxation time on the pulse interval observed in the Curr-Purcell-Meiboom-Gill (CPMG) experiment indicates an exchange of nuclei between the states differing in local magnetic fields. By fitting of the Luz-Meiboom equation to the CPMG results, the mean lifetime of sites and modulation frequency δω were determined. The rather slow exchange (lifetimes changing with temperature in the range 20-90 ms) is suggested as manifesting the local order fluctuations in the pretransitional zone of the LC. A simple two-site model of a pretransitional zone was considered (cluster ⇔ isotropic surrounding). Dipole-dipole interactions in clusters are unaveraged due to the local ordering, whereas in the isotropic subphase local magnetic fields are motionally averaged. Therefore, local order fluctuations are accompanied by the exchange observed in the CPMG sequence. Correspondence of the temperature dependence of δω to the Curie-Weiss law was established: δω-2 ∝ T - T*, (T* = Tc - 1), thus providing proof of our interpretation.

Effect of the network of hydrogen bonds on proton magnetic relaxation in solutions of copolymers of acrylic and methacrylic acids with styrene

It was established that the formation of a fluctuation network of hydrogen bonds inhibits intramolecular mobility, which determines the rate of longitudinal and transverse magnetic relaxation of protons. The effect of fluctuation network on both relaxation processes is intensified with a reduction in temperature which may be due to an increase in life of hydrogen bonds. The life of the hydrogen bond was evaluated. A comparison was made of the effect on intramolecular mobility of networks formed by chemical crosslinks, hydrogen bonds and van der Waals contacts or chains. An assumption was suggested that the main factor which determines the variation of intramolecular mobility in network formation is the increase in the mutual local density of units. It was established that, in contrast to the network of chemical crosslinks, the fluctuation network does not impose constant restrictions on the movement of chain elements, which may be regarded as isotropic for a length of time of the order of T2.

Distribution of the correlation times and patterns of 13C nuclear magnetic relaxation and the overhauser effect

The dependences of the spin lattice relaxation time T1c and the Overhauser effect for different correlation time distributions used to describe the relaxational properties of polymers are examined. The dependence of Tc and the value of the nuclear Overhauser effect has been established (as a function of ωcτ where ωc is the resonance frequency and τ is the time characteristic of the given spectrum) on the form, width and asymptotic behaviour of the correlation time spectra for short and long times. These values are sensitive to the parameters of the relaxation spectra and may serve for their discrimination.

Features of segmental motion in the diblock copolymers of styrene and butadiene

A study was made of samples of diblock copolymers using methods of DSC, NMR and dynamic mechanical tests. The copolymer samples differed as to the length of the interphase layer, i.e. the region where there is a blending of components in superstructures formed by segregated PS and PB domains. The nature of additional relaxation transitions appearing in the interphase layers is related to the segmental motion of PS, and the degree of decooperativity of the segmental motions is determined by the concentration of the surrounding PB segments.