Dipole-Echo Formation in Three-Pulse-Sequence NMR Experiments on Polymer Systems

Data from three-pulse NMR experiments, namely, the Goldman-Shen pulse sequence and the stimulated-echo sequence, on partially crystallized poly(ethylene glycol) and tetrafunctional dimethylsiloxane networks are presented. A dipole echo has been observed after a third radiofrequency pulse. Quantum mechanical calculations of the response of the spin system to the three-pulse sequences have been performed, considering only thez-zcomponent of the dipole-dipole spin coupling. The resulting expression suggests dipole-echo formation and is in agreement with experimental data. © 1995

Magnetic field pulse gradient NMR determination of geometric pore parameters and correlation functions for forces of interaction between molecules of a liquid and pore surface

Pore volume/surface ratio is determined by means of NMR with magnetic field pulse gradient g (MFPG) for standard porous media (glass spheres with the diameter of 58 to 63 μm and two fractions of sand with particle diameter under 90 and 200 to 250 μm) upon addition of polar (water and dimethylsulfoxide) and nonpolar (decane) liquids. Experimental and calculated values are compared, and the deviations explained. The type and values of correlation functions for interaction between the molecules of the studied liquids and the pore surface within the time interval of 3 to 100 ms were determined. The method of MFPG NMR enables us to study only the tails of the correlators. The diffusion decays are reduced at high values of g > 17 T/m under the Porod-Debye g-4 law. © 1996 MANK Hayka/Interperiodica Publishing

Influence of aerosil on the nuclear magnetic relaxation and translational mobility of dimethyl sulfoxide

The transverse (T2) and longitudinal (T1) proton magnetic relaxation times and the self-diffusion coefficients D of the dimethyl sulfoxide (DMSO) molecules in aerosil mixtures with different specific surface area were measured at different temperatures and liquid contents. The large difference between the times T1 and T2 in the systems examined is explained by the hypothesis that the molecules adsorbed on the solid surface (especially in micropores) lose some translational degrees of freedom. At high temperatures, the translational displacement of DMSO molecules is determined by the relative time of their residence in the vapor phase. In contrast, it is the surface diffusion that determines this displacement at low temperatures, when the lifetimes of the adsorbed molecules are longer than the times of the measurement of the self-diffusion coefficient. © 1998 MAK Hayka/Interperiodica Publishing

Mobility of liquid molecules within aerosil pores

Self-diffusion coefficients (D), times of longitudinal (T1) and cross (T2) magnetic relaxations of protons of polar (dimethylsulphoxide, dimethylphthalate) and nonpolar (tridecane) liquids within aerosil pores were measured using NMR spectroscopy. Measured T2 values of polar molecules are less by two orders and more than the values of nonpolar molecules. D coefficients of liquids are independent on their polarity. The results obtained can be explained by the orienting action of aerosil particles surface on polar molecules which leads to anisotropy of their rotating ability. Anisotropy parameter and times of isotropic reorientation equal to 1.6×10-7 and 6.8×10-7 s respectively for DMSO and DMP molecules were estimated

Temperature dependences of tridecane self-diffusion coefficient in porous media

Effective self-diffusion coefficients (D) of tridecane added to caolinite and dispersed magnesia were measured by NMR spectroscopy with pulse gradient of magnetic field over 258-415 K temperature range. Temperature relationships in lg D - 1÷T coordinates are presented as straight lines for the samples with θ<1 (where θ is degree of pores filling by liquid). It is suggested that in a such samples the part of liquid is in vaporous state and the condition of fast exchange (from NMR point of view) is obeyed. Explanation of the obtained temperature relationships is given and the values of activation energies are calculated

Self-diffusion in fluids in porous glass: Confinement by pores and liquid adsorption layers

Diffusion coefficients of 10 different polar and nonpolar liquids filled in porous glasses with mean pore diameters of 4 or 30 nm were determined with the aid of the NMR field-gradient technique. In the time scale of these experiments (0.3 to 500 ms) diffusion coefficients were found to be time independent. Within the experimental error, no influence of the polarity of the adsorbate can be stated. The diffusion coefficients of all investigated fluids in glass with 4 and 30 nm pores were reduced by factors of 0.17 and 0.63, respectively, relative to the bulk values. This relatively weak reduction can be explained by considering the known porosities of the adsorbents. The second objective of this study was to examine the diffusion behaviour below the melting point of adsorbates in porous glass. Fluids confined in pores do not freeze at the bulk freezing temperatures. In this respect, two phases must be distinguished. A maximal two monolayer thick film adsorbed on the inner surfaces does not crystallize at all, whereas the 'free' fraction of the fluid in the pores freezes at reduced temperatures according to the Gibbs-Thompson relation. The nonfrozen surface layers form a network in which self-diffusion can be investigated. Experiments have been carried out with cyclohexane. A reduction factor of 0.06 was found relative to the extrapolated values of the entirely unfrozen fluid in porous glass with a mean pore diameter of 30 nm. It is, thus, demonstrated that molecules in adsorption layers virtually retain their translational degrees of freedom along the surfaces. The lowering of the diffusivity is mainly due to the geometric restriction rather than to the interaction with the surface

Molecular Mobility in a Poly(ethylene glycol)-Poly(vinyl pyrrolidone) Blends: Study by the Pulsed Gradient NMR Techniques

The molecular mobility in PEG-PVP blends as a function of the time of system storage and the PVP molecular mass is studied by the pulsed-field gradient NMR method. The distribution of PEG molecules over their mobilities is found in a blend containing 36 vol % of PEG with the molecular mass of 400 g/mol. As the storage time of the system increases, the spectrum of diffusion coefficient values varies, thereby indicating the redistribution of PEG400 molecules in the blend with PVP. An anomalous (partly restricted) diffusion of PEG400 molecules is discovered, reflecting the influence of PVP macromolecules on the motion of short PEG chains. It is shown that, during the redistribution of PEG molecules in the blend, they are involved in a complex with PVP, which is characterized by its own transport properties. The data obtained by the NMR relaxation technique are in agreement with the results of NMR diffusion measurements in the studied systems

Influence of aerosil on the nuclear magnetic relaxation and translational mobility of dimethyl sulfoxide

The transverse (T2) and longitudinal (T1) proton magnetic relaxation times and the self-diffusion coefficients D of the dimethyl sulfoxide (DMSO) molecules in aerosil mixtures with different specific surface area were measured at different temperatures and liquid contents. The large difference between the times T1 and T2 in the systems examined is explained by the hypothesis that the molecules adsorbed on the solid surface (especially in micropores) lose some translational degrees of freedom. At high temperatures, the translational displacement of DMSO molecules is determined by the relative time of their residence in the vapor phase. In contrast, it is the surface diffusion that determines this displacement at low temperatures, when the lifetimes of the adsorbed molecules are longer than the times of the measurement of the self-diffusion coefficient. © 1998 MAK Hayka/Interperiodica Publishing

Studies of domain morphology in segmented polyurethanes by pulsed NMR

A new technique based on the Goldman-Shen pulse sequence with varying preparation interval is proposed for the study of domain morphology of segmented polyurethanes. The results of numerical calculations of the magnetization recovery in different models of domain morphology show that the method provides new information about the domain morphology which is beyond the reach of the conventional Goldman-Shen experiment. A close agreement of the theoretical predictions with the experimental data on samples of segmented polyurethanes with a fixed molecular mass of the hard blocks and variable molecular mass of the soft blocks reinforces the above statement. The resulting structural parameters obtained by this new NMR technique are compared with the data from the small-angle X-ray scattering (SAXS) method

Magnetic field pulse gradient NMR determination of geometric pore parameters and correlation functions for forces of interaction between molecules of a liquid and pore surface

Pore volume/surface ratio is determined by means of NMR with magnetic field pulse gradient g (MFPG) for standard porous media (glass spheres with the diameter of 58 to 63 μm and two fractions of sand with particle diameter under 90 and 200 to 250 μm) upon addition of polar (water and dimethylsulfoxide) and nonpolar (decane) liquids. Experimental and calculated values are compared, and the deviations explained. The type and values of correlation functions for interaction between the molecules of the studied liquids and the pore surface within the time interval of 3 to 100 ms were determined. The method of MFPG NMR enables us to study only the tails of the correlators. The diffusion decays are reduced at high values of g > 17 T/m under the Porod-Debye g-4 law. © 1996 MANK Hayka/Interperiodica Publishing