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.

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.