FIELD-CYCLING NUCLEAR-MAGNETIC-RESONANCE RELAXOMETRY AND FIELD-GRADIENT NUCLEAR-MAGNETIC-RESONANCE DIFFUSOMETRY OF POLYMERS CONFINED IN POROUS GLASSES - EVIDENCE FOR A RESTRICTED-GEOMETRY EFFECT

The chain dynamics of oligomer and polymer melts confined in porous gl asses (pore diameters 4 and 30 nm) was studied with the aid of proton field-cycling NMR relaxometry (total frequency range: (2 x 10(3))-(3 x 10(8)) Hz) and field-gradient NMR diffusometry. The frequency depende nce of the spin-lattice relaxation time, T-1, is discussed in terms of restrictions due to adsorption on the pore surfaces and to the geomet ry of the pore space. The behavior of polymers larger than the pores ( strong adsorption as well as geometry effects) was compared with that of oligomers (strong adsorption but vanishing geometry effects). A ten tative ansatz for the separation of the geometry dependent relaxation rate is proposed. The resulting frequency dependence of polymers excee ding the pore size is much steeper than in bulk melts and approaches t he proportionality T-1 proportional to nu(0.75) above 5 x 10(5) Hz, in accordance with the reptation model. The molecular weight dependence of T-1 suggests a critical molecular weight which may be defined by th e coincidence of the coil and pore diameters. Field-gradient NMR diffu sometry indicated a dramatic slowing down of self-diffusion when the c oil dimension approaches that of the pores.