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

摘要

The chain dynamics of oligomer and polymer melts confined in porous glasses (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 dependence of the spin-lattice relaxation time, T-1, is discussed in terms of restrictions due to adsorption on the pore surfaces and to the geometry 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 tentative ansatz for the separation of the geometry dependent relaxation rate is proposed. The resulting frequency dependence of polymers exceeding the pore size is much steeper than in bulk melts and approaches the 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 the coincidence of the coil and pore diameters. Field-gradient NMR diffusometry indicated a dramatic slowing down of self-diffusion when the coil dimension approaches that of the pores. [References: 25]