Molecular glass formers in hard and soft confinement probed by 31P and 2H NMR

Low molecular glass formers confined in nanoporous silica matrices (hard confinement) are investigated by different 31P and 2H NMR methods such as spin-lattice-, spin-spin relaxation, line-shape and stimulated echo decay. Decreasing the radius of the pores pronounced dynamic heterogeneities are observed. For example, the correlation function revealed by the stimulated echo exhibits a quasi-logarithmic decay in contrast to Kohlrausch decay in the bulk. As shown by 2D spectra the dynamic heterogeneities are transient in time, i.e., we observe exchange between slow and fast molecules. The effects are explained by assuming dynamics being inhomogeneous in space; that is the dynamics given by a correlation time τ(r) depend on the distance r from the confining wall. Similar NMR features are found for low molecular additives dissolved in polymer matrices (soft confinement). The additive dynamics are decoupled from those of the polymer, and liquid-like additive dynamics are revealed below Tg, i.e., in a solid polymer matrix. Again, strongly stretched correlation functions are observed.