Type A versus Type B Glass Formers:  NMR Relaxation in Bulk and Confining Geometry

We report 2H NMR spin−lattice relaxation times T1 and spectra of two fragile molecular glass forming liquids, deuterated o-terphenyl-d 14 (OTP) and toluene-d 5, confined in a nanoporous SBA-15 type matrix with a pore diameter of 7.1-nm size. We compare these results with bulk data of several glass formers. The temperature dependence of T1 allows us to differentiate type A (without secondary β process) from type B glass formers (with β process). We find that the α process of both OTP (type A) and toluene (type B) in confinement is governed by a broad heterogeneous distribution of correlation times, not seen in the bulk. Additionally, there is evidence that the β process in toluene changes under confinement conditions, where it exhibits a distribution of spin−lattice relaxation times that is substantially broadened toward faster times. In contrast, the excess wing for OTP is not altered in confinement.