Molecular Mobility and Phase Transformations of Several Low Molecular Weight Glass Formers Confined to Nanoporous Silica Matrices

Abstract

The dynamical behavior of three low molecular weight glass formers confined to nanostructured mesoporous silica (100 % Si) with pore sizes ranging from 2.8 to 8.6 nm, is probed by Dielectric Relaxation Spectroscopy (DRS). The confined guests embrace different classes of materials: a surfactant, Triton X-100, a liquid crystal, E7 nematic mixture and the pharmaceutical drug, Ibuprofen. All three glass formers show two distinct dynamical domains inside the pores, as revealed by the detection of both bulk-like and surface processes. The latter is characterized by a slower mobility than the bulk-like process and the temperature dependence follows the characteristic Vogel-Fulcher-Tammann-Hesse (VFTH) law indicating a glassy dynamics of the molecules anchored to the pore surface. In the case of E7, the Vogel temperature (T\(_{0})\) of this process is size dependent, decreasing with increasing pore size, which is taken as a finite size effect. Concerning the bulk-like process, assigned to the glassy dynamics of the molecules in the middle of the pore, the confinement effect becomes stronger depending on the material as follows: (1) Triton X-100, undergoing almost no change in the glass transition temperature (T\(_{g})\), only a slight increase \(\sim \)3 K is observed; (2) E7, with a maximum decrease of 10 K in T\(_{\mathrm{g}}\) compared to the bulk temperature for a pore size of 6.8 nm; and (3) Ibuprofen, which shows not only a higher decrease in the glass transition temperature, \(\sim \)30 K when confined to a pore size of 3.6 nm (MCM-41), but also its temperature dependence of relaxation times varies from VFTH to Arrhenius like, which is interpreted as 3.6 nm being a dimension that interferes with the length scale of cooperativity. Moreover, two secondary relaxations are detected in the pharmaceutical drug, the more local one being insensitive to confinement while the second process, taken as the precursor of the glass transition, becomes more mobile relatively to the respective process in bulk Ibuprofen. The results confirm that molecular dynamics of the probed low molecular weight guests confined into nanostructured mesoporous hosts is controlled by a counterbalance between confinement and surface effects.KeywordsTriton X-100E7IbuprofenNanostructured mesoporousGlass transitionConfinementSurface processCooperativityDebye process