Counterbalance between Surface and Confinement Effects As Studied for Amino-Terminated Poly(propylene glycol) Constraint in Silica Nanopores

Abstract

Broadband dielectric spectroscopy (BDS) and orthopositronium annihilation lifetime spectroscopy (PALS) are combined to study the molecular dynamics and the free volume of poly(propylene glycol) terminated with amino end groups (PPG-NH2) in the bulk state and when confined in native and silanized unidirectional silica nanopores with average diameters of 4, 6, and 8 nm. In the bulk state, three dielectric relaxation processes are observed: (i) the fast β-relaxation assigned to the librational fluctuations of the −O–NH2 moiety, (ii) the α-process corresponding to the dynamic glass transition, and (iii) the (slower) chain dynamics or normal mode (NM) relaxation. Under confinement in native nanopores, the β-process becomes slower, while the α and the normal mode relaxation processes become faster and broader and demonstrate a lower dielectric strength with decreasing pore diameter. In silanized nanopores the normal and β-processes are nearly bulklike, but the α-process still remains faster than bulk closer to ...