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

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 beta-relaxation assigned to the librational fluctuations of the -O-NH2 moiety, (ii) the alpha-process corresponding to the dynamic glass transition, and (iii) the (slower) chain dynamics or normal mode (NM) relaxation. Under confinement in native nanopores, the beta-process becomes slower, while the a 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 beta-processes are nearly bulklike, but the alpha-process still remains faster than bulk closer to the T-g. All these findings can be comprehended as controlled by the counterbalance between surface and confinement effects. The former are caused by attractive interactions with the solid walls of the nanopores (resulting in an additional slower process which is removed after silanization), and the latter are caused by an increase of the free volume of the polymer segments due to a less efficient packing as proven by orthopositronium annihilation lifetime spectroscopy. These results conform to the cooperative free volume model (CFV).