The Role of Interfacial Energy and Specific Interactions on the Behavior of Poly(propylene glycol) Derivatives under 2D Confinement

The effect of the chemical modification of poly(propylene glycol) (PPG) end groups on the molecular dynamics under 2D confinement and the polymer/matrix interactions (including interfacial energies) was investigated by a combination of differential scanning calorimetry (DSC), broadband dielectric spectroscopy (BDS), surface tension and contact angle measurements. The replacement of -OH groups in native PPG allowed to modify the interactions with the hydroxyl groups attached to the pore walls of nanoporous aluminum oxide (AAO) membranes of various pore diameter. It was found that the observed reduction in the glass transition temperature (T-g ) of the core polymers correlates well with a general trend (the higher the solid-liquid interfacial tension, gamma(sL), the lower T-g,T-confined) reported earlier. Moreover, we demonstrated that although the interfacial solid-liquid energy seems to be almost the same for each studied herein material, a clear change in the crossover temperature (T-c), related to the vitrification of the polymers adsorbed to the pore walls, is noted. Interestingly, the shift in T-c with respect to the glass transition temperature of the bulk polymer scales well according to the decreasing ability in the formation of H bonds in the order PPG-OH -> PPG-NH -> PPG-OCH3 for the constant, gamma(sL). One can add that no such effect is found for the glass transition of the core polymers, where a similar shift of the T-g was recorded. This finding has been discussed in the context of various sensitivity of the studied materials to the density fluctuations, equilibration phenomena occurring below T-c, etc. We believe that our finding will help in a better understanding of an interplay between interfacial and core molecules and contribute significantly to the discussion on the impact of interfacial interactions on the molecular dynamics of polymers under 2D confinement.