Effect of nanoconfinement on polymer chain dynamics

Recently, Hor et al. [Hor, J. L., Wang, H., Fakhraai, Z., & Lee, D. (2018). Macromolecules 51 (14), 5069-5078] reported a set of interesting results for the viscosity of unentangled polystyrene of two different molecular weights determined from capillary infiltration into a confining matrix of silica nanoparticles. They found that the viscosity and glass transition temperature (T_g) increase as the pore size of the matrix decreases, that is, the confinement effect becomes stronger. They attributed the change in viscosity to a change in segmental relaxation, that is, an increase in T_g. However, their interpretation did not consider that a changing T leads to a change in the monomeric friction coefficient and viscosity-molecular weight relations should be compared at constant friction factor, that is, at a constant distance from the glass transition temperature. We reanalyzed the results at constant friction factor under a framework of changing chain dynamics as confinement strength increases rather than being due only to the changing glass transition temperature. Our reanalysis of the data from Hor et al. as well as data from other literature shows that as unentangled polymers become increasingly confined, the chain dynamics change from Rouse-like to those of an entropic barrier regime. For entangled polymers, the chain dynamics change from those of a reptation-like regime to those of an entropic barrier regime.