Anomalous Sorption in Thin Films of Fluoroalkyl-Ended Poly(ethylene glycol)s

Absorption, desorption and swelling behavior of polymers play important roles in polymer processing and performance, ranging from issues of dimensional stability and permeability to controlling adhesion. Rich, nonlinear phenomena are associated with transient swelling and drying of glassy and semicrystalline polymers. Anomalous, non-Fickian behaviors are broadly associated with the nonlinear effects of the penetrant on the dynamics of the polymers. Here we describe anomalous behavior of a different type that is associated with a sorbent-triggered first-order phase transition. The system of interest is a hydrophilic, semicrystalline polymer with hydrophobic end groups, placed in contact with humid air.Dueto the strong tendency to absorb water, the melting transition of the crystallites can be induced by contact with sufficiently humid air; during desorption, a sufficient supersaturation mustbe reached before nucleation and regrowth of crystals occur. This leads to very different hysteresis in the sorption isotherm and overshoot in transient sorption than has been reported previously in polymers.