In situ dissolution and swelling of confined lamellar polymer crystals through exposure to humid air

Induced by exposure to humid air, we observed in-situ the temporal and spatial changes in thickness and morphology of sandwich-like polymer platelets of a block copolymer. These micron-size platelets consisted of a crystalline poly(ethylene oxide) (PEO) lamella confined between two glassy poly(styrene) (PS) layers, formed via crystallization of a hydrophobic-hydrophilic PS-b-PEO diblock copolymer. We observed first the dissolution of the confined crystalline PEO layers characterized by sharp dissolution front propagating at a constant velocity as described by case II diffusion of a liquid into a solid. The resulting hydrated PEO brushes absorbed further water until equilibration of the respective chemical potentials, accompanied by a change from a planar to a dome-shaped morphology which could be fully reversed by exposure to dry air. Repeated swelling/de-swelling cycles demonstrated stability and reproducibility of these developments taking place at different transport rates. We discuss the underlying processes of water permeation into and efflux from the PEO layers.