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

Brahim Bessif, Thomas Pfohl, Barbara Heck, Yaser Alshetwi, Emna Khechine, Jun Xu, and Günter Reiter. Macromolecules 55, 6015–6022, 2022

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.

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