During drying of a chemically crosslinked poly(vinyl alcohol) gel, we observed, by atomic force microscopy (AFM) and scanning electronic microscopy, micron-order wrinkling which remains until the end of drying. We show that while at high humidity drying is homogeneous and no surface instability is observed, fast drying at low humidity induces the formation of a glassy skin layer on the soft bulk that wrinkles because the skin is compressed as the bulk contracts. The role of the glass transition is not only to create a mismatch of the elasticity with the bulk but also to remove the in-plane tensile tension which stretches the skin before drying. We proved experimentally the presence of this thin glassy skin on the wet bulk by approach-retract curve measurements in AFM. We also comment on the effect of mechanical restraint and the observation of hierarchical wrinkling over different length scales.

Blends of two immiscible polymers dissolved in a common solvent exhibit phase separation during drying of the solvent. This leads to different morphologies depending on the physical and physicochemical conditions of the systems. Here we report the final morphology of blend films of a glassy polymer (polyacrylamide, PAAm) and a crystalline polymer (poly(ethylene glycol), PEG) in water, prepared by blade coating with a controlled thickness from several to hundreds of micrometers. By changing the blend composition, we observed the inversion in morphology from mounds to holes via multiscale phase separation. A systematic study of PAAm-rich films having a morphology with holes revealed that two regimes may be distinguished depending on the drying rate: a heterogeneous drying regime where a glassy skin of PAAm is formed at the surface and fixes the morphology and a slow homogeneous drying regime where coalescence of holes is observed. Formation of monolayer of PEG crystals having fingerlike branched patterns at the surface of the PAAm matrix at different drying conditions is discussed.