Skin Formation and Water Distribution in Semicrystalline Polymer Layers Cast from Solution:  A Magnetic Resonance Imaging Study

Magnetic resonance imaging (MRI) using FLASH and GARField is used to measure the water distribution as a function of time in layers of poly(vinyl alcohol) (PVOH) cast from aqueous solution. It is shown that the water distribution remains uniform in thin layers dried slowly but is nonuniform in thicker layers dried more quickly. Moreover, it is shown that layers dried slowly in comparison to a time characteristic of the amorphous−crystalline transition develop a crystalline skin whereas other layers do not. These results are described in terms of a parameter space defined by a (dimensionless) Peclet number for the layer and a dimensionless drying time. The magnetic resonance imaging results and interpretation are supported by pulsed field gradient measurements of polymer and water diffusion coefficients and also by differential scanning calorimetry (DSC) measurements of dried layers.