Desorption of Liquids from Polymers

Desorption is one diffusion process that has been given little attention, primarily because of the lack of adequate analytical techniques. Desorption measurements above the glass transition temperature of an unswollen polymer are expected to follow Fickian characteristics. Likewise, a polymer swollen so that the T, is below the experimental temperature initially exhibits Fickian desorption. The solvent is thought to desorb rapidly from the surface of the polymer and raise the T, of the surface layer. After the surface is above the experimental temperature, the desorption process slows, and the process is controlled by the diffusion through the glassy surface layer. NMR imaging provides the spatial distribution of solvent in the polymer and also the spatial distribution of the rate of desorption [23]. 

The desorption process can be related to Ta. which is the inverse of the rate of net solvent loss for a given pixel through the equation  

A nonlinear least-squares fit of the experimental data is used to calculate a image on a pixel-by-pixel basis [23]. 

We have reported some results on the NMR imaging of the desorption process [23]. Images of the desorption of methanol from swollen rods of PMMA were obtained [23]. The methanol volume fraction was 0.26. The rods were then placed in fully deuterated cyclohexane. The first image acquisition began 6 min after initial submersion. Images were collected in 1 h increments over a 104 h period. The signal intensity decreased with time, the maximum intensity of the 

F nre 6.8 A 7 image calculated from 20 images at 5 h increments over a 100 h interval. Reprinted with permission from [22]. Copyright 1990 American Chemical Society 

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