Experimental modeling of solvent‐casting thin polymer films

Abstract

An apparatus was designed and assembled to study the solvent removal from solution-cast thin polymer films. The computer interfacing of a thermogravimetric analyzer, spectrophotometer, electronic flowmeters, and control valves for the apparatus enabled the preprogramming of the carrier gas velocity, carrier gas solvent content, and temperature profiles to simulate the environment experienced in large parallel flow industrial driers. The apparatus has also been designed and operated to enable the visual observation of the drying film with a n optical microscope. Initial experimental studies conducted with the apparatus involved the effect of temperature on solvent removal. The results indicate that high dryer gas temperatures can apparently cause skinning of the film surface resulting in slower solvent removal rates. The skin formation can be suppressed by higher solvent concentration in the carrier gas. The visual observations revealed the formation of standing waves in the film surface during drying at high gas velocities (>200 cm/min). The wave formation at least partially overcomes the effect of skinning by increasing the surface area of the film, and may be the manifestation of flow instabilities involving circulation within the film.

Skinning During Desorption of Polymers: An Asymptotic Analysis

“…First, a glassy skin can form at the exposed surface. Since the molecular diffusion coefficient is smaller in the glassy region [1], [2], [3], the formation of such a skin slows desorption [7]. In extreme cases, trapping skinning can occur, and an increase in the driving force will decrease the total flux through the exposed surface.…”

Trapping Skinning in Polymers: Theoretical Predictions

“…An increase in n will increase F, as it should since we are increasing the diffusion coefficient. However, for fixed C,, an increase in k, which would normally increase the amount of penetrant desorbed, will actually decrease F. This embodies the very essence of trapping skinning (Cairncross and Durning, 1996;Powers and Collier, 1990).…”

“…Trapping skinning is an anomalous special case of the skinning effect in which an increase in the force driving the desorption will actually decrease the accumulated flux through the boundary (Cairncross and Durning, 1996;Powers and Collier, 1990). Crank (1950) and Crank and Park (I95 I) showed that this behavior cannot be fully explained by the lower Fickian diffusion coefficient in the glassy region.…”

A Spatially Nonlocal Model for Polymer Desorption

“…In trapping skinning, an increase in the force driving the desorption will actually decrease the accumulated flux through the boundary [3,11,[17][18][19]. These various anomalous features of the skinning process cannot be explained by simple Fickian diffusion, which postulates that the chemical potential of the system is a function only of the penetrant concentrationC.…”

“…This is physically reasonable, because here the leading order is O( −1/2 ). Since the skin is known to slow desorption [16,17], it is clear that the flux through the skin should be no larger than O(1). The last condition needed for our layer problem is a matching condition; we note from (3.3) that the appropriate one for our problem is…”