Solvent diffusion and dissolution of glassy polymers

Jianchu Wu, Purdue University

Abstract

Polymer dissolution in a solvent involves solvent diffusion and polymer dissolution. A solvent diffusion model was developed to account for the effect of polymer viscoelastic properties on solvent transport behavior. The model equations consisted of two nonlinear partial differential equations. The diffusional Deborah number was identified as a major model parameter affecting the transition from Fickian diffusion to Case II transport. A new polymer dissolution model was developed by incorporating the polymer chain disentanglement mechanism into the relevant transport equations. An expression for the disentanglement time was derived from the reptation theory and the concept of dissolution clock was introduced as a material clock controlling the moving position of the solvent-polymer boundary. A dimensionless dissolution number was defined as the ratio of the characteristic polymer disentanglement time to the characteristic solvent diffusion time; the dissolution number was shown to be proportional to the square of the gel layer thickness. Scaling law expressions for the dependence of the gel layer thickness and the polymer dissolution rate on polymer molecular weight were also derived. Numerical simulation of one-dimensional dissolution showed three distinct dissolution stages and confirmed the proposed scaling law relations for the gel layer thickness and the dissolution rate. Experimental studies of dissolution of polystyrene and poly(methyl methacrylate) in methyl ethyl ketone (MEK) was performed. A critical angle illumination microscope equipped with a dissolution cell was built to examine the surface layer structure during dissolution. Two types of polymer dissolution behavior were observed. For dissolution of polystyrene in MEK, the solvent diffusion behavior was quasi-Fickian and a constant gel layer thickness was observed during the stationary dissolution stage. The effect of polymer molecular weight on the gel layer thickness was investigated for nine monodisperse samples, with M$\sb{\rm n}$ ranging from 28,000 to 2,830,000. The experimental results showed the dependence of the gel layer thickness on molecular weight is more prominent in the high molecular weight region. The dissolution of PMMA in MEK was controlled by crack propagation and no significant gel layer was formed. The polystyrene data verified the new dissolution model.

Degree

Ph.D.

Advisors

Peppas, Purdue University.

Subject Area

Chemical engineering

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