On the Importance of Chain Reptation in Models of Dissolution of Glassy Polymers

Polymer dissolution was described by chain reptation incorporated into penetrant transport. The penetrant concentration field was divided into three regimes which delineate three different transport processes. Solvent penetration through the polymer was modeled to occur as a consequence of a diffusional flux and an osmotic pressure contribution. Species momentum balances were written that coupled the polymer viscoelastic behavior with the transport mechanism. Transport in the second penetrant concentration regime was modeled to occur in a diffusion boundary layer adjacent to the rubbery-solvent interface, where a Smoluchowski type diffusion equation was obtained. The disentanglement rate of the polymer is given by the ratio between the radius of gyration of the polymer and the reptation time. This rate was used to write the mass balance at the rubbery-solvent interface. Scaling law expressions for the disentanglement rate were derived. The model equations were numerically solved, and the effect of the polymer molecular weight and the diffusion boundary layer thickness on the dissolution mechanism was investigated for polystyrene dissolution in methyl ethyl ketone. The results showed that upon increasing the polymer molecular weight, the dissolution became disentanglement-controlled. Decrease in the diffusion boundary layer thickness led to a shift in the dissolution mechanism from disentanglement control to diffusion control.