A General Free Volume Based Theory for the Diffusion of Large Molecules in Amorphous Polymers above T_g. 4. Polymer-Penetrant Interactions

We previously advanced a mainly free volume based, penetrant molecular shape dependent theory for the diffusion of relatively large molecules in amorphous rubbery polymers. When tested against experimental evidence for plasticizer-in-PVC diffusion, our theory predicted diffusion coefficients (D) that were in reasonable agreement, although the trends with penetrant molecular size and temperature did not display adequate sensitivity. In this work, we have refined the theory to include a more realistic and molecular-based accounting of important polymer-penetrant interactions. The general diffusion equation has been appropriately modified to include an explicit activation energy for diffusion. Activation energies were determined by using a new “reverse solvation” model, which employs molecular mechanics based groupwise intermolecular energy calculations. Plasticizer-in-PVC intermolecular energies were calculated in this way for a series of dialkyl phthalate plasticizers and were then employed in the modified diffusion equation to predict diffusion coefficients. The results show a remarkable improvement over our previous calculations in that the calculated D’s follow the experimental data more closely and the shapes of the DvsT and D vs molecular size curves are more accurately reproduced.