Stress Assisted Diffusion in Polymers

Citation

Shimabukuro, Sy Ross (1991) Stress Assisted Diffusion in Polymers. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/2g5v-v304. https://resolver.caltech.edu/CaltechETD:etd-09222006-155328

Abstract

A model for diluent diffusion into a polymer which includes mass flux contributions from both a concentration gradient and pressure gradient is proposed. The pressure is a result of the diluent-induced swelling of the polymer and a constitutive law is incorporated that is analogous to that used in nonlinear thermo-viscoelasticity. The concept of free volume is used as the common basis to relate the relaxation processes for a viscoelastic material to the diffusion of the diluent molecules. The resultant strains (swelling) and stresses must satisfy a strain-displacement relationship and equilibrium equations, respectively.

The governing equations are solved using the finite element technique. An iterative scheme is developed to solve the stress-coupled diffusion process as two separate problems: one for the diffusion process and one for the mechanical response. The two problems are solved individually where the solution from one problem is used as input to the other.

Numerical simulations are performed for both one-dimensional and two-dimensional axisymmetric problems in which the diluent is assumed to diffuse along the axis of the cylinder. The results suggest that certain anomalous experimental diffusion results can be explained through the relaxation behavior of the polymer. There are indications that Case II behavior is included in the model by an appropriate choice of material parameters. The results also indicate that typical diffusion experiments may be inadequate to uniquely define the physical model for the diffusion process when more than one type of diffusion driving force exists.

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