A continuum thermodynamics theory for transport in polymer/fluid systems
Abstract
A predictive theory for fluid transport in polymers was established using the foundations of Continuum Thermodynamics. This framework is founded on mass, momentum and energy conservation and the entropy inequality. The thermodynamic properties of multicomponent mixtures depend on the strain, temperature histories, instantaneous temperature and concentration gradients and velocities. Component mass fluxes include the effects of temperature gradients, chemical potential gradients, and mechanical forces. This transport theory applies to all admissible thermodynamic processes and is not limited to equilibrium states or reversible processes. Constitutive equations were derived based on the restrictions imposed on these quantities by the entropy inequality. Thermodynamic constitutive equations for the polymer were derived using a fading memory representation. The nonlinear response is primarily due to the thermodynamic state accelerating/decelerating the relaxational time scale. This theory generalizes and improves previous models which describe ductile yield, aging and nonisothermal viscoelastic behaviors in real materials. In applying this transport theory to specific polymers, the mechanical properties must be determined as a function of composition. The complex shear moduli of crosslinked poly(2-hydroxyethyl methacrylate) and poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) were measured as a function of water content, temperature and stress/strain frequency using dynamic mechanical spectroscopy. The data could be described by multiple mechanism, time-temperature superposition. With as little as two percent water, the glass transition relaxation spectra and shift functions became independent of water content when properly referenced to the observed glass transition temperature. These results justify the time-temperature-concentration superposition proposed in the viscoelastic model. Numerical predictions for elastic and viscoelastic mixtures provided remarkable agreement with phenomena observed experimentally. Penetrant sorption in an elastic mixture is Fickian although the kinetics are retarded at high swelling due to increased diffusional path length. When transport is accompanied by the viscoelastic mixture's glass transition, several types of non-Fickian behaviors are predicted, including anomalous, Case II and super Case II kinetics. Polymer stresses induce a swelling interface, primarily due to the change in mechanical properties at the glass transition. The visoelastic properties also govern the time-dependent penetrant composition at the polymer/fluid interface.
Degree
Ph.D.
Advisors
Peppas, Purdue University.
Subject Area
Chemical engineering|Materials science|Mechanics
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