| dc.description.abstract | Molecular diffusion is the transfer or movement of individual molecules through a fluid or a solid by means of random, individual movements of the molecules. Even though moisture can diffuse into all materials above the absolute zero temperature, polymers are particularly susceptible to moisture ingress in highly humid environments. Fickian or non-Fickian diffusion models have often been utilized to characterize the effect of moisture absorption. Type, orientation, and homogeneity of reinforcement and type, microstructure, and dimensions of polymer control the effectiveness of each model on describing the moisture absorption dynamics.
In Chapter 3 of this dissertation, effects of additive content in I.30E nanoclay/DGEBA epoxy and I.30E nanoclay/DGEBF epoxy (EPON 862) nanocomposites, and APS or nBS treated glass sphere/DGEBA epoxy composites are investigated using moisture absorption data and one-dimensional hindered diffusion model. Impermeable additives in epoxy resulted in a linear decrease in diffusion coefficient with respect to the additive content following the rule of mixture. Impermeable additives also caused a linear decrease in the maximum moisture content of the composites. I.30E nanoclay addition into DGEBA epoxy lead to a more-Fickian absorption whereas increasing additive content in I.30E nanoclay/DGEBF epoxy (EPON 862) nanocomposite, and APS or nBS treated glass sphere/DGEBA epoxy composites shifted the absorption behavior to a more non-Fickian behavior.
In Chapter 3, capabilities of multilayer hindered diffusion model, a modified version of hindered diffusion model considering the discrete change in absorption properties, are tested by using the moisture absorption mass gain data of hollow glass sphere/EPON 862-EPON 862 multilayer composites. It was shown that the multilayer hindered diffusion model accurately recovered the absorption behavior of multilayer composites having different layer thicknesses.
In Chapter 4, hindered diffusion model was extended to a new void filling hindered diffusion model to illustrate the effect of porosity and microstructure on moisture absorption. Moisture absorption tests on ROHACELL® WF-71 closed-cell foam showed up to 62% change in the maximum absorbed moisture due to the effect of thickness of the foam. Thickness of closed-cell foams changes the foam porosity by the open-cells on the surface to the closed-cell ratio. The distinct absorption behaviors due to the thickness of foams are accurately predicted by the void filling hindered diffusion model by defining the moisture absorbed by the pores as trapped moisture. Effects of heat treatment and pressure on moisture absorption behavior of foams are also investigated. Results of void filling hindered diffusion model predictions are found to be consistent with the experimental observation such as decrease in cell size, porosity, and thickness.
Moisture induced damage such as swelling, micro cracks, delamination, residual stresses, and plasticization of the polymer limit the intended design life due to subsequent electrical, optical, thermal, and mechanical property changes. In Chapter 5, a new comprehensive modulus degradation model where the modulus varies inversely linear with the moisture concentration is introduced and validated by experimental data of tensile or flexural modulus and moisture absorption from three different published studies. | en_US |
