Date of Graduation
Master of Science in Mechanical Engineering (MSME)
Douglas E. Spearot
Po-Hao Adam Huang
Second Committee Member
Applied sciences; Crosslinking; Diffusion; Glass transistion temperature; Molecular dynamics; Polymer nanocomposites; Williams-landel-ferry equation
Molecular dynamics simulations are used to study diffusion of O2 molecules in pure polydimethysiloxane (PDMS), crosslinked PDMS, and PDMS-based nanocomposites. The PDMS chains and penetrates are modeled using a hybrid interatomic potential which treats the Si-O atoms along the chain backbone explicitly while coarse-graining the methyl side groups and penetrates. By tracking the diffusion of penetrates in the system and subsequently computing their mean-squared displacement, diffusion coefficients are obtained. In pure PDMS models of varying molecular weight, diffusivity of the O22 penetrates is found to have an inverse relationship with chain length. Simulation models with longer chains have more entanglements which restrict the evolution of free volume in the system necessary for diffusion of the penetrants, thus reducing their diffusivity. In agreement with experiment, the crosslinked models studied in this work maintain a PDMS to crosslink molecule weight ratio of 5:1 or 10:1. In order to satisfy this weight ratio criterion, the crosslinked models in this study are oversaturated (number of crosslink molecule ends exceeds PDMS chain ends). Despite crosslinking, the presence of these unbonded crosslink molecules in the system enhances diffusivity for the crosslinked cases in comparison to the pure PDMS models. In the nanocomposite models, diffusivity of O22 has an inverse relationship with volume fraction. Nanoparticles act as geometric obstacles for diffusion of the atmospheric penetrates, reducing the available porosity for diffusion. In models with the smallest gap between nanoparticles, a "crossover" behavior is observed at the lowest temperatures examined, resulting in diffusivities higher than the crosslinked and pure PDMS models. This is attributed to the preferential diffusion of the penetrants through localized regions of low density within the PDMS matrix. The creation of these low density regions is due to a combination of the limited mobility of the PDMS chains at temperatures near glass transition and the close proximity of nanoparticles at 20% volume fraction. For all models, the role of temperature on diffusion is captured using the Williams-Landel-Ferry (WLF) equation. The relationship between WLF parameters and molecular weight or nanoparticle volume fraction is studied.
Ullal, Varun, "Molecular Dynamics Study of Diffusion of O2 Penetrates in Uncrosslinked Polydimethysiloxane (PDMS), Crosslinked PDMS, and PDMS-based Nanocomposites" (2012). Theses and Dissertations. 417.