Browsing by Subject "Free volume"

Now showing 1 - 4 of 4

Computational modeling of transport through polymer membranes and globular proteins
(2012-08) Jiang, Yingying, doctor of chemical engineering; Sanchez, Isaac C., 1941-; Paul, Donald R.; Freeman, Benny D.; Truskett, Thomas M.; Elber, Ron
Within a polymer thin film, free-volume elements have a wide range of size and topology. This broad range of free-volume element sizes determines the ability for a polymer to perform molecular separations. Herein, the free volume and transport properties (diffusion, permeability, and selectivity) in both rubbery and glassy polymers were simulated using fully atomistic models. Extension of the computational tool to study the void structure in proteins is also included in this thesis. Six permeable thermally rearranged (TR) polymers and their precursors were studied. Using atomistic models, cavity size (free volume) distributions determined by a combination of molecular dynamics and Monte Carlo methods were consistent with experimental observation that TR polymers are more permeable than their precursors. The cavity size distributions determined by simulation were also consistent with free volume distributions determined by positron annihilation lifetime spectroscopy. The diffusion, solubility and permeation of gases in TR polymers and their precursors were also simulated at 308 K, with results that agree qualitatively with experimental data. A new hybrid Monte Carlo/Molecular Dynamics method is developed for estimating the slow diffusion processes of light gases transporting in glassy polymers. Diffusion coefficients, as small as 10⁻⁵ to 10⁻⁹ cm²/s are estimated for penetrants in four different polymers at 298 K. In all cases, agreement between literature experimental data and values obtained from the fast hybrid molecular dynamics method ranges from good to excellent. A new technique is developed using Monte Carlo methods to characterize the cavity size distribution and surface atoms in globular proteins. New statistical metrics have been defined for the structural characterization of globular proteins. Some of these metrics include volume, surface area, asymmetry ratio, interior cavity size distribution, and the identification of percolation channels. Wild-type (WT) myoglobin (Mb) and 5 Mb mutants have been studied in this research as examples. An analysis of cavity statistics provides an efficient method to quantify local properties such as packing density and transport pathways. The average cavity sizes of WT Mb and its mutants are around 4.0-5.0 Å.
Effect of moisture on mixed-mode TSR on a glass/epoxy interface
(2017-12) Ferreira Vieira de Mattos, Daniel; Liechti, K. M.; Huang, Rui; Rodin, Gregory J; Ravi-Chandar, Krishnaswa; Bonnecaze, Roger T
The understanding of interfacial failure in adhesively bonded structures is important for several sectors including transportation and infrastructure. This problem has motivated studies for several decades. Adhesives are polymeric and, as such, present time, temperature, strain rate and moisture dependence. The effect of moisture on interfacial adhesion and fracture is still an open problem and demands a deep multi-disciplinary study considering nonlinear viscoelasticity, fracture mechanics, diffusion, chemistry and surface science. This is justified through the mechanisms through which moisture can affect interfacial adhesion. The presence of moisture can degrade the interface integrity. The absorbed moisture also modifies the mechanical properties of the bulk adhesive as well as its interactions with substrates, which introduces changes in the response of the adhesively bonded structure as it is subjected to an external load. An additional complication for interfacial cracks constrained to grow along the interface is that crack growth is governed by the tensile and shear stresses at the interface as well as the interfacial interactions (adhesion energy, strength and range) embodied in traction separation relations and giving rise to the term mixed-mode fracture. This research investigates the effect of moisture on interfacial fracture for different mode-mixes. The content is developed in four parts. First, the adhesive is experimentally characterized via the following tests: mechanical loading, water diffusion, thermal and hygral expansion. These results introduce the second part: a nonlinear viscoelastic model calibrated considering all those measured properties. This model captures the effect of time, temperature, strain rate and moisture on the mechanical behavior of the adhesive. The third part deals with the fracture behavior of a glass/epoxy interface over a range of mode-mixes and moisture conditions. This is complemented by analyses including optical profile measurements of the fractured surfaces and extraction of traction and separation relations and toughness. Finally, a significant emphasis was placed on the numerical analysis which was required for each of the three components outlined above.
Impact of humidity and polymer blending on the gas transport properties of polybenzimidazoles
(2019-07-17) Moon, Joshua David; Freeman, B. D. (Benny D.); Paul, Donald R.; Sanchez, Isaac C; Riffle, Judy S; Lynd, Nathaniel A
Polybenzimidazoles (PBIs) are attractive polymers for gas separation membranes due to their high chemical and thermal stability and rigid, size-selective molecular structures. Opportunities exist for using PBIs for high temperature H₂/CO₂ separation, among other separations, where significant amounts of water are often present. However, PBIs are uniquely hydrophilic glassy polymers, and the impact of humidity on PBI gas transport properties is not well understood. Highly sorbing penetrants like water are often considered to affect molecular transport in polymers through phenomena such as competitive sorption, antiplasticization, and plasticization, but greater fundamental understanding is needed to relate these phenomena to other key concepts in polymer transport like free volume. Additionally, opportunities exist to improve low PBI gas permeabilities through material modification. This study investigates fundamentals of water sorption, dilation, and diffusion in PBIs to develop a systematic understanding of how water uptake affects molecular transport in hydrophilic glassy polymers. Water vapor sorption and swelling in PBIs were experimentally measured, which enabled direct evaluation of polymer free volume changes arising from water uptake. Gas transport properties were measured across a range of humidities using a custom experimental apparatus and correlated with humidity-induced free volume changes. This analysis enabled unique insight into the tradeoff between competitive sorption, antiplasticization, and plasticization effects of water sorption on PBI transport properties. Similar analysis could be used to investigate fundamentals of mixed penetrant sorption and diffusion in other polymers. Finally, a method of improving PBI gas separation properties by blending PBIs with a more permeable polymer was investigated. Commercial PBI was blended with an ortho-functional polyimide capable of undergoing thermal rearrangement at high temperatures. Films of PBI blended with a small fraction of polyimide exhibited matrix-droplet morphologies that enabled synergistic combination of PBI and polyimide gas separation properties. Heat treatment caused thermal rearrangement of the polyimide phase, increasing blend H₂ permeabilities, while also increasing structural order in the PBI phase, increasing blend H₂/CO₂ selectivities. The net result of heat treatment was simultaneous improvement in both H₂ permeability and H₂/CO₂ selectivity at ambient temperatures, surpassing the 2008 H₂/CO₂ upper bound
Preparation and characterization of disulfonated polysulfone films and polyamide thin film composite membranes for desalination
(2011-12) Xie, Wei, 1982-; Freeman, B. D. (Benny D.); Paul, Donald R.; Sanchez, Isaac C.; Bielawski, Christopher W.; McGrath, James E.
The current reverse osmosis desalination membrane market is dominated by aromatic polyamide thin film composite (TFC) membranes. However, these polyamide membranes suffer from poor resistance to continual exposure to oxidizing agents such as chlorine in desalination applications. To overcome these problems, we have synthesized and characterized a new generation of materials, disulfonated poly(arylene ether sulfone) (BPS) random copolymer, for desalination membranes. A key technical feature of these new materials is their high tolerance to chlorine in feed water and their excellent reproducibility in synthesis. In this study, water and sodium chloride solubility, diffusivity and permeability in BPS copolymers were measured for both acid and salt form samples at sulfonation levels from 20 to 40 mol percent. The hydrophilicity of these materials, based on water uptake, increased significantly as sulfonation level increased. The water and salt diffusivity and permeability were correlated with water uptake, consistent with expectations from free volume theory. In addition, a tradeoff was observed between water/salt solubility, diffusivity, and permeability selectivity and water solubility, diffusivity and permeability, respectively. The influence of cation form and degree of sulfonation on free volume, as probed via positron annihilation lifetime spectroscopy (PALS), was determined in BPS random copolymers in both the dry and hydrated states. PALS-based free volume data for hydrated polymers were correlated with water and salt transport properties. The influence of processing history on transport properties of BPS films was also studied. Potassium form BPS films having a 32 mol% sulfonation level were acidified using solid state and solution routes. Additionally, several films were subjected to various thermal treatments in the solid state. The influence of acidification, thermal treatment, and counter-ion form on transport properties was investigated. Finally, the influence of synthesis methods of polyamide TFC membranes from m-phenylenediamine (MPD) and trimesoyl chloride (TMC) via interfacial polymerization on transport properties is reported. Then, a disulfonated diamine monomer (S-BAPS) was used instead of MPD to prepare TFC membranes. The resulting membranes exhibited reduced chlorine tolerance than those prepared from MPD. However, introduction of S-BAPS to the MPD/TMC polymerization system increased the fouling resistance of the resulting polyamide TFC membranes.