Decontamination of VX from Silicone: Characterization of Multicomponent Diffusion Effects

Highlighting Thermodynamic Coupling Effects in the Immersion Precipitation Process for Formation of Polymeric Membranes

In the immersion precipitation process for membrane formation, a polymer casting film is placed in contact with a nonsolvent in a coagulation bath; an essential feature of the membrane formation process is the foray into the metastable region of the ternary phase diagram for the nonsolvent/solvent/polymer system. The primary objective of this article is to trace the origins of such forays. The Maxwell-Stefan diffusion formulation is combined with the Flory-Huggins description of phase equilibrium thermodynamics to set up a model for describing the transient equilibration trajectory that is followed in the polymer casting film. Four different systems are analyzed: water/acetone/CA, water/DMF/PVDF, water/NMP/PSF, and water/NMP/PEI (CA = cellulose acetate; PVDF = poly(vinylidene fluoride); PSF = polysulfone; PEI = polyetherimide, DMF = dimethyl formamide; NMP = N-methyl-2-pyrrolidone). The analysis shows that diffusional forays are mainly engendered due to thermodynamic coupling effects; such effects are quantified by the set of thermodynamic factors , where a _i , the activity of species i, is dependent on the volume fractions, ϕ _i and ϕ _j , of both nonsolvent (i) and solvent (j). In regions close to phase transitions, the off-diagonal elements Γ _ij (i ≠ j) are often negative and may attain large magnitudes in relation to the diagonal elements Γ _ii . Strong thermodynamic coupling effects cause the transient equilibration trajectories to be strongly curvilinear, causing ingress into the metastable region. If thermodynamic coupling effects are ignored, no such ingress occurs. It is also shown that analogous diffusional forays may lead to emulsion formation in partially miscible liquid mixtures.

Highlighting Thermodynamic Coupling Effects in Alcohol/Water Pervaporation across Polymeric Membranes

The pervaporation of binary alcohol/water mixtures across polymeric membranes is modeled by combining the Maxwell-Stefan (M-S) diffusion formulation with the Flory-Huggins (F-H) description of sorption equilibrium. The combined M-S/F-H model shows that the flux of each penetrant species is coupled to the driving force of its partner penetrant. Two types of coupling contributions can be distinguished: (i) coupling arising out of correlated motions of penetrants in the polymer matrix and (ii) thermodynamic coupling. The focus of this article is on the contribution of thermodynamic coupling, which is quantified by the set of coefficients , where a _i , the activity of species i, is dependent on the volume fractions ϕ _i ,ϕ _j , of both penetrants in the polymeric membrane. Detailed analyses of published experimental data for pervaporation of ethanol/water feed mixtures of varying compositions in both hydrophobic (poly(dimethylsiloxane)) and hydrophilic (cellulose acetate, polyimide, and polyvinyl alcohol/polyacrylonitrile composite) membranes show that in all cases, the cross-coefficients Γ _ij (i ≠ j) are negative and may attain large magnitudes in relation to the diagonal elements Γ _ii . The net result is that the permeation fluxes of each penetrant are suppressed by its partner, resulting in mutual slowing down of permeation fluxes. If thermodynamic coupling effects are ignored, significantly higher fluxes are anticipated than those that are experimentally observed.

Solvent-Assisted Desorption of 2,5-Lutidine from Polyurethane Films

A fundamental understanding of chemical interactions and transport mechanisms that result from introducing multiple chemical species into a polymer plays a key role in the development and optimization of membranes, coatings, and decontamination formulations. In this study, we explore the solvent-assisted desorption of a penetrant (2,5-lutidine) in polyurethane with aprotic (acetonitrile) and protic (methanol) solvents. Chemical interactions between solvent, penetrant, and polymer functional groups are characterized via time-resolved Fourier transform infrared spectroscopy (FTIR) during single and multicomponent exposures. For both solvents, an increase in the extraction rate of the penetrant is observed when the solvent is applied during desorption. Inspection of the FTIR spectra reveals two potential mechanisms that facilitate the enhanced desorption rate: (1) penetrant/solvent competition for hydrogen donor groups on the polymer backbone and (2) disruption of the self-interaction (cohesive forces) between neighboring polymer chains. Finally, the aprotic solvent is found to generate an order of magnitude greater desorption rate of the penetrant, which is attributed to a greater disruption of the self-interaction during penetrant desorption compared to the protic solvent and the inability of an aprotic solvent to form larger and potentially slower penetrant-solvent complexes.