Francesco Fornasiero

Solubilities and diffusivities of water vapor in poly(methylmethacrylate), poly(2-hydroxyethylmethacrylate), poly(N-vinyl-2-pyrrolidone) and poly(acrylonitrile)

Abstract Sorption and diffusion data were obtained for water vapor in four different polymers: poly (methylmethacrylate) (PMMA), poly (2-hydroxyethylmethacrylate) (PHEMA), poly (N-vinyl-2-pyrrolidone) (PVP) and poly (acrylonitrile) (PAN) at 35 °C using a gravimetric sorption method. Highest sorption was for PVP, followed by PHEMA. PMMA and PAN sorbed very little water. All the polymers exhibit a BET type III sorption isotherm; the large upturn at high activity for PVP and PHEMA is probably due to plasticization of the polymers by water vapor. Sorption data were interpreted using Flory–Huggins theory and the Zimm and Lundberg cluster integral. Fickian diffusion is observed for PHEMA. For PVP, the fractional uptake Mt/M∞ is linear with the square root of the time up to Mt/M∞=0.6−0.8 for all water activities aw, but it shows a clear water sorption overshoot at aw=55.3% and aw=72.1%, probably due to macromolecular relaxation. PMMA sorption kinetics is also characterized by a maximum in the water uptake. The diffusion coefficient increases significantly with water concentration for PVP and PHEMA, weakly for PMMA, but it is independent of concentration for PAN.

Sorption and transport of water vapor in thin polymer films at 35 °C

Using a gravimetric technique, experimental data at 35u2006°C are reported for isothermal sorption equilibria and sorption kinetics of water vapor in poly(N,N-dimethyl methacrylamide) n (PDMAA), poly(2-dimethyl aminoethyl methacrylate) n (PDMAEMA), poly(acrylic acid) n (PAA), and in a typical membrane of a commercial soft-contact lens made of poly(2-hydroxyethyl methacrylate) n (PHEMA). The highest sorption of water vapor is in PDMAA. The Flory–Huggins model and the Zimm–Lundberg clustering theory are used to interpret the equilibrium data. Over the entire range of water activity, the four materials show clustering functions larger than −1 indicating that water molecules cluster together in the polymers. The least hydrophilic polymer, PDMAEMA, and the most hydrophilic one, PDMAA, show respectively, the highest and the lowest tendency of water molecules to form clusters. Water diffusion into the polymer matrix is faster in PDMAEMA (diffusion coefficient, Du2006=u200610–20u2006×u200610−8 cm2 s−1) and in PDMAA (Du2006=u20064.9–8.7u2006×u200610−8 cm2 s−1) than in PHEMA-lens material (Du2006=u20060.55–3.4u2006×u200610−8 cm2 s−1) and PAA (Du2006=u20060.98–3.5u2006×u200610−8 cm2 s−1). The measured diffusion coefficients increase with water activity in the PHEMA-lens, decrease with activity in PDMAEMA, and show a more complex concentration dependence in PDMAA and PAA. By writing the diffusion coefficient as a product of an intrinsic mobility, Đ, and a non-ideality thermodynamic factor, Γ, the concentration dependence of D is explained by the interplay of two factors: rising plasticization with water activity, which causes an increase in Đ, and the effect of composition on Γ, decreasing with water activity.

Molecular thermodynamics of precipitation

Abstract Using a simple theory for fluids and a simple theory for a solid, it is possible to construct a semi-quantitative corresponding-states phase diagram where a reduced temperature is plotted as a function of a reduced density. The reducing parameters are molecular size ( σ 3 ) and molecular potential energy ( e / k B ); the phase diagram includes both low-density and high-density fluid regions and the solid region. These calculations apply to a pure substance or, of more interest, to a solute dissolved in a continuous solvent. The qualitative nature of the phase diagram depends strongly on the range of attractive intermolecular forces as indicated by an exponential parameter n ; when coordination number z =8 and n is about 6, we obtain the usual phase diagram where the fluid–solid region lies to the right of the fluid–fluid coexistence curve. But when n is about 7 or 8, the fluid–solid region lies above the fluid–fluid coexistence curve. Applications are discussed for aqueous solutions of a colloid or a globular protein that may also contain a salt or a polymer to induce precipitation.