Dilation kinetics of glassy, aromatic polyimides induced by carbon dioxide sorption

Over the past years, the equilibrium sorption of gases in polymers has been intensively studied. Mostly, glassy polymers were investigated because of their excellent selective mass transport properties. This work does not focus on the equilibrium sorption but on the kinetics to reach the equilibrium. We developed a new experimental method measuring the sorption-induced dilation kinetics of a polymer film. Carbon dioxide and glassy, aromatic polyimides were chosen as model systems. Low-pressure experiments demonstrate that the measured dilation kinetics represent the sorption kinetics. A significant delay between the sorption and dilation kinetics is based on the fact that dilation kinetics occurs simultaneously with the concentration increase in the center of the polymer film. High-pressure experiments reveal significant differences in dilation kinetics compared to low-pressure experiments. Generally, three regimes can be distinguished in the dilation kinetics: a first, fast volume increase followed by two much slower regimes of volume increase. The magnitude of fast and slow dilation kinetics strongly depends on the swelling history of the polymer sample. The results of the experiments are analyzed in the light of a model relating the fast dilation kinetics to a reversible Fickian dilation and the slower dilation kinetics to an irreversible, relaxational dilation

The formation of nodular structures in the top layer of ultrafiltration membranes

The formation of nodular structures in the top layer of ultrafiltration membranes is considered. A critical review of mechanisms described in the literature is given. Flat-sheet poly(ether sulfone) membranes and hollow-fiber poly(ether sulfone)/polyvinylpyrrolidone membranes were made by coagulation of a polymer solution in a nonsolvent medium under different circumstances. From these experiments, a number of empirical rules are found to describe the resulting morphology of the top layer. A new mechanism for the formation of a nodular structure is proposed. It is based on the small diffusion coefficient of the polymer molecules compared to the diffusion coefficient of solvent and nonsolvent combined with a high degree of entanglement of the polymer network. For unstable compositions, phase separation will proceed by growth in amplitude of concentration fluctuations. The rapid diffusional exchange of solvent for nonsolvent in the top layer leads to vitrification of the maxima of the concentration fluctuations that form the nodules. Complete disentanglement of the polymer chains between the nodules is not reached, which explains the small pores and the low porosity of ultrafiltration membranes

Salt Effects in Free Nonionic Films

The thickness has been measured of macroscopic aqueous films, stabilized by nonionic surfactants consisting of a hydrophobic part and a poly(ethy1ene oxide) moiety. Special attention has been paid to the influence of electrolytes. In the fiis, the surfactants behave like coiled polymers. Addition of electrolytes increases the rate of drainage. As a function of the electrolyte concentration, the equilibrium thickness passes through a maximum. These maxima are similar to those for the Huggins constant for the corresponding micelles

Characterization of clean and fouled ultrafiltration membranes

Much research into the fundamentals of membrane formation and separation has been performed in order to improve the efficiency of the manufacture of ultrafiltration membranes. Determination of the membrane characteristics is a key problem in these investigations. In this paper, we report on a study of membrane morphology by fractional rejection measurements, using low molecular weight saccharides as the test solute, and by electron microscopy. Using a simple model for solute/solvent transport through cylindrical pores, a “characteristic pore size” was derived from saccharide rejection data. This pore size of a hypothetical isoporous membrane, interpreting the measured separation characteristics, provides a promising means of describing differences between membranes with respect to pore size and pore size changes caused by solute adsorption. From high resolution electron micrographs, information was obtained on the skin layer morphologies and, for some membranes the sizes of the larger pores could be estimated