Independent Pores Research Articles

Free Volume and Swelling Dynamics of the Poly[(2-dimethylamino)ethyl methacrylate]-l-polyisobutylene Amphiphilic Network by Positron Annihilation Investigations

The poly[(2-dimethylamino)ethyl methacrylate]-l-polyisobutylene (PDMAEMA-l-PIB) amphiphilic network (APN), a new class of cross-linked systems, was synthesized by the use of methacrylate−telechelic PIB obtained via quasiliving carbocationic polymerization. The swelling dynamics and the free volume changes of this APN was followed by simultaneous swelling and positron annihilation measurements. It was found that the lifetime of ortho positrons (o-Ps) increases with increasing swelling ratio (R), reaches a maximum at relatively low R, and decreases to a constant value at equilibrium swelling. These findings indicate the collapse of the hydrophobic PIB domains and expansion of the hydrophilic PDMAEMA phase in the network upon contacting with water. After reaching the maximum, the decrease of the lifetime parameter is caused by filling the free volume with water in the network. A striking observation was obtained for the o-Ps formation intensity as a function of time (or swelling ratio): the intensity rapidly decreases, and it reaches a minimum at very low R, at ∼10% of the equilibrium swelling ratio, and then increases to a constant value. These phenomena reveal important aspects of the structure of the free volume in the APN and provide fundamental information about the swelling dynamics. The minimum of the o-Ps intensity is reached at around 1:1 water/monomer units molar ratio in the network. Surprisingly, a similar phenomenon was observed when the monomer itself was mixed with water. Molecular modeling by ab initio calculations indicates that a 1:1 ringlike cluster may be formed between water and DMAEMA. On the basis of these results it is concluded that the surface of the free volume units in the APN become covered with water molecules quickly by interacting with the PDMAEMA chains at the beginning of swelling. This indicates that the free volume in the PDMAEMA-l-PIB APN is not composed of independent pores but of interconnected channels which allow rapid diffusion of water molecules to cover the surface of the free volume units. This fast process results in a quick surface structure reorganization, i.e., enrichment of the surface of the channels by PDMAEMA segments leading to the corresponding rapid decrease of o-Ps formation intensity, and to the simultaneous collapse of PIB domains yielding increase of free volume in the network and positron lifetime at the beginning of swelling. Results of this study also allow us to predict that the combined method presented here, i.e., simultaneous swelling kinetics and positron annihilation investigations, can be widely applicable for gaining new information on free volume structure, swelling dynamics, and interactions between material components and swelling agents for a large variety of networks, gels, polyelectrolytes, amorphous materials, blends, composites, etc.

Monte Carlo study of liquid-crystal ordering in the independent-pore model of aerogels.

We have performed Monte Carlo simulations of a confined Lebwohl-Lasher nematic model with different boundary conditions to verify the appropriateness of the independent pore approximation for liquid crystals in silica aerogel systems. We find evidence for the need of going beyond independent pores to account for calorimetric and turbidity data at the same time. \textcopyright{} 1996 The American Physical Society.

Two physically distinct pores in the dimeric ClC-0 chloride channel.

The Torpedo chloride channel ClC-0 is the prototype of a large family of chloride channels that have roles in transepithelial transport and in regulating electrical excitability and cell volume. ClC-0 opens in bursts with two identical conductance levels of approximately 8pS. Hyperpolarization slowly increases the probability of bursts ('slow gating'), and depolarization increases channel opening within bursts ('fast gating'). Replacing serine 123 by threonine changes rectification, ion selectivity and gating, but retains the typical bursting behaviour with two identical independent albeit reduced, conductance states (approximately 1.5 pS). Coexpression with wild-type ClC-0, either as covalently linked concatamers or as independent proteins, leads to bursting channels with two different pores. Our experiments strongly suggest that conductance, ion selectivity and 'fast' gating are determined only by the single subunit forming a single pore, independent from the attached pore; in contrast, 'slow' gating is a function of both subunits. Thus ClC-0 is a homodimer with two largely independent pores.

The three-dimensional structure of human erythrocyte aquaporin CHIP.

Water-permeable membranes of several plant and mammalian tissues contain specific water channel proteins, the 'aquaporins'. The best characterized aquaporin is CHIP, a 28 kDa red blood cell channel-forming integral protein. Isolated CHIP and Escherichia coli lipids may be assembled into 2-D crystals for structural analyses. Here we present (i) a structural characterization of the solubilized CHIP oligomers, (ii) projections of CHIP arrays after negative staining or metal-shadowing, and (iii) the 3-D structure at 1.6 nm resolution. Negatively stained CHIP oligomers exhibited a side length of 6.9 nm with four-fold symmetry, and a mass of 202 +/- 3 kDa determined by scanning transmission electron microscopy. Reconstituted into lipid bilayers, CHIP formed 2-D square lattices with unit cell dimensions a = b = 9.6 nm and a p422(1) symmetry. The 3-D map revealed that CHIP tetramers contain central stain-filled depressions about the fourfold axis. These cavities extend from both sides into the transbilayer domain of the molecule leaving only a thin barrier to be penetrated by the water pores. Although CHIP monomers behave as independent pores, we propose that their particular structure requires tetramerization for stable integration into the bilayer.

Coupling-constant description of coupled flow and diffusion

We show how the effects of a complicated membrane structure on the transport of solute through the membrane can be represented rather accurately by a single new constant. This constant, which lies between 0 and 1, represents the coupling between the transport of solute by diffusion and by convective flow, as caused by the membrane structure. For a structural model of independent parallel pores, the coupling constant can be interpreted as representing a certain bound on the pore size distribution. By means of theorems on lower and upper bounds, we show that the errors resulting from this coupling-constant approximation are at most a few percent. The same theorems are also used to establish error bounds for a coupling-constant description of the rejection curves ( R vs. Q v) for sieving membranes.

Effect of heteroporosity on membrane rejection coefficients

Statistical heteroporosity theory is applied to the case of ultrafiltration across a membrane composed of an array of independent channels or pores. The primary question addressed is: For given values of the reflection coefficient σ, and diffusive permeability P s, measured independently for a membrane of unknown heteroporosity, how accurately can the rejection coefficient R be predicted by using the Spiegler—Kedem expression for rejection, derived without concern for heteroporosity? For the case of applied pressure much greater than back-osmotic pressure, limits are analytically derived which show a maximum 30% relative error in R at σ → 0, and a maximum absolute error of 0.06 at σ ≅ 0.5. Experimental regions where heteroporosity effects should be largest are given explicitly. It appears that, except for these regions, the Spiegler—Kedem equation can be used to describe accurately rejection across membranes of arbitrary heteroporosity. p]For the case of appreciable solute concentration in the feedstream, a proposed test of heteroporosity consists of measurements of rejection vs. volume flux, at small and large solute concentrations. For a particular model of membrane structure called the two-pipe model it is shown that, in principle, heteroporosity can result in significant changes of rejection with concentration at a given volume flow, but for three real membranes used as examples the effects are predicted to be relatively small.

Immobilization of enzymes by radiation-induced polymerization of glass-forming monomers: 2. Effects of cooling rate and solvent on porosity and activity of immobilized enzymes

The effect of cooling rate of a monomeric system on the porosity and activity of an immobilized enzyme prepared by radiation-induced polymerization of 2-hydroxyethyl methacrylate at low temperatures has been studied. Slow cooling gave the same effect on porosity of the polymer as decreasing the monomer concentration. A glass-forming solvent such as diethylene glycol was added to water to study the effect of the supercooling tendency of the solvent. Addition of diethylene glycol decreased porosity and also enzymic activity. Water was replaced by the miscible solvent p-dioxane and the immiscible solvent n-decane in order to clarify the effect of solvent. p-Dioxane had a similar effect to water on the relation between the monomer concentration, porosity and activity. On the other hand, polymer prepared from the system containing n-decane showed different immobilization properties owing to the presence of independent pores in the matrix.