Preparation and analysis of asymmetric membranes

Abstract

It has been shown, theoretically' and experimentally,* that a polymer membrane possessing an internal, fixed gradient of inhomogeneity (e.g., structure, composition, etc.), will exhibit anisotropic transport behavior, such as penetrant mass flow. The rate of flow of a soluble penetrant through such a membrane due to a chemical potential (concentration, vapor pressure) gradient across the membrane depends on the direction of that gradient relative to the direction of the membrane's fixed gradient of inhomogeneity. The same is true for other transport properties, e.g. heat transfer, etc.This directional preference for mass flow of penetrant can be utilized in separation processes; the "valve effect,"* superimposed on the natural separation capabilities of a homogeneous membrane4 will make the separation process more efficient. The analysis and elucidation of membrane structure is essential for the study and prediction of such behavior. Thus, a well-characterized membrane (as to structure, type, and amount of inhomogeneity) is a prime requisite.This note describes some methods for preparation and characterization of polymer membranes possessing a fixed gradient of composition from one surface to the other. The primary technique uses a high-energy electron source to initiate polymerization of monomer sorbed in a prescribed manner in a p o l F e r film. The resultant gradient of composition may be determined by spectroscopic analysis of slices obtained by microtoming the f i l m sample perpendicular to the diffusion axis.A typical diffusion-polymerization cell used for graded-membrane preparation is shown in Figure 1. The original polymer membrane (A) is placed above a monomer (B) reservoir and under a sintered stainless steel plate (void fraction >50%), to form a twocompartment chamber. The membrane acts as a barrier between the two compartments. Any transport of monomer from the lower compartment to the top one, which will take place when the top compartment is evacuated, will occur through the polymer membrane. The sintered stainless steel plate is rigid and prevents deformation of the polymer membrane due to the monomer vapor pressure differential across the membrane during the process.After assembly, the top compartment is evacuated first, thus pulling the membrane against the porous plate-a position it keeps throughout the experiment. The bottom compartment, which contains the monomer, is then outgassed, insuring an atmosphere

Asymmetric Mass Transport through Dense Heterogeneous Polymer Membranes: Fundamental Principles, Lessons from Nature, and Artificial Systems

“…[69] As mentioned earlier, Rogers and colleagues used preformed poly(ethylene) films in which vinyl monomers were diffused directionally followed by radiation-induced polymerization thus forming a compositionally graded membrane structure. [36,57] The same authors speculated that more elaborate membrane architectures can presumably be manufactured using graded graft copolymerization strategies of different monomers at the same or different directions, or by employing non-monomers which may react with the preformed membrane (e.g., by acting as radical chain-transfer reagents, swelling, or cross-linking agents) resulting in spatial structural gradients (see schematic Figure 3b). [36] In other studies, transversal compositional gradients leading to asymmetric transport properties were achieved by directional chemical treatment of preformed membranes with hydrogen peroxide, [42,59] alkaline hydrolysis, [8] and quaternization using methyl bromide vapors (see schematic Figure 3c).…”

“…[11][12][13][14][15][16]29,30] Such gradients of inhomogeneity may be structural and/or compositional and are commonly found in many living organisms, [17][18][19][31][32][33][34][35] including plants and insects. [17][18][19]35] Asymmetric mass transport, also known as "valve effect", [12,36] "anisotropic flow", [37,38] "vectorized flow", [37,39,40] "flow reversal" effect, [11,16,41] and "directional transport", [11,42] means that the permeation rate of chemical species diffusing under the same conditions (driving force, temperature, etc.) through the same membrane depends on the direction in which they pass the barrier.…”

“…Similarly to layered designs, the volume fraction and the distribution of the phases affect the anisotropy and the transport properties in the case of compositionally graded heterogeneous membranes. [11,25,36] Therefore, a prerequisite of asymmetric mass transport is the presence of structural and/or compositional gradients leading to a spatial-dependency of the permeability coefficient of the membrane. Based on the pioneering work of Rogers, Stannett, and Szwarc, and later studies, the direction and magnitude of transport asymmetry in polymer-based artificial dense membranes is dictated by the component with superior deviation from "ideality", i.e., pressure-sensitive permeability caused by a non-ideal diffusion and/or sorption.…”

Sorption and diffusion in asymmetric membranes

“…Details of the method of preparation and characterization of such membranes are described elsewhere. 8 The membrane used for this study was a polyethylene ( p = 0.92 g./ml.) film possessing a very nearly linear gradient of grafted poly(viny1 acetate) from the PVAc-rich face of 9% by volume to the face of nil-grafted polymer.…”

Surface hydroxylation of styrene–butadiene–styrene block copolymers for biomaterials

“…Odian and Kruse (1969) studied diffusion effects in radiation induced graft polymerization but since saturation of the reactive sites in the polymer does not occur, their mathematical solutions are not applicable here. (Rogers, et al, (1968) used these diffusion effects to prepare assymmetric membranes.) Goddard, et al, (1970) considered a reversible second order reaction, near equilibrium in their analysis of facilitated transport in membranes.…”