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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

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