Last update:
08/30/2006
Recent years have been marked by a growing
interest in surface modification of pervaporation membranes by covalent
end-attachment of polymer chains. The modification of inorganic membranes
for pervaporation applications has been of special interest since the
polymer chains alter the surface chemistry of the substrate (i.e.,
providing selectivity) while the mechanical strength of the membrane is
retained. Covalently bonded polymers can be used in good solvent
environments since dissolution of the polymer is prevented by its
attachment to the substrate. In order to tune the performance of the
membrane one has to consider both the chemical and topological properties
of the modifying polymer layer.
Past studies have been devoted to the macroscopic properties of the
resulting hybrid organic/inorganic materials. However, interest is growing
in the nanoscale features which result from the modification process.
Since polymers are becoming an important tools in applications such as
self-assembled monolayers and micromechanical devices, it is increasingly
important to understand and quantify the characteristics of surface-bonded
polymers so that surfaces can be engineered for specific applications.
Poly(vinyl acetate) was chosen as a model polymer in order to analyze the
surface properties resulting from surface modification by a two-step
free-radical graft polymerization method. A variety of tools such as
atomic force microscopy, thermogravimetric analysis, and scanning electron
microscopy were used to examine the dependence of surface features on
polymerization reaction conditions such as temperature and monomer
concentration. Typical Flory radius of the polymers grafted in the study
were found to be 110-170Å, while starting pore sizes of the inorganic
membrane substrates used were 50-500Å. It was determined that polymer
brush layer of 399Å was able to form on the membrane surface for the
specific size and graft density (2.0-3.5 mg/m2 surface) of the polymers
produced by the present free radical graft polymerization.
Liquid separation membranes created using the above graft polymerization
methods were found to efficiently separate organic mixtures of methanol
and methyl-tert-butyl-ether with separation factors up to 100, and aqueous
mixtures of TCE and water with separation factors of up to 370. Separation
could be increased by increasing the polymer graft yield. Modified
membranes displayed remarkably different behavior depending whether the
polymer chain size was smaller or larger than the membrane pore size,
suggesting that graft polymer size is as important a consideration as
polymer chemistry in the modification of porous membrane materials.
References:
- Jou, Jeng-Dung, W. Yoshida and Y. Cohen, "Pervaporation with Ceramic-Supported Polymer Membranes", J. Membrane Sci., 162, 269-284 (1999).
- Yoshida, W. and Y. Cohen, "Ceramic-Supported Polymer Membranes for Pervaporation of Binary Organic/Organic Mixtures", submitted (2002)
Schematic representation of a laboratory pervaporation system
Separation of Methanol/MTBE Mixtures. Comparison Pervaporation (using nano-structured ceramic-supported polymer membranes) with Isobaric Distillation. Source: Yoshida W., and Y. Cohen (2002).
