North American Membrane Society Annual Meeting 2006
Chicago, IL, 13th – 17th May
Abstracts of Attendees
Brandon Rowe’s Abstract
Tracking Physical Aging of Thin Glassy Polymer Films by Ellipsometry
B. Rowe, University of Texas
D. Paul, University of Texas
B. Freeman, University of Texas
Current gas separation membranes are typically formed from glassy polymers because of their exceptional permeability-selectivity properties. Glassy polymers are non-equilibrium materials whose properties (e.g., density, permeability, etc.) spontaneously, but usually slowly, evolve over time towards an equilibrium state. This process is known as physical aging. Interestingly, the physical aging rate becomes orders of magnitude more rapid if the thickness of the film is decreased below about one micron[1]. This phenomenon is an intrinsically fascinating scientific issue, and understanding physical aging is valuable for the gas separation industry.
Commercially used membranes for gas separation have a thickness of approximately 0.1 microns or less and may continue to decrease as the technology for making these films develops. Films ranging from 400 nm to 1000 nm have recently been shown to have rapid and thickness dependent aging rates as determined by gas permeability measurements. In addition to studying the gas transport properties of polymer films, physical aging is being examined by tracking the change in density of polymer films over time using high precision ellipsometry. Ellipsometry is a sensitive optical technique for determining properties of surfaces and thin films by measuring the changes in the polarization state of light when it is reflected from a sample. The refractive index of a polymer sample, measured by ellipsometry, is related to the polymer density through the Lorentz-Lorenz equation. Studies show an increase of polymer density with time, which correlates with the reduction in size and/or concentration of free volume elements in the sample. The effect of humidity on ellipsometry measurements has been studied and a direct relationship between refractive index and relative humidity has been found. The influence of humidity on physical aging has also been investigated.
1. Huang, Y. and D.R. Paul, Physical aging of thin glassy polymer films monitored by gas permeability. Polymer, 2004. 45(25): p. 8377-8393.[/tab]
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Hobum Park’s Abstract
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Novel Sulfonated Poly(arylene ether) Copolymer Membranes and Their Use in Reverse Osmosis: Fundamental Water and Salt Transport Study, Chlorine Stability and Anti-Fouling Characteristics
H. Park, University of Texas at Austin
B. Freeman, University of Texas at Austin
J.E. McGrath, Virginia Tech University
Z. Zhang, Virginia Tech University
G. Fan, Virginia Tech University
M. Sankir, Virginia Tech University
A. Roy, Virginia Tech University
H.S. Lee, Virginia Tech University
A Badami, Virginia Tech University
Recently, the interest in preparing new RO materials having excellent chlorine tolerance, similar or higher water flux and rejection and markedly reduced fouling relative to the conventional RO membranes is increasing. In the late 1980s, a few sulfonated polymers were reported to have promising RO properties and excellent chlorine tolerance, but the lack of control of the sulfonation process made the preparation of reproducible the materials challenging1. Recently, a systematic series of sulfonated poly(arylene ether) copolymer membranes developed at Virginia Tech have been prepared by directly copolymerizing a disulfonated monomer into the polymer backbone2. The present study focuses on exploring fundamental structure-property relationships in these materials. These polymers have excellent chlorine tolerance, low fouling in oily or protein-containing waters, and high water flux with moderate salt rejection. A fundamental study of salt diffusivity, solubility and permeability in these sulfonated copolymer membranes is providing basic information useful for molecular engineering these materials towards better property profiles.
1. R.J. Petersen, “Composite Reverse Osmosis and Nanofiltration Membranes” J. Membrane Sci. (1993), 83, 81-150. 2. M.A. Hickner, H. Ghassemi, Y.S. Kim, B. Einsla and J.E. McGrath, “Alternative Polymer Systems for Proton Exchange Membranes”, Chem. Rev. (2004), 104, 4587-4611.
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Roy Raharjo’s Abstract
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A Fundamental Study of Mixture Permeability, Solubility, and Diffusivity in Vapor Selective Polymers
R. Raharjo, University of Texas at Austin
B. Freeman, University of Texas at Austin
E. Sanders, Air Liquide, Newport, DE
Membrane separation technology has recently emerged as a potential alternative technique to remove higher hydrocarbons (C3+) from natural gas1. The material that could be suitable, and, in fact, is currently being considered, for such separation is silicone rubber or poly(dimethylsiloxane) (PDMS). Aside from its high gas permeabilities (i.e., highest among rubbery polymers), this polymer is more permeable to the larger, more condensable higher hydrocarbon than to smaller, less condensable permanent gases, such as methane. As a result, methane, the major constituent in natural gas, can be kept at high pressure, which eliminates the cost of recompression that would be incurred if a methane-selective membrane were used for this separation.
The gas sorption and transport properties in PDMS have been studied in the past. Most of the references in the literature, however, report only its pure gas properties; only a few actually investigate its mixture properties. To accurately estimate the membrane separation performance, the use of mixtures in the study is required. This paper presents the n C4H10/CH4 mixed gas permeability, solubility, and diffusivity in PDMS at various temperatures from -20oC to 50oC. The dilation isotherms of the mixture are also reported to complement the mixture sorption data.
The CH4 permeability, solubility, and diffusivity increase as the n C4H10 fugacity in the mixture increases. The n C4H10 sorption and transport properties in mixtures are unaffected by the presence of CH4 and are similar to those in pure gas conditions. The overall n C4H10/CH4 selectivity increases as n C4H10 activity in the mixture increases and temperature decreases. This report represents the first combined presentation of gas mixture permeability, solubility, and diffusivity in PDMS, which is the most widely used vapor separation polymer.
Another polymer whose properties will be discussed is poly(1-trimethylsilyl-1-propyne) (PTMSP), an ultra-high free volume, solubility selective, glassy polymer. These studies will help illustrate the fundamental mechanism of the competitive permeation, sorption, and diffusion in glassy polymers.
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Xiaoyan Wang’s Abstract
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Physical aging in poly[1-(trimethylsilyl)-1-propyne] (PTMSP)membranes: Effects on free volume and gas permeability
X. Wang, University of Texas at Austin
R. Raharjo, University of Texas at Austin
B. Freeman, University of Texas at Austin
I. Sanchez, University of Texas at Austin
Poly(1-trimethylsilyl-1-propyne) (PTMSP), the most permeable polymer known, is a fast aging material. The permeability of PTMSP to gases and vapors decreases dramatically with physical aging. Cavity size (free volume) distributions were calculated in as-cast and aged PTMSP using an energetic based cavity-sizing algorithm. The large cavities found in as-cast PTMSP disappear in aged PTMSP, which is consistent with the positron annihilation lifetime spectroscopy (PALS) measurements. We also characterized the connectivity of cavities both in as-cast and aged PTMSP membranes. Cavities are more connected in as-cast PTMSP than in aged PTMSP. The average cavity sizes calculated from computer simulation are in good agreement with PALS measurements. The transport and sorption properties of gases in as- cast and aged PTMSP are also measured by molecular simulation. Computer simulations showed the decrease of permeability and the increase of permeability selectivity in PTMSP membranes with physical aging, which agrees withe experimental observations. The reduction in gas permeability with physical aging results mainly from the decrease of diffusion coefficients. Solubility coefficients show no significant changes with physical aging.[/tab]
