Chapter 9
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Gas Permeation Properties of SilaneCross-Linked PoIy(propylene oxide) Kazukiyo Nagai and Tsutomu Nakagawa Department of Industrial Chemistry, Meiji University, 1-1-1 Higashi-mita, Tama-ku, Kawasaki 214-8571, Japan
Gas permeation properties o f rubbery silane-crosslinked poly(propylene oxide) films were evaluated and compared to those of polydimethylsiloxane (PDMS). The permeability, diffusivity, and solubility of poly(propylene oxide) [PPO] to various gases, except for carbon dioxide, obey common transport behavior in rubbery polymers, such as P D M S . The carbon dioxide diffusivity of PPO is lower than expected based on the trend o f the data o f other gases, whereas the carbon dioxide solubility is higher than expected. Because P P O is a very hydrophilic polymer, strong interactions exist between the propylene oxide units and carbon dioxide. For all gases, except for carbon dioxide, both permeability and diffusivity in PPO are lower than those o f polydimethylsiloxane. On the other hand, the solubility in PPO is almost the same as that in PDMS.
© 2004 American Chemical Society
Pinnau and Freeman; Advanced Materials for Membrane Separations ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
139
140 Carbon dioxide separation is one o f the most important industrial gas separation applications, including the following: (i) carbon dioxide removal from flue gas and landfill gas, (ii) natural gas sweetening, and (iii) enhanced oil recovery. Because C O is a greenhouse gas according to the 1997 Kyoto United Nations Protocol on Climate Change, most countries have tried to reduce carbon dioxide release into the atmosphere. Carbon dioxide separation from either air or methane are, therefore, target applications for the development of advanced membrane materials and novel processes. Gas transport through dense polymer membranes is based on a solutiondiffusion mechanism (1). Hence, the gas selectivity o f the membrane is the product of the diffusivity selectivity and the solubility selectivity. To increase the gas selectivity, materials with improved diffusivity selectivity and/or solubility selectivity are desirable. Carbon dioxide is a polar gas relative to air (i.e., oxygen and nitrogen) and methane. For carbon dioxide/inert-gas separation applications, it is possible to enhance the overall selectivity by increasing the solubility selectivity of the membrane. In the past, several membranes have been developed that contain chemical structures which exhibit preferential interactions with carbon dioxide, especially, polyether-based materials (2-4). Poly(propylene oxide)-based elastomers are used as sealing, coating, and adhesion materials (5). However, very few data on gas permeability, diffusivity, and solubility of this elastomer are available. In this study, gas permeation properties o f silane-crosslinked poly(propylene oxide) are reported and compared to those of polydimethylsiloxane. The structures of the polymers are shown in Figure 1. 2
Experimental Film Preparation Linear poly(propylene oxide) having a trimethoxysilyl group at both polymer chain ends was cured (crosslinked) using a tin-catalyst (Scheme 1) according to previous work (6). The average number of the repeat units of poly(propylene oxide) was 51. Characterization The geometric density of the silane-crosslinked polypropylene oxide) films was determined by measuring the film weight and volume at ambient conditions. The fractional free volume, FFV, of the films was estimated from F F V =
V
-
L
3
V
*
(1)
V
where Vis the polymer specific volume (reciprocal of geometric density) and F is the specific van der Waals volume estimated from Bondi's group contribution w
Pinnau and Freeman; Advanced Materials for Membrane Separations ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
141
CH, 3
I
{ c H
2
- C H -
CH
0
Si-04
}
I
CH
3
Poly(propylene oxide)
n 3
Polydimethylsiloxane
Figure 1. Repeat units ofpolyfpropylene oxide) and polydimethylsiloxane.
H co
0CH3
3
H CO-Si-C H -0«f CH2-CH-O *jC H -S!-OCH 3
3
6
H3CO
3
CH
3
"
e
OCH
3
3
H OinAir 2
CH3OI-H O-
1
^O^i^He^fCH^H^f-CsHrSi-O--— O
CH3
O
—
Scheme 1. Preparation of silane-crosslinked polyfpropylene oxide) films.
Pinnau and Freeman; Advanced Materials for Membrane Separations ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
142 method (7). The polymer cohesive energy density, CED and the solubility parameter, 5, of PPO were estimated using the group contribution method of 9
Fedors (
