Abstract
Although various polymer membrane materials have been applied to gas separation, there is a trade-off relationship between permeability and selectivity, limiting their wider applications. In this paper, the relationship between the gas permeation behavior of polyphenylsulfone(PPSU)-based materials and their chemical structure for gas separation has been systematically investigated. A PPSU homopolymer and three kinds of 3,3′,5,5′-tetramethyl-4,4′-biphenol (TMBP)-based polyphenylsulfone (TMPPSf) copolymers were synthesized by controlling the TMBP content. As the TMPPSf content increases, the inter-molecular chain distance (or d-spacing value) increases. Data from positron annihilation life-time spectroscopy (PALS) indicate the copolymer with a higher TMPPSf content has a larger fractional free volume (FFV). The logarithm of their O2, N2, CO2, and CH4 permeability was found to increase linearly with an increase in TMPPSf content but decrease linearly with increasing 1/FFV. The enhanced permeability results from the increases in both sorption coefficient and gas diffusivity of copolymers. Interestingly, the gas permeability increases while the selectivity stays stable due to the presence of methyl groups in TMPPSf, which not only increases the free volume but also rigidifies the polymer chains. This study may provide a new strategy to break the trade-off law and increase the permeability of polymer materials largely.
Highlights
The membrane-based separation technology is superior to other conventional processes for gas separation in terms of energy consumption, footprint, environmental impact, maintenance cost, and easy operation [1,2,3,4,5,6]
We aim to investigate the relationship between the gas permeation behavior and the effects of methyl groups in PPSU-based copolymers
PPSU-0 is the homopolymer of PPSU and the other three samples, PPSU-4, PPSU-6, PPSU-7, are the copolymers of PPSU and TMPPSf, which have the additional four methyl groups
Summary
The membrane-based separation technology is superior to other conventional processes for gas separation in terms of energy consumption, footprint, environmental impact, maintenance cost, and easy operation [1,2,3,4,5,6]. Challenges still exist, in developing advanced polymeric membrane materials, because of the existing trade-off relationship between gas permeability and selectivity [17,18,19,20,21]. Synthesis of new polymeric materials [6,7,10,11,12,13,16,22,23,24], cross-linking [7,24,25,26], development of mixed matrix membranes [3,11,27,28,29,30,31,32], and polymer blends [9,33,34]. Two breakthroughs have occurred in the last two decades; one was the polymers of intrinsic microporosity (PIMs) [10,35,36] while the other was the thermally rearranged (TR)
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