Photopolymerized thiol-ene networks for gas barrier and membrane applications

Luke Kwisnek

Abstract

Gas transport and free volume properties of photopolymerized thiol-ene networks for various applications are reported. For basic, commercially-available thiol-ene formulations, oxygen permeation was strictly dependent on the difference between Ttest (room temperature for this work) and network T g . Networks with Tg near Ttest demonstrated the lowest values of permeability, diffusivity, and solubility. A robust, high-barrier network was selected for further modification. New networks with embedded functionality were formed using a two-step approach. Thio-Michael addition of a tetrathiol to monofunctional acrylates formed new functional thiol monomers. These functional thiols were combined with an isocyanurate-based triene and UV-cured forming robust crosslinked networks with varying functionality. For barrier applications, cyano, hydroxyl, and amide groups were introduced to the network. Primary amide groups showed the most notable decrease in oxygen permeability due to strong hydrogen bonding. Alkyl and fluoroalkyl acrylates were also introduced to the network to increase permeability toward breathable membrane applications. Oxygen permeability increased substantially as both alkyl and fluoroalkyl length increased. Both alkyl and fluoroalkyl side groups acted as pillars or spacers which prevented packing and expanded free volume. Fluoroalkyl-modified networks demonstrated higher permeability compared to alkyl-modified networks of corresponding alkyl length. In addition to a structural effect from the bulkiness of the fluoroalkyl chains, a thermodynamic effect related to the frustration of fluorine in the network may have also contributed. Hydrophobocity also increased as alkyl and fluoroalkyl length increased. Measurement of water vapor permeation for the fluoroalkyl-derivatized series revealed the potential of these materials as UV-curable, water-repellant, breathable membranes. Photopolymerized thiol-ene chemistry was also used to fabricate membranes for various CO2 separation applications. An initial approach was made to improve a basic photopolymerized poly(ethyleneglycol)diacrylate (PEGDA) membrane by incorporating a small amount of various multifunctional thiols. Thiol-modified PEGDA membranes demonstrated improvements in gas permeability, tensile elongation, and processing in air all without sacrificing CO 2 selectivity. These improvements were all due to the stepgrowth nature of the thiol-ene reaction. Processing improvements would especially be critical for the UV-curing of gas separation membranes where the thickness of the membrane layer is on the nanometer scale. Such thin acrylate layers are extremely sensitive to oxygen inhibition. An alternative to the brittle PEG-acrylate membranes was also developed using pure thiol-ene chemistry. Specifically, a PEG-containing diene and a PEG-containing dithiol were combined, along with a trithiol for crosslinking, to form membranes with varying crosslink density and ethylene glycol content. Permeability increased by a factor of four as crosslink density decreased. No increases in CO 2 solubility or CO2 selectivity were observed for these materials despite nearly doubling in PEG content. Physical crosslinks, via entanglements, prevented further increases in permeability for networks with the lowest crosslink density. Tensile testing was used to investigate the mechanical behavior of this material family. Regarding membrane applications, flexibility and elongation may be important in the development of films for modified atmosphere food packaging. The elastomeric thiol-ene materials demonstrated strain at break approaching 200%, a vast improvement over the brittle PEG-acrylate membranes.