Network parameters such as cross-link density or intermolecular interactions were used as effective parameters to control polymer chain packing or free volume and thereby control the mass transport properties of networks.
A series of polyethylene glycol (PEG) based thiol-ene elastomeric networks having a broad range of molecular weight between cross-links, Mc, (inverse of cross-link density) was prepared. The specific volume of the networks was studied as a function of temperature and pressure using high pressure dilatometry, and the obtained PVT data was fit using Simha-Somcynsky equation of state (S-S eos) analysis. Fractional free volume, quantified through S-S eos analysis, changed linearly as a function of cross-link density of the networks proving that the system obeyed Fox and Loshaek model. Average free volume hole size, h\u3e, of the networks was directly probed using positron annihilation lifetime spectroscopy (PALS). h\u3e of the networks also changed linearly as a function of cross-link density. Typically, in traditional elastomers, changing cross-link density also causes simultaneous changes in chemical nature or polarity of the system, therefore those systems deviate from Fox and Loshaek model. Internal pressure Pi of the networks, calculated from PVT data, showed similar values for all networks, and the same was the case for storage permittivity values obtained from dielectric spectroscopy analysis. This proved that the chemical nature of the networks was unaffected and cross-link density was the only parameter controlling the free volume of the networks. Gas diffusivity of the networks obeyed Cohen Turnbull model, and thus the experimental gas diffusivity trends were modeled as a function of molecular weight between cross-links of the networks.
The effect of intermolecular (repulsive) interactions on free volume and gas transport was studied in a series of perfluorinated thiol-ene elastomers. Perfluorinated thiol-ene elastomers were prepared via 2-step synthesis. In the first step, a four-functional thiol was converted into a three functional thiol containing a perfluorinated dangling moiety via Thio Michael addition reaction with a perfluorinated acrylate. The modified thiol monomer was then reacted with triene to obtain perfluorinated thiol-ene networks. Fluorine content was varied by changing the length of perfluorinated moiety used in Thio Michael addition and this enabled the synthesis of a series of perfluorinated networks containing perfluorinated dangling moieties of different lengths. Fast reaction kinetics of thiol-ene chemistry prevented the phase separation of perfluorinated moieties and locked-in the thermodynamically frustrated perfluorinated moieties within the thiol-ene scaffold. The repulsive interactions between highly non-polar perfluorinated moieties and polar thiol-ene backbone created huge free volume pockets in the network. For the biggest perfluorinated dangling moiety used in this study, h\u3e increased by four times in comparison to an unmodified network. The gas transport properties showed a significant improvement as a function of the length of the dangling moiety. But gas diffusivity of the networks did not obey Cohen Turnbull model. The deviation was explained by the static nature of free volume pockets around perfluorinated moieties, and percolation of free volume pockets as the sizes of free volume cavities increased.
The 2-step synthetic approach was taken a step further to prepare a series of hybrid thiol-ene elastomers containing varying concentrations of polar PEG moieties and non-polar perfluorinated moieties. The effect of PEG and perfluorinated concentrations (or attractive and repulsive interactions) on chain packing was studied. PEG moieties improved CO2 gas permeability and selectivity because of the Lewis acid-base type interactions between PEG and CO2.
In the final chapter, the effect of moisture sorption on free volume, oxygen, and water vapor transport was studied on five different epoxy-amine networks having Tgs in the range between -11 °C to 227 °C. Water sorption did not show any effect on h\u3e of elastomeric networks. Whereas, glassy networks showed a V-shaped trend when h\u3e was plotted as a function of relative humidity or water content. The decrease in h\u3e was due to water molecules filling free volume holes. Increasing water concentration in the networks beyond 75% relative humidity (RH) resulted in swelling of the networks and thereby h\u3e increase. Effect of water sorption on oxygen permeability of the glassy networks also showed a similar V-shaped trend, but the mechanism was more complex as water sorption affected both oxygen diffusivity and solubility. Water vapor permeability of the glassy networks was unaffected until 75% RH. Beyond 75% RH, free volume increase due to swelling resulted in water vapor permeability increase
