Impact of morphology on O2 permeability in silicone hydrogel membranes: new insights into domain percolation from experiments and simulations

The transport of oxygen through silicone-hydrogel (SiHy) materials is of great interest in bio-materials' applications. In this study O2 permeability of hydrogels made from UV-cured polyacrylamide containing different siloxane co-monomers were analyzed through the traditional coulometric flux method, as well as by 1H NMR T1 relaxometry. It was shown by coulometric flux methods that the crosslinked polyacrylamide with short, linear siloxane side chains (polydimethylsiloxane (PDMS)) has higher O2 permeability as well as O2 solubility, as calculated from the composite diffusivity, than that with branched siloxane (tris-(trimethylsiloxysilyl-) (TRIS)). On the other hand, based on direct measurement from NMR T1 analyses, a slightly higher O2 solubility was observed for the samples with branched siloxane as expected from its larger free volume. In order to understand the reason behind this discrepancy, a series of morphological studies using 29Si and 1H NMR T2 relaxometry, as well as small-angle X-ray scattering (SAXS) were undertaken. These results are discussed in the context of molecular dynamic simulations undertaken on the same systems which together strongly suggest that morphology plays a critical role in O2 permeability. It is proposed that samples made from the linear silicone, PDMS, has a more pronounced siloxane phase separation than its bulky counterpart, TRIS. Significantly percolating hydrophobic silicone domains with twice the siloxane content are found for the PDMS sample with reduced tortuosity of the hydrophilic domain's percolating path. The mobility in the linear siloxane was also much higher which can be attributed to the molecular structure and to fewer interfacial and topological constraints arising from the different domain structure. Hence, we propose a framework for O2 permeability in SiHys, emphasizing the importance of percolated hydrophobic domain in designing SiHy with optimized O2 permeability. The study demonstrates that while a simple permeability measurement can be used for materials' comparison, the use of multiple orthogonal techniques is critical for developing a more complete understanding of small molecule transport in heterogeneous materials.