Fundamental molecular mechanisms of moisture transport in epoxy

The epoxy resins used in high performance aerospace composites typically absorb significant quantities of water, compromising the physical properties of the component. In this research, the ill-defined contributions of molecular topology to the moisture transport process are examined. Topology refers to the spatial arrangement, or packing, of the molecular segments within the epoxy. By combining this understanding with the known contributions from resin polarity, a molecular model of transport is proposed to provide scientists with the knowledge needed to create moisture-resistant epoxies and composites.; Positron annihilation lifetime spectroscopy (PALS) quantifies the amorphous topology in terms of the size and volume fraction of nanometer-sized voids. Through novel epoxy networks, the topology is systematically altered while the polarity is held constant. For the transport studies, the subsequent nano-pores comprise 3 to 7 volume percent of the epoxy, with radii of 2.5 to 3.1 A (water has a radius of 1.3 A). Extrapolation of the PALS data to absolute zero identifies the intrinsic hole volume fraction, with approximately 1/5 to 3/5 of the nano-pores ascribed to the intrinsic component. The PALS nano-pores are a local measure of the molecular packing and furnish information not afforded by macroscopic parameters, such as specific volume.; A limited number of correlations exist between the transport properties and the nano-pore characteristics. Both the equilibrium moisture uptake and the initial absorption rate (initial flux) increase with the intrinsic hole volume fraction. The diffusion coefficient exhibits a mild exponential dependence on the average nano-pore size, but only at low temperatures. As a general rule, these correlations between transport and topology, as quantified in the dry resin, are strongest under conditions where the moisture is ineffective at altering topology, such as low temperatures and the initial stages of absorption.; A molecular model of the moisture transport process is proposed. Water appears to traverse the epoxy through the nano-pores, which establish dynamic, interconnected pathways of unoccupied volume. The nano-pores effectively open the matrix and provide access to the polar hydroxyls and amines. These interactions between water and the polar sites ultimately dictate both the rate and amount of moisture absorbed.