Effect of photoinitiator concentration on hygroscopic stability of UV cured precision replicated composite optics

Replicated composite optics is a promising technique to fabricate high-quality mirrors with reduced weight and processing time compared to conventional glass mirrors; however, the optical layer is organic and susceptible to environmentally-induced dimensional changes, specifically to moisture exposures. Generally, to enhance polymer stability, thermal curing is necessary to maximize the cure state. Because replications are bonded, thermal exposures generate residual stresses that degrade optical quality. In this paper, the cure state of a UV-cured epoxy with RT processing was varied by changing the photoinitiator (PI) concentration, and the replication stability was evaluated in different humidity environments by laser interferometry. Increasing the PI concentration transformed the epoxy microstructure from homogenous to a more phasic network, as evidenced by both DMA and AFM, resulting in significant changes to the Tg, modulus, and moisture absorption. When replications were exposed to moisture, they experienced initial swelling followed by stress relaxation to net-zero regardless of initial processing stress. Reduced PI concentrations exhibited higher moduli and shorter swelling periods, with dimensional changes as small as 35 nm and complete stress relaxation in days. TTS master curves describing stress relaxation behavior correlated well with the observed behavior in replicated mirror samples. Furthermore, it was shown that the relieved stress persisted through multiple humidity cycles between 100% to 0% RH. These results show that high-humidity conditioning treatments can be utilized to eliminate residual stress in as-fabricated replicated mirrors over just a few days, providing a viable manufacturing and processing route for highly precise and stable replicated composite mirrors.