Replicated composite optics (RCO) are a promising technique to fabricate high-quality mirrors with reduced weight and processing time compared to conventional glass mirrors for space imagery technology, however, the optical layer is organic and susceptible to environmentally induced dimensional distortions that critically degrade performance. Therefore, environmental stability is the critical barrier to entry, and due to the lack of parametric material studies, little progress regarding critical controlling factors has been achieved. Conventional solutions, such as thermal curing to enhance polymeric cure state are not feasible due to the generation of CTE mismatch stresses in the bonded RCO structure that degrade optical quality. In this study, optical quality and stability are balanced by the utilization of a UV-cured epoxy resin; furthermore, significant modifications to the cure state are attained at room temperature. Replication surface quality was monitored as a function of processing conditions, cure temperatures, and exposure to different environments. Optical distortion in each was directly linked to a change in the residual stress state. It was found that replication optical quality degraded from that of the glass master due to the formation of residual curing stress, however, values were an order of magnitude smaller than those induced by a thermal cure. In response to hygrothermal environments, optical distortion was believed to be controlled by conventional material properties, specifically CTE and CME. However, stress relaxation was found to dominate optical stability by dictating dimensional drift and optical hysteresis in response to a hygrothermal environment. Even storage at RT can induce degradation over long periods of time, i.e. years. It was discovered that the relaxation rates are significantly accelerated by high stress, temperature, and humidity. As a result, maximized optical quality and stability are achieved in systems with a zero-stress state, high degree of crosslinking to resist relaxation, and enhanced properties.
By utilizing photopolymerization and manipulating its formulation and processing, a zero-stress replication with high thermal stability was fabricated at RT. This optimized processing protocol is very specific to this material system, and in general, all properties characteristic of maximized replication quality and stability are not simultaneously achieved. Several techniques were employed to minimize optical distortion; increasing CFRP laminate stiffness to reduce impact of stress on SFE change, performing gamma irradiation to induce additional crosslinking at RT and slow relaxation rates, and developing a novel processing technique to accelerate built-in residual stresses at room temperature with humidity cycling. The humidity cycling post-process is specifically desirable to relieve large distortions from thermal curing in order to enhance optical quality, material properties, cryogenic performance, and long-term stability. Furthermore, this processing protocol is globally applicable to all resin systems and curing techniques.
