An experimental study of recovery in embossing of polycarbonate below the glass transition temperature

The hot embossing process is a cost-effective replication technology for the fabrication of polymeric micro/nano-structures. Nevertheless, there is a need to reduce the embossing temperature as industrial applications of the hot embossing process are limited by the high temperature required. For amorphous thermoplastic polymers, the hot embossing process can be conducted below the polymer's glass transition temperature (Tg), which is promising for many industrial applications. However, it is generally difficult to achieve a high replication fidelity in such a below-Tg embossing process. The large resistance to plastic flow and the significant deformation recovery of the glassy polymer (the amorphous thermoplastic polymer in its glassy state) are two main causes. The problem of plastic flow may be solved by designing and developing a hot embossing system with large force capability, but the recovery problem may not and needs further study. In this research, the recovery of a polycarbonate (PC) glass in the below-Tg embossing process was quantified and investigated systematically. The effects of embossing depth, holding time, temperature, and loading rate on the recovery were discussed and analyzed. The research results reveal that it is the recovery (usually more than 20%) that severely limits the replication fidelity of the below-Tg embossing process. The recovery is strain-dependent due to the strain-dependent chain tension of the PC glass. As the chain tension generated in the strain-hardening region cannot be effectively reduced by increasing the embossing force, holding time, and temperature, the recovery problem poses a great challenge to be overcome. Considering the strain-rate dependence of the segmental mobility of glassy polymers, a strategy of increasing the loading rate to reduce the recovery was proposed and verified. Hence, this research advances our understanding of the below-Tg embossing process and provides new insights into how to improve the hot embossing process.