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 molecular (or 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.

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玻璃化转变温度以下聚碳酸酯压花恢复的实验研究

热压印工艺是一种用于制造聚合物微/纳米结构的具有成本效益的复制技术。然而，由于热压印工艺的工业应用受到所需高温的限制，因此需要降低压印温度。对于无定形热塑性聚合物，可以在低于聚合物的玻璃化转变温度 (Tg) 的条件下进行热压印工艺，这对于许多工业应用来说是很有希望的。然而，在这种低于 Tg 的压花工艺中通常难以实现高复制保真度。大的塑性流动阻力和玻璃态聚合物（玻璃态的无定形热塑性聚合物）的显着变形恢复是两个主要原因。塑性流动问题可以通过设计和开发具有大受力能力的热压印系统来解决，但恢复问题可能不会并且需要进一步研究。在这项研究中，对聚碳酸酯 (PC) 玻璃在低于 Tg 的压花工艺中的回收率进行了量化和系统研究。讨论和分析了压花深度、保持时间、温度和加载速率对恢复的影响。研究结果表明，正是回收率（通常超过 20%）严重限制了低于 Tg 压花工艺的复制保真度。由于 PC 玻璃的应变相关链张力，恢复是应变相关的。由于在应变硬化区产生的链张力不能通过增加压花力、保持时间和温度来有效降低，恢复问题是一个需要克服的巨大挑战。考虑到玻璃态聚合物分子（或链段）迁移率对应变率的依赖性，提出并验证了增加加载率以降低回收率的策略。因此，这项研究增进了我们对 Tg 以下压花工艺的理解，并为如何改进热压花工艺提供了新的见解。