The clinical effectiveness of clear orthodontic aligners mainly depends on the thermomechanical stability of the polymers in this challenging hydrothermal environment. In this study, we compare the water-induced viscoelastic changes and glass transition temperature (Tg) stability of four polymers with different microarchitectures. Specifically, we examined directly printed photopolymer networks (Tera Harz TC-85 and LuxCreo 4D Aligner), a monolithic thermoplastic (Duran+), and a multilayer thermoplastic (ClearCorrect). Samples were immersed in physiological saline (0.9 wt.% NaCl) at 37 °C for 7 days, and Dynamic Mechanical Analysis (DMA) was performed in three conditions: dry, after immersion, and after a 2 h desorption step, mimicking a typical clinical 22:2 wear cycle. All polymers showed a decrease in Tg after immersion, with TC-85 exhibiting the greatest reduction relative to the dry baseline. Tg recovery after a 2 h ambient desorption step was incomplete and was significantly associated with the amount of water retained after 2 h drying (expressed as % of initial uptake; R2 = 0.419), whereas total water absorption after 7 days was not associated with short-term thermal recovery.
Šimunović et al. (Tue,) studied this question.