Water Desorption Governs Glass Transition Recovery in Aligner Polymers

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.