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Cooling Rate Dependent Ellipsometry Measurements to Determine the Dynamics of Thin Glassy Films
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Water Desorption Governs Glass Transition Recovery in Aligner Polymers.

Luka Šimunović1, Luka Brenko2, Ana Marija Miličević1

  • 1Department of Orthodontics, School of Dental Medicine, University of Zagreb, 10000 Zagreb, Croatia.

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Clear aligner polymers degrade in the mouth. Water absorption lowers their glass transition temperature (Tg), impacting stability. Recovery is incomplete and linked to retained moisture, not total water uptake.

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Area of Science:

  • Biomaterials Science
  • Polymer Science
  • Orthodontics

Background:

  • Clinical effectiveness of clear orthodontic aligners relies on polymer thermomechanical stability.
  • The oral environment presents a challenging hydrothermal setting for aligner materials.

Purpose of the Study:

  • To compare water-induced viscoelastic changes and glass transition temperature (Tg) stability of four different clear aligner polymers.
  • To investigate the impact of simulated oral conditions on polymer properties.

Main Methods:

  • Four polymers were tested: two directly printed photopolymer networks (Tera Harz TC-85, LuxCreo 4D Aligner) and two thermoplastics (Duran+, ClearCorrect).
  • Samples underwent immersion in saline at 37°C for 7 days.
  • Dynamic Mechanical Analysis (DMA) assessed Tg in dry, wet, and post-desorption states, simulating clinical wear cycles.

Main Results:

  • All polymers exhibited a decrease in Tg after water immersion.
  • Tera Harz TC-85 showed the most significant Tg reduction compared to its dry state.
  • Tg recovery after 2 hours of desorption was incomplete and correlated with retained water, not total absorption.

Conclusions:

  • Water absorption significantly affects the Tg and thermomechanical stability of clear aligner polymers.
  • The incomplete Tg recovery highlights potential long-term performance limitations in vivo.
  • Material microarchitecture influences water-induced viscoelastic changes and thermal stability recovery.