Thermo-responsive shape memory polymers (polyurethane) with the ability to return from a deformed state to the original shape with a temperature change have been developed recently ( 18, 19 ). The mechanical properties of thermoplastic materials vary with the molecular and crystal structures, which influence the glass transition behaviour ( 15 ). Because the oral environment temperature may be increased up to 57☌ with the intake of a hot drink and several minutes may be required for it to return to its original temperature ( 17 ), the influence of such temperature increases and moisture on the mechanical properties of thermoplastic materials should be investigated. Another previous study reported that the residual stress within all materials decreased with time and that this process was accelerated significantly in a 37☌ water bath, compared with at ambient temperature ( 16 ). Previous study investigated the effects of water absorption, thickness changes by thermoforming, and tensile tests at 23 and 37☌ and reported that the mechanical properties of thermoplastic materials were influenced by thermoforming, water absorption, and temperature changes ( 15 ). The mechanical properties of thermoplastic materials may be influenced by environmental temperature changes. Previous in vitro studies have investigated the force delivery and the mechanical properties of thermoplastic appliances with different materials and thicknesses ( 10–14 ). To expand the clinical indications of thermoplastic appliances, it is important to understand the relevance of orthodontic forces from these appliances. Some patients can be treated with thermoplastic appliances alone however, in many cases, these appliances must be modified or used with auxiliaries or fixed appliances ( 7–9 ). Because improved aesthetic orthodontic appliances, especially for adult patients, are highly desirable ( 1, 2 ), recently developed transparent thermoplastic removable appliances have become widely accepted in clinical orthodontics ( 3–6 ).
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