## 1 Introduction  
Retention is the final stage of orthodontic therapy after achieving the desired teeth position. The primary purpose of any retention appliance is to maintain the obtained results and avoid relapse, as teeth tend to return to their pre-treatment positions [1, 2]. In addition, maintaining the results is crucial to determining long-term patient satisfaction following orthodontic treatment [3]. In the absence of scientific evidence, the selection of the retainer material is typically guided by the patient's and the dentist's preferences [4]. The aesthetics and invisibility of thermoplastic retainers are favored by many patients. A thermoplastic orthodontic retainer is a removable device that can be made of different types of polymers. These retainer materials are more cost-effective and easier to produce than Hawley or fixed retainers [5, 6]. Widespread use of thermoplastic retainers has been linked to increased patient satisfaction and adherence [7]. Retention methods vary greatly across practitioners, and no consensus exists [8]. However, the necessity of lifelong retention is now well recognized. Multiple studies have shown that stability can only be ensured through long-term retention [1; 9; 10; 11; 12]. The long-term use of thermoplastic retainers for extended periods can cause the material to deteriorate [13]. The disadvantages of thermoplastic retainers are their short lifespan and frequent replacements [14; 15; 16]. To lower the cost and preserve the results of orthodontic treatment, thermoplastic retainers should be fabricated from materials that exhibit high durability and stability. Fabricating thermoplastic retainers involves either vacuum forming or pressure forming that forms the plastic sheets into the patient model [17]. Since the forming procedure involves heating the plastic sheets, changes in their properties are anticipated. So, the effects of thermoforming on the water sorption and solubility and surface molecular analysis of different commercially available thermoplastic retainer materials were evaluated in the current study. Furthermore, thermocycling was employed in the current study to elucidate their clinical endurance and persistence [18]. Moreover, as the water uptake into the polymeric material leads to physiochemical degradation in the material itself, water sorption and solubility were used in the present study to assess the durability and stability of six commercially available thermoplastic retainer materials [19; 20; 21]. The material's surface molecular structure can be assessed with the help of Fourier transform infrared (FTIR) spectroscopy. It is possible to assess the effects of various aging methods on the surface molecular structure by comparing the FTIR spectra of the samples taken before and after thermoforming with or without thermocycling. Therefore, this was employed in the present study to identify the surface molecular analysis of the investigated thermoplastic retainer materials and the effects of different aging methods used (thermoforming and thermocycling) [22]. Although many studies evaluated the water sorption and solubility and FTIR spectroscopy of thermoplastic aligner materials, few have evaluated thermoplastic retainer materials. However, since they are prescribed for life, their stability in wet environment and durability are crucial factors determining their cost-effectiveness. We are unaware of any published research that has studied the water sorption and solubility and FTIR spectroscopy of Imprelon S pd, Essix Plus, Duran Plus, and Tru-Tain DX. Not only that, but we are unaware of any research that has evaluated the effects of thermoforming and thermocycling on the investigated thermoplastic materials' water sorption and solubility and FTIR spectroscopy. Therefore, in the current study we evaluated the water sorption and solubility of six commercially available thermoplastic retainer materials before thermoforming, after thermoforming, and after thermoforming and subsequent thermocycling.