## 1 Introduction  
Due to their high thermochemical stability, transmittance in the UV-VIS-IR spectral range, and resistance to aging, fluoropolymer materials have applications in various industries, including the piping, textiles, aerospace, automotive, and medical sectors [1, 2, 3]. Recently, they have begun to be applied in microelectronics and micro-optics, where they can replace micro-optical structures fabricated from expensive materials such as fused silica [4, 5], or be used as nanostructured fuel cell or gas sensor membranes [6, 7]. Fluoropolymers are classified into two major categories: fully fluorinated polymers (e.g., polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), and fluorinated ethylene propylene (FEP)) and partially fluorinated polymers (e.g., ethylene tetrafluorothylene (ETFE), polyvinylidene fluoride (PVDF), and ethylene chlorotrifluoroethylene (ECTFE)). The high strength of the interatomic bonds and the nonpolarity of the structural components result in a high degree of crystallinity and superior performance in extreme environments of fully fluorinated polymers. However, harsh environmental or operating conditions can alter the level of crystallinity, as well as other properties [8]. Fluoropolymeric materials exhibit a complicated nonlinear response when subjected to external loads. At small deformations, the material response is linear viscoelastic. At higher strains, the material undergoes distributed yield, unrecoverable deformation, viscoplastic flow, and finally gradual stiffening at large deformations until the ultimate failure occurs [9]. Therefore, it is important to understand the effects of extreme physical factors (e.g., heat and UV radiation) on the structural and mechanical properties and degradability of fluoropolymer materials at a micro-level scale. FEP is a melt-processable semicrystalline copolymer of tetrafluoroethylene and hexafluoropropylene. It is a transparent and durable material widely used due to its unique chemical, mechanical, and thermal properties [10; 11; 12]. FEP films transmit more UV, visible, and infrared radiation than ordinary window glass [13]. The occasional methyl side groups attached to the backbone act as defects, reducing the melting point as well as the cold flow of the polymer. The crystallinity of molded FEP parts may vary between 30% and 50%, depending on processing conditions [14]. Thus, FEP films are particularly promising for use in the optical systems and photovoltaic industry. Recent research on FEP films has focused on modifying their inherently inert surfaces through heat and UV treatments to enhance their functionality in advanced applications. Heat treatment, typically performed between 100 \({}^{\circ}\)C and 250 \({}^{\circ}\)C, can induce molecular rearrangement and increase crystallinity [15]. Controlled annealing may also improve adhesion by inducing minor surface restructuring or partial defluorination [16]. In contrast, UV treatment, particularly in the presence of ozone or reactive gases, promotes surface activation by breaking C-F bonds and introducing polar functional groups such as hydroxyl and carboxyl groups [17]. This modification enhances surface energy, wettability, and compatibility with coatings or biomaterials [18]. Techniques such as UV-induced photografting have been employed to functionalize FEP films with hydrophilic moieties, while exposure to UV/ozone has enabled nanostructuring for microfluidic and biomedical applications [19]. Combined heat and UV treatments have demonstrated synergistic effects in tuning surface properties for sensor interfaces and dielectric layers [20]. However, the influences of heat- and UV-induced microstructural transformations on functional performance shifts in FEP films are still underestimated and require more attention because they are relevant for applications like space materials, optics, or microfabrication. In this work, crystallinity changes in heat-UV-treated FEP films were investigated via X-ray diffraction (XRD).