In recent years, with the rapid development of the flexible displays and wearable electronic devices, higher requirements have been placed on the performance of electronic devices, such as lightweight and flexible. Polymer films such as transparent polyimide (CPI) and polyethylene terephthalate (PET) have gained widespread attention in the field of flexible optoelectronic materials owing to their high transparency, light weight, good bendability and high mechanical strength. However, due to the characteristics of their molecular structure, polymer films generally have the problems of low surface hardness and poor abrasion resistance. Therefore, enhancing the surface hardness and abrasion resistance of polymer films without compromising their characteristics such as transparency, light weight and flexibility become an urgent problem to be solved. Among the various enhancement strategies that have been developed, applying optically transparent coatings with abrasion and bending resistance to the surface of polymer films is one of the most effective and promising approaches. Hard flexible coatings are mainly achieved by organic-inorganic hybridization to achieve both hardness and flexibility. Organic polysilazanes can retain their organic groups while forming the Si—O—Si network structure by hydrolysis condensation reaction, which can realize organic-inorganic hybridization and providing a basis for constructing hard flexible coatings. In this work, an organic-inorganic hybrid hard flexible coating was prepared at 80 ℃ 90%RH by hybridizing organic polysilazane OPZ181 with triethoxysilane (KH550). The effect of KH550 content on the conversion process of OPZ181 was investigated, and the structure and properties of the hard flexible coatings were systematically characterized.

Using n-butyl ether as solvent, OPZ181 and KH550 were mixed homogeneously at room temperature to obtain a colorless, transparent and clarified OPZ181/KH550 polymer precursor solution. OPZ181/KH550 hybrid coatingswere prepared by liquid-phase coating methods, including spin-coating, scraping and drenching. The chemical structure and compositions of the coatings were characterized by using FT-IR, ²⁹Si NMR and XPS. The micro-morphology of the surface and cross-section of the coatings was observed by SEM. The optical transmittance of the samples was tested using a UV-Vis-NIR spectrophotometer. The mechanical properties of the coatings were characterized using a pencil hardness tester and nano-indenter. The bending test was carried out using a homemade experimental setup. The abrasion resistance of the coatings was tested on the reciprocating abrasion meter using 0000^# type steel wool.

The experimental results and analysis of FT-IR test show that when the ratio of OPZ181 to KH550 in the hybrid hard flexible coating is more than 2∶1, the coatings conversion efficiency to Si—O structure under hygrothermal conditions is increased, and the converted coating structure is dominated by Si—O bonds. The XPS results show that KH550 can effectively promote the conversion of OPZ181 to the Si—O structure at higher content, and —CH₃ can be retained in the coating after the conversion, which is consistent with the results of the FT-IR. Further deconvolution of Si 2p data shows that the coating is dominated by the CH₃SiO₃(T) structure with a high degree of crosslinking by the Si—O—Si structure. The ²⁹Si NMR results similarly show that OPZ181 transforms to the Si—O—Si network structure under hygrothermal conditions, and —CH₃ remains in the coating after the transformation. In addition, as the content of KH550 in OPZ181 increases, the degree of conversion and cross-linking of the Si—O—Si structure increases, and CH₃SiO₃ is the main structure of the hybrid coating. The pencil hardness as well as the nanoindentation hardness and modulus of the coatings increased with the increase of KH550 content, which is attributed to the fact that the coatings are dominated by the CH₃SiO₃ structure with a denser Si—O—Si network at the higher content of KH550. The coating has a pencil hardness up to 6 H, a nanoindentation hardness up to 0.7 GPa, and a modulus up to 5.7 GPa, indicating that the coating has good mechanical properties. Calculated from the load-displacement curves, the H/E of the coating is >0.1 and We >60%, indicating that the coating has good hard flexible properties. In the visible light range, the transmittance of the coated PET film is almost unchanged relative to that of the blank PET film, which can be kept above 90%, indicating that the coating has good optical transparency. From the SEM image, it can be observed that the surface and cross-section of the coating are flat and smooth, and there are no obvious defects such as holes and cracks produced during the transformation process. From the microscopic photographs and SEM surface morphology of the coatings after bending test 10,000 times with a bending radius (R) of 1.5 mm and 1 mm, respectively. The surface is kept flat and smooth, and there is no damage, which indicates that the coating is flexible and has excellent bending resistance. Blank PET film shows visible scratches on its surface after 20 abrasions cycles with 0000^# steel wool. In contrast, the surface of the coated PET film remained flat and smooth after 200 abrasion cycles without any scratches. This shows that the OPZ181/KH550 hybrid coatings have excellent abrasion resistance.

In this work, an organic-inorganic hybrid hard flexible coating was prepared at 80 ℃ 90% RH by hybridizing organic polysilazane OPZ181 with triethoxysilane (KH550). KH550 promotes the hydrolysis condensation reaction of OPZ181 under hygrothermal conditions and improves its degree of conversion. The hybrid hard flexible coatings are dominated by Si—O—Si network structure and contain organic groups such as—CH₃. The degree of cross-linking of the Si—O—Si network in the coatings increases with the increase of KH550 content, resulting in the mechanical properties of the coatings enhanced. The coating has a pencil hardness up to 6 H, nanoindentation hardness up to 0.7 GPa, and a modulus up to 5.7 GPa. The H/E of the coating is >0.1 and W_e>60%, indicating that the coating has good hard flexible properties. The surface of the coating is flat and smooth, and has excellent optical transparency. The coating has excellent bending resistance over 10 000 times under the bending radius of 1.5 mm and 1 mm. The coating can effectively improve the abrasion resistance of PET film. The above experimental results show that organic polysilazane is one of the potential materials for the preparation of organic-inorganic hybrid hard flexible coatings, which is expected to be applied to the protection of flexible devices such as flexible displays and wearable electronic devices.