Research

High refractive index polymers (HRIP) are an incredibly important class of materials given their potential applications in optoelectronic devices, such as anti-reflective components for displays, encapsulants for light-emitting diodes, optical sensors, and lithography. Because typical commodity polymers possess refractive indexes in the range 1.3 – 1.7, obtaining polymers with a refractive index greater than 1.8 while maintaining high optical transparency is an important challenge. Halogen atoms such as chlorine, bromine, and iodine have high atomic refractivity ranging from 5.97 to 13.90, making them useful components in the development of HRIPs.

In this work, a series of halogenated polymer thin films with high refractive indexes (n =1.7 – 2.0) and outstanding optical transparency in the visible and infrared range, were prepared via a unique vapor phase process – initiated chemical vapor deposition (iCVD). iCVD technique is a versatile and delicate technique that has been used to fabricate a wide range of polymer coatings and thin films for many applications. In this study, poly(4‐vinylpyridine) (P4VP) films with a thickness of 200nm were prepared using iCVD, then treated by halogen vapor to form halogen-containing polymer thin films with high refractive indexes. The charge transfer complex (CTC) formed between P4VP and halogen compounds, prepared by a simple vapor phase infiltration of halogen compounds, increases the refractive index of the P4VP thin film from 1.58 to 2.0 or higher, while maintaining the conformal and smooth nature of as-deposited polymer thin films. Specifically, the P4VP-I₂ complex is demonstrated to have a refractive index of 2.0 and is transparent above a wavelength of 600nm. In another formulation, P4VP complexed with ICl achieved a refractive index up to 1.77, while still retaining the outstanding optical transparency throughout visible and infrared range. The refractive index of the halogenated polymer films can be further fine-tuned by controlling the concentration of CTC in the polymer film via copolymerization with other monomers that are inert to halogen compounds. In this work, a series P4VP thin films copolymerized with 1H,1H,6H,6H-perfluoro-1,6-hexyl diacrylate (PFHDA) with refractive indexes ranging from 1.50 to 1.98 were prepared.

The formation of charge-transfer complex (CTC) in halogenated polymer thin film was confirmed via both Fourier-transform infrared and UV-Vis spectroscopies. The optical performance of these halogenated polymer coatings was characterized by spectroscopic ellipsometer and specular reflectometry. The thermal resistance and environmental stability of these high index polymer films were also investigated as a function of temperature. The refractive index of P4VP-I₂ film decreased by 16% after 96 hours at 30°C. In comparison, the refractive index of P4VP-ICl film decreased by 0.56% over 96 hours at 30°C. These halogenated polymer thin films are expected to have wide applications in sensors and optoelectronic devices.

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