Control of Fluoropolymer Crystallinity for Flexible, Transparent Optical Thin Films with Low Refractive Indexes

Abstract

Fluoropolymers possess among the lowest indexes of refraction for dense, continuous materials, but their crystallinity typically leads to light scattering and haze. In this work, we studied poly(1H,1H,2H,2H-perfluorodecyl acrylate) (pPFDA) as a low-index fluoropolymer and successfully suppressed its crystallization while preserving its desirable low index of refraction (1.36 at 633 nm wavelength) and hydrophobicity (water contact angle of 122°). This was achieved through copolymerization between the hydrophobic 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) and N-vinylpyrrolidone (NVP) using initiated chemical vapor deposition (iCVD). The resulting copolymer p(PFDA-co-VP) film was smooth (roughness <2 nm), highly transparent, thermally robust, and mechanically flexible. This contrasted with pPFDA homopolymer films, which were rough (roughness >30 nm), hazy, and disintegrated at 70 °C due to melting. Moreover, the copolymerization resulted in a 16-fold improvement in the deposition kinetics. To demonstrate its excellent performance in practical applications, the low-index copolymer was paired with a high-index poly(divinylbenzene) (pDVB) (n₆₃₃ = 1.59) to build a six-layer interference coating. A six-layer fully polymeric interference coating with precise, independent control of each individual layer’s thickness was prepared for the first time by iCVD. Optimized for broadband antireflection, it reduced the surface reflectance to 1% over the entire visible spectrum, while withstanding large mechanical strain.