Porous polymer films with tunable refractive index

A simple, energy efficient, and cost-effective method is presented for the fabrication of polymeric films with a low refractive index in the visible spectral range, suitable as insulators in flexible optical and optoelectronic devices. Thin films based on a commercial polymer with high thermal stability (up to 200 °C), such as poly (2,6-dimethyl-1,4-phenylene) oxide (PPO), presenting both microporosity in the nanoporous crystalline (NC) phases as well as a tunable macroporosity in the amorphous phase, have been prepared and characterized by several techniques. Refractive index of PPO thin films in the visible spectral range, as measured by spectroscopic ellipsometry, varies from a maximum of 1.58 (at 560nm) for fully amorphous samples to a minimum of 1.39 for NC films exhibiting both micro- and macroporosity. This latter value is close to that of polytetrafluoroethylene (PTFE), one of the polymers exhibiting the lowest refractive index value, and has been obtained through a straightforward, very easy and low energy consuming process.