On the interaction of infrared radiation and nanocellular polymers: First experimental determination of the extinction coefficient

Among the various mechanisms playing a role in the heat transfer through nanocellular polymers, the radiation contribution remains the most unknown, since there is a lack of experimental data about how infrared light interacts with such structures. In this work, we present the first experimental measurements of the transmittance in the infrared region of nanocellular polymers. Infrared transmittance spectra of a collection of polymethylmethacrylate (PMMA)-based micro- and nanocellular polymers with a constant density and a wide range of cell sizes (from 14 nm to 20 mu m) were obtained and evaluated to calculate the extinction coefficient. Results show that, as expected from theoretical considerations, a reduction of the cell size increases the amount of infrared radiation transmitted, that is, the scattering is reduced as cell size reduces. Nanocellular polymers were proved to act as Rayleigh-like scattering points, showing the transmittance both an intense wavelength and cell size dependence. As a consequence, the extinction coefficient reduces in the nanoscale. From these data, it is possible to conclude that the scattering due to the cellular structure can be neglected for very small cell sizes (smaller than 200 nm), but it must be considered for larger cell sizes. The obtained results were used to model the thermal conductivity including the radiation contribution, showing that at low relative densities and small cell sizes this heat transfer term becomes significant in nanocellular polymers.