Cellular polymers are well known for their use as thermal insulators in buildings. Since the reduction of cell size to the nanometric scale and the discovery of the Knudsen effect, nanocellular polymers have been seen as a potential alternative to current thermal insulators due to the reduced thermal conductivity that they present in comparison with conventional or microcellular polymers. Despite the efforts are centred in the insulation of the walls, it is important to notice that an important part of the heat losses (around 15%) produced in buildings are through the windows.
In addition to their low thermal conductivity, nanocellular materials, with cell sizes below 50 nm, produced from amorphous polymers are expected to keep up to some extent the transparent character of the former solid, fact that has been recently proved. These are unique materials that combine transparency and thermal insulation and that could be used to create transparent thermal insulating windows for the construction sector and transport industries, leading to substantial energy savings.
In this work the optical properties of semi-transparent nanocellular polymethylmethacrylate (PMMA) samples produced by means of the gas dissolution foaming process has been studied for the first time. Samples with a relative density of 0.4, cell sizes from 14 nm to 225 nm as well as different thickness have been characterized. The transmittance of these samples was characterized by light scattering measurements as well as with an visible-ultra-violet spectrometer. These measurements have led to obtaining the dependence of the transmittance with the cell size, the thickness of the sample and the wavelength of light, dependences that have been analysed by using the Beer-Lambert law and the equations for Rayleight scattering.
In addition, the interaction of light with the nanocellular PMMA has been modeled by Finite Element Method using Comsol Multiphysics, in order to understand the light transmission process and support the experimental results.