Variation of refractive indices in self-assembling honeycomb patterned PMMA films

Abstract

A novel solvent casting method for poly methyl methylacrylate (PMMA) thin films which allows for morphology and refractive index manipulation is presented in this work. The effect of solvent casting on the morphology and the optical properties of PMMA is studied as a function of solute concentration and the casting relative humidity. Acetic acid is used as the PMMA solvent and is presented as an alternative to toluene, tetrahydrofuran, and chloroform. Casted PMMA films annealed with relative humidities above 30% showed self-assembled nonporous honeycomb patterns. The refractive index of the cast films was observed to decrease from 1.49 to 1.41 as the casting relative humidity increased from 20% to 100%. Optical microscopic images revealed that the concentration of PMMA in the solution is strongly correlated to the size of honeycombs in the pattern. The depth of the honeycombs decreased from 3000 nm to 400 nm as the annealing relative humidity increased from 20% to 100%. Whereas the diameter of the honeycombs increased from 0.75 mm to 2.7 mm with increasing PMMA concentration. These results demonstrate the ability to produce PMMA thin films with specific structural dimensions and optical properties using acetic acid, which can be used to modify surfaces for various sensing applications.