Synthesis and Study of the Optical Properties of PMMA Microspheres and Opals

Polymethylmethacrylate (PMMA) microspheres were synthesized by surfactant-free emulsion polymerization. These microspheres were used to obtain opals by the self-assembly method. Monomer and initiator quantities were varied systematically to monitor the size of PMMA microspheres. From SEM and DLS measurements, a trend was observed showing as the monomer and initiator amounts increased the average diameter of PMMA microspheres increased except when a minimum monomer amount was reached, for which the size of the microspheres remained practically constant. Diffuse reflectance spectra were processed by the Kubelka-Munk treatment to estimate the energy band gap (E_g) of the PMMA microspheres. It was found that PMMA microspheres present an indirect transition. From SEM micrographs, it is seen that PMMA opals photonic crystals are formed by microspheres in a uniform periodic face-centered cubic (fcc) array. Variable-angle specular reflectance spectra show that the opals possess a pseudo photonic band gap (PBG) in the visible and near-IR regions. Furthermore, it was found that PBGs shift towards larger wavelengths as the average diameter of the PMMA microspheres increases.