Preparation of planar graded refractive index nanocomposites using microfluidics

In this paper, a microfluidics-based approach for preparing graded refractive index polymer composites is described. These graded refractive index polymers have potential for optical applications due to the fidelity and variety of the profiles that can be achieved using microfluidics technology. In this study, microfluidic devices, fabricated in poly(dimethylsiloxane) (PDMS), dispersed TiO2 nanoparticles in a prepolymer solution consisting of low molecular weight poly(ethylene glycol diacrylate) (PEGDA). Following dispersion of the particles to produce a controlled gradient profile in the liquid pre-polymer solution, UV light polymerized the solution to produce graded refractive index nanocomposites. Initially, as a means of understanding the flow parameters and straightforward elucidation of the gradient profiles, fluorescent polystyrene nanoparticles were dispersed. After a general method for dispersing nanoparticles was developed, anatase TiO2 was synthesized and dispersed to produce nanocomposites. However, transmission electron microscope (TEM) analysis and optical microscopy revealed that the particles were agglomerated and, consequently, the amount of TiO2 that would remain in the pre-polymer solution was severely limited. To address the problem of particle settling, rutile TiO2 was synthesized using a different precursor and dispersed. Rutile TiO2 was chosen because the synthesis scheme was expected to yield a well-dispersed solution. Also, rutile TiO2 is less likely to cause interference with the light used both for the polymerization and for demonstrating the material for optical applications. Following synthesis of the rutile TiO2, TEM analysis and zeta potentials exceeding 30 mV confirmed that the nanomaterials were well dispersed. After fabrication of the corresponding nanocomposite, optically transparent images were obtained and a graded profile with Δn = 0.1 was achieved.