Discrete combinatorial investigation of polymer mixture phase boundaries

We report a combinatorial approach for mapping bulk polymer mixture phase behaviour. The technique consists of a parallel cloud point detection scheme using discrete composition libraries, which are scanned across a temperature range and optically imaged. The sample substrates are microwell arrays fabricated by contact photolithography on a glass coverslip. Polymer blend libraries are generated using a custom built, programmable liquid dispenser system. The sample arrays are placed in a uniform, but continuously varying, temperature field, causing the mixture to traverse its phase boundary. Optical turbidity is detected by imaging the entire array and the cloud point curve is determined through automated parallel image analysis. The entire process is simple, flexible, inexpensive and rapid. In this demonstration, we investigate two binary mixtures of low molecular mass poly(styrene) and poly(butadiene), exhibiting upper critical solution temperature (UCST) phase behaviour. The cloud point curves obtained closely approximate the bulk binodal line, and have a high composition resolution of deltaphi = 0.01 (volume fraction), using 10 x 10 sample arrays. We discuss the potential and limitations of this high throughput methodology for other polymeric mixtures, including multi-component systems.