Understanding the origin of micro/nanoporous thin films of PMMA

Micro/nanoporous thin films of poly(methyl methacrylate) (PMMA) have been prepared by spin-deposition onto Si substrates from solutions of high molecular weight (∼996 kg/mol) PMMA and solvent tetrahydrofuran (THF) at a solution concentration of 15.0 mg/ml. Mean pore areas are <0.1 μm² as determined by atomic force microscopy and vary significantly depending on preparation conditions of PMMA/THF solutions. THF is a poor solvent for PMMA using Lewis acid-base and Gutmann's acceptor number theories. We propose that the nature of THF-PMMA interactions in solution produce the porous structure as a result of a complex interplay between the polymer, the solvent, the antioxidant stabilizer butylated hydroxytoluene (BHT), and ambient humidity. Controlling the BHT at concentration ranges between 0 and 1500 ppm and H₂O concentrations using a N₂ glovebox and rigorous drying procedures during solution preparation is critical to reproducibly create such nanoporous films. The resulting porous films have similarities to breath features (BF) reported in the literature but lack the highly ordered hexagonal pore structure characteristic of BF films resulting from humidity effects. We have also observed that the color of thin film optical interference effects seen using optical microscopy can be correlated to the size and density of micropores in the cast films. Finally, we propose a model using Lewis acid-base theory to understand the molecular interactions that result in the nanoporous film microstructure.