Physical Aging\nof Poly(methyl methacrylate) Brushes\nand Spin-Coated Films

While there is significant attention aimed at understanding\nhow\none-dimensional confinement and chain confirmations can impact the\nglass transition temperature (<i>T</i><sub>g</sub>) of polymer\nfilms, there remains a limited focus on similar effects on sub-<i>T</i><sub>g</sub> processes, notably, structural relaxation.\nUsing spectroscopic ellipsometry, we investigated the combined influence\nof confinement and molecular packing on <i>T</i><sub>g</sub> and physical aging, i.e., the property changes that accompany structural\nrelaxation, at select film thicknesses and aging temperatures (<i>T</i><sub>a</sub>). We used poly(methyl methacrylate) (PMMA)\nfilms in the brush and spin-coated morphologies as model systems.\nWe found that whether a PMMA film exhibited a decrease or increase\nin physical aging rate with confinement was dependent on the morphology.\nNotably, PMMA brushes exhibited higher physical aging rates compared\nto similarly thick spin-coated films at all values of <i>T</i><sub>a</sub>. These intriguing findings reveal the strong effects\nof confinement and molecular packing on the structural relaxation\nof polymer films. Results from this study have the potential to aid\nin the design of thin-film materials with controllable long-term glassy-state\nproperties.