Physical aging of polystyrene blocks under three-dimensional soft confinement in PS-b-poly(n-butyl methacrylate) diblock copolymer: Two equilibrations on the way

Polystyrene-block-poly(n-butyl methacrylate) (PS-b-PnBMA) was used to investigate three-dimensional (3D) soft confinement effect on physical aging of the PS block therein. The soft confinement is constructed by phase-separated PnBMA domains, as PnBMA is liquid on the aging temperatures of PS blocks due to its low glass transition temperature. In enthalpy recovery, aging response of PS blocks is represented by a low and broad heat capacity peak associated with an enhanced aging rate with respect to homo-PS, when the aging temperature is relatively low. However, the aging response exhibits opposite characteristics at relatively high temperatures, compared with the results of homo-PS. The phase-separated morphology and thus the soft confinement on PS blocks was confirmed by atomic force microscope imaging using the Peak Force quantitative nanomechanical mapping (QNM) technique. Two local maximums of recovered enthalpy versus aging temperature indicate that two equilibration processes exist during aging of confined PS blocks, within a substantially shorter timescale to the bulk. The 3D soft confinement effect on aging of PS blocks is attributed to dual equilibration mechanisms: one dominates at higher aging temperatures, leading to a restrained aging rate, while the other plays a key role at lower aging temperatures, resulting in accelerated physical aging.