Fundamental Understanding of Polystyrene Template Sintering Process for Tailoring Morphology of Well-Connected Inverse Opal Structures

An inverse opal (IO) structure is a highly ordered porous structure that has broad applications in fields, including optics, thermal management, and chemical catalysis. The morphology and properties of IO structures primarily depend on the sintering process applied to the IO spherical template. In this research, we provide a new fundamental understanding of the polystyrene template sintering process for IO structure formation through both experimental verification and polymer melt theory development. Through experiments, we investigated 15 sintering cases using three types of polystyrene templates formed from 3.0, 4.0, and 5.3 μm diameter colloidal particles to study the optimal sintering conditions for achieving self-organized and well-connected copper inverse opal (CIO) structures. The theory underlying a "generalized" version of the Mark-Houwink equation is applied for the first time to understand polystyrene bead template sintering and CIO formation. From experiments, it was found the square of the IO neck diameter-to-pore diameter ratio, (d_n/d_p)², an important parameter to indicate the IO morphology, is proportional to the sintering time at the initial stage of sintering in agreement with the developed theory. The study also clarifies relationships among the polymer recipe weight-average-molecular weight (M_w), d_n, and d_p on the IO morphology. Specifically, the slope of the linear relation between (d_n/d_p)² and the sintering time is proportional to (M_w^1.34·d_p)^-1 under 100 °C process conditions, a commonly used sintering temperature. As a result, the morphology and features of the IO structure were well predicted using polystyrene templates with different properties. Findings from this research enable the design of IO structures for a range of applications using empirically fit coefficients in the Mark-Houwink equation.