Stimuli responsive optical polymers through omnidirectional and reconfigurable porosity

Abstract

Manufacturing of porous materials with reconfigurable porosity is beneficial to systems with tailored material properties. This study describes the design and characterization of dynamic omnidirectional porosity in thermoplastic porous networks and their associated applications as novel indicators of temperature and chemical vapors. Herein we report on porous thermo-responsive films that undergo considerable pore size reduction (~40%) and pore density reduction (~86%) when heated to an activation temperature (Ta = 100 °C). These films are synthesized using a solid-state, high-pressure gas saturation and phase separation process on physically crosslinked block copolymers such as poly (styrene-butadiene-styrene) (SBS), and poly (styrene-ethylene/butylene-styrene) (SEBS). The pores act as light-scattering centers, leading to optical opacity of the film. Upon activation, there is a noticeable volume change (~18%) and a concomitant optical opaque-to-transparent transition (OTT) by virtue of the omnidirectional shrinkage of pores. The transparency of these films changes from 45% to 95%, with modulation occurring indiscriminately across a broad spectrum (e.g., UV, visible, and IR). We detail the underlying mechanism for the thermo-responsive dynamic porosity and its interlinked relationship with OTT, and control of OTT temperature onset and its associated shape recovery time. Furthermore, we demonstrate the utility of dynamic porosity for actuation and chemical vapor sensing purposes.