Structural and Optical Response of Polymer-Stabilized Blue Phase Liquid Crystal Films to Volatile Organic Compounds

Abstract

Engineering useful mechanical properties into stimuli-responsive soft materials without compromising their responsiveness is, in many cases, an unresolved challenge. For example, polymer networks formed within blue-phase liquid crystals (BPs) have been shown to form mechanically robust films, but the impact of polymer networks on the response of these soft materials to chemical stimuli has not been explored. Here, we report on the response of polymer-stabilized BPs (PSBPs) to volatile organic compounds (VOCs, using toluene as a model compound) and compare the response to BPs without polymer stabilization and to polymerized nematic and cholesteric phases. We find that PSBPs generate an optical response to toluene vapor (change in reflection intensity under crossed polars) that is sixfold greater in sensitivity than the polymerized nematic or cholesteric phases and with a limit of detection (140 ± 10 ppm at 25 °C) that is relevant to the measurement of permissible exposure limits for humans. Additionally, when compared to BPs that have not been polymerized, PSBPs respond to a broader range of toluene vapor concentrations (5000 vs <1000 ppm) over a wider temperature interval (25–45 vs 45–53 °C). We place these experimental observations into the context of a simple thermodynamic model to explore how the PSBP response reflects the effect of toluene on competing contributions of double-twisted LC cylinders, disclinations, and polymer network to the free energy that controls the PSBP lattice spacing. Overall, we conclude that the mechanical and thermal stability of PSBPs, when combined with their optical responsiveness to toluene, make this class of self-supporting LCs a promising one as the basis of passive and compact (e.g., wearable) sensors for VOCs.

3D Chiral Photonic Nanostructures Based on Blue‐Phase Liquid Crystals

“…[ 96 ] Recently, BP‐based polymeric films were found to show an optical response to toluene vapor that was sixfold faster in sensitivity than the polymerized nematic or cholesteric phases and with a limit of detection. [ 165 ] Free‐standing BP film with 3D chiral nanostructures that respond to diverse external stimuli as well as mimic natural coloration and functions could serve as a platform for the development of functional and intelligent materials toward diverse applications, however, scalable‐manufacture of these flexible optical films remains scarce. [ 181–183 ] It is worth noting that BPLCs are significantly promising as a template for the self‐assembly of nanoparticles or polymers due to their inherent 3D disclination networks.…”

“…Yang et al determined the response of PSBPs to VOCs (using toluene as a model compound) and compared it with the optical responses of BPLCs without polymer stabilization and polymerized cholesteric and nematic phases. [ 165 ] They found that compared with polymerized cholesteric and nematic phases, PSBPs generated a greater response (change in reflection intensity) to toluene vapor with the limit of detection (140 ± 10 ppm at 25 °C) close to the permissible human exposure limit. In the PSBPs, the introduction of a guest (i.e., polymer) into the defects of the BP systems not only stabilized the BPLCs, but also enabled interaction with the targeted molecular species to realize high levels of selectivity.…”

Areas of opportunity related to design of chemical and biological sensors based on liquid crystals

“…Chiral LC phases are promising for sensing applications due to their intrinsic optical properties, such as Bragg reflection (Figure 5). A number of past studies demonstrate that chiral LCs exhibit measurable optical responses when volatile organic compounds absorb into them [4,9,[37][38][39][40][41][42]. For example, cholesteric phases have been demonstrated to respond to tetrachloroethylene, with the response arising from an expansion of the pitch of the cholesteric phase [9].…”

“…For example, cholesteric phases have been demonstrated to respond to tetrachloroethylene, with the response arising from an expansion of the pitch of the cholesteric phase [9]. More recent studies have extended investigations of chiral LC phases for VOC sensing to BPs, including BPI (body centre cubic) and BPII (simple cubic) phases (Figure 5(a)) [37,39]. Interestingly, when exposed to toluene vapour, the lattice expands in BPI while it shrinks in BPII [37], revealing that the influence of the toluene on the BP is not simply to swell the material, as occurs with cholesteric LCs.…”

“…Interestingly, when exposed to toluene vapour, the lattice expands in BPI while it shrinks in BPII [37], revealing that the influence of the toluene on the BP is not simply to swell the material, as occurs with cholesteric LCs. Polymerised BPs, which exhibit stable BP phases over a wide range of temperatures [39], have also been explored for VOC sensing. Inspection of Figure 5(b) shows that a polymer-stabilised BP can generate a larger optical response to toluene vapour than either a polymerised cholesteric or nematic phase.…”