[Paper Review] Retention and Deformation of the Blue Phases in Liquid Crystalline Elastomers

Kyle R. Schlafmann, Timothy J. White|arXiv (Cornell University)|Mar 1, 2021

Advanced Materials and Mechanics52 references62 citations

TL;DR

This paper demonstrates the first retention of blue phase I, II, and III in fully solid, elastomeric networks via one-step photopolymerization of chiral liquid crystalline diacrylates with a dithiol chain transfer agent, enabling reversible, tunable photonic responses to mechanical strain, thermal changes, and chemical swelling. The resulting liquid crystalline elastomers (LCEs) exhibit stable, reversible lattice deformation with minimal thermal sensitivity but strong color-shifting under mechanical and chemical stimuli.

ABSTRACT

The blue phases are observed in highly chiral liquid crystalline compositions that nascently organize into a three-dimensional, crystalline nanostructure. The periodicity of the unit cell lattice parameters is on the order of the wavelength of visible light and accordingly, the blue phases exhibit a selective reflection as a photonic crystal. Here, we detail the synthesis of liquid crystalline elastomers (LCEs) that retain blue phase I, blue phase II, and blue phase III. The mechanical properties and deformation of LCEs retaining the blue phases are contrasted to the cholesteric phase in fully solid elastomers with glass transition temperatures below room temperature. Mechanical deformation and chemical swelling of the lightly crosslinked polymer networks induces lattice asymmetry in the blue phase LCE evident in the tuning of the selective reflection. The lattice periodicity of the blue phase LCE is minimally affected by temperature. The oblique lattice planes of the blue phase LCEs tilt and red-shift in response to mechanical deformation. The retention of the blue phases in fully solid, elastomeric films could enable new functional implementations in photonics, sensing, and energy applications.

Motivation & Objective

• To develop a method for stabilizing highly chiral blue phases in fully solid, elastomeric networks with Tg < 20°C.
• To overcome the challenge of retaining 3D photonic nanostructures in elastomeric matrices due to high crosslink density and network rigidity.
• To enable dynamic, reversible optical responses in solid-state photonic materials through mechanical, thermal, and chemical stimuli.
• To demonstrate that blue phase LCEs exhibit minimal thermal dependence but strong, tunable reflection shifts under deformation and swelling.

Proposed method

• One-step free-radical photopolymerization of chiral diacrylate monomers (C6M, SLO4151), chiral dopant R811 (15 wt%), dithiol BDMT (14.2 wt%), and photoinitiator Omnirad 819.
• Use of photoalignment cells with 405 nm light exposure to achieve planar alignment of the cholesteric and blue phase structures.
• Controlled cooling from isotropic phase at 90°C to target phase temperatures (e.g., 70°C, 60°C) at slow rates (0.25–2°C/min) to enable phase-specific polymerization.
• Photopolymerization at 365 nm (50 mW/cm²) for 10 minutes to form lightly crosslinked, branched elastomeric networks with Tg ≈ 15°C.
• Characterization via polarized optical microscopy (POM), Kossel diffraction, DSC, stress-strain testing, and UV-Vis transmission/reflection spectroscopy.
• Solvent swelling experiments using toluene, acetone, and dichloromethane to probe lattice expansion and optical response.

Experimental results

Research questions

• RQ1 Can blue phase I, II, and III be stabilized in fully solid, elastomeric networks with Tg < 20°C using a single-step photopolymerization process?
• RQ2 How do mechanical deformation, thermal cycling, and chemical swelling affect the lattice periodicity and selective reflection in blue phase LCEs?
• RQ3 What is the role of the dithiol chain transfer agent (BDMT) in enabling elastomeric behavior while preserving blue phase nanostructure?
• RQ4 How does the optical response of blue phase LCEs compare to that of cholesteric LCEs under identical stimuli?
• RQ5 To what extent is the lattice deformation in blue phase LCEs reversible and tunable?

Key findings

• Blue phase I (BPI) and blue phase II (BPII) were successfully retained in fully solid, elastomeric films with Tg ≈ 15°C using one-step photopolymerization.
• The lattice periodicity of BPI and BPII LCEs remained minimally affected by temperature, showing <10 nm shift in reflection wavelength when heated from room temperature to 150°C.
• Mechanical deformation induced oblique lattice plane tilting and a red-shift in selective reflection, with reversible optical response observed.
• Chemical swelling with toluene and dichloromethane induced symmetric lattice expansion, resulting in large red-shifts into the near-infrared (e.g., >200 nm shift with dichloromethane).
• Photoalignment enabled spatial patterning of BPI lattice orientation, with Kossel diffraction confirming improved lattice homogeneity in patterned regions.
• The branched, lightly crosslinked network architecture suppressed crosslink density, enabling elasticity while preserving 3D photonic structure.