Retention and deformation of the blue phases in liquid crystalline elastomers

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 spacing 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 that retain blue phase I, blue phase II, and blue phase III. The mechanical properties and optical reconfiguration via deformation of retained 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 evident in the tuning of the selective reflection. The lattice periodicity of the blue phase elastomer is minimally affected by temperature. The oblique lattice planes of the blue phase tilt and red-shift in response to mechanical deformation. The retention of the blue phases in fully solid, elastomeric films could enable functional implementations in photonics, sensing, and energy applications.


Introduction
Liquid crystalline elastomers (LCEs) are compelling responsive materials 1 . Recently, they have drawn considerable research interest for their utility as machine-like mechanical actuators 2 as well as the spatial programmability of their material properties 3 . Analogous to small-molecule liquid crystalline systems, which have been prevalent in display applications for some time, LCEs also are relevant to a variety of optical and photonic applications including solid-state optical sensing 4 , tunable diffraction 5 , and adaptive lasing 6 .
Here we detail an approach to prepare fully solid photonic materials that are robust, yet elastic and capable of dynamic reconfiguration. The fluidic nature of small molecule liquid crystalline systems enables the response time and performance of liquid crystal displays 7 . In certain functional implementations, the realization of dynamic optical response in fully solid films is particularly desirable.
Further, as evident in biological systems, dynamic reconfiguration of solid skins naturally induces multifunctionality even under ambient conditions 8 . Ranging from regulated structural color and diffuse reflectance in cephalopods 9,10 to glare-reducing corneal nipple arrays on nocturnal insect eyes 11,12 and photonic crystalline structures responsible for coloring in butterfly wing scales 13,14 , the natural environment provides countless examples of dynamic light control. Many synthetic material systems have incorporated bio-inspired mechanisms to imitate their photonic properties for technological applications 15 .
Multi-dimensional photonic crystals such as those observed in the structural coloration in certain animals are of particular interest for optical applications such as non-linear optics and waveguides 16 .
These unique periodic nanostructures exhibit photonic band gaps in multiple propagation directions.
Three-dimensional photonic crystals are nano-scale emulators of crystal structures typically observed in atomic lattices. Nanostructures such as those arising from inverse opals or lithographic deposition exhibit three-dimensional photonic crystallinity, however the intensive fabrication of such materials is complex and costly 17 . The optics and photonics community has increasingly looked towards self-assembly and liquid crystals as light-manipulating media. Liquid crystalline materials can be formulated to adopt a catalogue of anisotropic phases. Chiral liquid crystalline phases in particular exhibit photonic band gaps due to their periodic anisotropy 18 .
The liquid crystalline blue phases are a subset of liquid crystalline phases in which calamitic mesogens align in a double-twist morphology associated with large concentrations of chiral species. The blue phases are frustrated phases and the nanostructure of these materials are a combination of defect packing and double-twist molecular arrangement of cylinders on the order of 100nm in diameter 19 . These cylinders stack upon one another to generate cubic lattices stabilized by disclinations. Blue phase I (BPI) organizes as a body-centered cubic (bcc) lattice, blue phase II (BPII) achieves a simple cubic (sc) lattice, and blue phase III (also called the "blue fog") is largely amorphous. In this way, BPI and BPII are threedimensional photonic crystals that are selectively reflective to visible light 19 .
These polymers exhibit largely static photonic properties due to their high crosslink density, however recently the selective reflections of these materials have been sensitized to humidity and pH through supramolecular bonds within the network 33 and to small amounts of mechanical deformation 34 . Polymerstabilized blue phase gels (32 wt.% polymerizable media) exhibit electro-optical response, along with limited photonic sensitivity to mechanical deformation 35 . However, the liquid crystalline blue phases have not been retained in truly elastomeric polymer networks with a glass transition below room temperature (Tg < 20°C).
We have reported a straightforward and scalable approach to prepare well-aligned cholesteric liquid crystal elastomers (CLCEs) in the planar orientation composed of main chain chiral mesogens 36 .
Here, enabled by a similar reaction chemistry, we prepare liquid crystalline elastomers that retain the blue phases upon photopolymerization. We characterize changes in the lattice spacing to mechanical deformation, heat, and chemical exposure.

Results and Discussion
Historically, liquid crystalline elastomers (LCEs) have been prepared by two-stage polymerization reactions in which alignment is enforced in the second stage by mechanical force. The preparation of hierarchical liquid crystalline phases, such as the cholesteric or blue phases, are not readily amenable to these processes. The helicoidal nanostructure of the cholesteric phase has been retained in LCEs by mechanical alignment coupled with anisotropic deswelling through either centrifugation [37][38][39] and/or anisotropic deswelling 40 . However, the intricate, 3-dimensional nanostructure of the blue phases have not been retained in LCEs by these methods. A recent report 36 details the development of a materials chemistry to prepare LCEs that originate from the desired chiral phase that are conducive to surface-enforced alignment and prepared from a one-step reaction. This chemistry was utilized to prepare and retain the cholesteric phase in LCEs of high optical quality (low haze) and considerable thermochromism.
The blue phases are observed in formulations that have a large concentration of chiral species.
The liquid crystalline molecules organize in cubic, nanostructured phases. The periodicity of the blue phases is defined by crystalline packing within unit cells. The blue phases are typically observed in very narrow temperature ranges, which can complicate the retention of these phases in polymer networks.
Here, we report the synthesis of LCEs that retain blue phase I (BPI), blue phase II (BPII), and blue phase III (BPIII). The composition is based on a mixture of the liquid crystalline diacrylate C6M and chiral liquid crystalline diacrylate SLO4151 (Figure 1a). The non-reactive chiral dopant R811 is added at 15 wt% to increase the concentration of chiral species. Notably, the addition of R811 both broadens the temperature range of the blue phases before polymerization ( Figure S1) and enables the formation of The formulation to prepare LCEs with 15wt% R811, 14.2 wt% BDMT, liquid crystalline monomers, and photoinitiator exhibits five liquid crystalline phases before polymerization. Accordingly, this study is uniquely able to prepare LCEs in the nematic (polydomain), cholesteric, BPI, BPII, and BPIII phases simply by varying the polymerization temperature. Upon polymerization, the LCEs retain the characteristic birefringent textures of these phases when imaged with polarized optical microscopy

Temperature (°C)
Exo down Figure 1c). To confirm the retention of cubic periodicity in LCEs, we undertook so-called Kossel diffractive measurements 24 . As expected, the cholesteric, BPIII, and nematic phases do not exhibit a diffraction pattern (Figure 1d). However, the cubic orientation of BPI and BPII are evident (Figure 1d).
The photographs of the LCE retaining the cholesteric, BPI, and BPII illustrate the retention of the selective reflection associated with these phases. A hazy blue texture indicates the amorphous BPIII phase, which is not cubic (evident in Figure 1d) and does not exhibit a coherent reflection. Extraction of the non-reactive chiral dopant from the LCE results in a slight blue-shift in reflection color, evident in the images of the free-standing LCE films (Figure 1e) when compared to the POM images in Figure 1c. (see also  properties in the x-y plane, the planar organization of these materials does differentiate the mechanical properties in the z axis (through the thickness). Thus, we conclude the phase of the LCE is the primary differentiator in stress-strain behavior reported here.
The mechano-optical response of the deformation of BPI and BPII was examined by UV-vis spectroscopy during an applied load. The selective reflection (λ !"# ) of the cubic blue phases (BPI and BPII) is defined by: where $ % is the average refractive index, & is the lattice constant, and ℎ, ), * are the Miller indices.
Uniaxial deformation lengthens the material in the loading axis while reducing the width of the film and more importantly, the material thickness. Again, the optical properties of BPI and BPII LCE are derived from periodic cubic nanostructures. The directional bias introduced by mechanical load affects the lattice spacing within the BPI and BPII which cause the LCE retaining these phases to undergo a sizable blue shift in the primary reflection. The magnitude of this shift (120 nm) is greater than that observed for the CLCE polymerized from the same formulation. The reflection of the BPI LCE is associated with the The influence of uniaxial deformation of BPI and BPII LCE to introducing cubic asymmetry is further elucidated by Kossel imaging. In this method, monochromatic light (440 nm) is incident upon the blue phase LCE through a high numerical aperture objective (Figure 3a). The rear focal plane is observed using a Bertrand lens. The incident light exhibits a characteristic diffraction pattern as it passes through the cubic lattice of the blue phase LCE. The two-dimensional projection of the reciprocal lattice space is captured (Figure 3b-c). Figure 3d,  Again, the liquid crystalline blue phases are cubic structures; their selective reflection originates from the periodicity of that cubic lattice. In that sense, each set of lattice planes act as a Bragg reflector which blue-shifts ∝ cos (1) as the viewing angle increases. To further explore the spectral effect of mechanical deformation of the lattice, Figure 4 measures the angular dependent optical properties of a BPII LCE during deformation. Evident in Figure 4a, at 0% strain the BPII LCE exhibits blue-shifting when rotated from orthogonal to the optical probe to angled 45°. At normal incidence (e.g., 0°), the extraction of the non-reactive chiral dopant after preparation of these films may impart some residual stress that could be the source of the lattice tilt that may be the origin of the small reflection apparent at blue to purple between the normal incidence and 45° sample angle.

Evident in
As the sample is uniaxially deformed, the reflection associated with the [100] lattice plane at normal incidence begins to blue-shift (as shown in Figure 2b) due to the decreasing film thickness. The Upon heating, the BPI and BPII LCE exhibit minimal change in reflection (Figure 6a). This is distinctive from an LCE that retain the cholesteric phase, that exhibits upwards of 200 nm shifts in selective reflection to changes in temperature 36 . As detailed above relating to the mechanical response, the effective macroscopic isotropy of BPI and BPII suppress the magnitude of the relative change in periodicity associated with the average thermal expansion coefficient of the polymer. Comparatively, LCEs retaining the CLC phase are isotropic in the x-y axes but anisotropic in the thickness, which results in a directional mechanical response to the thermotropic disruption of order. The branched, lightly crosslinked polymer network prepared from the liquid crystalline monomer mixture reported here can retain the three-dimensional, nanostructured architecture associated with either the body-centered cubic BPI or the simple cubic BPII. Mechanical, thermal, and chemical stimuli
Alignment cells were made by spincoating two Corning EXG glass slides with 22 μL of photoalignment dye PAAD-22 (Beam Co.) diluted (2 to 1 by volume) with DMF. The coated glass slides were dried at 100°C for 30 minutes to drive off any residual solvent. Once dried, the coated sides of two glass slides were adhered together with an epoxy adhesive mixed with 20 μm glass spacers. Photoalignment was achieved by irradiating the alignment cell with a linearly polarized 405 nm laser at an intensity of 10 mW/cm 2 for 10 minutes. Spatial variation in alignment utilized a photomask.
To prepare the liquid crystal elastomers retaining the various phases detailed hereto, the monomer mixture was melted to an isotropic phase at 90°C and filled via capillary action into a 20 μm photopatterned alignment cell. The filled cell was then cooled to the desired temperature (phase) using an Instec HCS 402 heat stage. The filled cells were cooled at 2°C/min to 70°C, then 0.5°C/min to 60°C, and 0.25°C/min at 0.5°C increments thereafter. Samples were equilibrated at the designated temperature for at least 2 minutes. Thereafter, the mixtures were photopolymerized for 10 minutes with 365 nm light at an intensity of 50 mW/cm 2 . The polymer networks were extracted from the alignment cell and subsequently washed with acetone to remove any unreacted components.

Materials characterization
Phase transition temperatures and micrograph textures were obtained via polarized optical microscopy (POM) on a Nikon Eclipse Ci-POL in reflection mode with inline Instec HCS 402 heat stage. Kossel diagrams were captured on the same microscope with 100x oil-immersion objective (1.25na), Bertrand lens, and 2 , = 440 nm x 10 nm bandpass filter.
Stress-strain measurements were performed on an RSA-G2 solids analyzer at a linear strain rate of 5% strain per minute. Glass transition temperatures were determined via differential scanning calorimetry (DSC) (TA Instruments Discovery DSC 2500). Transmission and reflection spectra of liquid crystal elastomers were collected with a Cary 7000 spectrometer (UV-Vis) utilizing a universal measurement accessory.
Gel fractions were obtained by recording the mass of the polymer sample before and after washing with dichloromethane for 24 hours.

Supplementary Information
Figure S1 | Blue phase stabilization. Thermotropic phase windows for monomer mixtures (0.8:1 thiol to acrylate functional group ratio) with constant weighted average helical twisting power. The chiral diacrylate SLO 4151 has a helical twisting power of 8μm and the non-reactive chiral dopant R811 has a helical twisting power of 11μm).