A facile photoinitiated polymerisation route for the preparation of photonic elastomers with chiral nematic order

ABSTRACT Photonic crystal elastomers that can change colour upon stretching or compression have potential applications in mechanical sensors and optical coatings. However, facile synthetic strategies are required for these materials to be made on a commercially viable scale. To address this issue, we report a photoinitiated polymerisation method to prepare stretchable chiral nematic cellulose nanocrystal (CNC) elastomer composites that exhibit reversible visible colour upon the application of mechanical stress. The initial CNC-elastomer composite is colourless, but when it is stretched (or compressed), the helical pitch of the chiral nematic structure is reduced to lengths corresponding to the wavelengths of the visible region, resulting in colouration. By increasing the percentage elongation of the material (ca. 50–300%), the structural colour can be tuned from red to blue. The colour of the material was characterised by reflectance optical spectroscopy and reflectance circular dichroism to confirm the wavelength and polarisation of the reflected light. GRAPHICAL ABSTRACT


Introduction
Photonic crystal elastomers are a class of responsive material that exhibit a colour change when mechanical stress is applied.These materials with dynamic optical properties are of interest for potential applications such as mechanical sensors [1][2][3][4], optical coatings [5,6], soft displays [7,8] and biosensors [9].The colour in these materials arises from the photonic crystal component, as the periodic structure alters how light interacts with the material, giving rise to structural colour.This structure-dependent colour is extremely beneficial as it can be tuned, will not fade, and does not require the use of toxic dyes [10,11].
Many examples of photonic elastomers have been constructed from molecular and polymeric liquid crystals, particularly where polymerisable mesogens are polymerised in the liquid crystalline state.A large proportion of the research on photonic crystal elastomers has focused on cholesteric liquid crystalline polymers.These materials have been investigated for their piezoelectricity [12], photonic properties [13], circular polarisation [14], laser emission [15] and mirrorless lasing [16].Meanwhile another important part of this research has employed synthetic opals, such as SiO 2 and polystyrene [17], and inverse opals as the photonic crystal component [18].CONTACT Mark J. MacLachlan mmaclach@chem.ubc.caSupplemental data for this article can be accessed online at https://doi.org/10.1080/02678292.2023.2200265.
However, we are not limited to synthetic materials to produce photonic crystals.Nature provides remarkable examples of biomaterials exhibiting structural colour as exemplified by chitin nanocrystals in the jewel beetle [19], guanine nanocrystals in the panther chameleon [20] and cellulose in Pollia condensata [21].Cellulose is a particularly attractive building block for photonic crystal applications as it is the most abundant biopolymer and research has demonstrated that we can prepare photonic crystals from ethyl cellulose, hydroxypropylcellulose [22,23], and cellulose nanocrystals (CNCs) [24,25].Interestingly, a series of other cellulose derivatives can be rendered through surface modifications, and their lyotropic cholesteric phases can be fixed by photo-crosslinking to yield chiral nematic films that form gels and change colour when they swell [26].These films can be prepared with high optical quality [27] and used as a medium for mirrorless lasing [28].
CNCs may be prepared by the acid-catalysed hydrolysis of biomass using sulphuric acid.This process yields nanocrystals that are~75-80% crystalline [29], ~3-50 nm in diameter, and 50-1160 nm in length [30].Furthermore, these nanocrystals with high aspect ratio are surface-functionalised with sulphate half-ester groups, which gives them a surface charge and allows them to form stable colloids in water.CNCs are chiral due to the D-glucose units in the cellulose, and this is manifested in the screw-shape morphology adopted by CNCs.One consequence of this chirality is that CNCs spontaneously form a chiral nematic liquid crystal in water [31].When aqueous suspensions of CNCs are evaporated to form films, the CNCs organise into a chiral, helical structure that resembles the long-range chiral nematic order of the lyotropic liquid crystal phase [32].If the helical pitch of the CNC films matches the wavelength of incident light, it is selectively reflected, resulting in colouration of the materials [33].Interestingly, because the chiral nematic structure of CNCs is always left-handed, the reflected light is always left-handed circularly polarised.This feature is appealing for applications as reflectors, security features and in optoelectronics [34][35][36][37][38][39][40][41].
Although CNCs exhibit exceptional strength, high surface area and are easy to functionalise, CNC films have several drawbacks: they can be susceptible to redissolution in water, are brittle and have static optical properties [42].To address this issue, CNCs have been employed as hard and soft templates to prepare materials and composites in which the chiral nematic structure is preserved [43,44].Recently, we reported a synthetic strategy that enabled us to embed chiral nematic CNCs in a poly(ethyl acrylate) elastomer composite (CNC-E) [45,46].When relaxed, the CNC-E composites are colourless, as the helical pitch of the chiral nematic structure is longer than the wavelength of visible light.However, when the film is stretched (or compressed), the helical pitch is reduced so that CNC-E reflects light in the visible region, resulting in colouration.By increasing the percentage elongation of the material (ca.50-300%), the structural colour can be tuned from red-to-green-to-blue. Thermal polymerisation conditions and a nitrogen atmosphere were required to prepare the CNC-E, which limits the scale and commercial utility of this approach.
Photoinitiated polymerisations are extensively used in both industry and academia for the preparation of coatings [47], adhesives [48], ceramics [49], optoelectronics [50], dental fillings [51] and biomaterials [52], to name but a few examples.The majority of photoinitiated polymerisations employ UV light, but the number of applications that use visible or infrared light is growing.Photoinitiated polymerisations have several advantages over their thermal analogues including fewer side reactions, easier to perform for large area applications and low reaction temperatures (normally room temperature), which increases the functional group tolerance of the reaction and types of monomers that can be used [53].Many UV light-initiated polymerisations use monomer formulations that can contain high amounts of acrylates or methacrylates [54].This is due to the high rate of polymerisation and the broad range of chemical structures that can be incorporated into (meth)acrylate monomers [55].
Here, we report a simple and expedient route to prepare chiral nematic CNC-elastomers with reversible visible colour.By utilising a photoinitiated polymerisation approach, we were able to prepare chiral nematic CNC-E under ambient conditions in less than 5 hours.The optimised CNC-E composites were characterised by optical spectroscopy and circular dichroism to assess colour and chirality of the material.

General experimental
All chemicals were purchased from Sigma Aldrich and used without further purification.The aqueous suspension of CNCs was obtained from FPInnovations (2.0% w/w, pH = 2.2).UV polymerisations were carried out using an Analytik Jena UVM-28 EL Series UV Lamp, 8 W, 302 nm.Samples were placed 1 cm away from the UV light.Circular dichroism (CD) spectroscopy was performed on a JASCO J-815 spectrometer (Jasco Inc.) using a powder CD attachment integration sphere DRCD-466 L (Jasco Inc.).Reflectance spectroscopy was performed using an OceanOptics setup with the following parts: light source DH-2000-BAL (Halogen Lamp), reflection probe R400-7-UV-VIS and spectrophotometer FLAME-S-XR1.Differential scanning calorimetry (DSC) was performed on a Netzsch DSC Polyma 214.Samples were held at 120°C for 10 min prior to heating/cooling cycles to remove any residual water.Samples were heated at a rate of 10°C/min for the chosen temperature range, with a 5 min isotherm between the heating and cooling cycles.Data were collected over three heating and cooling cycles.

Preparation of chiral nematic glucose-CNC films (g-CNC)
D-glucose (75 mg) was added to an aqueous CNC suspension (2.0% w/w, pH = 2.2, 15.0 g) and sonicated for 1 min.The obtained CNC suspension was cast in a 5 cm diameter polystyrene Petri dish, lined with a cellulose acetate disc.The CNC suspension was then allowed to evaporate at room temperature and ca.50% relative humidity.Once the film was dry, as judged by the film exhibiting homogeneous colour, the film was stored at 75% relative humidity for at least 24 h or until required.

Preparation of chiral nematic CNC elastomer (CNC-E)
In a 20 mL vial, a CNC film (1.5 × 1.5 cm) placed on top of a glass slide was soaked with 1.25 mL of a 5 mg/mL solution of 2,2-dimethoxy-2-acetophenone (DMPA)/ dimethyl sulphoxide (DMSO) for 30 min.The excess DMPA/DMSO solution was removed by syringe.A mixture of ethyl acrylate (EA, 1.2 mL), 2-carboxyethyl acrylate (0.12 mL), poly(ethyleneglycol) dimethacrylate (5 μL) and DMPA (ca. 5 mg) was slowly added to the vial so as not to introduce air bubbles into the swollen film.After the sample stood for 2 h at room temperature, a glass slide was placed on the monomerinfused film.Polymerisation was initiated with UV-B light for 1 h.

Results and discussion
In our previous route to prepare the chiral nematic CNC-E using a thermal initiator, a glucose-CNC (g-CNC) film was prepared by water evaporation of a 4% w/w CNC aqueous suspension that contained 25% w/w glucose (Figure S1).A piece of the g-CNC film was then soaked in a 2,2′-azobis(2-methylpropionitrile) (AIBN)/dimethylsulfoxide (DMSO) solution to swell the film to aid monomer infiltration (SOAK1).The AIBN/DMSO solution was then removed and the monomer solution (containing ethyl acrylate, 2-hydroxyethyl acrylate and AIBN) was added (SOAK2).The mixture was then polymerised at 60°C to yield the desired CNC-E composite with reversible visible colour [46].Modifying this procedure to be compatible with a photoinitiated polymerisation approach requires changing the radical initiator from the thermally activated AIBN to a photoinitiator.The composition of SOAK2 will also most likely need to be adjusted or changed, especially as we want to perform the polymerisation under atmospheric conditions.Scheme 1 shows the process of preparing photonic elastomers using photopolymerisation.
For the photoinitiator, we selected the acetophenone Type I (cleavage) radical initiator 2,2-dimethoxy-2-acetophenone (DMPA).Acetophenone-based initiators exhibit high reactivity and quantum efficiency, and can be stored at room temperature for long periods [53].Furthermore, acetophenones do not cause significant yellowing of the material, which is important for our system as we want the elastomer component to be colourless and have good optical clarity.Initial tests with these conditions were performed by irradiating a mixture of monomers (ethyl acrylate and 2-hydroxyethyl acrylate) and different concentrations of DMPA (2, 5, 10 and 20 mg/mL) with UV-B light for 1 h (Figure S2).All samples produced polymeric material, though all elastomers prepared were somewhat tacky.We attributed this to incomplete polymerisation due to oxygen inhibition and it was most pronounced in the lowest DMPA concentration sample (2 mg/mL).
In an attempt to address this issue, we then further modified the SOAK2 composition by introducing a crosslinking agent; we selected to test two crosslinkers, N,N'-methylenebis(acrylamide), and poly(ethylene glycol) dimethacrylate (M n = 550 g/mol).The addition of both crosslinkers improved the efficiency of the polymerisation and produced elastomers that were no longer tacky (Figure S3).However, the mechanical properties of the elastomer were also altered, with higher amounts of crosslinker unsurprisingly producing stiffer elastomers (Table S1).Specifically, for the samples crosslinked with N,N'-methylenebis(acrylamide), the introduction of this crosslinker produced a more brittle material that tore easily when mechanical stress was applied.Therefore, we selected poly(ethylene glycol) dimethacrylate as the crosslinker to use in SOAK2.
For the final optimisation of the SOAK2 conditions, we tested how the crosslinkers performed in the preparation of CNC-E.For this we set the SOAK1 concentration at 5 mg/mL DMPA/DMSO solution (to match the concentration in SOAK2) and tried two different crosslinking loadings (1 and 2 μL), which improved the polymerisation without making the elastomer too stiff.However, the resulting CNC-Es were very sticky and it was virtually impossible to remove the film from the glass slides without damaging the material (Figure S4a).The increased complexity of the reactions of making CNC-E versus the pure SOAK2 test reactions highlighted that we needed to modify the composition of SOAK2 further to obtain the desired CNC-E.Therefore, we investigated how we could modify the SOAK2 composition to reduce oxygen-inhibition.In 2016, Pietrzak and co-workers reported that replacing a 2-hydroxyethyl acrylate (HEA) with 2-carboxyethyl acrylate (CEA) in alumina slurries reduced oxygeninhibition [56].They postulated that the carboxylic acid group of the CEA can form more hydrogen bonds with the alumina than the alcohol group of the HEA.This produced a more viscous solution, reducing oxygen penetration.Recognising the hydrogen-bonding capacity of CNCs, we investigated whether replacing HEA with CEA would also improve our photoinitiated polymerisation.For our system, by modifying SOAK2 to include CEA and increasing the amount of PEGDA to 5 μL, we were able to obtain a CNC-E that was easy to remove from the glass slides and was not tacky (Figure S4b).
CNC-E is colourless in the relaxed state, but when it is stretched the colour changes from red-to-green-toblue with increasing mechanical stress (Figure 1(a,b)).When the mechanical stress is removed, the elastomer returns to its colourless, relaxed state.The colour of the CNC-E at different elongation lengths was characterised by reflectance optical spectroscopy (Figure 1(c)) and circular dichroism (CD) (Figure 1(d)) spectroscopy.In addition, the reflectance CD spectra produced a positive peak of high intensity, which corresponded to the observed colour of the CNC-E.These data confirmed that the chiral nematic structure was preserved and that the helical pitch of the CNC is reduced when mechanical stress is applied.As expected, this is consistent with the data obtained for the CNC-E with reversible visible colour prepared using thermal polymerisation conditions [46].In both cases, the mechanism of colouration is due to compression of the helical pitch of the CNC chiral nematic organisation into the visible wavelength when mechanical stress is applied.
Next, we tried to expand this protocol to other acrylates that can form elastomers including methyl acrylate, butyl acrylate and 2-hydroxyethyl acrylate, as well as the thermoplastic-forming methyl methacrylate.Initially for these reactions we replaced the ethyl acrylate in SOAK2 with the new acrylate and kept all other components in SOAK2 the same.The methyl acrylate and 2-hydroxyethyl acrylate polymerised successfully under these conditions, while butyl acrylate and methyl methacrylate needed to be irradiated for 3 and 16 h, respectively, which we attributed to the slower reaction kinetics of these monomers [57].Methyl acrylate and 2-hydroxyethyl acrylate produced colourless CNC-E, which were comparable to what we obtained for the ethyl acrylate CNC-E (Figure S5).While both butyl acrylate and methyl methacrylate produced coloured materials (Figure S6), the colouration of these samples is indicative of low monomer impregnation into the g-CNC film, resulting in a smaller increase in the helical pitch of the film.For both acrylates, we think the low monomer impregnation is probably a result of the increased hydrophobicity of the monomer arising from the longer alkyl chain length.To overcome this issue, we adjusted the SOAK1 time to 1 h, as we know that increasing the SOAK1 time facilitates greater monomer infiltration into the film [46].We also added ca. 15 vol% DMSO to SOAK2 to reduce the hydrophobicity of the monomer solution.When these new conditions were used, we were able to obtain a colourless CNC-E for butyl acrylate and a colourless chiral nematic CNC-plastic for methyl methacrylate (Figure S6).Differential scanning calorimetry (DSC) was used to determine the glass transition temperature T g of the chiral nematic CNC-composites (Table 1), with the prepared composites having T g s that span over 120°C.
Characterisation of the chiral nematic CNC-E composites prepared from methyl acrylate, 2-hydroxyethyl acrylate and butyl acrylate when mechanical stress was applied, revealed different photonic behaviour with changing the acrylate (Figure 2).The methyl acrylate monomer behaved in a similar way to the original ethyl acrylate analog (Figure 2(a,e); however, the higher T g of the elastomer meant it was stiffer and had poorer recovery, as well as the visible colour being less intense.The 2-hydroxyethyl acrylate CNC-E could only really be stretched to give an orange colour (Figure 2(b,f)) and the butyl acrylate analog yielded no visible colour, even when stretched to over 17 times its length (Figure 2(c)).Some of these changes may be due to differences in the monomer penetration, volume change during polymerisation, SOAK1 and SOAK2 times and maybe even the refractive index of the final polymer matrix.We have previously demonstrated that by tuning these parameters, one can successfully access chiral nematic CNC-E with reversible visible colour [46], but not all applications require a visible colour readout.Therefore, by expanding the scope of monomers that can be used in this approach we aim to demonstrate the utility of this synthetic approach to prepare novel chiral nematic

Conclusions
In summary, we have developed a photopolymerisation route to chiral nematic CNC-E from a range of acrylate monomers.The elastomers prepared from methyl acrylate, ethyl acrylate and 2-hydroxyethyl acrylate all exhibit reversible visible colour and were characterised by reflectance optical microscopy and reflectance CD spectroscopy.As the CNC-E is elongated, the colour changes from red to blue due to compression of the helical pitch of the chiral nematic organisation upon the application of mechanical stress.We also demonstrated that methyl methacrylate can be used in this synthetic approach to prepare chiral nematic CNC-plastics, although it does require longer polymerisation times than the acrylate monomers.The photopolymerisation method we have developed is faster and requires less stringent conditions than the thermal polymerisation route we have previously reported for the preparation of chiral nematic CNC-E and enables a range of different chiral nematic CNC-Es to be synthesised.

Disclosure statement
No potential conflict of interest was reported by the authors.

FundingC
.E.B. thanks the Banting Postdoctoral Fellowship and Killam Postdoctoral Fellowship for funding.The authors acknowledge financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada (Discovery Grant).

Table 1 .
Reaction conditions and characterisation of the resulting chiral nematic CNC-composite.