Concentric chiral nematic polymeric fibers from cellulose nanocrystals

Hierarchical biological materials, such as osteons and plant cell walls, are complex structures that are difficult to mimic. Here, we combine liquid crystal systems and polymerization techniques within confined systems to develop complex structures. A single-domain concentric chiral nematic polymeric fiber was obtained by confining cellulose nanocrystals (CNCs) and hydroxyethyl acrylate inside a capillary tube followed by UV-initiated polymerization. The concentric chiral nematic structure continues uniformly throughout the length of the fiber. The pitch of the chiral nematic structure could be controlled by changing the CNC concentration. We tracked the formation of the concentric structure over time and under different conditions with variation of the tube orientation, CNC concentration, CNC type, and capillary tube size. We show that the inner radius of the capillary tube is important and a single-domain structure was only obtained inside small-diameter tubes. At low CNC concentration, the concentric chiral nematic structure did not completely cover the cross-section of the fiber. The highly ordered structure was studied using imaging techniques and X-ray diffraction, and the mechanical properties and structure of the chiral nematic fiber were compared to a pseudo-nematic fiber. CNC polymeric fibers could become a platform for many applications from photonics to complex hierarchical materials.


Introduction
Biological materials, such as osteons and plant cell walls, have complicated hierarchical structures, with organization over many length scales. [1][2][3] Each level in the hierarchy represents an increase in organizational complexity. At the rst length scale, molecular-and nano-sized units co-assemble to form highly ordered micron-sized units. These micron-sized units interact in sophisticated ways to form larger structural units. This hierarchical fabrication process continues to the macroscopic scale, resulting in a material with unique properties. Mimicking the bottom-up construction of hierarchical materials at the nano/micron scale has remained elusive, but we show here that combining liquid crystals and polymerization techniques within conned spaces could be a strategy to recreate such complex structures in articial systems.
Cellulose nanocrystals (CNCs) are nano-sized spindleshaped crystalline cellulose particles obtained by treatment of cellulosic biomass with strong mineral acids. When they are prepared with sulfuric acid, the CNC particles are covered with sulfate half-ester groups that give them a surface charge and lead to colloidal stability in water. 4 Importantly, CNCs spontaneously form a chiral nematic (also called cholesteric) lyotropic liquid crystal above a critical concentration in water. Initially, liquid crystalline anisotropic droplets, 5 called tactoids, spontaneously nucleate at CNC concentrations above $3 wt%. 6 CNC tactoids are the intermediate state bridging the isotropic phase and the macroscopic liquid crystalline phase with longer range order. 7 Upon further concentration, the CNCs form a continuous chiral nematic phase. In this chiral nematic structure, CNC spindles are aligned such that the liquid crystal director rotates through the liquid crystal along the cholesteric axis, as shown in Fig. 1a. The cholesteric pitch (p) is the distance needed for the director to rotate 360 .
When the lyotropic liquid crystal suspension of CNCs is dried into a lm, the chiral nematic organization of the CNCs can be preserved in the solid state. If the pitch matches the wavelength of visible light, then the lms can appear iridescent. Interesting materials have been developed based on the liquid crystalline nature of CNCs, including lms, 8,9 microcapsules, 10 3D printer inks, 11 hydrogels, 12,13 aerogels, 14 and hybrid composites. 15,16 The chiral nematic structure of CNCs has also been used as a template to form other chiral nematic materials such as chiral nematic silicate glasses. 17 CNC structures developed by evaporation-induced selfassembly lack long-range order, do not have a uniform pitch value, and are multi-domain with cholesteric axes pointing in different directions in different regions. 18,19 This is largely due to evaporation-induced ow, non-uniform evaporation, and uncontrolled liquid crystalline nucleation sites and growth. Magnetic elds 20,21 and slow evaporation 22 have been applied to improve the long-range order in CNC-based structures, but both techniques are slow and only simple planar structures have been reported. [20][21][22] The organization of CNC particles can be  At 72 h, the tube was also imaged after rotating 90 around its long axis as schematically shown. The apparent inner diameter of the tubes in these images is 0.55 mm while the actual inner diameter is 0.4 mm. This discrepancy originates from light refraction through the cylindrical object (see Fig. S1, ESI †). manipulated by conning CNC suspensions in small spaces. For example, uniformly aligned photonic lms were reported for CNC suspensions dried in a thin rectangular capillary. 23 Furthermore, concentric spherical multi-shell structures with radially oriented cholesteric axes were reported for CNCs conned to microdroplets. 24,25 In this paper, we show that by conning CNC aqueous solutions inside a cylindrical capillary tube, a longrange-ordered single domain of CNCs with chiral nematic structure can be obtained. The ordered structure is then locked in place by a polymerization technique, resulting in bers with a concentric chiral nematic structure (Fig. 1b). We studied the effect of tube orientation, CNC concentration, CNC type, salt addition, capillary tube size, and time on the organization of the material. Polarized optical microscopy (POM), electron microscopy, confocal microscopy, 2D X-ray diffraction (2D-XRD), and tensile test measurements were applied to demonstrate the ordered structure and its properties. We discuss how a simple connement approach could be applied to obtain well-ordered hierarchical structures with a wide range of potential applications.

Results and discussion
This study stemmed from the observation that lling a capillary tube (0.4 mm inner diameter) with 4.5 wt% CNC solution resulted in the formation of birefringent periodic lines parallel to the length of the tube when observed between crossed polarizers using POM (Fig. 2). The lines are evenly spaced and resemble the characteristic ngerprint lines observed in chiral nematic CNC structures using POM, where the distance between the lines is equal to one-half pitch (p/2). 26 Temporal POM images of the capillary tube lled with 4.5 wt% CNC suspension show that it takes time for the ordered structure to form (Fig. 2). Immediately aer lling the capillary, the solution was completely isotropic and the sample appeared dark when viewed between crossed polarizers. Aer 3 h, tactoids became visible, but disappeared over 72 h as they coalesced into a continuous liquid crystalline phase, which was evident in POM images as periodic dark lines that continue throughout the length of the tube. The same part of the tube was then imaged aer rotating the tube 90 around its long axis, as shown in Fig. 2. Interestingly, the top half of the tube was dark (isotropic) and the bottom half showed the straight parallel lines. This observation suggests that tactoids settled to the bottom of the tube due to gravity before coalescence, and the periodic structure is only occupying the bottom half of the tube interior. Tactoid sedimentation is also observed in vertical orientation of the tube as will be discussed in the next paragraph. In Fig. 2, the distance between periodic lines in both views are equal to 14 AE 2 mm. Color that is apparent in the rotated view is due to the increase of the thickness as more CNCs are in the light path, as described by the Michel-Levy chart for birefringence. 27 The orientation of the tube during liquid crystal formation is also important in this low concentration regime of 4.5 wt%. In the experiments described in the previous paragraph, the tube was le in a horizontal orientation while the liquid crystal developed. When the tube was instead positioned vertically during liquid crystal formation, tactoid formation was again evident aer 3 h (Fig. 3). Aer 24 h, larger tactoids had settled and fully lled the bottom of the tube, coalescing and producing a relatively disordered ngerprint texture. Moving toward the top of the tube, a progressive reduction in the size of the tactoids is clear, suggesting faster sedimentation for larger tactoids. No tactoids were observed aer 72 h, and highly uniform lines were visible in the bottom third of the tube, above which a completely isotropic phase existed (Fig. 3).
As shown in Fig. 4, the distance between the periodic lines observed by POM decreased signicantly as CNC concentration increased. The lines only appeared at CNC concentrations greater than ca. 4 wt%, and between 4-9 wt% the distance decreased from 19 AE 3 to 4 AE 0.2 mm. No lines were observed above ca. 9 wt% CNC concentration, but the sample looked colorful and highly birefringent by POM. At 10 wt%, the successive colors from the tube edge to its center follows the successive order of interference colors in the Michel-Levy chart 27 and, as predicted, it ends in pink (the highest-order interference color) in the central region, where the sample is thickest. These results have been attributed to higher CNC interactions that enhance the intrinsic twist, resulting in a smaller pitch and agrees with the reported inverse relation between chiral nematic pitch and concentration in CNC suspensions. 28 At high CNC concentration (>ca. 10 wt%), the high viscosity of the solution prevents liquid crystal formation as it limits the free motion of CNC particles.
We examined a second batch of CNCs with a smaller average particle size (92 AE 4 nm). We found that it behaved similarly, but the concentration at which structural order was observed increased to 8-12 wt% for the smaller sized CNC batch (Fig. 4) as opposed to 4-9 wt% for the larger one (193 AE 14 nm). The smallest distance between periodic lines observed in the smaller sized CNC batch was 1.5 AE 0.2 mm (Fig. 4), which occurred at 12 wt%. Previous reports have shown reducing the length of the CNC particles decreases the cholesteric pitch and increases the critical concentration required for cholesteric phase formation, in agreement with our result; 29,30 however, the observed difference between CNC batches may not be due solely to the particles size, 31 and instead could be caused by CNCs being obtained from different cellulose sources or due to variable reaction conditions during CNC extraction. The effect of salt addition was also tested by adding NaCl (0-12 mM) to a 9 wt% CNC suspension prior to capillary tube lling. Salt addition slightly reduced the distance between the lines from 4 AE 0.2 to 3 AE 0.3 mm (see Fig. S3, ESI †). At high salt concentration (12 mM), the liquid crystal phase was disrupted, and periodic lines were not observed by POM.
We investigated the effect of the capillary tube diameter on the liquid crystal structure by using tubes with 0.4 and 1.2 mm inner diameter. Fig. 5 shows time-series POM images of these tubes lled with 6 wt% CNCs. In the small (0.4 mm) tube, the ngerprint lines gradually appeared and the highly ordered single-domain structure formed within 3 days as exemplied by uniform birefringent color and evenly spaced parallel lines throughout the length of the tube (see also Fig. S4, ESI †). In contrast, CNCs conned in the large tubes (1.2 mm) did not form long-range order, even aer 21 days. Instead, the sample showed a multidomain structure with birefringent colors and periodic lines with various orientations visible by POM in some regions of the tube (see Fig. 5 and also Fig. S5, ESI †). These ndings suggest a small inner diameter of the capillary tube is important to obtaining a well-ordered monodomain structure of CNCs in the tube. It is the capillary tube glass surface that directs the CNC chiral nematic structure formation and, for the smaller tube, the higher surface to volume ratio helps with the uniform structure formation.
Aer understanding the evolution of CNC liquid crystalline domains inside the capillary tube, we sought to capture this structure in a polymer matrix. There are previous reports of capturing CNC structure through polymerization techniques such as photopolymerization of magnetically aligned CNCs in hydroxyethyl methacrylate (HEMA) or polyacrylamide hydrogels. 32,33 Here we used hydroxyethyl acrylate (HEA) instead to obtain an elastic ber. The tube was lled with a mixture containing CNCs and HEA monomer, N,N 0 -methylenebisacrylamide crosslinker, and 2hydroxy-4 0 -(2-hydroxyethoxy)-2-methyl propiophenone initiator. Aer the sample aged horizontally for 72 h to form the ordered structure, polymerization of the organic monomers was initiated with UV irradiation, converting the HEA and crosslinker into poly(hydroxyethyl acrylate), PHEA. The resulting CNC/PHEA ber could be easily removed from the capillary tube using a small needle (see Video 2, ESI †). The ber has high water content; when a 6 wt% CNC solution is used, the resulting ber has $85 wt% water, 9 wt% PHEA, and $6 wt% CNC.
The 3D structure of the CNC/PHEA composite ber was studied using the scattering mode of laser scanning confocal microscopy (LSCM). Fig. 6a shows the 3D structure of a CNC/ PHEA ber reconstructed from LSCM data. Videos showing the complete 3D structure, Z-stacks, and cross-sections are provided in the ESI (Videos 3-5 †). The 3D structure and crosssection images clearly show concentric rings spanning the length of the ber (Fig. 6a and b). In Fig. 6b, the intensity of scattered light from the concentric rings is maximum at the top of the ber and gradually fades toward the center of the ber. The other half of the ber was completely dark (not shown in the image), suggesting an absence of ordered structure there. Observed scattering artefacts (indicated by red arrows in Fig. 6b) are possibly caused by nonlocal out-of-focus backscattering at these locations, a well-known issue with the scattering mode of LSCM. 34 Presumably, the concentric rings (the white arrow in Fig. 6b) represent half helical pitches of the chiral nematic structure of the CNCs inside the ber, which was further investigated by carrying out light microscopy and SEM imaging on the cross-section of the bers (Fig. 6c-f). Remarkably, evenly spaced concentric rings were observed. For the CNC/PHEA ber prepared using low CNC concentrations, the concentric rings observed are uniform and nearly defect-free, but they do not cover the whole cross-section ( Fig. 6c and d). In contrast, at higher CNC concentrations, the concentric rings cover the whole cross-section (Fig. 6e), but more defects are apparent. The light microscopy image of the high CNC concentration ber is provided in Fig. S6, ESI. † At high concentration, the distance between the concentric rings (i.e., pitch) is smaller. The smallest pitch was observed at the highest CNC concentration (Fig. 6f), in agreement with POM images (Fig. 4), but there are some regions without the ngerprint texture. Distances between the periodic lines in POM imaging, and concentric rings in SEM and confocal microscopy are in good agreement (Table S1, ESI †).
Overall, based on these observations, a concentric cylindrical chiral nematic structure is proposed for the CNC/PHEA hybrid ber as illustrated in Fig. 1b. The cholesteric axis is radial, going from ber edge to its center. This type of structure is known as a radially twisted-axial conguration, or double twist cylinder geometry, and was previously reported for chiral nematic liquid crystals inside capillary tubes, 35-37 but has not been captured using photopolymerization. There are two requirements for formation of this structure: 36 (1) a cylinder with diameter larger than the intrinsic cholesteric pitch of the liquid crystal; and (2) parallel anchoring of the liquid crystal mesogens to the inner cylinder surface. In our case, both conditions are met: the CNC pitch is at least an order of magnitude smaller than the tube diameter, and CNC particles anchor parallel to the hydrophilic glass substrate. 38 The previous report on connement of CNCs inside a thin rectangular capillary tube has suggested a very different chiral nematic structure with cholesteric axis parallel to the length of the capillary tube. 23 In contrast, the chiral nematic structure of our bers is concentric showing a radial cholesteric axis. This discrepancy originates from a very different approach taken in the reference, 23 where structure formation was induced through evaporation. In that reference, unlike our work, the capillary tube was not sealed. This led to evaporation from the capillary tube end which induced ow in that direction. CNCs were transferred to the evaporation boundary where they formed a planar chiral nematic structure. In that structure, the cholesteric axis was parallel to the capillary length because the nucleation site was not the glass capillary surface, but instead it was the air-liquid interface at the evaporation edge. In our case, the nucleation site is the glass capillary surface, which in combination with the round geometry of our capillary tubes, led to the observed concentric chiral nematic structure. Fig. 7a and b show the 2D-XRD measurements of a CNC/ PHEA ber that was polymerized immediately aer lling the tube, where the CNCs did not have time to organize into a chiral nematic liquid crystal. Data are shown for the sample in a relaxed condition and during 2Â stretching parallel to the long axis. Diffraction intensities I(f) with respect to the azimuthal angle (f) at (200) plane (2q ¼ 22.9 ) are maximum at the equatorial position, indicating CNC alignment parallel to the ber long axis. The alignment improves upon stretching, with an increase of the Hermans order parameter (S) from 0.46 to 0.80. This suggests that the shear force during the lling of the capillary tube caused the CNC spindles to partially align parallel to the length of the tube and stretching improved the alignment, effectively transforming them from a pseudonematic to a nematic arrangement. S values close to 0.8 have only been reported for highly aligned CNC particles obtained at high shear forces. 39 Next, a capillary tube was lled with CNCs and the PHEA precursors, and was polymerized aer 72 h of horizontal aging, allowing the concentric structure to form. Fig. 7c and d show the 2D-XRD pattern of this ber while relaxed and during 2Â stretching, respectively. Similar to the pseudo-nematic sample, the (200) diffraction ring of this chiral nematic structure shows high intensity at equatorial positions, giving S values of 0.51 and 0.60 for relaxed and stretched bers, respectively. Fibers kept their chiral nematic structure aer stretching and the chiral nematic structure did not unwind into a nematic structure. This was also conrmed by the persistence of the ngerprint lines in the ber at different extents of stretching (see Fig. S7, ESI †). Numerical simulations of chiral nematic liquid crystals inside capillary tubes have predicted a value of $0.6 for a perfectly concentric chiral nematic structure. 37 The CNC/PHEA bers are highly stretchable (see Video 6, ESI †). Tensile tests were carried out on the concentric chiral nematic CNC/PHEA ber and compared to the pseudo-nematic CNC/PHEA ber and a pure PHEA ber (without CNCs). Fig. 7e shows the stress-strain curves for the three ber types. Interestingly, mechanical properties of the chiral nematic and the pseudo-nematic bers are very similar (Fig. 7f-h). This is unexpected as in the pseudo-nematic ber the CNC particles are mostly aligned parallel to the ber length (the direction of tension), but in the chiral nematic ber the director rotates, and the CNC particles are not generally aligned parallel to the direction of tension. Both bers have similar tensile moduli and high strength, but they are not as stretchable as the pure PHEA ber. Addition of CNCs clearly has a reinforcing effect on the polymer, but it also decreases elasticity. In contrast to the pure PHEA ber, CNC/PHEA composite bers underwent a permanent plastic deformation and did not rebound to their original shape upon release aer stretching (see Videos 6 and 7, ESI †).
The structure of the single-domain chiral nematic concentric CNC/PHEA ber is very similar to biological hierarchical structures, such as the twisted plywood architecture of collagen bers in the osteon of cortical bone lamellae. 40 In wood and plant cells walls, cellulose microbrils wind around the hollow core, where cells reside, forming multiple concentric layers, known as wood cell units. 2 We showed that polymerization of a liquid crystal system conned inside a capillary tube could achieve similarly complex structures. Many applications could be envisioned based on the CNC ber platform. Fibers could be bundled or weaved to develop larger, higher dimensional structures with multiple levels of hierarchy. These bers could also be used as a template to develop other concentric chiral nematic organic or inorganic materials. An interesting application of CNCs is in the eld of photonics. Chiral nematic structures reect circularly polarized light when the wavelength matches their helical pitch. 18 One of our goals was to prepare stretchable CNC/PHEA bers that show structural color upon stretching, but the smallest pitch we obtained was $3 mmfar from the required sub-micron range (Fig. 4). Both drying and stretching decrease the ber diameter and consequently decrease the pitch value to the same extent. Fig. S7 and S8 (ESI †) show the decrease of the distance between ngerprint lines aer stretching and drying. The distance between the layers decreases upon drying or stretching the ber, and consequently the pitch decreases. For our smallest pitch ber, a $4Â reduction in the ber diameter is required to bring the pitch into sub-micron range. Aer drying and stretching, however, this level of ber thinning was not feasible, and we could not observe structural color from the bers. We are currently exploring modication of the polymer ber composition and using CNCs from other sources that have a smaller aspect ratio (and pitch) to develop bers with pitch closer to this sub-micron range. Another possible application for the concentric CNC bers is in the eld of ber optics. Existing optical bers rely on total internal reection or diffraction from a periodic structure. A concentric chiral nematic CNC optical ber would rely on a completely different phenomenon. It should be possible to transfer le-handed circularly polarized light with a wavelength matching the CNC ber's pitch. Depending on the transmitted light wavelength, a ber of desired pitch could be designed, and the inner core could be hollow. This approach may lead to optical bers covering a large range of wavelengths, high exibility, and minimal manufacturing cost. In practice, translating these bers into optically applicable bers faces several challenges, including controlling the pitch size and the ber length, and developing a uniform distribution of chiral nematic structure along the ber circumference instead of a ber structure with only half of the cholesteric liquid crystal phase.

Conclusion
Chiral nematic CNC polymer bers with controllable pitch were fabricated. We combined capillary connement, photopolymerization and CNC liquid crystals to develop these uniquely ordered bers. We evaluated a series of variables to understand and control the self-assembly behavior of CNCs in capillary connement. Through photopolymerization, the structure of the chiral-nematic phase was locked to obtain chiral nematic bers, which were investigated with respect to their structural features as well as their mechanical properties. In this work, we tried to understand and demonstrate the feasibility of forming these highly ordered bers and to understand the limitations and challenges. Addressing these challenges would bring us closer to applying these bers to the eld of optics.

Materials
Aqueous suspensions of CNCs prepared from sulfuric acid hydrolysis of Kra sowood pulp were supplied by FPInnovations (4 and 6 wt%, acid form of CNCs with pH ¼ 2.0). Two types of CNCs were used; a larger sized CNC batch with an average hydrodynamic radius of 193 AE 14 nm (n ¼ 18) and zeta potential of À51.2 AE 2.3 mV (n ¼ 7) (measured using a Brookhaven NanoBrook Omni Dynamic Light Scattering instrument), and a smaller sized CNC batch with an average hydrodynamic radius of 92 AE 4 nm (n ¼ 18) and zeta potential of À49.2 AE 2.2 mV (n ¼ 7). Experiments described throughout the paper used the larger sized CNC batch unless it is explicitly specied that the smaller size was used. Higher concentrations of CNC suspension were prepared from the original 4 or 6 wt% CNC suspensions using rotary evaporation. The CNC concentration was conrmed by measuring the weight of a known volume of CNC suspension aer drying in triplicate. Hydroxyethyl acrylate (HEA) (Aldrich, 98%), hydroxyethyl methacrylate (HEMA) (Aldrich, 98%), N,N 0 -methylenebisacrylamide (Aldrich, 99%), and 2-hydroxy-4 0 -(2-hydroxyethoxy)-2methylpropiophenone (Aldrich, 98%) were used as received.

Formation of CNC chiral nematic structure
Before lling the capillary tube, the CNC solution was bathsonicated for 15 min. Silicate glass capillary tubes with inner diameter of 0.4 mm (7.6 cm in length, Drummond Scientic Co.) and 1.2 mm (Kimble Chase) were lled with CNC suspension based on capillary action. Alternatively, at concentrations higher than 9 wt%, the CNC suspension was injected into the capillary tube. The suspension inside the lled capillary tube was adjusted to leave air gaps at both ends of the tube. The tube ends were sealed using 5 min epoxy (Devcon), xed on a glass slide, oriented horizontally or vertically, and aged to monitor formation of the chiral nematic structure. Different CNC concentrations and CNC types were tested. The effect of salt addition was tested by adding NaCl (0-12 mM) to a 9 wt% CNC suspension prior to capillary tube lling.
Formation of CNC/PHEA polymeric ber 1 g of HEA monomer, 20 mg of N,N 0 -methylenebisacrylamide (crosslinker), and 6 mg of 2-hydroxy-4 0 -(2-hydroxyethoxy)-2methylpropiophenone (initiator) were dissolved in a 10 mL aqueous CNC suspension. This solution was used to ll the silicate glass capillary tubes as described above. Different CNC concentrations and CNC types were used. The lled capillary tubes were irradiated with a 300 nm ultraviolet-B-light source (8 W) for 1 h to complete the photopolymerization of the monomers. The polymerization was carried out either immediately aer capillary tube lling, resulting in a pseudo-nematic CNC/PHEA ber, or aer 72 h aging, resulting in a chiral nematic CNC/PHEA ber. The polymerized ber was removed from the capillary tube using a LUTER 0.35 mm 3D printer nozzle cleaning needle.

Polarized optical microscopy (POM) imaging
CNC lled capillary tubes and polymeric bers were imaged between crossed polarizers using an Olympus BX53M microscope. Images were collected on a rotating stage using a white light source without a waveplate. Images were collected either at parallel or diagonal orientation of the tube with respect to polarizer/analyzer (see Fig. S9 †). The periodic distance between lines (chiral nematic pitch) was measured by determining RGB intensity distribution along the ber diameter using a custom algorithm developed in MATLAB® (MathWorks Inc.). The periodic lines in the POM images result in an RGB intensity distribution curve with an oscillating pattern (see Fig. S10, ESI †). The distance between the peaks in this oscillating pattern was measured for >20 replicates, reporting the average and standard deviation. The apparent inner diameter of the tubes in POM images is 0.55 mm while the actual inner diameter of the tube is 0.4 mm (see Fig. S1, ESI †). The true distance between the periodic birefringent lines is the apparent distance divided by 1.37 (¼0.55/ 0.4), and this corrected value is reported throughout the paper.
Scattering mode laser scanning confocal microscopy imaging A 6 wt% CNC/PHEA ber was removed from the capillary tube aer polymerization, xed at on a glass slide and imaged using a Zeiss LSM 510 confocal microscope. The local scattering throughout the sample was imaged using scattering mode of the confocal microscope using a 63Â objective lens. The sample was scanned on the xy-plane, and 53 stacks were collected along the z-axis, through the ber thickness. The 3D reconstruction and videos were developed using ImageJ (NIH Image, Bethesda, MD). Maximum intensity cross-section projection showing concentric rings was used to measure the distance between the rings using a custom algorithm developed in MATLAB® (MathWorks Inc.) (see Fig. S11, ESI †).

Scanning electron microscopy (SEM) imaging
SEM images were collected on the cross-section of the bers. A HEMA solution was prepared by dissolving 7.4 mg of 2-hydroxy-4 0 -(2-hydroxyethoxy)-2-methylpropiophenone (initiator) in 2.14 g of HEMA. CNC/PHEA bers with 0.4 mm diameter were removed from the capillary tube aer polymerization, placed in a 1.4 mm capillary tube, lled with the HEMA solution, sealed, and le overnight for HEMA to replace water in the CNC/PHEA ber. The contents of the capillary tube were photopolymerized as described before. The polymerized capillary tube was broken and the cross-sections were xed on a stub, sputter coated using a Leica EM ACE600 (5.6 nm iridium) and imaged using FEI Nova NanoSEM430 with an accelerating voltage of 5-15 kV. Crosssections were also imaged using light microscopy.

2D X-ray diffraction (2D-XRD) studies
The pseudo-nematic CNC/PHEA ber and the chiral nematic CNC/PHEA bers were removed from the capillary tubes, xed using a paper clip, and 2D-XRD patterns collected. The 6 wt% (larger-sized CNC batch) was used to prepare the bers. The ber long axis was positioned vertically for the XRD (positioning the ber horizontally only rotates the 2D-XRD pattern). It was necessary to remove the polymerized bers from the glass tube otherwise the amorphous silicate diffraction would obscure the CNC diffraction peaks. Fibers were measured at both a relaxed and a 2Â stretching position. Images were recorded on a Bruker APEX DUO with APEX II CCD detector using Cu Ka 1 X-ray beam with a wavelength of 0.154 nm at 0.6 mA, 45 kV for 480 s at 40 and 60 mm from the detector in transmission mode. Multiple rings are visible corresponding to X-ray diffraction of different cellulose crystal lattices with the brightest diffraction ring at 2q ¼ 22.9 corresponding to the (200) diffraction of the cellulose Ib crystal, 41 where cellulose polymer chains in a single CNC are aligned parallel to the CNC rod length. 39 Diffraction intensities I(f) with respect to the azimuthal angle f were determined at 2q ¼ 22.9 by tting a circle on the corresponding ring, and measuring pixel intensity along the circumference. The north pole of the ring corresponds to f ¼ 0 in the measurements. Hermans order parameter (S) was calculated from the (f, I(f)) values using eqn (1) and (2), 39,42 through a numerical analysis in MATLAB® (MathWorks Inc.), aer subtracting the background. S can range between 0 and 1, with 0 indicating an isotropic structure and 1 indicating a perfectly aligned structure.
Tensile mechanical testing When CNC/PHEA bers were initially removed from the capillary tube, they had high water content and, despite being sturdy and exible, they were not stretchable. PHEA with high water content acts more like a hydrogel and lacks any stress-releasing mechanism, leading it to break under tension. 43 A completely dry ber is also not suitable as it is brittle, especially at high CNC content. This is likely due to strong hydrogen bonding between CNC particles. 44 The ideal ber should retain some water or contain a plasticizer. To obtain bers with good mechanical properties, glycerol was added; glycerol is commonly applied in industry to prevent cracks in cellulose-based dialysis tubes. 45 Aer polymerization, bers were removed from the capillary tubes and placed in a 2.5 v% glycerol solution for 1 h and then dried. Glycerol remains as a plasticizer and prevents complete drying due to its hygroscopic nature. The ber has $35 wt% CNC, $35 wt% PHEA, $15 wt% glycerol and $15 wt% water. Tensile testing was performed on 9 wt% (larger sized CNC batch) pseudo-nematic CNC/ PHEA, chiral nematic CNC/PHEA ber, and pure PHEA (no CNCs) bers using a 5960 Series Universal Testing System (Instron) equipped with a 50 N load cell. The two ends of the ber were glued to square shaped Teon pieces, and positioned between the grips. The strain rate was 5 mm min À1 and seven replicates were measured for each ber. Data were smoothed using a low pass lter with cut-off frequency of 60 Hz in MATLAB® (MathWorks Inc.). The smooth curves were used to determine the maximum stress-at-break, strain-at-break, and tensile modulus. Tensile modulus is reported by measuring the slope of the stress vs. strain curve. The cross-section area of the bers was determined using light microscopy.

Author contributions
A. M. is credited for the conception, design, analysis and writing of the study. C. M. W. is credited for carrying out the mechanical tests and authorship. Y. T. X. is credited for the conception and authorship. W. Y. H. is credited for providing CNC and authorship. M. J. M. is credited for the conception, design, supervising and authorship.

Conflicts of interest
There are no conicts to declare.