Introduction

Nowadays, intensive research focuses on the development of biobased polymers (Kimura 2009; Nakajima et al. 2017; Piorkowska 2019) as it plays an important role in struggling with carbon dioxide emissions causing global warming, and in diminishing dependence on fossil resources. Cellulose, a biodegradable polysaccharide present in plant cell walls, is the most abundant natural polymer on Earth (Pauly and Keegstra 2008). Recently, cellulose rod-shaped nanocrystals isolated from native cellulose draw increasing attention. Due to cellulose abundance, its low weight, high strength and stiffness, and renewability, numerous studies have been reported on the isolation of cellulose nanocrystals (CNCs) from different sources, their morphology, and properties, as well as their use in high-performance applications (Mendoza-Galvan et al. 2019; Querejeta-Fernandez et al. 2015; Sanchez-Botero et al. 2018). The applications of CNCs include green electronics and optoelectronics. For example, a nanocellulose/epoxy composite with improved thermal stability, flexibility, and high optical transparency of ca. 90% was described as suitable for application as a substrate for polymer-based flexible solar cells (Wang et al. 2020). CNCs can form nematic or chiral nematic structures in water (Cherpak et al. 2018; Habibi et al. 2010; Parker et al. 2018). This allowed obtaining CNC films with the order preserved on a mesoscopic scale using processing methods, such as drop-casting, spin-coating, and dip-coating (Diaz et al. 2013; Kontturi et al. 2007; Mendoza-Galvan et al. 2019; Rofouie et al. 2018; Sanchez-Botero et al. 2018; Tran et al. 2020). Interestingly, Droguet et al. (Droguet et al. 2022), optimized the self-assembly of CNC suspensions into photonic films using a continuous roll-to-roll technique and obtained meter-scale structurally colored CNC films. Alignment of CNCs leading to nematic order was induced by shearing of CNC suspension (Chowdhury et al. 2017; Edgar and Gray 2003; Khelifa et al. 2013; Parker et al. 2018). The nanometer and micrometer thick films were prepared with CNCs aligned in the shearing direction, also with the assistance of an electric field (Csoka et al. 2011; Diaz et al. 2013; Hoeger et al. 2011). In turn, shearing during spin coating resulted in the radial orientation of CNCs from the centre of the sample (Edgar and Gray 2003).

However, the preparation of CNCs films with the order on a macroscopic scale is still a problem.

We hypothesized that it would be possible to orient CNCs using a zone-casting technique. The zone casting technique was developed to obtain from solutions highly oriented networks of the molecular metal, tetrathiotetracene-tetracyanoquinodimethane (TTT-TCNQ), embedded in a polymer matrix (Tracz et al. 2007). During the casting process, the solution was continuously supplied through a flat nozzle onto a moving substrate. Appropriate rates of solvent evaporation and solution supply resulted in a stationary gradient concentration within the meniscus, as shown in Fig. 1, which enforced directional crystallization. To achieve stationary conditions, a solution supply rate, substrate velocity, initial solution concentration, solvent evaporation rate, and crystallization rate had to be chosen properly. The last two parameters could be influenced by a choice of solvent and a control of casting temperature. Depending on the casting conditions, different crystal orientation in the obtained layers was achieved (Tracz et al. 2007). This method was used in the past to obtain films of materials with structures oriented over large areas (Makowski et al. 2014; Tracz et al. 2007). Many materials were efficiently processed for an organic field-effect transistor (Duffy et al. 2008; Pisula et al. 2005), organic photovoltaic devices (Liu et al. 2013), and even thin films of diblock copolymer with a large-scale alignment of lamellae (Tang et al. 2005). In each case, the oriented structure was formed from a solution of the respective compound. So far, the zone casting technique was not used to obtain films with oriented crystals from their suspension.

Fig. 1
figure 1

Scheme of zone-casting

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In the study, the zone-casting method was used to achieve a large-scale orientation of CNCs. By optimization of the casting conditions of CNC aqueous suspension, the transparent film with the uniaxial orientation of crystals perpendicular to the casting direction was obtained. During the classical zone-casting, the crystal orientation is achieved through a crystallization process, whereas during the process with CNC suspension the ordering occurred at large length scales through alignment of the nanocrystals. The orientation of the nanocrystals was confirmed by polarized light microscopy, X-ray diffraction, and atomic force microscopy. The film exhibited optical anisotropy, with the strongest transmission of light polarized in the direction perpendicular to the crystal orientation direction.

Experimental

Materials

8 wt.% aqueous suspension of CNCs (BGB Ultra), produced with a transition metal-catalyzed oxidative process from a viscose grade dissolving pulp (wood source aspen and maple) was purchased from Blue Goose Biorefineries Inc. (Canada). According to the supplier (Blue Goose Biorefineries Inc. 2020), CNCs were of type I, with a crystallinity degree of 80%, determined by the Segal method, and able to form a chiral nematic network in water. The crystals were 100–150 nm long with transverse sizes of 9–14 nm, as determined by transmission electron microscopy. The carboxyl content was 0.15 mmol/g, and sulfate half ester moieties were absent.

Cover glasses, 35 mm × 50 mm, from Fisher Scientific (United Kingdom) were used as substrates for the films. The glasses were cleaned in distilled water for 10 min using an ultrasonic bath, then rinsed with ethanol and dried under nitrogen flow.

Film casting

The CNCs-based films were prepared using the zone casting method as shown schematically in Fig. 1. The CNC aqueous suspension was diluted with water to obtain a concentration of 2.5 mg/ml, and sonicated in an ultrasonic homogenizer Hielscher UP 200S (Germany) at 200 W, 15% amplitude, frequency 24 Hz at room temperature for 30 min.

To measure the ζ-potential of CNCs, the suspension was diluted with deionized water and admixed with an aqueous NaCl solution to obtain 0.05% CNC and 5 mM NaCl concentration. pH was varied by the addition of NaOH. The ζ-potential values are shown in Table S1 in Supplementary Information (SI). The values obtained for the CNC suspension ranged from −26 to −35 mV, and were similar to those provided by the producer (Blue Goose Biorefineries Inc. 2020).

The casting conditions were optimized to achieve a continuous and stable process allowing to obtain 2 μm thick films, 4 cm long and 2 cm wide, with oriented CNCs. The temperature of the aqueous CNC suspension and CNC concentration were varied from 25 to 60 °C and from 1 to 4 mg/ml, respectively. In addition, substrate movement rates ranging from 5 to 9 μm/s were tested. Too low or too high suspension temperature resulted in the lack of CNC orientation due to the unstable meniscus. Insufficient CNC concentration resulted in too thin and discontinuous layers. Excessive concentration caused self-assembly of CNCs starting during deposition, whereas too slow substrate movement resulted in too small meniscus, both worsening the uniaxial orientation.

Finally, at the optimized conditions, the films with uniaxial orientation of CNCs were obtained. The suspension with a CNC concentration of 2.5 mg/ml, kept at 50 °C, was pushed by the piston of a 1 ml injector to flow through a slit-like nozzle at a constant rate of 1.3 × 10−4 ml/s. The suspension was deposited on the glass substrate, also kept at 50 °C, and moving at a rate of 9 μm/s. The evaporation of water resulted in the formation of the solid CNC film.

Characterization

The zone-cast films were examined with polarized light microscopy (PLM) under a Nikon Eclipse E400 Pol microscope equipped with a SANYO VCC-3770P camera.

The crystalline structure of the films was studied with wide-angle X-ray diffraction (WAXD), using an X-ray diffractometer coupled to a sealed-tube source of filtered CuKα radiation (λ = 0.154 nm), operating at 40 kV and 8 mA, PANalytical XRD Aeris from Malvern Panalytical (United Kingdom). 2θ scans were collected in a reflection mode with a step of 0.02°.

Moreover, the orientation of CNCs in the films was investigated with 2D-WAXS in a transmission mode. Malvern Panalytical (the Netherlands) diffractometer was used, with nickel-filtered CuKα operating at 50 kV and 30 mA. The 2D-WAXS patterns were collected with a Pilatus 100 K detector from Dectris (Switzerland).

The morphology was investigated using atomic force microscopy (AFM) in a tapping mode at room temperature. Images were recorded under an ambient atmosphere using a Nanoscope IIIa MultiMode from Digital Instruments (USA), with a scanner sampling resolution of 512 × 512 data points. Probes with rectangular silicon cantilevers, RTESP from Veeco (USA) were used, with a nominal radius of curvature of 7–10 nm, a spring constant of 20−80 N/m, and a resonance frequency between 264 and 369 kHz. Image analysis was performed using SPIP Image Metrology software (Denmark). The thickness of the films was determined with AFM in a height mode. Three filmswere measured in at least three places to determine an average value.

Results and discussion

2 cm × 4 cm films with CNC uniaxial orientation were obtained. The thickness of CNC films, measured with AFM, was 2 µm, as illustrated in Fig. 2a. The film thickness was uniform and varied no more than ± 0.1 μm.

Fig. 2
figure 2

a AFM height profile of edge of CNC film recorded to measure film thicknesses, b photograph of CNC film

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The cast films were transparent, as shown in Fig. 2b. The micrographs shown in Fig. 3 evidence orientation of CNCs. The film was birefringent and exhibited strong optical anisotropy. The incident beam, polarized in the direction parallel or perpendicular to the casting direction (CD), was extinguished by the analyzer. The maximum intensity of transmitted light was observed at 45° angle between CD and the analyzer and polarizer axes, as is also shown in Fig. 3.

Fig. 3
figure 3

PLM micrographs of CNC film differently oriented in respect to CD indicated by arrows

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WAXD curve of CNC film, in Fig. S1 in SI shows two overlapping peaks at 2θ of 14.75° and 16.5°, and a peak at 22.5° as it was observed for the same CNCs by others (Delepierre et al. 2021). These peaks are typical of the I form of cellulose crystals. 2D-WAXD pattern of CNC film is shown in Fig. 4a. The equatorial reinforcement of the 22.5° reflection evidences the orientation of CNCs in the film with the planes containing chain axes [(110)Iα and (200)Iβ (Kim et al. 2013)] perpendicular to the CD. The Hermans orientation factor calculated for this reflection was 0.7. The AFM image presented in Fig. 4b confirms the CNC orientation perpendicular to CD. Exemplary AFM height profiles are shown in Fig. S2 in SI. The mean transversal size of CNCs was 15.6 nm. In turn, Fig. S3 in SI shows exemplary vector field image, obtained using the OrientationJ plugin for ImageJ software, confirming the orientation of CNCs in the films perpendicular to CD. The coherency (C) was calculated with this software, based on the eigenvalues of the so-called structure tensor, which elements are convolutions of a tapering function and products of partial derivatives of the brightness function (Puspoki et al. 2016). The measurements were conducted for three films and the results were averaged. The details of the calculation of C are given in SI. The obtained value of C was 0.82. C is 1 when the local structure is oriented in one direction and C is 0 if the image is isotropic, thus the value of 0.82 confirms the good orientation of the nanocrystals.

Fig. 4
figure 4

2D-WAXD pattern (a) and AFM height image (b) of CNC film. Arrows indicate CD

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In general, the ability of CNCs to form stable suspensions and their ability to self-organize are governed by the orientation dependent attractive interactions due to van der Waals forces and the repulsive interactions either steric or electrostatic. The alignment of CNCs, favored by a higher concentration, results from a compromise between the rotational and translational entropies of individual nanocrystals and electrostatic effects (Parker et al. 2018).

The orientation of CNCs occurred during the zone-casting process due to the concentration gradient, resulting in the highest concentration in the vicinity of the zone of film formation.

In addition, it was undoubtedly influenced by the shape anisotropy of CNCs. Most research on evaporative film formation has focused on CNCs stabilized by strong acid sulfate half ester surface groups, which have a strong tendency to form chiral nematic films. Here, the CNCs used were stabilized by weak carboxyl groups, which minimized the tendency to form chiral nematic structures, and facilitated the observed uniaxial orientation. Moreover, the low CNC concentration, low flow rate, and low substrate movement rate permitted to avoid any shear-induced orientation along the motion direction, again facilitating the observed orientation.

One can expect that the transmission of light may depend on its polarization direction in respect to the orientation of CNCs in the film. Indeed, Fig. 5 shows a dependence of the intensity of light transmitted through the CNC film on an angle between the orientation direction of the crystals and the incident beam polarization direction. The transmission was the strongest for the light polarized in the direction perpendicular to the orientation direction of the crystals, that is in the CD.

Fig. 5
figure 5

Dependence of transmitted light intensity on its polarization direction in respect to orientation direction of CNC in film

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Conclusion

8 cm2 CNC films, 2 µm thick, with uniaxially oriented nanocrystals were obtained from CNC aqueous suspension with the zone-casting technique, in an environmentally friendly process. Optimization of the zone-casting conditions permitted us to achieve the uniaxial orientation of the nanocrystals and the continuity and reproducibility of the production process of the novel films with the preserved crystal orientation on a macroscopic scale. It must be emphasized that there is a difference between the classical zone-casting process, where the crystal orientation was achieved through a crystallization and the process with CNC suspension, where the ordering occurs at larger length scales through alignment of the nanocrystals. It is worth mentioning that the zone-casting process was continuous and stable, and much longer films can be easily obtained on substrates of appropriate length. Moreover, the zone-casting was a one-step process on the glass substrate, which did not require any special preparation, and the obtained films did not require post-casting drying to remove water.

The uniaxial orientation of nanocrystals in the films perpendicular to the zone-casting direction was evidenced by PLM, AFM, and 2D-WAXD. The films were transparent and exhibited optical anisotropy. The transmission of light depended on its polarization direction in respect to the crystal orientation direction and was the strongest for the light polarized perpendicularly to the crystal orientation direction. The properties of the novel CNC films obtained in an environmentally friendly process make them interesting materials for optoelectronics.