Effect of A Limited Amount of D-Sorbitol on Pitch and Mechanical Properties of Cellulose Nanocrystal Films

: A cellulose nanocrystal (CNC) suspension can form liquid crystal ﬁlms with unique self-assembly behaviors. This gives CNC ﬁlms a special iridescence, which has potential in many aspects, but the brittleness of pure CNC ﬁlms limits their application. In this work, we propose a simple physical mixing method to obtain CNC ﬁlm by adding D-sorbitol as a plasticizer. We ﬁrst found that low D-sorbitol content (less than 6 wt% in CNC/DS composite solution) did not make a signiﬁcant difference compared with pure CNC ﬁlms in optical performance and, at the same time, the mechanical properties of the CNC ﬁlms were improved. The various low contents of D-sorbitol can be well dispersed in CNC aqueous suspension, and the wavelength of the selectively reﬂected phenomenon is relatively stable and slightly decreased at 5 nm for concentrations from 0 to 6%. This phenomenon is opposite to that generally reported, where the wavelength of the selective reﬂected phenomenon increases obviously with the increase in plastic content. The pitch of the chiral structure decreased from 406 to 362 nm with an increase in D-sorbitol concentration. When the content of D-sorbitol reached 4%, the tensile strength, elongation at break, and Young modulus increased to 39.9 Mpa, 3.00%, and 2.99 GPa, respectively.


Introduction
Liquid crystals (LC) can be regarded as a mesophase. They exhibit some degree of isotropy, like a liquid, and can maintain the order of crystal in at least one direction, so anisometric molecules only have orientation order but not position order [1]. The LC assembly behavior results in corresponding optical properties [2], and they show chiral or achiral structures in nature [3][4][5][6]. These characteristics have been considered as a compelling discipline and applied in many optical-related fields [7][8][9][10].
Many substances can form liquid crystal phases in daily life, and they have special properties due to the structure of liquid crystal molecules. Compared with petrochemical resources, cellulose as a green, renewable, and cheap material has aroused widespread interest [11,12]. Cellulose nanocrystals (CNC), some of the most versatile and abundant biomaterials, are derived from natural cellulose [13,14]. The CNC suspension can selfassemble to form left-handed chiral liquid crystals when it reaches a certain critical concentration [15,16]. This helix structure can be maintained in the dried film through the slow evaporation of water [17]. Thus, the CNC solid film can exhibit unique optical properties, and when the incident light wavelength is in the visible range, the material appears colorful to the naked eye [18,19]. To date, the research on this optical film has aroused far-ranging interest, such as in optical anti-counterfeiting, sensors, and some composite devices [20][21][22][23][24]. However, the pure CNC film is fragile, which seriously limits its application field and value [25]. Therefore, it is important to overcome this shortcoming, while also maintaining its optical properties.
In recent years, there have been many studies conducted on the optical characteristics of flexible CNC films. Tatsumi et al. (2012) used 2-hydroxyethyl methacrylate (HEMA) monomer mixed with CNC suspension to enhance the mechanical properties. However, the pitch of the composite film is too large, which leads to the color of the film being unable to be directly observed by naked eyes [26]. He et al. (2018) reported the fabrication of CNC films by introducing glycerol as an additive. However, with the increase in glycerol concentration, the wavelength of the selectively reflected obviously increases [27]. Compared with these additives, D-sorbitol, as a green and nontoxic non-solvent plasticizer, is usually used in starch-based materials [28,29]. The six hydroxyl groups in D-sorbitol can act outside the crystalline area of CNC films and interact with hydroxyl groups that can be contacted on the CNC surface. Csiszar et al. (2017) proved that the plasticizing behavior of D-sorbitol plasticizers in CNC films is very similar to that of starch film, as reported previously [30]. However, they mainly explored the mechanical properties of CNC composite films with a higher content of gradient-increased D-sorbitol but did not study the influence of D-sorbitol on the pitch of the CNC spiral structure and the optical properties of CNC composite films under low limited content.
In the present work, we controlled the content of sorbitol in CNC/DS composite solution below 6 wt%, and we found that sorbitol can improve the flexibility and tensile strength of CNC films without destroying the self-assembly behavior of CNC. The various contents of D-sorbitol can be well dispersed in CNC aqueous suspension, and the wavelength of the selectively reflected phenomenon is opposite to that generally reported, where the wavelength of the selective reflected phenomenon increases with the increase in plasticizer concentration. The pitch of the chiral structure decreased from 406 to 362 nm with the increase in D-sorbitol content.

Preparation of the CNC Suspension
CNC aqueous suspension was prepared from MCC by sulfuric acid hydrolysis (Gebauer et al. 2011). In detail, 5 g MCC was hydrolyzed in 64% sulfuric acid solution (88 mL) for 70 min at 50 • C. The reaction suspension was diluted 10 times with deionized (DI) water and dialyzed against DI water for several days until the pH was close to 6.0-7.0. The solid content of the prepared CNC suspension was about 1.0 wt%. The as-prepared CNC suspension was condensed to 6.0 wt% as the final used concentration.

Preparation of Pure CNC and CNC/DS Films
The pure CNC and CNC/DS films were all prepared from CNC aqueous suspension with a concentration of 6.0 wt% and cast into a polytetrafluoroethylene (PTFE) disk with a diameter of 60 mm. The pure CNC film was dried at 25 C for 4-7 days. The CNC/DS films were formed by adding various amounts of 10 wt% D-sorbitol solution and prepared by a similar process. The D-sorbitol percentage in the modified CNC film was 2%, 3%, 4%, 5%, and 6%, the corresponding materials marked as CNC/DS2, CNC/DS3, CNC/DS4, CNC/DS5, and CNC/DS6. The thicknesses of all the films were approximately the same, at 0.08 mm.

Characterization
The mechanical properties of samples were tested by a servo material multifunctional high and low temperature control testing machine (AI-7000-NGD, Goodtechwill). Before testing, samples were cut into 30 mm × 5 mm strips. The stretching speed was 10 mm/min under 0.01 N, and the working distance was 10 mm. The test was repeated 5 times for each sample and the average value was taken. UV-vis spectra of samples were measured with an ultraviolet-visible spectrophotometer (Cary 5000, Agilent) from 300 to 800 nm. The sample interval was 1 nm, and the scanning speed was 300 nm/min. The samples' optical characteristics were observed using a polarized optical microscope (POM, SMART-POL, OPTEC). The Bruker VERTEX 70 Fourier transform infrared spectrum (FT-IR) was used for testing, with a resolution of 2 cm −1 , scanning rate of 16 times/s, and a testing range of 400-4000 cm −1 . The X-ray diffraction (XRD) patterns of samples were measured by an X-ray diffractometer (D8 Advance, Bruker). The scanning rate was 6 • /min at a diffraction angle of 5 • to 50 • (2θ). An atomic force microscope (AFM, AFM5100, Agilent) was used in contact mode to test the mica sheet. Before the test, the CNC and CNC/DS suspensions were diluted 600 times, dropped on the mica sheet surface, and dried completely at room temperature. Cross-sections of the film were observed by placing samples sprayed with gold using scanning electron microscopy (SEM, S4800, Rigaku), and the acceleration voltage was 3 kV.  The XRD patterns of pure CNC and CNC/DS films are shown in Figure 2a. The diffraction peaks of these six films are almost same, and they all possess the characteristic of cellulose I peaks at 14.9 • , 16.5 • , and 22.7 • , which correspond to the (1-10), (110), and (200) crystal planes, respectively [31], indicating that the addition of D-sorbitol will not change the crystal structure of natural CNC films.  Figure 2b illustrates the pure CNC and CNC/DS films and D-sorbitol FT-IR spectra. The characteristic absorption peak of pure CNC and CNC/DS at~2900 cm −1 is the C-H stretching vibration of methylene [32]. The absorption peak at~1640 cm −1 corresponds to the vibration of adsorbed water molecules [33,34]. The absorption peak at~1050 cm −1 is attributed to the C-O-C pyranose ring skeletal vibration of cellulose [35]. Additionally, the O−H stretching within 3300−3500 cm −1 suggests the existence of hydrogen bonding. From the FT-IR spectra of Figure 2b, the hydroxyl peak at 3200-3500 cm −1 shifts towards a low wavenumber and becomes narrow, and its characteristic peak does not change. It was proven that with the increase in D-sorbitol content, the number of hydrogen bonds in the film increases, and there is no chemical reaction between them. Figure 3. All films have a bright color, manifesting a birefringence phenomenon of the pure CNC and CNC/DS films. It is also obvious that the CNC/DS films exhibit liquid crystal structures and there is no big difference in structure and color compared with pure CNC films. This indicates that the plasticizing modification of D-sorbitol does not destroy the inherent liquid crystal structure of the CNC/DS films.   (Figure 4a). The points in Figure 4b correspond to the values of maximum wavelengths of selective reflection in Figure 4a, and the line represents the trend of maximum wavelengths of selective reflection. The total trend gradually decreases. For the CNC/DS6 film, the wavelength of maximum selective reflection is at 376 nm, which is only a 5 nm shift compared with the pure CNC film (Figure 4b). Therefore, the D-sorbitol content has little influence on the wavelength of selective reflection compare with the pure CNC film; even in the low content range (less than 6 wt% in CNC/DS composite solution), the maximum reflection wavelength decreases with the increase in D-sorbitol content.  Figure 5c shows that the elongation at the break of CNC/DS films continues to increase with the increase in sorbitol content. By adding D-sorbitol, the elongation at break of CNC/DS2, CNC/DS3, CNC/DS4, and CNC/DS5 films is 1.13%, 2.46%, 3.00% and 3.48%, respectively. Additionally, the incorporation of 6% D-sorbitol obtained the elongation at break of 9.4%, which is much larger than that of the CNC/DS film with 5% D-sorbitol. This is because D-sorbitol, as a non-solvent plasticizer (called an auxiliary plasticizer), continuously enhances the plasticizing effect with the increase in its concentration, this resulting in an upward trend of the elongation at break.

Mechanical Properties of CNC/DS Films
Those results show that the addition of D-sorbitol can improve the tensile strength and deformability of CNC/DS films. Additionally, the combination of CNC and 4% D-sorbitol has the best effect on the mechanical properties of CNC/DS films. The tensile strength, elongation at break, and Young modulus are 39.9 MPa, 3.00%, and 2.99 GPa, respectively ( Figure 5).
The reason for this phenomenon may be that D-sorbitol forms hydrogen bonds with hydroxyl groups on CNC. With this increase in the content, hydrogen bonds also form between D-sorbitol. D-sorbitol content of 4% can properly combine with hydroxyl groups on CNC and form hydroxyl groups with D-sorbitol itself. When the content of D-sorbitol is higher than 4%, due to the increase in the content of D-sorbitol, the proportion of hydrogen bonds formed by themselves increases, which affects the formation of hydrogen bonds between D-sorbitol and CNC.

Morphology of the CNC and CNC/DS4 Film
The typical morphologies of CNC and CNC/DS4 are presented in Figure 6. The morphologies of the CNC and CNC/DS4 are the same, with the rod-like CNC nanoparticles being randomly oriented and dispersed in the suspension. In some places, CNC nanoparticles are piled up and connected with each other, resulting in slight local aggregation. The occurrence of this phenomenon may be caused by the small size, large specific surface area, and abundant hydroxyl groups on the surface of CNC during the drying process. It can be seen that the addition of the D-sorbitol does not affect the orientation and dispersion morphology of the CNC aqueous suspension, which ensures that CNC nanoparticles can normally self-assemble during film formation.  Figure 7 are SEM cross-section images of CNC, CNC/DS4, and CNC/DS6 films. Parallel and periodically arranged layers can be clearly seen from the cross-sectional view of the pure CNC film in Figure 7a, which indicates that the film is a chiral nematic structure. Additionally, the pitch (P) can also be obtained from this image (P/2 = 203 nm). Figure 7 shows the structure of the CNC/DS4 and CNC/DS6 films similar to the pure CNC film, and the obtained P/2 = 194 nm ( Figure 7b) and 181 nm (Figure 7c), respectively. It can be seen from the change in P/2 that the addition of D-sorbitol does not cause a significant change in the CNC structure and P; the P was even reduced by 44 nm with the increase in D-sorbitol concentration. This provides evidence for a blue shift in the wavelength of the selectively reflected phenomena.

Discussion
Scheme 1 shows incorporations of the CNC and D-sorbitol mechanism. The green shapes and red dots represent the CNC layer and the D-sorbitol molecule, respectively. The pure CNC film has intermolecular hydrogen bonds in the layer structure, which is due to the free hydroxyl groups on the surface of CNC. Adding D-sorbitol to CNC suspension does not produce a chemical reaction, but only provides more hydroxyl [27]. The addition of sorbitol in CNC solution will "capture" the intramolecular hydrogen bond in CNC and form the extramolecular bond between D-sorbitol and CNC, which leads to the increase in hydrogen bonds. These new hydrogen bonds provide more force In addition, due to the high initial concentration of CNC suspension, the already-existing liquid crystal phase in the suspension may prevent D-sorbitol from entering the inner layer of the layered spiral structure, so D-sorbitol molecules are mainly hydrogen bonded with groups on the surface of CNC at the edge of the layer. The hydrogen bond formed at the edge of the layer provides more force. This brings the CNC layers closer together and finally leads to a decrease in the pitch. More hydrogen bonds weaken the rigidity of CNC rods, thus improving the mechanical properties of CNC/DS films.

Conclusions
In summary, we successfully used D-sorbitol as a plasticizer to obtain CNC/DS films without changing the structure of CNC. The addition of D-sorbitol with a low concentration can improve the mechanical properties and have less influence on optical properties. The wavelength of the selectively reflected phenomenon decreased by 5 nm and the pitch of the chiral structure decreased from 406 to 362 nm, with the D-sorbitol concentration increasing up to 6%. The CNC/DS4 films can bend under a certain force and appear as a smooth surface, which solves the brittleness problem of pure CNC film. Therefore, the unusual pitch change after adding a small amount of sorbitol has potential value for the future application and development of CNC optical film materials.