2.1 Fabrication of the Hollow Pseudo Catenoid (PC) Structure

The detailed process for the preparation of the hollow PC structure was illustrated in Figure 1. This special structure was fabricated via mechanical stretching and then air-drying process of dynamic hydrogels, which were prepared by in situ polymerization of acrylamide in borax-NaOH buffer solution (pH = 10) with 3-(acrylamide) phenylboronic acid (PBAAm)/dopamine methylacrylate (DMA) complexes as dynamic crosslinkers (Figure 1b). CNCs with 90–300 nm in length and about 10 nm in diameter were prepared via TEMPO-mediated oxidation and could be added into the hydrogel precursor (Figure S1, Supporting Information). They contained carboxyl groups of 1.50 ± 0.02 mmol g⁻¹ and aldehyde groups of 0.12 ± 0.02 mmol g⁻¹ on the surface, allowing them to be well dispersed in neutral or weak alkaline aqueous solutions for the preparation of dynamic hydrogels.^[20] The abundant hydroxyl and carboxyl groups on CNCs could interact with amide groups on polyacrylamide chains through hydrogen bonding (Figure S2, Supporting Information). Obtained dynamic hydrogels showed the typical shearing thinning property (Figure S3, Supporting Information), which is because of the quick temporary dissociation and reconstruction of PBAAm/DMA complexes.^{[20, 21]} This reversible transition of dynamic solution-gel states with corresponding tunable viscosities was maintained during the programming and shaping of dynamic hydrogels (Figure 1b). This property allowed the preparation of the hollow PC structure by using hydrogel tubes via mechanical stretching. During the further long-time drying process of stretched hydrogel tubes, the evaporation of water drove shrinking hydrogels with smaller volumes, and the surface tension led to the smallest surface area in the resulted xerogels. Indeed, this air-drying process is a self-organization process of the hydrogel tube under certain strain stress to promote the spontaneous formation of the hollow PC structure. Figure 1c presents the typical hydrogel tube (containing 2 m acrylamide with PBAAm/DMA complexes and 2 wt% CNCs) after stretching with the elongation ratio (λ) of 2 and hollow PC2/2 xerogel after air drying. With the same method, more transparent PC0/2 xerogel without CNCs also can be achieved. (PCx/y, x means the weight percent of CNCs in the dynamic hydrogel; y means elongation ratio (λ)).

2.2 Regulation of the Morphology of Hollow PC Structure and their Optical Property

There are two key factors that affected the formation of the PC structure. One is the hollow geometry of the precursor hydrogel tube, which is critical for the air-drying process. Generally, the drying of bulk hydrogels starts from the surface and proceeds slowly to the interior, while the shape change is largely governed by internal stress and surface tension. The influence of these parameters can be strongly offset within the hydrogel tubes, because the water loss from the thin hydrogel wall will happened on both inside and outside at the same time. Another key factor is the appropriate morphology control. During the stretching process of the dynamic hydrogels, the tensile stress increased dramatically up to λ = 3 and increased slowly with higher λ (Figure 2a). Therefore, the morphology control of PC structures will be highly affected during the stretching process. Although the hollow shape could favor the formation of PC structure, the wall of hydrogel tubes still has a certain thickness. This thickness is generally much higher than the soap bubble layer (Figure 1a) and can generate different curvatures of the inside and outside surface of the tubes. Compared to the dimension of hydrogel tubes, the wall thickness needs to be small enough to minimize its influence, especially for the structures with higher elongation ratios. Consequently, the morphology of PC0/10 xerogel with much larger outlayer diameter (D = 26 mm) and comparably much smaller wall thickness (T = 2 mm) was more optimized than that with D/T = 13/2 (Figure 2b). Besides, higher elongation ratio than λ = 2 with the same diameter/thickness (D/T) ratio drove the structure away from the ideal catenoid shape. Consequently, the larger D/T ratio and appropriate elongation ratio led to hollow PC structures with a concaved surface approaching the ideal catenoid.

Thanks to the mechanical stretching process during the fabrication of the PC structure, the CNCs in the dynamic hydrogels can be unidirectionally aligned, leading to vivid birefringence phenomenon and providing the additional optical property for this structure. Moreover, the subsequent air-drying process further promoted the orientation of CNCs within the polymer matrix. As shown in Figure 2c, the hydrogel tube of PC1/10 after mechanical stretching presented light yellow interference color between the crossed polarizers. After air-drying, the interference colors of the xerogel became more obvious (blue), which shows further alignment of CNCs in addition to the optimization of the curved surface during the air-drying process. Besides, PC structure with different CNCs contents showed different birefringence as shown by PC1/10 and PC2/10 (red), indicating the different alignments of CNCs within the xerogels.

Importantly, larger elongation ratios (λ) of stretched dynamic hydrogels could promote the alignment of CNCs in the xerogels, generating more vivid birefringence colors. As verified in Figure 2d,e, the PC structures showed much stronger angle dependence in the diffraction patterns by increasing the elongation ratios from 1 to 10. The corresponding orientation indexes of CNCs in the PC xerogels increased from 0.66 to 0.89, showing enhanced alignment of CNCs. Only with sufficient elongation ratio of larger than λ = 2, the PC structure contained well aligned CNCs and exhibited adjustable birefringence, generating diverse vivid colors of obtained PC xerogels (Figure S4, Supporting Information). Therefore, the optical property of the PC structure can be a visual evidence of the alignment of CNCs, which is highly affected by the contents of CNCs and the elongation ratio.

2.3 Mechanical Properties of the Pseudo Catenoid Structure

In general, the alignment of CNCs within the materials could highly affect the mechanical property.^{[22, 23]} Herein, the mechanical properties of PC structure with various CNCs contents and elongation ratios were investigated.

On the one hand, stronger stretching of initial hydrogels tubes will largely reduce loading capability of the resulting PC xerogels, even though the orientation index of CNCs within the structure enhanced (Figure S5, Supporting Information). As shown in Figure 3a,b, the ultimate fracture forces of the PC xerogels without CNCs or with 2 wt% CNCs decreased dramatically with increasing λ. The specific parameters of PC xerogels with various λ were presented in Table S1 (Supporting Information). When λ = 1 and 2, the ultimate fracture force of the PC xerogels was not significantly affected by the CNCs contents (Figure 3a,b,d), indicating the decisive role of geometry in the xerogel tubes obtained with low elongation ratios. Interestingly, the concave surface can transfer the vertical deformation to the confinement toward the center of PC. Therefore, all the rupture of PC xerogels without CNCs were broken toward the inside of the tubes, as illustrated in Figure 3c. On the other hand, CNCs will reinforce the mechanical properties of the xerogels, and switch the plastic rupture to the brittle rupture mode, as shown in Figure 3b. In particular, CNCs effectively improved the ultimate fracture force in the largely stretched PC xerogels with λ = 5 or 10 (Figure 3d), but little enhancement in the short xerogel tubes (λ = 1, 2). Furthermore, CNCs also enhanced the Young's modules in the largely stretched PC structures, which can be verified from the slopes of the curves in Figure S6 (Supporting Information), showing the enhanced stiffness in PC xerogels containing CNCs. In addition, the brittle rupture on one side of PC xerogel with CNCs (Figure 3c) also demonstrated the enhanced stiffness by CNCs incorporation. The enhanced mechanical property of largely stretched PC structure can be attributed to the enhanced alignment of CNCs (caused by increased CNCs contents) within the xerogels, which can compensate the geometry disadvantage resulted by strong stretching.

Consequently, the mechanical property of PC structure can be divided into two stages. As illustrated in Figure 3d, the first stage was controlled by the inherent geometry of PC structure, while the second stage was mainly taken over by the contents of aligned CNCs. Figure 3e presented the fracture energies (G_f) of PC xerogels. Herein G_f was defined as the work needed in creating the fracture surface. Generally, G_f decreased with increasing λ regardless of the presence of CNCs, indicating the gradually weaker mechanical strength caused by larger λ. However, G_f of PC xerogels containing CNCs were much higher than that without CNCs for PC with larger λ. Higher G_f further verified the effective enhancement of the mechanical strength by aligned CNCs and this alignment further compensated the morphological defect caused by the larger λ. Therefore, the mechanical properties of PC xerogels were primarily controlled by geometry (small λ) and contents of aligned CNCs (large λ), respectively. In addition, PC xerogels with λ = 2 seem to be highly suitable parameters for PC structures, considering their good mechanical property, alignment of CNCs and continuous PC morphology.

The curvature surface of hollow PC generated by the effects of tension and self-organization of CNCs could give rise to the special mechanical performance. For better understanding the geometry of PC structure, the cylinder xerogels without tension were prepared, where water loss only happened on the outside of hydrogel tube (Figure 4a). Herein, the finite element analysis was employed for better illustrating the stress distribution and deformation of various samples. As shown in Figure 4b-c, the catenoid samples could avoid the stress concentration in local region compared with cylinder samples, endowing the catenoid samples with higher elongation at break and better energy dissipation ability. This result also can be demonstrated by mechanical test. As shown in Figure 4d, the cylinder xerogels with/without CNCs exhibited much higher ultimate fracture forces than PC, but their compression curves presented several turning points before the structure completely broke or the force disappeared, as demonstrated in Figures S7 and S8 (Supporting Information) for the details. Besides, the cylinder xerogels without CNCs presented the typical yield point before it completely broke, as shown in Figure 4e. The bottom of the cylinder xerogels became irregular which caused many turning points on the curves during compression. After incorporated with CNCs, the cylinder structure almost maintained its original shape during the compression process, and the structure presented a brittle rupture in the vertical direction, thereby exhibiting higher stiffness than that without CNCs. Especially, compared with cylinder structure without CNCs, the curve of cylinder xerogels containing CNCs tended to exhibit a small rupture around 250 N due to the enhanced stiffness caused by CNCs. These small yield points of cylinder xerogels can be adapted by themselves during the compression process, leading to the higher ultimate fracture forces. This can be mainly attributed to the vertical surface of cylinder that could bear larger force along the axis than the curvature surface of PC. Nevertheless, these results indicated that the cylinder xerogels suffered from an irreversible and nonfatal deformation under this compression. To be strictly accurate, the fracture energy of cylinder xerogels will be calculated before the first yield point of the compression curves, instead of the highest ultimate fracture force. Consequently, the fracture energy G_f of cylinder xerogels didn't present more advantages than PC, especially the one with CNCs (Figure S9, Supporting Information).

In addition to this first-cycle static compression process, PC geometry presented a distinct advantage during the multiple compression cycles. Figure 4f and Figure S10 (Supporting Information) showed the cyclic compression curves of as-prepared PC and cylinder xerogels at almost the same strain (≈5%). Without intervals between each compression cycle, the second hysteresis loop of cylinder xerogels became much smaller and narrower compared with those of PC. Cylinder structures were actually impaired after the first compression. Importantly, the third hysteresis loop of PC xerogel was almost overlapped with the second hysteresis loop, showing the stable mechanical property of PC structure. Accordingly, the hysteresis losses of three cycles for various samples have been presented in Figure 4g, with two key points that can be concluded. Firstly, from the perspective of geometry, like C0/1 and PC0/2, though cylinder structure showed higher hysteresis loss than PC in the first compression cycle due to the much higher compression force, the hysteresis losses of its second and third cycles decreased dramatically. Instead, PC geometry demonstrated a better energy dissipation system, because the hysteresis losses of the second and third compression cycles for PC geometry were higher than that of cylinder geometry, and the hysteresis losses of the second and third compression cycles for PC can keep about 50% of the first cycle, while for cylinder geometry, they only maintained about 10% of the first cycle. In addition, this more efficient energy dissipation system of PC structure also can be verified in Figure 4h. The energy dissipating efficiency of cylinder xerogels C2/1 was only less than 5%, while the energy dissipation of PC2/2 was about 40%. Thence, the PC geometry shows a better energy dissipation system by relaxing most of compression energy. This should be due to the fact the inwardly curved surface of PC structure can transfer the laterally inward extrusion to resist the vertical deformation (Figure 4a–c), leading to the better energy dissipation. Secondly, different than the geometry, the incorporation of reinforcing stiff CNCs did not favor the energy dissipation in both PC and cylinder structures. Specifically, the hysteresis losses of the three cycles for cylinder structure with CNCs was much lower than that without CNCs. Moreover, CNCs only slightly increased the hysteresis loss of PC xerogels, showing little effects of CNCs on energy dissipation. In a word, though the PC geometry did not bear as much compression force as cylinder structures did during the first compression, it showed more efficient energy dissipation and can stably withstand loading of multiple cycles.

4.1 Materials

Microcrystalline cellulose (MCC), 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), 3-(acrylamido) phenylboronic acid (PBAAm), methacrylic anhydride, sodium hydrochloride, phosphotungstic acid hydrate, 2,4,6-trimethylbenzoyl chloride were purchased from Sigma-Aldrich (Germany). Dopamine hydrochloride, dimethyl phenylphosphonite, and 2-butanone were obtained from Alfa Aesar (Germany). Acrylamide, sodium sulfate anhydrous, disodium tetraborate were bought from Merck KGaA (Germany). Sodium hydroxide was received from VWR (Germany). The borax-NaOH buffer (pH = 10) was bought from TH Geyer (Germany). All solvents used as received were supplied by TH Geyer (Germany). Dopamine methylacrylate (DMA) and lithium phenyl-2,4,6-trimethylbenzoyl-phosphinate (LAP) were synthesized according to previous reports.^{[24, 25]}

4.2 Preparation of CNCs

4.3 Characterization

The tensile test of dynamic hydrogels and the compression tests of xerogels were carried out on a Z3 micro tensile test machine from Grip-Engineering Thümler GmbH. The rheological properties of hydrogels were investigated with a stress-controlled bulk rheometer (Anton Paar MCR501) by using a 25 mm parallel-plate setup at room temperature (23 °C). Storage modulus (G′), loss modulus (G″), damping factor, and complex viscosity were recorded. The wide-angle X-ray scattering (WAXS) of the xerogels was carried out at the Shanghai Synchrotron Radiation Facility (SSRF) on the BL16B beamline with an X-ray wavelength of 0.124 nm. The orientation index (π) of CNCs was calculated according to the method reported in the previous work.^[21]