Electrical switching of high-performance bioinspired nanocellulose nanocomposites

Nature fascinates with living organisms showing mechanically adaptive behavior. In contrast to gels or elastomers, it is profoundly challenging to switch mechanical properties in stiff bioinspired nanocomposites as they contain high fractions of immobile reinforcements. Here, we introduce facile electrical switching to the field of bioinspired nanocomposites, and show how the mechanical properties adapt to low direct current (DC). This is realized for renewable cellulose nanofibrils/polymer nanopapers with tailor-made interactions by deposition of thin single-walled carbon nanotube electrode layers for Joule heating. Application of DC at specific voltages translates into significant electrothermal softening via dynamization and breakage of the thermo-reversible supramolecular bonds. The altered mechanical properties are reversibly switchable in power on/power off cycles. Furthermore, we showcase electricity-adaptive patterns and reconfiguration of deformation patterns using electrode patterning techniques. The simple and generic approach opens avenues for bioinspired nanocomposites for facile application in adaptive damping and structural materials, and soft robotics.

This manuscript contributed by Jiao et al. suggests a straightforward strategy to spatially control the mechanical properties of a composite material composing of cellulose nanofibrils (CNFs) as the fillers and copolymers with hydrogen bonding agent UPys as the matrix that is coated with carbon nanotubes to generate heat under direct current. The elevated temperature results in softening of the hybrid material with distinct mechanical performances such as breaking stress and stress relaxation, when compared to that at room temperature. Sophisticated control of the mechanics is achieved by using selective gating of the electrode patterns prepared by spray coating. Although embedding nano/micron wires has been used for electro-heating in various polymeric materials such as shape memory polymers and liquid crystalline elastomers/networks, combination of electrothermal effect and supromolecular polymer materials, especially the spatially heating and programmed mechanics, is informative for the design of adaptive systems. The fabrication of the hybrid material and characterization/demonstration are completed. I think this work could be considered for publication in this journal. Other comments/suggestions are listed below.
--The tailored mechanics of the hybrid material is mainly attributed to the breaking and reformation of self-complemented UPy dimers. But in page 6 the authors mentioned the interaction at the CNF/polymer interface and CNF delinking. These interactions and the change with temperature should be characterized, which might be significant for the mechanical reinforcement of the fillers to the polymer matrix.
--Thermal-induced elastomeric-to-melt transition is mentioned at several places of the main text. If this is true, the material should experience plastic deformation during the stress relaxation test (Fig  2b). After turning off the electro-heating, the stress should not rise again. In Fig 2d the material at high temperature up to 120 oC can still maintain loading stress. These results suggest the material might be in a rubber state. This point should be considered.
--How does the content of CNF influence the mechanical properties of the hybrid materials?
--DSC is used to characterize the Tg of the copolymers, which are lower than -40 oC. But the materials are mechanically robust with extremely high Young's modulus. The state of polymer matrix should be carefully reconsidered. If it is in a soft rubber state, the hybrid material should be not so strong. I suggest extending the temperature range in DSC measurement to above 100 oC to check if there is thermal transition at ~60 oC. At room temperature the material with dense hydrogen bonds might be in a glassy state due to the presence of a large amount of UPy dimers. A key question is whether this concept provides stronger effect than heating of a polymer composite based on amorphous inorganic particles, or short (<1mm) fibers, where an amorphous thermoplastic polymer matrix goes from glassy to rubbery state ? Perhaps the nano fiber network structure and applications to films is the key, but a micro composite example may be helpful to clarify the advantage with the concept, in which way would such a material be inferior? For the patterning, and for films, it is apparent that a nanocomposite provides advantages, but is it feasible for thicker structures? What are some estimated thickness limitations, and how could they be overcome? There appears to be not so much materials science in this study but rather an emphasis of the "invention". Perhaps the materials science key should be the matrix. Has this concept been used before? Can the nanostructure, and mechanisms for the softening in the composite be given a stronger focus? Referee #1: This manuscript contributed by Jiao et al. suggests a straightforward strategy to spatially control the mechanical properties of a composite material composing of cellulose nanofibrils (CNFs) as the fillers and copolymers with hydrogen bonding agent UPys as the matrix that is coated with carbon nanotubes to generate heat under direct current. The elevated temperature results in softening of the hybrid material with distinct mechanical performances such as breaking stress and stress relaxation, when compared to that at room temperature. Sophisticated control of the mechanics is achieved by using selective gating of the electrode patterns prepared by spray coating. Although embedding nano/micron wires has been used for electro-heating in various polymeric materials such as shape memory polymers and liquid crystalline elastomers/networks, combination of electro-thermal effect and supromolecular polymer materials, especially the spatially heating and programmed mechanics, is informative for the design of adaptive systems. The fabrication of the hybrid material and characterization/demonstration are completed. I think this work could be considered for publication in this journal. Other comments/suggestions are listed below.
1. The tailored mechanics of the hybrid material is mainly attributed to the breaking and reformation of self-complemented UPy dimers. But in page 6 the authors mentioned the interaction at the CNF/polymer interface and CNF delinking. These interactions and the change with temperature should be characterized, which might be significant for the mechanical reinforcement of the fillers to the polymer matrix.
In page 6, we state that the inclusion of UPy motifs leads to promoted interactions in the polymer phase as well as at the CNF/polymer interface, allowing stiffening and strengthening in mechanical properties of nanocomposites. In principle, the UPy motifs share the ability to form hydrogen bonds with the different CNF surface groups.
While we believe that the thermo-reversible de-linking is most efficient in the matrix, we cannot exclude that interactions also occur and change at the interface -this is why we point to them. However, we suggest that the bulk phase is the major player because the thermal transition in the composite coincides rather well with the transition found in pure bulk. Unfortunately, changes at the interface cannot be characterized, e.g. spectroscopically, as the abundance of different interactions leads to non-specific or non-selective bonding. We had attempted this before (eg. Using FTIR). It is also not possible to go into the direction of single fiber pullout measurements, as the CNFs are nanoscale. In summary, although the reviewer raises an interesting point, the requested data can to the best of our understanding not be obtained.
We do however not think that this is a negative point for the overall understanding of the material system concept. We added a comment that the interactions may also change at the interface, but that this can unfortunately not be analyzed (page 6).
2. Thermal-induced elastomeric-to-melt transition is mentioned at several places of the main text. If this is true, the material should experience plastic deformation during the stress relaxation test (Fig 2b). After turning off the electro-heating, the stress should not rise again. In Fig 2d the material at high temperature up to 120 oC can still maintain loading stress. These results suggest the material might be in a rubber state. This point should be considered.
The elastomeric-to-melt transition exclusively refers to the polymer (please see also new Sup. Fig. 4 for photographs); the bioinspired nanocomposite can of course not enter a melt phase due to the high fractions of reinforcements (50 wt%). We checked the position in the manuscript again and believe to be accurate in this statement.
Here we are dealing with highly-reinforced nanocomposites with 50 wt% of CNFs. The polymers are nanoconfined in the CNF network. When the polymers are molten at high temperature, the CNF network is still holding the overall structures, thus maintaining the loading stresses. The polymer melt provides nanoscale lubrication for CNF network leading to strong relaxation. Once the electro-heating is turned off, the stress increases again, which is linked to the reassociation of the hydrogen bonds and potentially thermal contraction during cooling. We added this on page 9.

How does the content of CNF influence the mechanical properties of the hybrid materials?
The inclusion of CNFs within the nanocomposites leads to substantial stiffening and strengthening, and a less ductile behavior. The details have been reported in previous articles and in summarized in previous reviews. (Adv. Funct. Mater. 2019, 1905309;Acc. Chem. Res. 2020, 2742-2748J. Mater. Chem. A, 2017,5, 16003-16024). We added a sentence to the MS to guide the reader to this literature (page 6).
4. DSC is used to characterize the Tg of the copolymers, which are lower than -40 oC. But the materials are mechanically robust with extremely high Young's modulus. The state of polymer matrix should be carefully reconsidered. If it is in a soft rubber state, the hybrid material should be not so strong. I suggest extending the temperature range in DSC measurement to above 100 oC to check if there is thermal transition at ~60 oC. At room temperature the material with dense hydrogen bonds might be in a glassy state due to the presence of a large amount of UPy dimers.
We believe there is a misunderstanding in polymer characterization and bioinspired nanocomposite characterization.
DSC shows a Tg at -46 °C (polymer bulk); polymer bulk rheology shows a dissociation of the UPy dimers at around 62 °C (crossover of G' and G'') which is associated from the rubbery state to the melt. Note that rheology in Sup. Fig. goes to 120 °C, and would also reveal any further transitions. This is the polymer level characterization. Due to the limited bonding strength of non-flanked UPy units (not flanked by urea, so they cannot crystallize into higher ordered structures, see work by Sijbesma and Meijer), they cannot crystallize, and simple supramolecular dissociation is too weak to be observed in DSC. Mechanical characterization is more sensitive, as it measures a different observable. That is why we use these complementary techniques. As requested by the reviewer performed DSC up to a 100 °C, but the line remains of similar slope in above the Tg as expected. (in Supplementary Fig. 2e).
Due to the inherent dynamics of UPy/UPy dimers, and the use of a low Tg backbone material, the material at room temperature is an elastomer and not a glass (Tg ≪ RT). We further added photographs to new Sup. Fig. 4. The polymer is elastomer at room temperature; when heated it turns into a melt (rheology clearly shows that).
The high Young's modulus in the composite arises from the inclusion of 50 wt% CNF in the bioinspired nanocomposites. Once in the nanocomposite state, the elastomer to melt transition of the polymer only translates into better lubrication of the CNF network and allows for easier movement and softening and toughening. The bioinspired nanocomposite cannot melt. Fig 4 should experience plastic deformations. It should be meaningful if the material can restore to the original state, so that it can be repeatedly tailor the mechanical properties. Is it possible to form a lightly crosslinked network by some covalent bonds? In addition, scale bars should be added to Fig 4. No, this is not possible, because the bioinspired nanocomposite is not a rubber and cannot be transformed into a composite rubber at such high fractions of entangling nanoscale reinforcements (50 wt% CNF). The use of high fractions of reinforcements is an essential criterion for bioinspired nanocomposites (Acc. Chem. Res. 2020, 2742-2748. The plastic deformation in bioinspired nanocomposites or CNF/polymer nanopapers occurs by realignment and frictional sliding of the CNF nanofibrils, not by exclusive polymer deformation. Hence such materials based on entangling nanofibrils cannot recover the original state, to our opinion no matter what kind of molecular engineering would be done to the soft matrix. The CNF network cannot relax back to its original position after inelastic deformation and disentanglement on a slow colloidal length scale.

The hybrid materials in
We added the scale bars in Fig. 4. We added the method in SI (Supplementary Fig. 6). Fig 3, temperature sweep is carried out at a strain of 30%. Is this strain in the linear region of the material?

In supplementary
Yes, the strain is in the linear region of the polymer. We added the amplitude sweep of EG-UPy29 at temperature before and after thermal transition in Supplementary Fig. 3.