Dear Sinan,

Thanks so much for your interests in this topic. In papermaking industry, surface chemical modification has long been used to strengthen paper by altering the nature of physicochemical interactions between cellulose fibers. The adhesion between fibers can be improved through chemical modification by increasing the enthalpy of interactions. One example along this line is the synthesis of ionic cellulose derivatives such as carboxymethyl cellulose. Hysteresis caused by unzipping and rezipping the network of ionic bonds in carboxymethyl cellulose intrinsically introduces new toughening mechanism to the system. Here I would like to point out one nice review paper "Unconventional methods in cellulose functionalization, Progress in Polymer Sciences, (2001)". I believe these experimental exercises from chemistry and materials science will pump new idea and momentum into our mechanics community.

Best,

Dear Sinan,

Thanks very much for your comments.

Regarding to surface functionalization, I would like to point out a prospect that the CNT films could be strengthened and toughened by adding surface functionalization. It is experimentally feasible to introduce functional groups on CNT surface, for example as illustrated in a recently paper (Nano Research, 8, 2242-2250). The carboxyl groups are functionalized on the sidewalls of carbon nanotubes. We expect that, by further adding of cellulose into these functionalized CNT systems, a strong and tough hybrid CNT/cellulose film could be potentially achieved, in comparison with the pure CNT films which often suffer from poor mechanical properties despite of the superior strength of individual CNTs. This is on-going research and we hope to be able to report with more details in near future. Thanks.

Best,

Dear Sinan,

Thanks for your interests in this topic. We studied your recent work on interfacial mechanics of cellullose nanocrystals with great interest.

You raised a few important aspects for the mechanical properties of cellulose-based paper, which well deserve further more comprehensive investigation. I'll try to elaborate and share my 2 cents on some of these aspects:

• Creep behavior of paper is of key importance to paper and packaging industry. Take the corrugated paper box as an example. When such boxes are stuffed with goods and then stacked on top of each other for storage over a long time, the sides of the boxes will bulge, and may eventually lead to the collapse of the stack. Such bulging deformation is a consequence of the creep of paper. Creep behaviors of paper are similar to that of many polymers, and strongly tied to the molecular feature of the consitituent fibers and the nature of the chemical process used in making paper. The environmental effects, such as temperature and moisture, can lead to the so-called "accelerated creep" in paper products, and have been systematically studied by paper industry over the years.
• Given the molecular nature as shown in Fig. 1, cellulose paper (regular or nanopaper) falls into the category of hydrogen bond dominated solids, whose mechanical properties are controlled primarily by the hydrogen bond density in such solids. For example, the Young's modulus of a hydrogen bond dominated solid is shown to be proportional to the hydrogen bond density (# of hydrogen bonds per unit volume) to the power of 1/3 [Nature (1955)]. Therefore, it is expected that the stiffness of paper decreases as the hydrogen bond density decreases. It has been shown that water molecules can cause hydrogen bond dissociation in such solids in a cooperative manner [Nature (1976)]: one bond breaking and triggering other bonds to break cooperatively, similar to the formation and breaking of hydrogen bonds in water.

Above said, the creep behavior of cellulose nanopaper, especially under the influence of moisture, remains as an open question so far, and awaits further investigation in light of existing insights from the studies of conventional paper. Hope this is helpful.

Dear Teng,

Thank you so much for a very nice overview of recent works on nanocellulose. It is very exciting to see the applications of the nanofibrils to paper systems where strength and toughness tradeoffs can be seen. Have you thought about studying cellulose nanopaper at different relative humidities? I was curious how moisture uptake influences the mechanical properties in relation to regular paper for example, including creep behavior. Also, what about surface functionalization with polymers or other chemical groups? Do you think other toughening mechanisms could be activated in such systems given that nanocellulose is amenable to these surface modifications?

Dear Ahmed,

It's very interesting to read your papers about sacrificial bonds.

For the rate effect, we would like to point out that, like all atomistic simulations, the effective loading rate (~100 m/s) is orders of magnitude higher than the real loading rate in the experiments (5 mm/min), due to the limit of computation cost. Nonetheless, our atomistic simulations show that, even at such a much higher loading rate, the hydrogen bond breaking and reformation can readily occur among neighboring cellulose fibers. It may be because of that hydrogen bond density per surface that is available to dissipate energy in cellulose nanopaper is rather always fixed and maximized (due to the crytal structure of cellulose chain, and such a feature might be different from that of sacrifical bonds?). When this density is high, the hydrogen bonding is so prevalent in every direction ( thinking of a compact entangled fiber network ) that it is hard to ignore certain bonds even if the loading rate is high.

On the other hand, there is room for improvement in the hydrogen bond density per volume that is available to dissipate energy by further and further decreasing feature size, but we don't have data to this end, as capped by our smallest available fiber diameter. Indeed it is intriguing to see if further increase in the hydrogen bond density per volume would result in saturating the gain in toughness.

Best,

Shuze

Xiaodong, thanks for bringing up this aspect (BTW, once again I'm amazed and wonder if there is any materials you HAVEN't studied, :) ). We should have cited your 2009 paper, and will do so in future related publications.

It's interesting to see that the nanoscale building blocks of bamboo show the feature of cobble-like polygonal nanograins. I wonder if you looked into the aspect ratio of such cellulose nanograins. If the findings in our study work for bamboo, one would expect longer cellulose nanograins (or nanofibers) would lead to more energy dissicpation under deformation, thus can lead to a higher toughness. This is an aspect that deserves further investigation. Thanks.

Teng, this is a great topic. Indeed we need to revisit natural materials. Bamboo is another example. Below is the paper I published years about on bamboo.

http://www.sciencedirect.com/science/article/pii/S0928493108003433

Also related to a couple of points you raised related to rate depedence as well as the density of hydrogen bonding, we have been doing some theoretical investigations on rate dependent response of non-collagenous proteins with sacrificial ionic bonds in bone. These materials show rate strengthening as well as time-depednent self-healing. I wonder if some of these techniques are relevant to nanocellulose films. http://journals.aps.org/pre/abstract/10.1103/PhysRevE.88.012703. In the case of sacrificial bonds, it does not seem that the more there are the better, in fact the gain in toughness saturates after a certain density of sacrificial bonds: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0056118. I wonder if a similar effect will be noticed for the hydrogen bonding.

Best,

Ahmed

Dear Zheng,

These are very fundamental questions and are very relevant to a variety of application whether using the fiber as a reinforcement or the nanocellulose film as an attachment to other surfaces. I am very much looking forward to seeing what the case for the nanocellulose fiber will tun out to be as this is a very interesting application. Ofcourse an integration of experiment and multiscale simulations is required to address these fundamental issues.

Best Wishes,

Ahmed

Dear Shuze,

Thank you very much for your reply and providing these very interesting references. Indeed, as early as ancient Egypt and China :) people have been using fibers (from wood and hay and other materials) to reinforce clay and mud bricks and stone and to overcome some negative thermal effects (e.g. shrinkage cracking). So thank you for pointing to that review paper on this aspect. I will read it with much interest. I am also personally interested in self-healing biological and bio-inspired materials and I find the toughening mechanism for the nanocellulose paper through multiple hydrogen bonding intriguing. We have modeled a similar phenomena in bone but in that case it was ionic bonding. I will share this work below.

Best Wishes,

Ahmed

Dear Teng,

I am glad you found some of these ideas of common interest and relevance. Indeed, the motivation is, as you mentioned, the unique properties these nanocellulose films rae exhibiting. In particular, the fact that the films could be made electroconductive might provide another tool for SHM through exploring how conductivity (or electric current flow) changes as a function of strain or through the evolution of damage. I am also quite intrigued by your proposal of combining SHM with energy harvesting.

Thanks for sharing the paper on correlating damage with color. In that paper chemistry plays the main role. It may be possible through the use of transparent films (like the nanocellulose one) to have a similar effect through light-material interaction and changing color due to modulating light diffraction by surface deformation.

Best, Ahmed

Dear Ahmed,

Thank you so much for your constructive comments. As you pointed out, composites reinforced with cellulose-based fibers have been intensively studied in the past decades. Compared with conventional cellulose-based fibers, the tough and strong nature of cellulose fibers made of NFC fibers indeed offer new opportunities to develop composites with enhanced mechanical properties. Since the mechanical and other physical properties of the composite are generally dependent on the fibres, to better design cellulose-reinforced composites, further mechanistic studies are needed to understand the unique mechanical behavior of nano-sized cellulose fibers:

1) Investigate the mechanical response of nano-sized cellulose fibers by micro-scale simulation. Mechanical behaviors including but not limited to yielding strength, hysteresis, rate-dependence need to be extracted from micro-scale simulations. Then the mechanical properties obtained can be implemented into FEM to reveal the macro-scale response of the composite.

2) Understanding the adhesion between cellulose fibers (made of NFC fibers) and matrix materials such as thermoplastics are also critical to design better cellulose-reinforced composites. The influence of increasing density of hydrogen bonds on the adhesion needs to be revealed. Moreover, as commented by Teng, cellulose nano-fibers can be readily functionalized by other materials, which implies that the adhesion between cellulose fibers and matrix materials might be tuned by adding functional groups. The effectiveness of the methods needs to be evaluated carefully by experiments and simulations. Just my 2cents.

Kindly let me know your thoughts.

Best,

Dear Ahmed,

Thanks for pointing out to use cellulose fibers as reinforcement in structural elements.

This review article, Composites reinforced with cellulose based fibres, Prog. Polym. Sci. 24 (1999) 221–274, (http://www.sciencedirect.com/science/article/pii/S0079670098000185) is a very classical review paper discussing reinforcement effect.

More recently, here is a report on making self-healing materials using cellulose and cationic poly (Bio-Inspired Multiproperty Materials: Strong, Self-Healing, and Transparent Artificial Wood Nanostructures, ACS Nano, 2015, 9 (2), pp 1127–1136, http://pubs.acs.org/doi/abs/10.1021/nn504334u). Hope this is of your interests.

Best regards,

Shuze

Dear Ahmed,

Glad to know you find this topic interesting. The two types of possible applications of cellulose in structural engineering you proposed are intriguing, particularly the one to use nanocellulose paper for structure health monitoring. The nanocellulose paper has some unique features that are well in line toward such applications:

• Nanocellulose paper is transparent. Unlike the cellulose fibers used in regular paper that are of diameters of 10s microns, nanocellulose paper is made of cellulose nanofibers of diameters of 10s nanometer, well below the wavelength of visible lights. As a result, it is highly transparent, with an average optical transmmitance above 95% in visible spectrum. These features make cellulose paper suitable as a surface coating or wrapping for a structural component, without changing much of its physical apprearance (texture, color, etc.)
• Nanocellulose paper is distinct from transparent plastic foils. Nanocellulose paper is made of a network of cellulose nanofibers that can be readily functionalized or coated with other materials. This gives a wide range of flexibility to fine tune the structure of the nanocellulose paper to achieve desirable properties. For example, certain areas of a nanocellulose paper can be potentially made elelctrically conductive, making possible to seamlessly embed electronic circuits in such a paper. By contrast, a plastic foil is often made of densely packed molecular chains. While it is possible to functionalize the constituent molecular chains, it often involves rather complicate processing procedures given its molecular-level size.
• Certain nanocellulose paper can be made to be of both high transparency and high haze, a pair of typically compromising properties. Such a unique combination of optical properties is desirable for solar cells, as detailed in Section 4.1. If the function of harvesting solar energy can be achieved in nanocellulose paper based structural health monitoring (SHM) components, it could poentially address the typical challenge of power supply in SHM systems, given the long service term and dispersed distribution of such systems.

I also find your idea of studing the changes in refractive index of the nanocellulose paper as a function of strain and use this as a guide for structural health evaluation inspiring. This is an aspect we haven't considered yet and I'm not aware of research progress on this. On a separate but relevant note, there have been progress on correlating deformation/damage with color change of materials (see below for an example), demonstrating the potential of using simple optical method for structural health monitoring.

Hope this is helpful.

Dear Teng et al.,

Thanks for sharing your excellent work on cellulose. Two applications related to structural engineering came to my mind ss I am reading through this. Let me know what you think :)

1. Since the nano cellulose paper is transparent it may have a potential application in structure health monitoring. If we imagine wrapping a structural component with this paper and the component experiences additional strain, the cellulose paper will deform (may wrinkle for example) and its interaction with light may change leading to change in color. Thus the color may be indicative of the state of strain in the substrate. One can further study the changes in refractive index of the nanocellulose paper as a function of strain and use this as a guide for structural health evaluation using optical methods.

2. A more classical use would be to implement cellulose fibers as reinforcement in structural elements. While the stiffness and yield strength are lower than steel, they have reasonable values for structural applications beside being more robust and not susceptible to corrosion. Indeed, fiber reinforced polymers FRP have witnessed a surge of use in strengthening and repair work of RC structures in recent years.

Let me know your thoughts.

Best,

Ahmed