Assembly, gelation and helicoidal consolidation of nanocellulose

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Invited Feature Article

Assembly, gelation and helicoidal consolidation of nanocellulose dispersions Yingxin Liu, Christina Schütz, German Salazar-Alvarez, and Lennart Bergström Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04013 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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Assembly, gelation and helicoidal consolidation of nanocellulose dispersions Yingxin Liu, 1,2† Christina Schütz1,2‡, German Salazar-Alvarez1,2, Lennart Bergström1* 1

Department of Materials and Environmental Chemistry, Stockholm University, 106 91 Stockholm, Sweden 2

Wallenberg Wood Science Center, KTH, 100 44 Stockholm, Sweden

ACS Paragon Plus Environment

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Abstract

The ability to probe the assembly, gelation and helicoidal consolidation of cellulose nanocrystals (CNC) dispersions at high concentrations can provide unique insight to the assembly and can assist optimized manufacturing of CNC-based photonic and structural materials. In this feature article, we review and discuss the concentration dependence of the structural features, characterized by the particle separation distance and the helical pitch, at CNC concentrations (c) that range from the isotropic state, over the biphasic range to the fully liquid crystalline state. The structure evolution of CNC dispersions probed by time-resolved small angle X-ray scattering during evaporation-induced assembly highlighted the importance of gelation and consolidation at high concentrations. We briefly discuss how the homogeneity of helicoidal nanostructures in dry CNC films can be improved and present an outlook for future work.


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Introduction Nanomaterials based on renewable resources are attracting a rapidly growing interest for many structural and functional applications, ranging from scaffolds for regenerative medicine to optical and devices for energy generation and storage.1,2 Full utilization of the intrinsic properties of nanosized materials of natural origin requires the development of robust and versatile isolation methods with a high yield, and the development of suitable processing routes.3 Recently developed methods to isolate the nanosized constituents of cellulose, the most abundant biopolymer on earth, on a relatively large scale has opened up new possibilities for the fabrication of cellulose-based high performance materials.1,4–6 The high surface area and low density as well as available surface hydroxyl groups, excellent mechanical performance and low thermal conductivity of nanocellulose offer a combination of properties for design of a wide range of structural and functional materials.1 Fabrication of nanocellulose into films, coatings and other bulk materials with outstanding mechanical, optical and heat transport properties demands control of the microstructure and alignment during assembly of the rod-like nanoparticles.7–10 Marchessault et al.11 and Gray and coworkers12 showed several decades ago that aqueous dispersions of cellulose nanocrystal (CNC) produced by acid hydrolysis can form a chiral nematic liquid crystalline phase, characterized by a helicoidal organization with a well-defined periodicity, called the pitch. The pitch is typically in the range of tens of micrometers in the fluid state but decreases upon drying, resulting in the characteristic iridescent optical properties of dried CNC films. Additionally, dissipation of fracture energy by the helicoidal organization of the fibrillar chitin matrix, another important rod-like polysaccharide nanoparticle, is related to the extraordinarily high fracture toughness of the mineralized tissue of many marine organisms like lobster13 and the damage-tolerant dactyl club of stomatopods.14 The attractive optical properties and helicoidal structure have triggered several


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attempts to utilize CNC-based materials in applications like optical encryption15 or as chiral templates.16 Hence, understanding and controlling the self-assembly and helical ordering of CNC is not only a topic of fundamental importance but also a tool for the production of novel materials with attractive optical and mechanical properties. In this feature article, we will review and discuss recent work on CNC assembly probed primarily with in situ small angle X-ray scattering (SAXS). The insights on the assembly at high concentrations where gelation and helicoidal consolidation were related to how to optimize the homogeneity of CNC-based materials. Chirality, twist and phase behavior of cellulose nanocrystals Cellulose nanocrystals are rod-like particles isolated usually by acid hydrolysis from the elementary cellulose fibrils in plant cell walls, or from tunicate and bacteria.1,3 The dimensions and aspect ratio (AR) of CNC depend on the origin of the cellulose and the hydrolysis conditions; bacterial CNC (length: 100-1000 nm, width: 10-50 nm and AR: 50-100); tunicate CNC (length: 500-2000 nm, width: 10-30 nm and AR: 50-200); and wood-derived CNC (length: 50-350 nm, width: 3-5 nm and AR: 15-80).1,3 Sulfuric acid was the most commonly used acid for producing colloidally stable CNC. The aspect ratio (AR) and polydispersity can be partially controlled by the acid concentration and hydrolysis time.17,18 Recently, some organic acids, e.g., dicarboxylic acids have been used to produce CNC with a high degree of crystallinity and thermal stability.19 CNC is molecularly assembled from chiral D-glucose and possess a well-defined right-handed chirality, verified by both experimental observations and simulations on single CNC or CNC aggregates/bundles.20–23 Figure 1 exemplifies the observed right-handed twist of CNC characterized by atomic force microscopy (AFM) and cryogenic scanning electron microscopy (Cryo-SEM).20 The details of the degree of crystallinity and number of cellulose chains in


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nanocellulose are debated but spruce-derived CNC is often considered to have a square crosssection of 24 packed cellulose chains as shown in Figure 1 c.24

Figure 1. Chirality and phase behaviour of wood-derived cellulose nanocrystals. (a) Highresolution AFM phase image and, (b) Cryo-SEM image of CNC. The inset arrows highlight the twisted structure. (c) Schematic illustration of CNC with a terminating surface structure. The inset refers to the cross-section consisting of 24 dense-packed cellulose chains in one fibril. Adapted with permission from Ref 17. Copyright 2015 Macmillan Publishers Ltd. (d) Liquid crystalline (LC) phase diagram of CNC of different origins as a function of the CNC concentration (vol%). The line connecting the points is a guide for the eye and the numbers behind the references are the aspect ratio range (AR) of the studied systems. Adapted with permission from Ref 3. Copyright 2014 Nature Publishing Group. Dispersions of colloidally stable CNC undergo an isotropic-chiral nematic phase transition with increasing particle concentration. The helical pitch (p) is normally determined by polarized optical microscopy (POM)25–28 and appears as extinction lines at distances of p/2 when the helix is observed perpendicularly to its main axis. Pitch values for CNC systems vary between 60 μm and 7 μm in dispersions17,25–27,29,30 and films,28 and can attain values in the range of the wavelength


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of visible light in dried films.31–34 The liquid crystalline phase behavior (Figure 1d) depends on the particle concentration,17 the aspect ratio,17,26 the surface chemistry of the CNC,7 the ionic strength, solvent 25,27,30 and the temperature.28,34 Packing of cellulose nanocrystals in the anisotropic phase

Figure 2. Concentration-dependence of the interparticle distance and twist angle. (a) Average center to center separation distance (d) and helical pitch (p) as a function of the CNC concentration. The top-right inset is a schematic representation of the CNC particle arrangement defining the helical half-pitch (p/2) and the separation distance. The images of CNC dispersions at corresponding concentrations was obtained between crossed polarizers. (b) Twist angle as a function of the CNC concentration. The inset illustrate the twist angle, . Adapted from Ref 33. Copyright 2015 American Chemical Society.


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The average separation distance (d) between the neighboring CNC in the isotropic and anisotropic phase can be determined by small angle X-ray scattering (SAXS) from the concentration-dependent position (q, nm-1) of the major scattering peak using d = 2π/q.35 Figure 2a gives an example of the relation between the helical pitch, inter-particle separation distance and phase behavior as a function of particle concentration for CNC dispersions obtained by sulfuric acid hydrolysis. The SAXS measurements showed that d gradually decreases from 51 nm at c  1.3 vol% to 24.5 nm at c  6.5 vol% and laser diffraction measurements showed that the pitch decreased from about 16 m down to 2 m when c increased from 2.5 vol% to 6.5 vol%. The twist angle, , in the chiral nematic phase could be obtained following:  = 360° d/p; Figure 2b shows that  increased from about 0.9 ° to about 3.7 ° as c increased from 2.5 vol% to 6.5 vol%. The increase of twist angle accompanied with CNC concentrations was related to the increasing magnitude of the electrostatic repulsion compared to the van der Waals attraction, which makes a parallel arrangement energetically less favourable.35 Additional studies involving e.g. molecular dynamic simulations may give important insight to how the range and magnitude of the nanoparticle interactions contribute to CNC assembly.36 Indeed, packing screw-like rods with an offset between their main axes is beneficial for minimizing the free energy of the system.37 A slight twist also increases the effective free volume, and a corresponding increase of the entropy of the system.38 Assembly of cellulose nanocrystals at high particle concentrations Fabrication of solid (dry) nanocellulose films with tunable optical properties by CNC chiral assembly requires that the transformation of the chiral nematic phase in the dispersion to the helicoidal nanostructures in the dry films can be controlled.39 Previous work has shown that the evaporation induced structural evolution of CNC is influenced by several and sometimes


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competing processes:39–41 e.g., liquid crystalline phase separation, (hydro)gelation or kinetic arrest, and capillary force-driven consolidation. However, while the effect of the ionic strength,28 magnetic and electric fields,42,43 and evaporation rate40,44 on the micrometer-scaled pitch in dispersions at relatively low concentrations (up to ~7 vol%) have been investigated in some detail, was nanoscale information of the structure evolution at significantly higher concentrations only recently obtained.45,46

Figure 3. In situ SAXS measurements during the evaporation induced assembly of CNC and structural evolution in nanoscale. (a) A CNC drop pinned on a patterned mica substrate. The substrate has a hydrophilic inner region and a hydrophobic outer region, preventing the drop sliding when oriented vertically, perpendicularly to the X-ray path. (b) SAXS profiles of CNC-H and CNC-Li drops with a time step of 1 min and 2 min, respectively. (c) Evolution of the center to center CNC separation distance as a function of the CNC concentration (vol%). Adapted from Ref 42. Copyright 2018 American Chemical Society. In one of our recent studies, the real-time structural evolution of aqueous dispersion of sulfonated CNC with hydrogen (CNC-H), and lithium (CNC-Li), counter ions was investigated using in situ SAXS in a configuration where the CNC drop was pinned on a mica substrate and dried at a constant temperature and relative humidity (25 °C and RH 40 %), as shown in Figure


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3a.45 The SAXS measurements yielded information (Figure 3b) on the change of the average separation distance with increasing particle concentration, starting from a fully liquid crystalline state. Figure 3c shows that the separation distance in CNC-H dispersions decreased with increasing particle concentration according to d ∝ c-1, which can be related to unidimensional contraction of the nematic phase.47 The assembly of CNC-Li shows a distinctly different pathway with a scaling of d ∝ c-1/3, which is typical for isotropic consolidation of randomly oriented particles. The specific counter ion effect on CNC assembly was related to the colloidal stability of CNC, where dynamic light scattering showed that the formation of aggregates is more pronounced for CNC-Li compared to CNC-H with increasing ionic strength.45 We have also investigated recently presented an in situ SAXS investigation of CNC assembly in a shrinking levitating drop by that covered a very large concentration range.46 The in situ SAXS study was performed on a single droplet without interference of a solid substrate or added surfactants, which may interfere with the phase transitions and assembly behavior. Figure 4a shows how a CNC aqueous drop (with an initial diameter of 1.5 mm) was levitating between the pressure nodes generated in the sonotrode. The shrinkage of the drop, i.e., both the equatorial and polar radii, were continuously recorded by a camera (Figure 4b) and used to estimate the particle concentration. The measurements in the levitating drop covered an unprecedented particle concentration range from 1 vol% to ~38 vol%, and the corresponding average CNC separation distances obtained from the SAXS data decreased from 50 nm to 5 nm. Scaling analysis of the concentration dependence of the separation distance (Figure 4c) suggests that assembly of CNC can be divided into three regions as a function of the concentration. Region I (1-2 vol%) corresponds to the isotropic state in the phase diagram where the CNC drop demonstrates a scaling of d ∝ c-1/3. Region III (above 23 vol%) is characterized by a d ∝ c-1 scaling, characteristic for the


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unidimensional compression of a nematic structure. The scaling exponents in regime II (2-23 vol%) increased gradually from -1/2 to -2/3 with increasing CNC concentration. The good correspondence between the scaling behavior in the levitating drop and sealed capillaries33 between 2 to 6 vol%, suggests that a scaling of d ∝ c-1/2 is characteristic for the biphasic state of CNC dispersions. The shift of the scaling exponent from -1/2 to -2/3 occurs around the apparent gelation threshold of CNC at 6 vol%. It has been shown that a consolidation of CNC amorphous hydrogels leads to a scaling exponent of -1/3.45 The intermediate scaling of d ∝ c-2/3 may thus be related to a combination of unidimensional compression of the nematic phase and a randomly oriented gel..

Figure 4. In situ SAXS measurements during the evaporation induced assembly of CNC and the corresponding structural evolution in nanoscale. (a) Schematic illustration of a levitating CNC


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drop illuminated by the incident X-ray beam. (b) Photograph of the shrinking drop at different evaporation time. (c) Evolution of the center to center CNC separation distance as a function of the CNC concentration (vol%) (: levitating drop; and : capillary data (ref. 33). The solid curves refer to a power law relation with exponents ranging from -1/3 to -1. Adapted with permission from Ref 43. Copyright 2018 Royal Society of Chemistry. Controlling the helix orientation and homogeneity in cellulose nanocrystal films

Figure 5. POM and SEM images of a CNC film dried under ambient conditions starting from (a) an isotropic dispersion (1.9 vol%) and, (b) a fully liquid crystalline dispersion (3.1 vol%). Insets refer to the possible orientation of liquid crystalline tactoids. SEM images of a fractured surface of CNC film (drying from 3.1 vol%) close to the, (c) film center and, (d) perimeter, respectively. Adapted with permission from Ref 31. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. There have been significant efforts to improve the homogeneity and optical properties of CNC films by controlling the drying conditions by, e.g., external field, temperature, solvent and additives3,39,41. Most, if not all, of these films have been cast from relatively dilute CNC dispersions


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within the isotropic phase concentration range28,48,49 or by the coexistence of an isotropic and a chiral nematic phase.31,34,50 Figure 5a shows that films cast from a dilute CNC dispersion dominated by the isotropic phase yield a mosaic-like polydomain texture with a significant variation of the chiral nematic pitch. The inhomogeneous texture originates from the incomplete fusion of tactoids that form as the system enters the biphasic region during the initial drying stage.26,51 The relaxation of the polydomain texture into a uniform helix is usually hindered since the evaporation of water will lock the system in an arrested state.3,34 In contrast, casting a film from a fully liquid crystalline dispersion results in a more homogeneous film with larger liquid crystalline domains (Figure 5b) where SEM images of fractured films (figure 5c) revealed the internal helical arrangement, with a helix mainly oriented along the film normal. Alignment of the tactoids has been reported using external fields,43,52 although they can also be achieved using a mild circular shear flow upon drying.33 The formation of dry films with large domain sizes is essential for improved optical properties40,41,53 and may also result in mechanical properties that match those found in biological specimens.13,14 Additionally, incorporation of the flexible polymer, e.g., poly(ethylene glycol)44 and plasticizer, e.g., glycerol54 can yield homogeneous CNC films with improved flexibility. Conclusion and outlook This feature article illustrates how recent studies using primarily in situ, time-resolved small angle X-ray scattering (SAXS) have provided important insights into the structural evolution of CNC dispersions at high particle concentrations. We show that analysis of the scaling of the particle separation distance as a function of particle concentrations provides important information on the assembly, gelation and helicoidal consolidation, which can assist to optimize the conditions when producing CNC films with well-defined and uniform photonic properties. Future work


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should combine the techniques to probe assembly at very high particle concentrations with in situ control of the interactions between the particles. The application of external magnetic fields, control of ionic strength, and CNC charge density during time-resolved SAXS measurements at high particle concentrations could provide quantitative information on the factors that influence the competition between assembly, gelation and helicoidal consolidation.39,55 Furthermore, the structural evolution and transition during the final drying stage when the pitch attains submicron values need to be better understood. AUTHOR INFORMATION Corresponding Author * [email protected] Present Addresses † John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, 02138 Massachusetts, USA ‡ Physics and Materials Science Research Unit, University of Luxembourg, 1511 Luxembourg, Luxembourg Author Contributions All authors have contributed to the writing and all authors have given approval to the final version of the manuscript. Funding Sources We acknowledge the Wallenberg Wood Science Center (WWSC) and the Swedish Research Council (VR) for financial support. Notes The authors declare no competing financial interest.


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Yingxin Liu obtained his MSc from Fudan University (China) and PhD from Stockholm University (Sweden). During his PhD, Yingxin’s research was centered on the colloidal assembly and liquid crystal phase behavior of nanocellulose under the supervision of Prof. Lennart Bergström. Now, he is a postdoctoral fellow at Harvard University.

Christina Schütz obtained her PhD in materials chemistry from Stockholm University (Sweden) supervised by Prof. Lennart Bergström and Assoc. Prof. Germán Salazar-Alvarez. She now works as a postdoctoral Humboldt fellow at University of Luxembourg and her current research focuses on the influence of surface functionality on the assembly process of cellulose nanocrystals.


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German Salazar-Alvarez obtained his Ph.D. degree in 2005 from the Royal Institute of Technology (KTH), and his Docent degree from Stockholm University in 2012, where he has been a group leader since 2010. His research activities are devoted to the fabrication of new hybrid systems based on nanocellulose, and synthesis of nanoparticles with e.g., magnetic, and ion transport functionalities.

Lennart Bergström is a professor of Materials Chemistry at Stockholm University. His research interests are in the area of sustainable materials processing and nanoparticle self-assembly. He is a recipient of the Humboldt Research Award and a member of the Royal Swedish Academy of Engineering Sciences.


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Assembly, gelation and helicoidal consolidation of nanocellulose

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