Cellulose Nanocrystals: Properties, Production and Applications

By Wadood Y. Hamad

Research into cellulose nanocrystals is currently in an exponential growth phase, with research into potential applications now strengthened by recent advances in nanomanufacturing. The possibility of routine commercial production of these advanced materials is now becoming a reality.

Cellulose Nanocrystals: Properties, Production and Applications provides an in-depth overview of the materials science, chemistry and physics of cellulose nanocrystals, and the technical development of advanced materials based on cellulose nanocrystals for industrial and medical applications. Topics covered include:

• A comprehensive treatment of the structure, morphology and synthesis of cellulose nanocrystals.
• The science and engineering of producing cellulose nanocrystals and the challenges involved in nanomanufacturing on a large industrial scale. 
• Surface/interface modifications of cellulose nanocrystals for the development of novel biomaterials with attractive structural and functional properties.
• The scientific bases for developing cellulose-based nanomaterials with advanced functionalities for industrial/medical applications and consumer products. 
• Discussions on the (i) reinforcing potential of cellulose nanocrystals in polymer nanocomposites, (ii) utilization of these nanocrystals as efficient templates for developing tunable photonic materials, as well as (iii) applications in sustainable electronics and biomedicine.

Cellulose Nanocrystals: Properties, Production and Applications will appeal to audiences in the physical, chemical and biological sciences as well as engineering disciplines. It will be of critical interest to industrialists seeking to develop sustainable new materials for the advanced industrial economies of the 21st century, ranging from adaptive “smart” packaging materials, to new chiral, mesoporous materials for optoelectronics and photonics , to high-performance nanocomposites for structural applications.

• Language: English
• Publisher: Wiley
• Release date: Mar 31, 2017
• ISBN: 9781118675700

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Table of Contents

Cover

Title Page

Series Preface

Foreword

Prologue

1 New Frontiers for Material Development and the Challenge of Nanotechnology

1.1 Perspectives on Nanotechnology

1.2 Societal Ramifications of Nanotechnology

1.3 Bio‐inspired Material Development: The Case for Cellulose Nanocrystals

1.4 A Glance at Bio‐inspired Hierarchical Materials

1.5 Concluding Thoughts

2 Assembly and Structure in Native Cellulosic Fibers

2.1 Physical and Chemical Characteristics of the Cellulose Molecule

2.2 Morphology and Structure of Native Cellulosic Fibers

2.3 Physical and Mechanical Properties of Native Cellulosic Fibers

3 Hydrolytic Extraction of Cellulose Nanocrystals

3.1 Introduction

3.2 The Liberation of CNCs Using Acid Hydrolysis

3.3 Reaction Kinetics of CNC Extraction

3.4 Processing Considerations for Sustainable and Economical Manufacture of CNCs

3.5 Micro/Nano Cellulosics Other Than CNCs

4 Properties of Cellulose Nanocrystals

4.1 Morphological Characteristics of CNCs

4.2 Structural Organization of CNCs

4.3 Solid‐State Characteristics of CNCs

4.4 CNCs Chiral Nematic Phase Properties

4.5 Shear Rheology of CNC Aqueous Suspensions

4.6 Thermal Stability of CNCs

5 Applications of Cellulose Nanocrystals

5.1 Prelude

5.2 The Reinforcing Potential of CNCs in Polymer Nanocomposites

5.3 CNC‐Stabilized Emulsions, Gels, and Hydrogels

5.4 Controlled Self‐Assembly of Functional Cellulosic Materials

5.5 Toward Bio‐inspired Photonic and Electronic Materials

5.6 CNCs in Biomedicine and Pharmaceuticals

5.7 Environmental, Health, and Safety Considerations of CNCs

5.8 Perspectives and Challenges

Epilogue—The Never‐Ending Evolution of Scientific Insights

Bibliography

Subject Index

End User License Agreement

List of Tables

Chapter 01

Table 1.1 Typical physical properties of different forms of cellulosic micro/nano materials and structures obtained using distinctly different processing—mechanical, chemical, and/or enzymatic—of the same raw material, softwood kraft pulp fibers.

Chapter 03

Table 3.1 Elemental analysis data, calculated sulphate group/anhydroglucose unit, yield, and degree of polymerization (DP) of the extracted cellulose materials from hydrolysis of softwood kraft pulp at various sulfuric acid concentrations and temperatures for 25 min.

Table 3.2 Elemental analysis data, calculated sulphate group per 100 anhydroglucose units, yield, and DP of the extracted cellulose materials from the hydrolysis of bleached softwood kraft pulp using 64 wt. % of sulfuric acid at 65°C for various times.

Chapter 04

Table 4.1 Calculated integrals (%) for most of the peaks observed in the ¹³C CPMAS spectra of CNC material in freeze‐ and air‐dried forms, as obtained from DMFIT.

Table 4.2 Parameters derived from powder XRD spectra of CNC materials described in Table 4.1.

Table 4.3 Typical data for the residual mass loss (as percentage) of H2SO4‐hydrolyzed CNC material of different pH at different temperatures.

Chapter 05

Table 5.1 Physical properties for CNCs compared to other high‐performance reinforcing materials.

List of Illustrations

Chapter 01

Figure 1.1 Pollia condensata fruit, another natural material exhibiting structural color, that is, internal color produced via the interference and diffraction of light passing through innate, ordered nanostructures (right image).

Figure 1.2 (a) A photograph of porous scaffolds of practical dimensions obtained by freeze‐casting of ceramic suspensions. (b) SEM image of the lamellar architecture of ice‐templated Al2O3/PMMA hybrid materials.

Figure 1.3 Schematic representation of the structure of egg‐box junction zones in TEMPO‐mediated oxidation of cellulose/alginate/calcium system: (a) coordination of Ca²+ in a cavity created by a pair of guluronate sequences along alginate chains; (b) laterally associated egg‐box multimer in the composite sponge; (c) oxidized cellulose nanocrystals (OCNs) in the cross‐linking of alginate sponge; and (d) construction of the semi‐interpenetrating polymer network (SIPN) composed of oxidized microfibrillated cellulose (OMFC) and alginate. The black solid circles in (a) represent the oxygen atoms possibly involved in the coordination with Ca²+.

Chapter 02

Figure 2.1 Cellobiose, the monomer of cellulose.

Figure 2.2 ¹³C NMR CP‐MAS spectra of Iα and Iβ cellulose.

Figure 2.3 Schemas of the micellar, continuous, fringed micellar, and fringed fibrillary structural theories describing crystalline and amorphous components in polymeric fibers.

Figure 2.4 Conceptual schematics—first proposed by Forgacs (1963)—of the cell wall layers of a typical fiber or tracheid, showing fibrillar and/or microfibrillar directions.

Figure 2.5 Schematic representation of a hemp stem (left), and SEM of the fractured end of kenaf bast (subjected to tensile loading) illustrating the fiber bundle morphology characteristic of nonwood plant fibers (right).

Figure 2.6 Models of a submicroscopic element—first proposed by Mark (1980)—in a layer of a fiber wall (left), and a basic unit cell wall (right). The unit contains a single rectangular anisotropic filament surrounded by a matrix of thickness, t.

Chapter 03

Figure 3.1 Pictorial representation of the ultrastructure of lignocellulosic fibers.

Figure 3.2 Schematic presentation of the polymeric structure of cellulose, showing highly ordered (crystalline) to least ordered (amorphous) regions. (a) Crystallites linked by disordered or amorphous regions and (b) isolated highly ordered crystallites.

Figure 3.3 Yield vs. DP of the extracted, H2O‐insoluble cellulose materials from the hydrolysis of bleached softwood kraft pulp (DP = 1178) at various sulfuric acid concentrations and temperatures for 25 min (Hamad and Hu 2010).

Figure 3.4 Effect of hydrolysis temperature on the degree of sulfation (DS), degree of polymerization (DP), and particle size of CNC extracted from bleached kraft hemlock pulp at a constant sulfuric acid concentration and hydrolysis time. (Note: Left y‐axis represents both mean size and DP, while right y‐axis DS.)

Figure 3.5 Time‐of‐flight secondary ion mass spectroscopy (ToF‐SIMS) spectra of CNC particles processed using 60 wt. % and 65°C (a) and 64 wt. % and 45°C (b). CNC samples were mounted on a 1 cm × 1 cm clean silicon wafer using a double‐sided tape, and analysis was performed using PHI TRIFT V nanoTOF SIMS instrument. Both positive and negative (not shown) secondary mass spectra of each CNC sample were measured using 30 keV Au+ primary ion beam at static mode with 400 µm × 400 µm analytical area. The spectra consistently indicate that the CNC samples are similar in their fragment regions (m/z up to 200), but the contrast becomes evident in their intact molecular regions. This indicates that CNC particles obtained using higher acid concentration and lower temperature (~64 wt. % and 45°C) have heat‐sensitive oligomers adsorbed onto the CNC surface.

Figure 3.6 A conceptual flow diagram depicting the principal steps for CNC manufacture—with a particular focus on the process design for CNC separation and acid recovery. The dialysis module can effectively combine more than one function (purification, homogenization, and concentration), and can be repeated as many times as necessary depending on scale‐up requirements and efficiency—that is, module surface area essentially defines efficiency and quality of the extraction process. (AP, acid preparation; BG, bioenergy generation; ET, effluent treatment; RM, raw material.)

Figure 3.7 SEM micrograph of microfibrillated cellulose, MFC.

Figure 3.8 SEM micrographs showing MCC morphology and structure. The arrows in the image point to cellulose crystallites in MCC particles. The scale bars in the images are 100 µm (left) and 5 µm (right)—after Mathew et al. (2005).

Figure 3.9 SEM micrograph of bacterial cellulose (Nakagaito et al. 2005).

Chapter 04

Figure 4.1 Experimental ¹³C cross‐polarization magic angle spinning nuclear magnetic resonance (CP/MAS NMR) spectrum of lyophilized CNC (black), acquired using 4096 scans (see text for further experimental details), with an overlay of a simulated spectrum for cellulose II (gray) obtained using DMFIT and the assignments reported for cellulose II.

Figure 4.2 Micrographs of CNCs in aqueous suspension at high (top left) and low (top right) concentrations imaged using a scanning transmission electron microscope (STEM), where the CNC spindles (or rods) are shown as discrete nano‐scaled particles. The bottom SEM micrographs show lyophilized CNC self‐assemblies, whose morphology is characterized by agglomerates of parallelepiped, rod‐like structures of nano‐scale cross sections. (CNCs were extracted from bleached softwood kraft pulp using 64% sulfuric acid concentration at 45°C for 25 min.)

Figure 4.3 Cross‐polarized transmission (left image) and reflection (right) optical images of air‐dried CNCs suspension clearly showing the fingerprint local periodic structure.

Figure 4.4 SEM of partially hydrolyzed pulp obtained using 40% H2SO4 at 65°C for 25 min.

Figure 4.5 SEM of evaporation‐induced self‐assembled (EISA) CNC films produced from suspensions at (a) 25°C, pitch, P = 2.57 µm ± 0.23, (b) 55°C, P = 1.32 µm ± 0.23, (c) 75°C, P = 1.07 µm ± 0.19, and (d) 100°C. Note the gradual reorganization of the chiral nematic order as temperature is increased, until complete randomness as 100°C. At ~75°C, some order is still notable in certain regions of the CNC film.

Figure 4.6 Chiral nematic organization remains exquisitely evident in lyophilized (or freeze‐dried) CNC material. (a) Macro image of the longitudinal section of freeze‐dried CNC isolated from a storage vial. (b)–(c) Reflected light microscopy of material obtained from (a). Note the fingerprint pattern. (d)–(e) SEM images clearly depicting the periodic layering and chiral organization within each layer of a segment of the freeze‐dried CNC material shown in (a).

Figure 4.7 (a) Comparison of the ¹³C CPMAS spectra for, from top to bottom, Sample 2 air‐dried, Sample 2 freeze‐dried, Sample 1 air‐dried, and Sample 1 freeze‐dried CNC. The increase in intensity for the lines assigned to C4 amorphous, C6 amorphous (see Table 4.2), and a line which may possibly be assigned to (β)/Iβ (Hesse and Jaeger 2005) correlate with the increase in the amorphous area (listed), as determined from powder XRD measurements. (b) T1 relaxation profile for all ¹³C types in air‐dried Sample 1. The spectra clearly indicate that the regions assigned as amorphous in Table 4.1 are more mobile than the crystalline components. (c) T1 inversion recovery data points and fits for the line assigned to C6 amorphous for Sample 1 freeze‐dried (data: +; fit: solid black line), Sample 1 air‐dried (data: ×; fit: dotted black line), Sample 2 freeze‐dried (data: *; fit: solid gray line), and Sample 2 air‐dried (data: ; fit: dotted gray line).

Figure 4.8 Wide‐angle X‐ray diffractograms for (freeze‐dried) CNC materials extracted from bleached kraft softwood pulp using 64 wt. % sulphuric acid for 25 min at the indicated temperatures.

Figure 4.9 Diffraction pattern for (freeze‐dried) CNC (64 wt. %, 45°C, 25 min) resolved into crystalline peaks and amorphous background (following the Ruland–Rietveld analytical approach). The residual plot (shown in gray) represents the difference between the actual diffraction pattern and the resolved (modeled) peaks.

Figure 4.10 Degree of polymerization (DP) vs. crystallinity (a) and crystallite size (b) of the extracted cellulose materials from the hydrolysis of the softwood kraft pulp (DP = 1178) with various sulphuric acid concentrations at various hydrolysis temperatures (for 25 min).

Figure 4.11 ²H/¹³C REDOR spectra (reference, dephased, and difference) for 2‐day liquid D2O‐soaked CNC after a dipolar evolution time of 3.2 ms.

Figure 4.12 (a–e) Model fits of ²H/¹³C REDOR data to dephasing curves calculated with the cellulose Iβ–B structure.

Figure 4.13 ¹³C/³¹P REDOR spectra, where the solid line is the ¹³C reference spectrum collected with ³¹P dephasing pulses. The dashed line (exchanged in the vertical axis by a factor of 50) shows the difference spectrum in the presence of the ³¹P dephasing.

Figure 4.14 The three levels of structure in CNC films: (a) Model of two cellulose I chains in crystalline domains, based on the findings of Witter et al. (2006). The cylinders represent the cellulose chains. (b) Assuming that the rods which come together to form the chiral nematic phase are 3 × 3 nm, then roughly nine of the cellulose I chains would form a rod (gray). As reported by Orts et al. (1998) and Lima and Borsali (2004), these rods are helically twisted along their length of 20–200 nm, in the presence of high salt, for example. (c) The (straight or helically twisted) rods come together in layers to form the chiral nematic phase of CNC.

Figure 4.15 Ionic strength influence on the phase separation in CNC aqueous suspensions at fixed concentration. Actual behavior is represented, for 3 wt. % CNC after 25 days of standing, at different NaCl concentrations (top images), and concomitant schematic illustrations (bottom) of these effects on CNC phase separation.

Figure 4.16 (a) CD spectra of films produced from CNC suspensions with concentrations of 0.1 (black), 0.2 (black dashed), 0.5 (dark gray), 1.0 (dark gray dashed), and 2.0 w/w % (gray) at room temperature. (b) Plot of ln(1/P) of the CNC films vs. ln(c) for CNC particles. The error bars represent the error associated with determining the maximum reflection wavelength in the CD spectra. The fit yielded the following parameters: y = (−0.225 ± 0.002)x − (5.602 ± 0.001), R² = 0.9999.

Figure 4.17 (a) Shear rate dependence of the order parameter, S, for CNC nanoparticles of L = 180 nm ( ) and L = 280 nm ( ), along with the viscosity profile for L = 280 nm as a function of shear rate ( ). A higher concentration was used for the shorter particles so that the viscosities of the two samples would be equivalent.(b) Viscosity as a function of nanorod concentration for shear rates of 0.01 and 100 s−1.

Figure 4.18 Effect of ultrasound energy input (level of sonication) on the viscosity material function at (a) 5 wt. % and (b) 7 wt. % CNC suspensions.

Figure 4.19 Polarized optical micrographs of 7 wt. % CNC suspensions at rest: (a) unsonicated sample, (b) sample sonicated at 500 J, (c) sample sonicated at 1000 J, and (d) sample sonicated at 2000 J energy applied per gram of CNC in suspension.

Figure 4.20 Steady‐state viscosity vs. shear rate for (a) unsonicated CNC suspensions and (b) CNC suspensions sonicated at 1000 J/g CNC with concentrations varying from 1 to 7 wt. %.

Figure 4.21 Steady‐state viscosity versus shear rate of 1 wt. % ( ), and 7 wt. % ( ) CNC suspensions before sonication (solid symbols) and after sonication at 1000 J/g CNC (open symbols).

Figure 4.22 Polarized optical micrographs of CNC suspension (5 wt. %, unsonicated) during steady shear tests, at shear rates of (a) 0.01 s−1, (b) 0.05 s−1, (c) 0.1 s−1, (d) 0.5 s−1, (e) 1 s−1, and (f) 10 s−1.

Figure 4.23 Polarized optical micrographs of CNC suspension (5 wt. %, 1000 J/g CNC sonicated) during steady shear tests, at corresponding shear rates of (a) 0.01 s−1, (b) 0.05 s−1, (c) 0.1 s−1, (d) 0.5 s−1, (e) 1 s−1, and (f) 10 s−1.

Figure 4.24 Complex viscosity │η*│ ( ), storage modulus G′ ( ), and loss modulus G″ ( ) vs. angular frequency of unsonicated (solid symbols) and sonicated at 1000 J/g CNC (open symbols) of (a) 3 wt. %, (b) 5 wt. %, (c) 7 wt. %, and (d) 10 wt. % of CNC suspensions.

Figure 4.25 Cox–Merz rule comparison of (a) unsonicated and (b) sonicated at 1000 J/g CNC samples at different concentrations. The open symbols represent the steady shear viscosity, and the solid symbols represent complex viscosity. Failure of this rule indicates significant structural formation.

Figure 4.26 Steady shear viscosity vs. shear rate at various temperatures from 10 to 50°C of CNC suspensions, sonicated at 1000 J/g CNC, with concentrations of (a) 3 wt. %, (b) 5 wt. % (c) 7 wt. %, and (d) 10 wt. %.

Figure 4.27 Polarized optical micrographs at (a) 20°C, (b) 30°C, and (c) 40°C of 5 wt. % CNC suspension under constant shear rate of 0.1 s−1.

Figure 4.28 Temperature effects on shear viscosity for a 5 wt. % CNC suspension (sonicated at 1000 J/g CNC) at different shear rates.

Figure 4.29 Effect of temperature on the complex viscosity of CNC suspensions at various concentrations and at a constant angular frequency of 1 rad/s.

Figure 4.30 (a, b) Viscosity vs. shear rate for CNC suspensions possessing 4.39 OSO3H/100 glucose units. All suspensions were sonicated for 1000 J/g CNC.

Figure 4.31 (a, b) Viscosity vs. shear rate for CNC suspensions possessing 3.55 OSO3H/100 glucose units. All suspensions were sonicated for 1000 J/g CNC.

Figure 4.32 POM micrographs of CNC suspension possessing 4.39 OSO3H/100 glucose units at different concentrations.

Figure 4.33 POM micrographs of CNC suspension possessing 3.55 OSO3H/100 glucose units at different concentrations.

Figure 4.34 Storage moduli, G′ (solid symbols), and loss moduli, G″ (open symbols), vs. angular frequency, ω, of CNC suspensions possessing (a) 4.39 and (b) 3.55 OSO3H/100 glucose units at different concentrations. All suspensions were sonicated to 1000 J/g CNC.

Figure 4.35 Shear rate‐dependent viscosity profiles for constant‐mass, isotropic CNCs suspensions at different electrolyte concentrations (up to 10 mM NaCl). At ~15 mM NaCl, the CNC suspension undergoes coagulation (not shown).

Figure 4.36 Shear rate‐dependent viscosity profiles for 7 wt. % (a) and 10 wt. % (b) CNC suspensions at different electrolyte concentrations.

Figure 4.37 POM images for 9 wt. % CNC suspensions at different NaCl concentrations, taken at shear rate of 0.01 s−1. The scale bar is 50 µm in all images.

Figure 4.38 Storage, Gʹ (filled symbols), and loss, Gʺ (open symbols), moduli vs. angular frequency, ω, at different CNC concentrations without NaCl.

Figure 4.39 Storage, Gʹ (filled symbols), and loss, Gʺ (open symbols), moduli vs. angular frequency, ω, for (a) 7 wt. % and (b) 10 wt. % CNC suspensions at different NaCl concentrations.

Figure 4.40 Storage, Gʹ (filled symbols), and loss, Gʺ (open symbols), moduli vs. angular frequency, ω, for (a) 12 wt. % and (b) 15 wt. % CNC suspensions at different NaCl concentrations.

Figure 4.41 Correlation functions for a 10 wt. % CNC sample under oscillatory shear measured from 0 to 300 s at (a) f = 1 Hz and (b) f = 10 Hz. The evolution of the fist echo is shown for various strains. The echo amplitude drops significantly from 1 at γ0 = 15% and f = 1 Hz and 27% and f = 10 Hz exhibiting shear‐induced irreversible rearrangement (yielding). The initial decay rate with stain at f = 10 Hz. The linear dependence indicates the absence of wall slip.

Figure 4.42 Thermogravimetric responses of H2SO4‐hydrolyzed CNCs is pH sensitive. (Solid line represents CNC films at pH = 7; dotted line, pH = 10; and dashed line, pH = 3.) Thermal stability improves at neutral and basic pH. Thermogravimetric testing was conducted using Perkin Elmer Pyris 6 instrument equipped with Pyris V6.0 software. The samples (approximate mass between 5 and 6 mg) were individually heated at a constant rate of 10°C/min in nitrogen (flow rate = 20 ml/min). Heating was continued until the sample attained constant mass.

Figure 4.43 Thermogravimetric responses for H2SO4‐hydrolyzed CNC films (control, dashed line), and solvolytically desulfated CNC film (solid line), and BuOH‐washed solvolytically desulfated CNC film (dotted line). Sulfation was reduced from 298 mmol/kg CNC for the control film to 18 mmol/kg CNC for the desulfated CNC film.

Figure 4.44 SEM micrographs (at three magnification scales) of solvolytically desulfated CNC films, where the particles completely lose their chiral nematic organization. This occurs regardless of the hydrolysis conditions used for CNC synthesis—see SEM images for sulfated and heated CNC films shown in Figure 4.5.

Figure 4.45 X‐ray diffractometric patterns of lyophilized (Na‐form) CNC indicating structural stability from approximately −125 to ~300°C and suitability for high‐temperature processing. Heating was carried out in capillary tubes using GADDS system operated at 50° intervals. (Typical X‐ray settings are described in Section 4.3.1.)

Chapter 05

Figure 5.1 CNCs are nanomaterials that possess the full suite of properties described in this illustration. As such, CNCs are the only functional nanomaterials derived from entirely renewable resources using selective strong mineral acid hydrolysis. It is apposite to note that other mechanical or chemi‐mechanical treatments of biomass produce cellulose nanostructures that only share some of the attributes described earlier.

Figure 5.2 Schematic illustrations of the stress–strain response and corresponding physical properties of reinforcement, matrix, and composite. Here, UTS, ultimate tensile strength; εf, strain at failure; E, modulus of elasticity (or Young’s modulus); and toughness, the shadow area under stress–strain curve (includes both elastic and plastic contributions).

Figure 5.3 Examples of chemical agents that can react with ─OH on cellulose surface.

Figure 5.4 Dynamic mechanical data—top plot represents the tan δ, and bottom one, the storage modulus, as functions of temperature—for nanocomposites of CNCs and PVA. It is evident the reinforcing effect of CNCs increases with increasing loading—rubbery storage modulus is increased twofold at around [CNCs] = 9.1 wt. % and fourfold at [CNCs] = 20 wt. %. The Tgof the polymer nanocomposites are practically unaffected. (DMA testing was performed at the same relative humidity and in tensile mode with a DMA Q800 TA Instrument. Measurements were made at 1 Hz constant frequency, 0.15% strain amplitude, 30–100°C temperature range, and 5°C/min heating rate.)

Figure 5.5 Experimental data and percolation predictions for the dynamic mechanical response for CNC–PVA nanocomposites as a function of CNC volume fraction. The top figure represents the logarithm of the storage tensile modulus at 35°C (below Tg), and the bottom at 80°C (above Tg). The experimental data present a better fit to the percolation model than to the Halpin–Kardos model—which assumes good dispersion of reinforcement, but no reinforcement–reinforcement interactions. For percolation modeling, the following assumptions were followed: Ef, modulus of the CNC network = 10 GPa; Em, modulus of the polymer matrix at 35°C = 4.76 GPa, and at 80°C = 0.227 GPa. (Refer to Section 5.2.8 for details.) The experimental data show equivalent, or better, correspondence to the percolation model, indicating the excellent compatibility between CNCs and PVA within the nanocomposite.

Figure 5.6 X‐ray diffractograms for CNC, PVA, and CNC–PVA nanocomposites at different CNC loadings. It is evident that the structure of CNCs remains intact throughout the nanocomposites, and CNC presence becomes apparent at loadings above 4 wt. %, which is approximately the critical percolating threshold. Diffraction was carried out using a Bruker D8 Advance powder X‐ray diffractometer equipped with a CuXα X‐ray tube, a diffracted beam graphite monochromator and NaI scintillation detector. (More specifics regarding XRD experimentation can be found in Hamad and Hu 2010.)

Figure 5.7 Scanning electron micrograph illustrating typical morphology of spray‐dried cellulose nanocrystals (SD‐CNCs).

Figure 5.8 Scanning electron micrographs of spray‐freeze dried CNCs (SFD‐CNCs) illustrating the interior structure. When low concentrations (0.5 wt. %) of aqueous CNC suspension are used, SFD‐CNCs have morphologies that are porous, agglomerated and filament‐like (a–c). However, for higher concentrations (6 wt. %), SFD‐CNCs appear as spherical, foam‐like structures (d–f).

Figure 5.9 Schematic depicting solution ring‐opening polymerization reaction of CNCs and L‐lactide in a suitable solvent, where the former is dispersible as individual nanoparticles and the latter easily dissolved (a). Alternatively, bulk ring‐opening polymerization reaction occurs with surface‐modified CNCs suspended in molten L‐lactide (b).

Figure 5.10 DSC thermograms of neat PLA, CNC, and CNC–PLLA nanomaterials, I–III (prepared by in situ ring‐opening polymerization according to Hamad and Miao 2014) carried out in nitrogen environment at 10°C/min. Tcand Tmincrease with increasing molecular weight, and the choice of catalyst used in the in situ polymerization process (at the same reaction conditions) affects the molecular weight—higher MW for Sn(oct)2.

Figure 5.11 Isochronal evolution of the storage modulus, E′, as a function of temperature for neat PLA films and nanocomposite films produced from extruding CNC–PLLA nanomaterials at different CNC loadings, prepared by in situ ring‐opening polymerization, and commercial PLA (Hamad and Miao 2014). (a) The graph illustrates crystallization and softening effects in highly crystallizable PLA, whereas (b) depicts the responses in poorly crystallizable PLA. In both cases, the role of CNCs as effective nucleating agents is clear.

Figure 5.12 (a) Neat PLA and CNC–PLA nanocomposite films. (b) SEM images illustrating CNC dispersion within an extruded CNC–PLA nanocomposite pellet from which the film in (a) is made. All films appear transparent; however, there appears a yellow haze with CNC–PLA nanocomposite films—due to the interaction between L‐lactide and Sn(oct)2 in small laboratory‐scale settings. The transparent nature of the CNC–PLA nanocomposite film indicates that dispersion—or nonaggregation—of the cellulose nanoparticles within the biopolyester is optimal and uniform following the in situ ring opening polymerization approach described in Hamad and Miao (2014).

Figure 5.13 Gas barrier performance global map for typical packaging materials—for example, poly(ethylene terephthalate), PET, PLA, and high‐density poly(ethylene), HDPE, as well as CNC and CNC–PLA nanocomposite films. CNCs, through influencing the crystallization state of the polymer resin, contribute to improving both gas and water vapor barrier properties (normalized with respect to film thickness).

Figure 5.14 Reaction scheme of the free‐radical, surface‐grafting polymerization of dimethylamino propyl methacrylamide (DPMA) in the presence of CNCs. The reaction is carried out at pH ≈ 11.5 using ammonium persulfate (APS) as initiator, at 50°C for ~18 h. The resulting nanomaterial, P(DPMA)–CNCs, can then be used with the epoxy‐hardener emulsion system to cast and cure the nanocomposite material.

Figure 5.15 Stress vs. strain behavior of the P(DPMA)–CNC–epoxy nanocomposite system indicating synergetic improvements in the mechanical performance. (Concentration of the P(DPMA)–CNC nanomaterial is 4 wt. % in the system, and of CNC is 3 wt. %.) Relative to the neat epoxy system (a pure gilycidyl ether of bisphenol A with an epoxide equivalent weight of 171–175 and a polyetheramine primary diamine curing agent with an average molecular weight of about 230), there is ~20, 30, and 80% improvement in ultimate tensile strength, Young’s modulus and strain‐to‐failure of the novel P(DPMA)–CNC–epoxy nanocomposite system (Hamad and Su 2012).

Figure 5.16 Dynamic mechanical response—storage (a) and loss (b) moduli—of the P(DPMA)–CNC–epoxy nanocomposite system described earlier. There is an order‐of‐magnitude change in storage modulus for the nanocomposite system relative to neat epoxy extending over the full range of the rubbery state (Hamad and Su 2012).

Figure 5.17 Schematic diagram of the series‐parallel model. R and S represent the rigid (reinforcement) and the soft (polymer matrix) phases, respectively, and ψ the volume fraction of the percolating rigid phase.

Figure 5.18 Theoretical plots, based on percolation theory, of the relationship between storage modulus and CNC volume, or weight, fractions illustrating the role aspect ratio (in the range 10–100) plays in predicting the mechanical properties of ideal CNC‐reinforced nanocomposites. Two variables are examined: CNC network strength, 10 GPa (a and c) and 5 GPa (b and d); the polymer matrix density is taken to be 0.9 g/cm³ (a and b) or 1.23 g/cm³ (c and d).

Figure 5.19 Examples of Pickering emulsions stabilized by CNCs at different concentrations. (a) Water : oil = 2 : 8 wt. %, [CNCs] in water = 1 wt. %, and (b) water : oil = 5 : 5 wt. %, [CNCs] in water = 5.5 wt. %. (Scale bars: (a) 1 mm, and (b) 10 microns.)

Figure 5.20 (a–c) Confocal laser scanning microscopy (CLSM) images of hexadecane‐in‐water emulsion stabilized by CNCs, indicating that above closed packing conditions the spherical droplets are transformed to a polygonal shape.

Figure 5.21 (a) Tensile stress–strain behavior for GMA‐g‐CNC/PAM nanocomposite hydrogel (20 wt. %) relative to typical chemically cross‐linked hydrogels (BIS/PAM). The response regions for GMA‐g‐CNC polymer nanocomposite hydrogels can be categorized as: linear elastic (region I), viscoelastic (region II), visco‐plastic response (region III), and yielding/relaxation (region IV). (b) The graph represents the tensile stress–strain curves for this polymer nanocomposite hydrogel at different functionalized CNC concentrations (10, 20, and 50 wt. % relative to acrylamide weight).

Figure 5.22 SEM (a, b) and polarized light (c, d) micrographs depicting the chiral nematic organization of EISA films prepared from CNC–DMAPS nanocomplexes (CNCs : DMAPS = 1 : 0.41)—cf. Figure 4.5 illustrating the same for neat EISA CNC films.

Figure 5.23 The stress–strain behavior of EISA CNC–DMAPS films depends on DMAPS dosage and pH. If the pH is changed from neutral (main plot) to ~2.5 (inset), the maximum stress of the film is dramatically reduced (comparison is made for CNC : DMAPS = 1 : 0.41). The stretch in both cases is only slightly affected. The photograph in the sub‐insert illustrates the flexibility of the film. Legend: CNC : DMAPS = 1 : 1 (solid black line) and 1 : 0.41 (dashed black line).

Figure 5.24 Synthetic route to photonic nanopaper or mesoporous photonic cellulose (MPC) films. Initially, an aqueous suspension of cellulose nanocrystals (CNCs) is combined with a urea‐formaldehyde (UF) precursor. Following evaporation‐induced self‐assembly (EISA), a CNC–UF composite with chiral nematic order is obtained. Thermal curing of the composite is followed by treatment with aqueous KOH to yield MPC films.

Figure 5.25 Structural characterization of MPC films. (a) Solid‐state ¹³C CP/MAS NMR spectra of the CNC–UF composite and MPC; (b) PXRD pattern of pristine CNCs (cellulose I), CNC–UF composite, and MPC; and (c) Isothermal N2 adsorption of MPC dried from EtOH with supercritical CO2 (BET surface area), as well as the corresponding BJH pore‐size distribution calculated from the adsorption branch of the isotherm (inset).

Figure 5.26 Optical characterization of the CNC–UF composite and resulting MPC film. (a) Photograph of composite film after curing with its SEM image on the bottom. (b) Photograph of MPC air dried from H2O with its SEM image on the bottom. (c) Photograph of MPC dried from EtOH with supercritical CO2 with its SEM image on the bottom.

Figure 5.27 Sensing performance of the functional MPC films. (a) Photographs of MPC soaked in different EtOH : H2O ratios as indicated, and (b) UV‐Vis (solid lines) and CD spectra (dashed lines). (c) Swelling kinetics of MPC: Curve shows the dependence of the wavelengths on time when a dry sample is immersed in water. (d) Pressure response of MPC at 0, 0.4, 0.8, 1.6, 2.7, 5.9, and 7.8 × 10⁶ N/m² (first and final value given in graphic) showing a clear blue‐shift of the peak reflectance wavelengths. (e) Peak reflection wavelengths vs. pressure plotted, the data were fit with an exponential curve.

Figure 5.28 Self‐assembly of CNCs with silica sol‐gel precursors indicates the fingerprint‐like texture characteristic of the formation of a chiral nematic phase as illustrated by polarized optical microscopy, POM (a). Evaporation of the mixture to dryness yields an iridescent composite (b, c) whose structural color depends on the silica loading. After calcination of the composite to remove the CNC template, mesoporous sol‐gel derived films are obtained that retain their intense structural color (d). The use of organosilica precursors yields flexible films after acid hydrolysis to remove the CNC template from the composite (e). Cracking observed during evaporation can be eliminated in the mesoporous silica films by the addition of polyols such as glucose, leading to large, crack‐free films (f).

Figure 5.29 Helium ion microscopy of chiral nematic mesoporous (a–d) ethylene‐bridged organosilica and (e, f) silica at varying magnification.

Figure 5.30 Effect of refractive index changes on the photonic properties of CNMS films. (a) Series of photographs showing the reversible color change that occurs upon wetting and drying a CNMS film. (b) Circular dichroism (CD) spectra of a CNMS film at different sucrose concentrations ranging from 0 to 50 wt. %. (c) Plots of peak intensity and position vs. refractive index for the series of CD spectra shown in (b).

Figure 5.31 Chiral nematic mesoporous titania thin films prepared by nanocasting appear iridescent through a left‐handed circularly polarized filter and opaque white through a right‐handed filter (a), illustrating the selective reflection in these materials. SEM images of the thin films (b, c) confirm mimicking CNCs’ helical, twisting structure, with globules likely introduced by titania crystallization.

Figure 5.32 Photographs and characterization of new hybrid materials templated by CNCs. (a) Silica films doped with Au and Ag NPs depicting characteristic colors arising from the plasmon resonance of the NPs.CD spectra (b) clearly demonstrate the induction of chirality to the plasmon resonance by the chiral nematic surrounding.(c) Photographs showing chiral nematic organosilica films before (I) and after doping with PPV (II) as well as fluorescence from PPV composites under UV illumination (III).(d) Fluorescence quenching of the PPV composites by diluted solutions of TNT demonstrating their potential use in sensing.

Figure 5.33 Infiltration of chiral nematic organosilica films with the thermos‐responsive liquid crystals (LCs) reversibly changes the color from green (a) to transparent (b) upon heating from the liquid crystalline phase to the isotropic phase. The change in color and its reversibility can be quantified by UV‐Vis studies showing the decrease of the photonic signal upon heating the LC‐infiltrated composite material to 50°C (c) and the return of the photonic signal upon cooling to room temperature (d).

Figure 5.34 Demonstration of the potential application of mesoporous photonic plastics in security features or optical sensors. (a) The mesoporous films appears brightly red‐colored under a left‐handed circular polarizer (a, left), while its color disappears under a right‐handed polarizer (a, right). (b) The swelling behavior of the mesoporous plastics in mixtures of water and ethanol is shown by the photographs (bottom) and schematic illustration (top). Swelling of the material leads to a change in the helical pitch that affects the color of the chiral nematic polymer films. (c) The swelling kinetics of the samples in water reveals a fast response of the material to solvent changes. (d) UV‐Vis spectra show a systematic shift of the reflection signal of the chiral nematic resin during swelling in different mixtures of water and ethanol.

Figure 5.35 (a) Images from static CA measurements of CNC‐templated phenol‐formaldehyde resin films treated with HCl (left), pristine (middle), and treated with CH2O (right); (b) photographs of a strip of the resin films illustrating changes in color in water/ethanol binary solvent mixtures of different proportions. The left and right ends of the strip were treated with HCl and CH2O, respectively, while the middle part remained untreated; (c) CD spectra of the untreated (green), HCl‐treated (blue), and CH2O‐treated (red) resin films in dry state (solid lines) and after swelling in water (dashed lines). As the CD spectrometer that was used could not measure beyond 900 nm, the reflection peak of the CH2O‐treated film in the swollen state was confirmed with complementary UV‐Vis spectroscopy (red dotted line). Inkjet printing of photonic patterns on mesoporous‐resin films: (d) pattern printed as letters UBC on the mesoporous resin film, (e) UBC patterned as an image, (f) and (g) more complicated images patterned on another resin film. The pattern in (f) was revealed by swelling in water while the pattern in (g) was revealed in 20/80 (v/v) water/ethanol mixture. The appearance and disappearance of the patterns upon swelling and drying, respectively, are completely reversible.

Figure 5.36 Synthesis of the mesoporous chiral nematic bilayer phenol‐formaldehyde (PF) resin films. A suspension of CNCs is mixed with an aqueous solution of the PF polymer precursor. Layer‐by‐layer film fabrication resulted in bilayer composite films with chiral nematic organization upon evaporation‐induced self‐assembly (EISA) of the CNC–PF mixture followed by thermal curing. Treatment with alkaline solution removes (most of) the CNC template, yielding highly iridescent, bilayer mesoporous photonic resins after supercritical drying.

Figure 5.37 (a) Schematic (left) and corresponding photographs (right) of bilayer film showing selective swelling of one layer causing actuation to the direction of the opposite layer. (b) Site‐selective actuation of the bilayer resin film cut in a hand shape. All fingers are straight when the film is swollen in water (left). Alternate fingers bend when drops of acetone are put on the respective fingers and allowed to dry (panels in the middle). All fingers curl when the film is completely dry (right). The light yellow color of the bilayer films originates from the resin, and is apparent when the films are viewed on a white background.

Figure 5.38 Formation of the chiral nematic structure in nanocomposite hydrogels at varying