Cellulose Nanocrystals Surface Research Articles

Facile Method to Determine the Molecular Weight of Polymer Grafts Grown from Cellulose Nanocrystals.

One of the main challenges associated with the modification of cellulose nanocrystals (CNCs) with polymers by surface-initiated polymerization is the characterization of the resulting products, notably the molecular weight of the grafts. The solid nature of the (modified) CNC nanoparticles limits the possibility to apply solution-based characterization techniques, and the cleavage of the macromolecules from the surface of the CNCs to enable their characterization using solution-based techniques is intricate. Here, we report that 1H NMR spectroscopy of the supernatant of the heterogeneous reaction mixture can be used to approximate the molecular weight of poly(hexyl methacrylate) grafts grown from the surface of CNCs via surface-initiated atom transfer radical polymerization. This was achieved using 1H NMR spectra to determine the monomer conversion from the change of the relative ratio of monomer and solvent signals in the 1H NMR spectra, which in turn allowed determining the weight of PHMA produced. The number-average molecular weight of the grafted polymer was then estimated by assuming that standard atom transfer radical polymerization kinetics are at play and using the initiator concentration on the CNC surface determined by elemental analysis. The method was validated by comparing the results with the gravimetric data and the data of free polymers that were synthesized with a sacrificial initiator.

THERMAL AND MECHANICAL PROPERTIES OF POLYMER COMPOSITES REINFORCED BY SULFURIC ACID-HYDROLYZED AND TEMPO-OXIDIZED NANOCELLULOSE: A COMPARATIVE STUDY

The main drawback of cellulose nanocrystals (CNCs) obtained by conventional sulphuric acid hydrolysis is their low thermal stability in consequence of pyrolysis catalyzed by sulfo-groups on the CNC surface. Replacement of surface sulfo-groups by carboxyl groups as a result of oxidation allows the thermal stability of CNCs to be enhanced significantly. Although a great number of studies have reported properties of polymer nanocomposites reinforced by CNCs, thermal properties of the composites compared to the neat polymers are discrepant and still poorly understood. In this work, CNCs were produced from microcrystalline cellulose by sulfuric acid hydrolysis and (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) oxidation. The CNC composites with water-soluble polymers – polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone and polyacrylamide – were obtained. The composites were characterized by various methods, i.e. transmission electron and scanning electron microscopies, energy-dispersive X-ray analysis, Fourier-transform infrared spectroscopy, X-ray diffraction and thermogravimetric analyses, differential scanning calorimetry, and tensile testing. A side-by-side comparison between the thermal and mechanical properties of the polymer composites reinforced by sulfuric acid-hydrolyzed and TEMPO-oxidized nanocellulose was conducted. Analysis of the thermal properties of CNC shows that the surface sulfonate groups replacement with carboxyl groups leads to significant increase of initial temperature of thermal degradation and temperature of the maximum decomposition rate of the CNC. However, the thermal behavior of the composites is much more complicated, and such thermal properties are discussed in detailed. The tensile properties analysis of the composites demonstrates that an addition of TEMPO-oxidized nanocellulose does not improve significantly the tensile strength and Young’s modulus as compared with sulfuric acid-hydrolyzed one.

Encapsulation of cellulose nanocrystals into acrylic latex particles via miniemulsion polymerization

Cellulose nanocrystals (CNCs) show promise for enhancing properties of waterborne latexes and coatings. Miniemulsion polymerization is a promising route to obtaining well-controlled latex distributions sufficient to encapsulate CNCs in the monomer phase. Here, we demonstrate the first miniemulsion polymerization resulting in encapsulation of CNCs that incorporates costabilizer, water soluble initiator, and comonomer mixtures, aspects that are important for practical implementation. CNCs were added to the monomer phase as either unmodified (umCNC) or one of two functionalized forms: modified with isocyanatoethyl methacrylate (mCNC, containing a polymerizable vinyl group) or grafted with butyl acrylate/methyl methacrylate copolymer (gmCNC). The gmCNCs were incorporated successfully into acrylic particles. In contrast, the umCNCs were located outside the formed latex particles. Interestingly, the mCNCs coagulated during the polymerization, highlighting the challenge of utilizing polar and reactive acryloyl CNC surface modifiers that lead to migration from the droplet phase.

Studies into interactions and interfacial characteristics between cellulose nanocrystals and bovine serum albumin

This study investigates the interactions between cellulose nanocrystals (CNCs) and bovine serum albumin (BSA) under different pH conditions. A multiscale technique was employed to characterize the CNCs and BSA at pH 7 and pH 3. ζ-Potential measurement and UV–vis spectroscopy demonstrated strong interactions between CNCs and BSA at pH 3, whereat they have opposite charges. Interfacial tensiometry showed a deficiency in the surface activity of the CNCs and indicated that BSA dominated the interface behavior in their complex. Quartz crystal microbalance with dissipation revealed that the sequential adsorption of BSA and CNCs produced viscoelastic bilayers at pH 3, and the mass adsorbed was ∼ 28 times that adsorbed at pH 7. Molecular dynamics simulations indicated that the key interactions between the two materials were produced between the hydrophobic CNC surface and the BSA domain IIA region. These results provide interesting insights into the design of complex food emulsions and fluid interfaces.

Effects of cellulose nanocrystals on the vulcanization of natural rubber/cellulose nanocrystals nanocomposite and corresponding regulating strategies

AbstractVulcanization is a vital process in rubber processing, it endows rubber with valuable physical and mechanical properties, making rubber a widely used engineering material. In addition to vulcanization agent, reinforcing fillers play a non‐ignorable influence on the vulcanization of rubber nanocomposites. Herein, the effects of cellulose nanocrystals (CNCs) on the vulcanization of natural rubber (NR)/CNCs nanocomposite was studied. It was found that even though the addition of CNCs can effectively improve the dispersion of ZnO in NR matrix, the vulcanization of NR was inhibited. This may be attributed to the CNCs' adsorption of vulcanizing agents (DM, ZnO) and the acidic chemical environment on the surface of CNCs. In order to improve the vulcanization properties of NR/CNCs nanocomposite, tetramethyldithiochloram (TMTD) and triethanolamine (TEOA) were used as a combination accelerator and curing activator, respectively, and polyethylene glycol (PEG) was introduced to screen hydroxyl groups on the surface of CNCs to prohibit the CNCs' adsorption of vulcanizing agents. The results indicate that TMTD and TEOA effectively improved the vulcanization rate of NR/CNCs nanocomposite and increased the crosslink density by an order of magnitude. Subsequently, the tensile strength, tear strength, and so forth. of NR/CNCs nanocomposite were significantly improved. However, PEG hardly help to improve the vulcanization properties of NR/CNCs nanocomposite. In addition, the control samples without CNCs were prepared and characterized, the comparation between NR and NR/CNCs nanocomposite shows that the synergistic effect of crosslink density and CNCs' reinforcement more effectively improve mechanical properties of NR. This work not only elucidates the inhibiting mechanisms of CNCs on the vulcanization of NR, but also provides practical strategies for improving the vulcanization and properties of NR/CNCs nanocomposite. It may accelerate the application of CNCs as rubber reinforcing filler.

Interactions in solvent–polycaprolactone–cellulose nanocrystals–polyvinyl pyrrolidone system: Experiment and molecular dynamics simulation

In this paper, physical adsorption of polyvinylpyrrolidone (PVP) was used as the method of surface modification of cellulose nanocrystals (CNCs). The work also considers the PVP molecular weight effect on dispersion of the surface-modified CNC in dichloromethane (DCM). The authors analyze the physicochemical properties of the composite with polycaprolactone (PCL) - CNC/PVP/PCL - prepared from a solution of PVP-modified CNCs in DCM, and conduct a molecular dynamics simulation of the interactions between the system (CNC, PVP, PCL) components in vacuum and in a solvent medium (water, DCM). According to the simulation results, the CNC/PVP/PCL composite has the following structure: PCL macromolecules have practically no direct contacts with the CNC surface but are located between the PVP macromolecules that, in their turn, are predominantly located near the CNC particle surface.

Fully Bio-Based Elastomer Nanocomposites Comprising Polyfarnesene Reinforced with Plasma-Modified Cellulose Nanocrystals.

This article proposes a process to prepare fully bio-based elastomer nanocomposites based on polyfarnesene and cellulose nanocrystals (CNC). To improve the compatibility of cellulose with the hydrophobic matrix of polyfarnesene, the surface of CNC was modified via plasma-induced polymerization, at different powers of the plasma generator, using a trans-β-farnesene monomer in the plasma reactor. The characteristic features of plasma surface-modified CNC have been corroborated by spectroscopic (XPS) and microscopic (AFM) analyses. Moreover, the cellulose nanocrystals modified at 150 W have been selected to reinforce polyfarnesene-based nanocomposites, synthesized via an in-situ coordination polymerization using a neodymium-based catalytic system. The effect of the different loading content of nanocrystals on the polymerization behavior, as well as on the rheological aspects, was evaluated. The increase in the storage modulus with the incorporation of superficially modified nanocrystals was demonstrated by rheological measurements and these materials exhibited better properties than those containing pristine cellulose nanocrystals. Moreover, we elucidate that the viscoelastic moduli of the elastomer nanocomposites are aligned with power–law model systems with characteristic relaxation time scales similar to commercial nanocomposites, also implying tunable mechanical properties. In this foreground, our findings have important implications in the development of fully bio-based nanocomposites in close competition with the commercial stock, thereby producing alternatives in favor of sustainable materials.

Coatable tannic acid-deposited cellulose nanocrystals for Fe(III) sensing and its application to a facile, scalable and portable sensing platform

Leveraging from the synergistic effect by combining biomass-based cellulose nanocrystals (CNCs) having high surface area and plant-based metal-chelating polyphenol coating was exploited. Herein, CNCs nanohybrid was fabricated by grafting tannic acid (TA) on the surface of CNCs (TA@CNCs). The TA@CNCs in the form of water dispersion were laminated on a variety of substrates ranging from hydrophobic to hydrophilic and flexible to rigid using a simple drop-casting process. The coated TA@CNCs exhibited excellent selectivity and sensitivity to Fe(III) in various samples including water, serum, and milk. The coated TA@CNCs displayed a remarkable color change from light gray to black on contact with Fe(III) in samples, with a limit of detection (LOD) of 0.52 and 0.54 ppb for serum and milk samples, respectively. The present approach of coating TA@CNCs onto various substrates can provide a large-scale, facile, and economical way for developing a portable sensing platform for Fe(III) with excellent biocompatibility.

The effect of sulfate half-ester groups on cellulose nanocrystal periodate oxidation

Periodate oxidation introduces aldehyde functionality to cellulose. The use of dialdehyde cellulose has been demonstrated for crosslinking and as a chemical intermediate towards functionalized cellulose. Commercially available cellulose nanocrystals (CNCs) typically carry a surface sulfate half-ester functionality, which results from their manufacture via sulfuric acid hydrolysis and subsequent esterification. The sulfate half-ester group is a bulky group carrying a net negative charge above pH 2 that modifies the colloidal and electro-chemical properties of the CNCs. Periodate oxidation is regioselective to the bond between carbons in positions 2 and 3 in the anhydroglucose unit while the sulfate half-ester groups are mostly considered to be located in carbon in position 6. This regioselectivity could be the reason why the role played by the sulfate half-ester group on modification by periodate oxidation has not previously been elucidated. Here, the influence of the sulfate half-ester on the oxidation of CNCs, which is shown to steer the oxidation kinetics and the properties of the resulting materials, is studied. Conventional physicochemical analysis of the oxidant consumption is accompanied by elemental analysis, Fourier-transform infrared, X-ray photoelectron and solid-state nuclear magnetic resonance spectroscopy, and wide-angle x-ray scattering analyses; the zeta potential is used to characterize the colloidal properties of the suspensions and atomic force microscopy for determining particle dimensions. The presence of the sulfate half-ester group decreases the rate of oxidation. However, the content of the sulfate half-ester groups decreases when degree of oxidation reaches approx. 50%. We demonstrate that the CNC surfaces are affected by the oxidation beyond the C2–C3 bond cleavage: insight into the kinetics of the oxidation process is a prerequisite for optimizing CNC oxidation.

Incorporating Hydrophobic Cellulose Nanocrystals inside Latex Particles via Mini‐Emulsion Polymerization

AbstractHydrophobic cellulose nanocrystals (CNCs) are encapsulated inside poly(butyl acrylate/vinyl acetate/acrylic acid) latex particles via mini‐emulsion polymerization (MEP). To achieve a genuine MEP, the effects of the concentration of surfactants (sodium dodecyl sulfate/Disponil A3065) and a hydrophobic agent (octadecyl acrylate (ODA)) are optimized in the presence of 0.5 wt% CNC. Using a combination of surfactant and ODA concentrations leading to a particle nucleation method restricted to the monomer droplets, the effects of CNC loading up to 1.5 wt% on the polymerization process and final nanocomposite properties are studied. Despite an increase in particle size and a lower rate of polymerization at higher CNC loadings, the droplet nucleation mechanism remains dominant up to 1.25 wt% CNC loading. Pressure‐sensitive adhesive (PSA) performance for nanocomposites produced using hydrophobic CNCs in MEP decreases whereas performance improves considerably when using hydrophilic and partially hydrophobic CNCs in conventional emulsion polymerization. These results shed light on emulsion polymerization technique selection and CNC surface properties for greener industrial production of water‐based PSAs.

Preparation of shape memory polyurethane/modified cellulose nanocrystals composites with balanced comprehensive performances

AbstractIn this work, we first synthesized modified cellulose nanocrystals (MCNC) with 2,4‐toluene diisocyanate and 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide via a two‐step in‐situ polymerization method, and then incorporated it with shape memory polyurethane (SMPU) to fabricate composite. It was found that the urethane chains and phosphorus‐containing flame retardant were covalently grafted onto the cellulose nanocrystals surface, and the MCNC exhibited good dispersion level in the SMPU matrix. Due to the crystallization‐induced and flame retardant effects of the prepared MCNC, it could be acted as a multifunctional reinforcer in SMPU composites. Noticeably, the most balanced improvements in crystallization, shape memory, mechanical, thermal stability, and flame retardancy properties of composites were observed with the incorporation of 3 wt% MCNC. Shape memory experiment demonstrated that the SMPU‐MCNC3 composite exhibited admirable thermal induced shape memory behavior, and its shape fixity and shape recovery ratios were maintained above 90% after five cycles. More importantly, the SMPU‐MCNC3 composite showed well balanced mechanical (σb = 13.1 MPa, εb = 189.3%), limiting oxygen index (23.8%), and UL‐94 rating (V‐2) among the prepared polyurethane films. In addition, the representative SMPU‐MCNC3 showed obviously reductions in heat and smoke production during combustion as compared to the neat SMPU. The present work offers a promising route to fabricate shape memory polyurethane composites with versatile functions.

Coating properties of UV-curable PUA nanocomposites with modified cellulose nanocrystals

A simple modification of cellulose nanocrystals (CNC) was intended for preparing ameliorated nanofillers for UV-curable PUA coatings. This modification was carried out by dispersing CNC in DMSO with 2-isocyanatoethyl acrylate using one-pot system to introduce acrylic groups on the surface of CNC. The dispersity of modified CNC was ensured by solvent exchange method in which can obtain CNC/solvent suspension without any drying step. FT-IR analysis was carried to identify the modification of CNC. The mechanical properties and surface hardness of the UV-cured composite coatings with different modified CNC contents were measured by universal testing machine and pencil hardness tester, respectively.

Green Hydrophilic Capsules from Cellulose Nanocrystal-Stabilized Pickering Emulsion Polymerization: Morphology Control and Spongelike Behavior.

Pickering inverse emulsions of hydroxyl oligoethylene glycol methacrylate were stabilized in isopropyl myristate, a biofriendly oil, using surface-modified cellulose nanocrystals (CNCs) as stabilizing particles. The emulsions were further polymerized by free or controlled radical polymerization (ATRP), taking advantage of the bromoisobutyrate functions grafted on the CNC surface. Suspension polymerization of the emulsion led to full bead or empty capsule morphologies, depending on the initiation locus. The thickness of the CNC shell surrounding the polymerized emulsions could be tuned by modulating the aggregation state of the CNCs after their surface modification. An increase from 6 to 40 CNC layers helped improve the compression moduli of the beads from a dozen to hundreds of kPa.

Tuning the Physicochemical Properties of Cellulose Nanocrystals through an In Situ Oligosaccharide Surface Modification Method.

The trend to replace petroleum-based products with sustainable alternatives has shifted research efforts toward plant-based materials such as cellulose nanocrystals (CNCs). CNCs show promise in numerous applications (e.g., composites and rheological modifiers); however, maximizing their performance often requires surface modifications with complex chemistries and purification steps. Presented here is a novel surface modification method with the potential to tune CNC properties through the in situ deposition of cellulose phosphate oligosaccharides during CNC production. This was achieved by leveraging the selective solubility of oligosaccharides, which are soluble at a low pH (during the CNC hydrolysis) yet become insoluble and precipitate onto CNC surfaces upon increasing pH during quenching. Oligosaccharide-coated CNCs demonstrated subtle changes including higher surface charge densities and lower water adsorption capacities and viscosities than their unmodified counterparts. CNC surface coverage was tuned by controlling the oligosaccharide degree of polymerization. Overall, this fundamental study introduces an easily scalable modification route that opens the door for expanded CNC functionality and applications.

Improvement of filtration and antifouling performance of cellulose acetate membrane reinforced by dopamine modified cellulose nanocrystals

A thin layer of polydopamine (PDA) was coated on the surface of cellulose nanocrystals (CNCs) by the self-polymerization of dopamine (DA) in water to improve the hydrophilicity and lipophilicity of CNCs. Then, the CNCs coated with PDA (PDA@CNCs) were blended with pore forming agent and cellulose acetate (CA), and the CA composite membrane reinforced by PDA@CNCs (PDA@CNCs/CA) was prepared by solution phase transformation method. The effects of PDA@CNCs content on the filtration performance, antifouling ability and mechanical properties of the composite membrane were studied. The results show that adding a small amount of PDA@CNCs can significantly improve the filtration capacity, antifouling performance and tensile strength of CA membrane. When the content of PDA@CNCs was 4 wt%, the hydraulic permeability, flux recovery ratio and tensile strength of PDA@CNCs/CA composite membrane increased by 33%, 15% and 76%, respectively. Simple DA modification improves the lipophilicity and dispersion of CNCs, enhances the binding force between CNCs filler and CA matrix, and improves the performance of CA composite membrane, thus widen the industrial application of CA materials, and shows a broad application prospect in the field of biological separation membrane and water treatment membrane.

Water-redispersible cellulose nanocrystals adsorption of glucose via alcohol precipitation

Cellulose nanocrystals (CNCs) are difficult to redisperse in water after completely dried due to their irreversible agglomeration during drying. Various alternatives have been proposed in the literature, some of which require energy-intensive methods of redispersion or might use an additive that is inadequate or toxic for food and pharmaceutical-related applications. The present work examined a novel approach that involved using glucose as redispersing agents to redisperse CNCs. The experimental data indicated that glucose could be adsorbed onto the surface of CNCs through alcohol precipitation. The oven-dried CNCs, which were adsorbed with glucose via alcohol precipitation, could be homogeneously redispersed in water. The water-redispersed CNCs showed similar particle size distribution and zeta potential to those of the CNCs without drying.

PAMAM-Functionalized Cellulose Nanocrystals with Needle-Like Morphology for Effective Cancer Treatment.

Gene therapy is used to correct or compensate for diseases caused by gene defects and abnormalities. Improving the transfection efficiency and reducing the toxicity of gene carriers are the keys to gene therapy. Similar to a typical cationic gene carrier—polyethylenimine (PEI, 25 kDa)—the polyamidoamine (PAMAM) dendrimer also has a large number of amino groups. These amino groups can be complexed with nucleic acids after protonation under physiological conditions. However, the concentrated positive charge can cause undesirable cytotoxicity. Cellulose nanocrystals (CNCs) have good biocompatibility and unique needle-like morphology, and have been proven to be efficiently taken up by cells. In this article, three-dimensional spherical PMAMA dendrimers are conjugated onto the surface of CNCs to obtain a kind of needle-like cationic carrier (CNC-PAMAM). PAMAM dendrimers act as anchors to bind the plasmid DNAs (pDNA) to the surface of the CNC. The prepared CNC-based carrier showed high transfection efficiency and low toxicity. The CNC-PAMAM can effectively deliver the suicide gene to the tumor site, enabling the suicide gene/prodrug system (cytosine deaminase/5-fluorocytosine (CD/5-FC)) to play an effective anti-tumor role in vivo. This research demonstrates that the functionalization of CNCs with PAMAM dendrimers is an effective method for developing novel gene delivery systems.

Luminescent films employing quantum dot-cellulose nanocrystal hybrid nanomaterials

Colloidal quantum dot (QD) materials are promising candidates for luminescent solar concentrator (LSC) technology thanks to their broadband absorption above semiconductor bandgap and tunable Stokes-shift for minimizing the light reabsorption. Previously reported QDs in LSC contain non-polar organic ligands that can be easily incorporated into common host matrix/waveguide materials – polyacrylates or polycarbonates, however, these organic ligands on QD surfaces introduce problems when some new matrix materials are employed. For example, cellulose nanocrystal (CNC), a wood-derived eco-friendly material, has the potential for LSC, but no solvents are available to dissolve both QD and CNC simultaneously. Here we report high-quality luminescent films employing QD-CNC hybrid materials. Such films are produced from stable QD-CNC suspension by applying solution-phase exchange to replace non-polar organic ligands on QDs to the metal halide complex, as well as performing acetylation on CNC surfaces. The fabricated QD-CNC hybrid film keeps similar photoluminescence quantum yield and lifetime compared to as-synthesized QDs in non-polar organic solvents. The developed method in this work has the potential to be widely used to mix QD with other materials having polar functional groups.

In Vitro and in Vivo Analyses of the Effects of Source, Length, and Charge on the Cytotoxicity and Immunocompatibility of Cellulose Nanocrystals.

Cellulose nanocrystals (CNCs) are an emergent, sustainable nanomaterial that are biosourced, abundant, and biodegradable. On account of their high aspect ratio, low density, and mechanical rigidity, they have been employed in numerous areas of biomedical research including as reinforcing materials for bone or tissue scaffolds or as carriers in drug delivery systems. Given the promise of these materials for such use, characterizing and understanding their interactions with biological systems is an important step to prevent toxicity or inflammation. Reported herein are studies aimed at exploring the in vitro and in vivo effects that the source, length, and charge of the CNCs have on cytotoxicity and immune response. CNCs from four different biosources (cotton, wood, Miscanthus x Giganteus, and sea tunicate) were prepared and functionalized with positive or negative charges to obtain a small library of CNCs with a range of dimensions and surface charge. A method to remove endotoxic or other impurities on the CNC surface leftover from the isolation process was developed, and the biocompatibility of the CNCs was subsequently assayed in vitro and in vivo. After subcutaneous injection, it was found that unfunctionalized (uncharged) CNCs form aggregates at the site of injection, inducing splenomegaly and neutrophil infiltration, while charged CNCs having surface carboxylates, sulfate half-esters, or primary amines were biologically inert. No effect of the particle source or length was observed in the in vitro and in vivo studies conducted. The lack of an in vitro or in vivo immune response toward charged CNCs in these experiments supports their use in future biological studies.

Self-healing and toughness cellulose nanocrystals nanocomposite hydrogels for strain-sensitive wearable flexible sensor

Recently, self-healing and high mechanical strength hydrogels have aroused much research due to their potential future in strain-sensitive flexible sensors. In this manuscript, we successfully designed self-healing and toughness cellulose nanocrystals (CNCs) nanocomposite hydrogels by grafted polypyrrole (PPy) on the surface of CNCs to enhance electrical conductivity. The obtained nanocomposite hydrogels exhibit outstanding self-healing and mechanical behaviors, and the optimal mechanical strength, toughness and self-healing efficiency can be up to 5.7 MPa, 810% and 89.6%, respectively. Using these functional nanocomposite hydrogels, strain-sensitive wearable flexible sensors were designed to monitor finger joint motions, bending of knee, and even the slight pulse beating. Surprisingly, the flexible sensors could evidently perceive body motions from large movements (knee bending) to tiny signals (pulse beating). In addition, it exhibited excellent durability after repeated cycles. This method of prepared self-healing nanocomposite hydrogels will have a potential prospect in the design of biomedical, biosensors, and flexible electronic devices.