Mechanically Robust Gels Formed from Hydrophobized Cellulose Nanocrystals (original) (raw)
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Cellulose
Utilization of reversible non-covalent interactions is a versatile design strategy for the development of stimuli responsive soft materials. In this study, hydrophobic interactions were harnessed to assemble water-soluble macromolecules and nanoparticles into a transient hybrid network forming thermosensitive hydrogels with tunable rheological properties. Hybrid hydrogels were built of biopolymer derived components: cellulose nanocrystals (CNCs), nanoparticles of high aspect ratio, and hydroxypropyl methylcellulose (HPMC). To enable polymer/CNC assembly via hydrophobic interactions, the surface of highly hydrophilic CNCs was modified by binding octyl moieties (octyl-CNCs). The amphiphilicity of Electronic supplementary material The online version of this article (
Hydrophobization of cellulose nanocrystals for aqueous colloidal suspensions and gels
Biomacromolecules, 2020
Surface hydrophobization of cellulose nanomaterials has been used in the development of nanofiller-reinforced polymer composites and formulations based on Pickering emulsions. Despite well-known effect of hydrophobic domains on self-assembly or association of water-soluble polymer amphiphiles, very few studies have addressed the behavior of hydrophobized cellulose nanomaterials in aqueous media. In this study, we investigate the properties of hydrophobized cellulose nanocrystals (CNCs) and their self-assembly and amphiphilic properties in suspensions and gels. CNCs of different hydrophobicity were synthesized from sulfated CNCs by coupling primary alkylamines of different alkyl chain lengths (6, 8 and 12 carbon atoms). The synthetic route permitted the retention of surface charge, ensuring good colloidal stability of hydrohobized CNCs in aqueous suspensions. We compare surface properties (surface charge, Zeta-potential), hydrophobicity (water contact angle, microenvironment probing using pyrene fluorescence emission) and surface activity (tensiometry) of different hydrophobized CNCs and hydrophilic CNCs. Association of hydrophobized CNCs driven by hydrophobic effects is confirmed by X-ray scattering (SAXS) and autofluorescent spectroscopy experiments. As a result of CNC association, CNCs suspensions/gels can be produced with a wide range of rheological properties depending on the hydrophobic/hydrophilic balance. In particular, sol-gel transitions for hydrophobized CNCs occur at lower concentrations then hydrophilic CNCs and more robust gels are formed by hydrophobized CNCs. Our work illustrates that amphiphilic CNCs can complement associative polymers as modifiers of rheological properties of water-based systems.
Carbohydrate Polymers, 2020
Rheological properties of hydrogels composed of TEMPO-oxidised cellulose nanofibrils (OCNF)-starch in the presence of cationic surfactants were investigated. The cationic surfactants dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB) were used to trigger gelation of OCNF at around 5 mM surfactant. As OCNF and DTAB/CTAB are oppositely charged, an electrostatic attraction is suggested to explain the gelation mechanism. OCNF (1 wt%) and soluble starch (0.5 and 1 wt%) were blended to prepare hydrogels, where the addition of starch to the OCNF resulted in a higher storage modulus. Starch polymers were suggested to form networks with cellulose nanofibrils. The stiffness and viscosity of OCNF-Starch hydrogels were enhanced further by the addition of cationic surfactants (5 mM of DTAB/CTAB). ζ-potential and amylose-iodine complex analyses were also conducted to confirm surface charge and interaction of OCNF-starchsurfactant in order to provide an in-depth understanding of the surfactant-induced gel networks.
Ion-Mediated Gelation of Aqueous Suspensions of Cellulose Nanocrystals
Biomacromolecules, 2015
Nanofibrillar hydrogels are an important class of biomaterials with applications as catalytic scaffolds, artificial extracellular matrixes, coatings, and drug delivery materials. In the present work, we report the results of a comprehensive study of nanofibrillar hydrogels formed by cellulose nanocrystals (CNCs) in the presence of cations with various charge numbers and ionic radii. We examined sol−gel transitions in aqueous CNC suspensions and the rheological and structural properties of the CNC hydrogels. At a particular CNC concentration, with increasing charge and cation size, the dynamic shear moduli and mesh size in the hydrogel increased. These effects were ascribed to a stronger propensity of CNCs for side-by-side association. The resulting hydrogels had an isotropic nanofibrillar structure. A combination of complementary techniques offered insight into structure−property relationships of CNC hydrogels, which are important for their potential applications.
Biomacromolecules, 2016
While injectable hydrogels have several advantages in the context of biomedical use, their generally weak mechanical properties often limit their applications. Herein we describe in situgelling nanocomposite hydrogels based on poly(oligoethylene glycol methacrylate) (POEGMA) and rigid rod-like cellulose nanocrystals (CNCs) that can overcome this challenge. By physically incorporating CNCs into hydrazone cross-linked POEGMA hydrogels, macroscopic properties including gelation rate, swelling kinetics, mechanical properties, and hydrogel stability can be readily tailored. Strong adsorption of aldehyde and hydrazide modified POEGMA precursor polymers onto the surface of CNCs promotes uniform dispersion of CNCs within the hydrogel, imparts physical cross-links throughout the network, and significantly improves mechanical strength overall, as demonstrated by quartz crystal microbalance gravimetry and rheometry. When POEGMA hydrogels containing mixtures of long and short ethylene oxide side chain precursor polymers were prepared, transmission electron microscopy reveals that phase segregation occurs with CNCs hypothesized to preferentially locate within the stronger adsorbing short side chain polymer domains. Incorporating as little as 5 wt % CNCs results in dramatic enhancements in mechanical properties (up to 35-fold increases in storage modulus) coupled with faster gelation rates, decreased swelling ratios, and increased stability versus hydrolysis. Furthermore, cell viability can be maintained within 3D culture using these hydrogels independent of the CNC content. These properties collectively make POEGMA-CNC nanocomposite hydrogels of potential interest for various biomedical applications including tissue engineering scaffolds for stiffer tissues or platforms for cell growth.
Magnetically tunable composite hydrogels with cellulose nanocrystals
Helsingin yliopisto, 2020
In this work, a series of biocompatible nanocomposite hydrogels was prepared by UV-initiated polymerization based on 2hydroxyethyl methacrylate (HEMA), using ethylene glycol dimethacrylate (EGDMA) as a crosslinker and 2-hydroxymethyl-2methylpropiophenone as a photoinitiatior, containing liquid crystals of cellulose nanocrystals (CNCs) doped with magnetic nanoparticles. The formation of liquid crystals was achieved thanks to the intrinsic property of CNCs to self-assemble above a critical aqueous concentration. By varying the preparation conditions, allowing different times for phase-separation between the nanoparticles and CNCs and exposing the polymerization mixture to small magnetic field, films with different size and orientation of CNC liquid crystal domains were synthesized. Subsequently, the hydrogel films were studied by dynamic mechanical analysis (DMA) to evaluate the effect of these parameters on the mechanical properties, specifically the Young's modulus and the ultimate strength. Also, the microstructure of the films was studied via polarized optical microscopy (POM) and scanning electron microscopy (SEM). The water uptake capacity was also analyzed. The results indicate that the presence of cellulose nanocrystals modulates the architecture of the prepared hydrogels. Cholesteric microdomains were embedded in PHEMA matrix and their uniaxial alignment was achieved upon exposure to small static magnetic field, already after several hours. Moreover, structural gradient in the distribution of the liquid crystalline microdomains, in dependence on their size, was obtained within the material. This originated from the direct proportionality between the size and the density of liquid crystals. Finally, it was shown that cellulose nanocrystals act as reinforcing structures of the hydrogels, when the degree of their self-assembly is sufficient, and therefore the resulting hydrogel exhibits both higher resistance to elastic deformation and also higher ultimate strength. Finally, we showed that mechanical performance of these nanocomposites can be enhanced by systematic orientation of the liquid crystalline domains.
The role of cellulose nanocrystals in biocompatible starch-based clicked nanocomposite hydrogels
International Journal of Biological Macromolecules, 2019
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Journal of the American Chemical Society, 2014
Cellulose nanocrystals (CNCs) are high aspect ratio colloidal rods with nanoscale dimensions, attracting considerable interest recently due to their high mechanical properties, chirality, sustainability, and availability. In order to exploit them for advanced functions in new materials, novel supracolloidal concepts are needed to manipulate their self-assemblies. We report on exploring multivalent interactions to CNC surface and show that dendronized polymers (DenPols) with maltose-based sugar groups on the periphery of lysine dendrons and poly(ethylene-alt-maleimide) polymer backbone interact with CNCs. The interactions can be manipulated by the dendron generation suggesting multivalent interactions. The complexation of the third generation DenPol (G3) with CNCs allows aqueous colloidal stability and shows wrapping around CNCs, as directly visualized by cryo high-resolution transmission electron microscopy and electron tomography. More generally, as the dimensions of G3 are in the colloidal range due to their ∼6 nm lateral size and mesoscale length, the concept also suggests supracolloidal multivalent interactions between other colloidal objects mediated by sugar-functionalized dendrons giving rise to novel colloidal level assemblies. S upramolecular chemistry and molecular self-assembly have been maturing to a rich variety of "bottom-up" approaches for advanced materials using molecular level construction units ranging from low to high molecular weights. Therein the structural information is encoded within the molecules and their balanced mutual interactions, geometrical sizes, and architectures to allow controlled assemblies and functions at molecular length scale. 1 Extension of these concepts to larger colloidal length scale has recently been pursued toward supracolloidal assemblies and new functions. 2 It involves subtleties, as the colloidal level units are more easily prone to aggregation than molecules, they require engineering of stronger and tunable but balanced interactions, and also the control of structural uniformity poses challenges. Several examples can be given on the recent progress: Rod-like metallic nanocrystals combining polymeric functional end groups which lead to colloidal level assemblies and functionalities have been introduced, such as for tunable plasmonics; colloidal assemblies architecturally resembling molecules have been formed; self-assembly between viral capsids and dendrimers have been explored; block-like colloidal level objects have been demonstrated by "living" block copolymer assemblies; and assemblies based on patchy colloids have been described. 2 Native nanocelluloses have a great promise in materials science as mechanically excellent, sustainable, widely available, and functionalizable 1D colloidal materials, 3 taken that the structures and self-assemblies could be fully mastered and transferred to advanced materials properties. Nanocelluloses can be cleaved from plant cell walls, which constitute the largest source of polymers on the Earth. Due to their native crystalline internal structure consisting of hydrogen-bonded parallel chains, they have extraordinarily high mechanical properties with modulus of ∼140 GPa and strength up to GPa range. 4 Two main forms of nanocelluloses can be extracted from plants, both having lateral dimensions in the nanometer range, where the rod-like cellulose nanocrystals (CNCs) have length of ∼100−500 nm, while the longer and entangled nanofibrillated celluloses reach micrometers. 3 Nanofibrillated cellulose has been pursued for, e.g., hydrogels, reinforcement of polymer composites, strong and transparent films, and to prepare ductile, lightweight, and functional aerogels. 5 The rod-like CNCs have been used to reinforce polymer blends, templating for chiral assemblies, and they have been decorated with polymer brushes or supramolecular groups for advanced functions, like self-healing material properties. 6 The CNCs are typically prepared by sulphuric acid hydrolysis of macroscopic cellulose fibers, thus leaving anionic sulfate ester groups on the CNC nanorod surface. 3 This allows colloidal stabilization via electrostatics in aqueous medium. The anionic groups have been used to bind cationic surfactants in order to tune the interfacial properties to low polarity medium as well as to prepare layer-by-layer assemblies using anionic CNCs and cationic polyelectrolytes. 7 Here we show that DenPols with dendritic side groups having maltose-based sugar peripheral units endure tunable generation-
Wet stable and mechanically robust cellulose nanofibrils (CNF) based hydrogel
Polymer, 2018
Freeze dried, highly porous materials made from cellulose nanofibrils (CNF) hydrogels are capable of absorbing and storing a significant quantity of liquid inside their 3D structure, with total absorption capacity increasing linearly with porosity. One of the challenges of freeze dried high porosity CNF gels is their propensity to break down rapidly in aqueous environments. Here we explore a method to overcome this deficiency by incorporating methacrylate functionalized carboxymethyl cellulose (MetCMC) into the CNF system followed by UV irradiation leading to crosslinking of the methacrylate groups of MetCMC. The resultant polymer composite matrix successfully maintains a robust 3D structure, without collapsing, even when rewetted and stored in water. When freeze dried, the CNF-MetCMC composite maintains its size and shape whereas air drying induces significant shrinkage. In contrast, air dried CNF-MetCMC hydrogels swell when rewetted. Swelling and shrinkage of CNF-MetCMC hydrogels were tuned by controlling the ratio between CNF and MetCMC in the composite. The crosslinking between the methacrylate groups of MetCMC also enhances the dry and wet modulus of CNF-MetCMC gels significantly. We invoke a simple model involving a balance between hydrogen bonding and crosslinking to explain these data. 1.0 Introduction Cellulose has recently gained significant attention for a broad range of applications due to the natural abundance and biodegradability of this polymer[1,2]. Cellulose can be comminuted to the nanoscale by the mechanical tearing of cellulose fibers in wood pulp, forming what are now referred to as cellulose nanofibrils (CNF)[3,4]. Freeze dried CNF based gels have been produced and are characterized by their 3D