Controlled production of patterns in iridescent solid films of cellulose nanocrystals

A method to produce predefined patterns in solid iridescent films of cellulose nanocrystals (CNCs) by differential heating of aqueous CNC suspensions during film casting has been discovered. Placing materials of different temperatures beneath an evaporating CNC suspension results in watermark-like patterns of different reflection wavelength incorporated within the final film structure. The patterned areas are of different thickness and different chiral nematic pitch than the surrounding film; heating results in thicker areas of longer pitch. Thermal pattern creation in CNC films is proposed to be caused by differences in evaporation rates and thermal motion in the areas of the CNC suspension corresponding to the pattern-producing object and the surrounding, unperturbed suspension. Pattern formation was found to occur during the final stages of drying during film casting, once the chiral nematic structure is kinetically trapped in the gel state. It is thus possible to control the reflection wavelength of CNC films by an external process in the absence of additives.     

Buildup mechanism of carboxymethyl cellulose and chitosan self-assembled films

Films with different numbers of layers have been built by alternating the adsorption of carboxymethyl cellulose (CMC) and chitosan (CHI) at different pH levels. The adsorption process was recorded by quartz crystal microbalance (QCM). The results showed that under all pH conditions considered, the growth of the films is nonlinear. The film construction performed at pH 4.0 (preferred assembly pH) with different numbers of bilayers (CMC/CHI as one bilayer) was also observed step by step by atomic force microscopy (AFM). Comparing the growth process from QCM with the surface morphological changes from AFM shows the existence of an inhomogeneous structure for the first nine bilayers, and, after a coalescence of islands, an increase in the number of bilayers was demonstrated. The possible growth mechanism was also evaluated.     

Langmuir-Blodgett films of cellulose nanocrystals: preparation and characterization

The goal of this work is the preparation of monolayers of cellulose I nanocrystals providing flat crystalline cellulose surfaces. Suspensions of cellulose nanocrystals were prepared by hydrolyzing ramie and tunicin fibers with sulfuric acid. Due to surface grafted sulfate groups, the negatively charged, rod-like cellulose nanocrystals were found to form stable layers at the air-water interface in the presence of a cationic amphiphilic molecule such as dioctadecyldimethylammonium (DODA) used in this work. These layers were formed at different cellulose-DODA weight ratios, compressed and analyzed by tensiometry, ellipsometry and Brewster angle microscopy. At low cellulose concentrations the layers are discontinuous, becoming dense and homogeneous upon reaching a critical weight ratio, which depends on the aspect ratio of the cellulose nanocrystals. After transfer onto silicon wafers, the surface composition and morphology as well as the thickness of the films were examined by X-ray photoelectron spectroscopy, ellipsometry and atomic force microscopy. The results indicate that they are monolayer films, well structured, relatively smooth and pure. These films offer a crystalline and easily reproducible model cellulose surface.     

Influence of high loading of cellulose nanocrystals in polyacrylonitrile composite films

Polyacrylonitrile-co-methacrylic acid (PAN-co-MAA) and cellulose nanocrystal (CNC) composite films were produced with up to 40 wt% CNC loading through the solution casting method. The rheological properties of the solution/suspensions and the structural, optical, thermal, and mechanical properties of the resulting films were investigated. The viscosity of the composite suspensions increased with higher CNC loadings and with longer aging times. PAN-co-MAA/CNC films maintained a similar level of optical transparency even with up to 40 wt% CNC loading. The glass transition temperature (Tg) increased from 92 to 118 °C, and the composites had higher thermal stability below 350 °C compared to both neat PAN-co-MAA and neat CNC. The mechanical properties also increased with higher CNC loadings, elastic modulus increased from 2.2 to 3.7 GPa, tensile strength increased from 75 to 132 MPa, and the storage modulus increased from 3.9 to 10.5 GPa. Using the Kelly and Tyson model the interfacial shear strength between the PAN-co-MAA and CNC was calculated to be 27 MPa.     

Reinforcement of hydroxypropylcellulose films by cellulose nanocrystals in the presence of surfactants

Hydroxypropylcellulose (HPC) films were prepared by casting with cellulose nanocrystals in the presence of anionic surfactant sodium dodecylsulphate (SDS) and cationic surfactant hexadecyltrimethyl ammonium bromide (CTAB). The cellulose nanocrystals were isolated from maize straw, a biomass source produced in huge quantities as an agrowaste in Brazil. These bionanocomposite films had good transparency and their surface hydrophilic character was evidenced by static contact angle measurements. Thermogravimetry (TGA) measurement revealed that nanocrystals and surfactants changed the thermal stability of the HPC films. Dynamic mechanical analysis (DMA) showed that the tensile storage and loss moduli of the HPC films increased by increasing the contents of cellulose nanocrystals and surfactants, especially in the case of CTAB. This good reinforcing effect of HPC matrix can be explained as due to electrostatic attractive interactions brought about by the presence of CTAB and the nanocrystals.     

Highly transparent nancomposite films based on polybutylmethacrylate and functionalized cellulose nanocrystals

Efficient surface functionalization of cellulose nanocrystals (CNC) with hydroxyl butyl acrylate monomer (HBA) was carried on under mild condition using N,N′-carbonyldiimidazole as an activator. The grafting of the acrylic monomer was shown to bring about the high yield grafting of polymer chains on the functionalized CNC during in situ polymerization process. Surface functionalization of CNC with HBA and the polymer grafting on the modified CNC were confirmed by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Nanocomposite film prepared from in situ polybutylmethacrylate polymerization process using HBA functionalized nanocrystals exhibited high transparency degree here assigned to improved dispersion. DMA analysis proved that the best mechanical/rheological performance is obtained for HBA–CNC contents of 4 %.     

Multiphoton photoemission of self-assembled silver nanocrystals

Silver nanocrystals, self-organized in compact hexagonal networks, on gold and graphite exhibit anisotropic optical properties. From polarized electron photoemission spectroscopy, a two-photon mechanism is demonstrated and an enhancement due to the surface plasmon resonance (SPR) of the nanocrystal film is observed. Two SPR peaks appear, due to dipolar interactions and induced by the self-organization of silver nanocrystals. This property is used to probe the substrate effect on the plasmon resonance. Its damping is related to particle–substrate interactions.     

Preparation of cellulose nanocrystals and carboxylated cellulose nanocrystals from borer powder of bamboo

Carboxylated cellulose nanocrystals (CCN) and cellulose nanocrystals (CNC) were prepared from borer powder of bamboo by two different kinds of procedures: one-step approach with ammonium persulfate for CCN and two-step approach with sulfuric acid for CNC. The obtained samples were characterizated by transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction and thermogravimetric analysis. The results show that the particles of CCN and CNC present spherical shape with diameters of 20–50 and 20–70 nm, respectively. The crystallinity of CCN and CNC is significantly improved after a series of chemical treatment, which is up to 62.75 and 69.84 %, respectively. The research indicates that the borer powder from bamboo could be an excellent raw material for manufacturing CNC in a low-cost and environmental-friendly way. Rational and sustainable utilization of the bamboo borer powder to develop new bioproducts holds great potential value for industry and offers many benefits and opportunities.     

Dielectrophoresis of cellulose nanocrystals and alignment in ultrathin films by electric field-assisted shear assembly

Ultrathin films of cellulose nanocrystals (CNCs) are obtained by using a convective assembly setup coupled with a low-strength external AC electric field. The orientation and degree of alignment of the rod-like nanoparticles are controlled by the applied field strength and frequency used during film formation. Calculated dipole moments and Clausius-Mossotti factors allowed the determination of the critical frequencies, the peak dielectrophoresis as well as the principal orientation of the CNCs in the ultrathin films. As a result of the combination of shear forces and low electric field highly ultrathin films with controlled, unprecedented CNC alignment are achieved.     

Highly ordered patterns of parabolic focal conics in lamellar lyotropic systems

In this paper, we report the experimental observation of the formation of highly ordered parabolic focal conical patterns in lamellar surfactant solutions. Predominantly, we investigated mixtures of sodium dodecyl sulfate, water, hexanol, and decane, located in the immediate vicinity of the region of the L3 and Lalpha phase coexistence. The experimental studies of the formation of the patterns and of their temporal development are described. We give a simple model picture for the underlying structure, corroborated by the experimental results. There appears to be only one independent length scale that controls the appearance of the whole pattern.     

Characterization of focal conics in chiral smectic C liquid crystals by X-ray microdiffraction

Focal conics consisting of an ellipse and a hyperbola in chiral smectic C liquid crystals aligned in surface-stabilized cells of medium thickness (about 25 µm cell gap) were characterized in relation to the chevron structure by synchrotron X-ray microdiffraction. The focal conic texture is embedded in the chevron structure with a relatively sharp interface. The deformed layer of the focal conics is a kinked bookshelf consisting of a few slightly bent segments for small focal conics, whereas it is a bookshelf layer for large focal conics. Around the focus of the ellipse toward the hyperbola, a complicated layer structure appears, although the core region of small layer curvature has been hardly observed within the present experimental sensitivity. The broad and narrow walls of a zigzag defect in the same cell are analyzed for comparison.     

Molecular wiring of nanocrystals: NCS-enhanced cross-surface charge transfer in self-assembled Ru-complex monolayer on mesoscopic oxide films

We report on rapid ambipolar cross-surface charge transfer within self-assembled monolayers (SAM) of the heteroleptic Ru-complexes cis-RuLL'(NCS)(2) (L = 2,2'-bipyridyl-4,4'-dicarboxylic acid, L' = 4,4'-dinonyl-2,2'-bipyridyl) (1) and cis-RuLL' '(NCS)(2) (L = 2,2'-bipyridyl-4,4'-dicarboxylic acid, L' = 4,4'-dimethyl-2,2'-bipyridyl) (2) on the surface of mesoscopic insulating oxide films. The bipyridyl ligands of the Ru-complex transport electrons, while the NCS groups plays a pivotal role in mediating surface confined hole percolation. Molecular dynamics calculations show the NCS ligands of 1 and 2 to orient in a fashion that enhances the overlap of the HOMOs of neighboring ruthenium complexes. Using ab initio Hartree-Fock calculations the electronic coupling matrix element for intermolecular hole exchange at the surface is estimated to be 0.13 eV. Cyclic voltammetry as well as spectroelectrochemical and impedance measurements performed with a series of other Ru-complexes confirmed the control of the cross surface charge transfer by the molecular structure. Complex 2 shows the highest percolation rate, the surface hole diffusion coefficient being 1.1 x 10(-8) cm(2)/s. The effects of the ligand properties, such as denticity, geometry, and size, on the intermolecular charge transport are discussed in detail.     

Preparation and characterization of bionanocomposite films based on wheat starch and reinforced with cellulose nanocrystals

Cellulose - In recent times, the attention of scientific community has been focusing on the replacement of petroleum-based polymers by others more environmentally friendly. In this sense,...     

Synthesis and self-assembled ring structures of Ni nanocrystals

Narrow size distribution Ni nanocrystals with average diameters from 5 to 13 nm ( approximately 20% standard deviation) and a face-centered cubic (fcc) structure were synthesized via rapid thermo-decomposition in the presence of surfactants in solution. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to characterize these nanocrystals. It was found that the solvent determined the rate of the decomposition of Ni precursors, while the surfactants controlled the size and shape of Ni nanocrystals. A three-step process was proposed to explain the synthesis. The purified Ni nanocrystals readily formed micrometer-sized ring structures on TEM grids after solvent evaporation (hexanes), and the magnetic field was found to increase the density of the rings.     

Dispersion of cellulose nanocrystals in polar organic solvents

In an attempt to prepare stable dispersions of cellulose nanocrystals in dipolar aprotic solvents, dilute aqueous suspensions of cellulose nanocrystals were prepared by sulfuric acid hydrolysis of cotton. The aqueous suspensions were freeze-dried, and then sonicated in the solvent of interest. Dispersions of 1 and 3% w/v concentration were prepared in polar organic solvents DMSO and DMF. The dispersions showed flow birefringence. The redispersion was incomplete, and there was some evidence for aggregation in the suspensions. A small amount of water appeared to be critical to suspension stability. Birefringent cellulose films were prepared from the dispersions by drying under vacuum and at ambient conditions.     

Layer-by-layer assembly of cationic guar gum,cellulose nanocrystals and hydroxypropyl methylcellulose based multilayered composite films

Eco-friendly sustainable materials provide an appealing template to replace contemporary synthetic-nonrenewable resource-based materials while maintaining the acceptable material properties to meet the performance requirements. Here, a layer-by-layer (LBL) self-assembly technique was used for fabricating multilayer composite films using all bio-based polymers/polysaccharides, i.e. cationic guar gum (CGg), carboxylated cellulose nanocrystals (cCNCs) and hydroxypropyl methylcellulose (HPMC). A five layered composite film was fabricated by depositing polymeric layers as follows: CGg→cCNCs→HPMC→cCNCs→CGg. The structural analysis of (CGg/cCNCs/HPMC)_5 L multilayered composite films indicated the existence of electrostatic interaction as well as H-bonding between polymeric layers that resulted in homogenous, dense and compact film surface with improved smoothness and strength properties. As compared to pure CGg film, the (CGg/cCNCs/HPMC)_5 L multilayered composite films showed improved tensile strength (84.8?% increment) and modulus (29.19?% improvement). Importantly, the deposition of HPMC layer contributed in achieving multilayer composite films with more flexible behavior (46.55?% improvement in elongation at break). Furthermore, owing to the high transparency (89.5?% transmittance), appreciable gas and oil barrier performance and resistance to various solvents (e.g. acetone, THF and DMAc), these multilayer films are promising candidates for various applications including renewable/sustainable packaging materials.

     

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

Vulcanization 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.     

Formation of hairy cellulose nanocrystals by cryogrinding

Cellulose - From a green chemistry perspective, cryogrinding of cellulose fibers conducted under mild conditions is introduced as a rapid, facile, and scalable methodology for the mechanochemical...