Gilberto Siqueira

Cellulose Whiskers versus Microfibrils: Influence of the Nature of the Nanoparticle and its Surface Functionalization on the Thermal and Mechanical Properties of Nanocomposites

In the present work, nanowhiskers and microfibrillated cellulose (MFC) both extracted from sisal were used to reinforce polycaprolactone (PCL). We report the influence of the nanoparticles nature on the mechanical and thermal properties of the ensuing nanocomposites. The surface of both the nanoparticles was chemically modified to improve their compatibilization with the polymeric matrix. N-Octadecyl isocyanate (C18H37NCO) was used as the grafting agent. PCL nanocomposite films reinforced with sisal whiskers or MFC (raw or chemically modified) were prepared by film casting. The thermal behavior (Tg, Tm, Tc, and degree of crystallinity) and the mechanical properties of the nanocomposites in both the linear and the nonlinear range were determined using differential scanning calorimetry (DSC), dynamical mechanical analysis (DMA), and tensile tests, respectively. Significant differences were reported according to the nature of the nanoparticle and amount of nanofillers used as reinforcement. It was also proved that the chemical treatment clearly improves the ultimate properties of the nanocomposites.

From interfacial ring-opening polymerization to melt processing of cellulose nanowhisker-filled polylactide-based nanocomposites.

In the present work, cellulose nanowhiskers (CNWs), extracted from ramie fibers, were incorporated in polylactide (PLA)-based composites. Prior to the blending, PLA chains were chemically grafted on the surface of CNW to enhance the compatibilization between CNW and the hydrophobic polyester matrix. Ring-opening polymerization of l-lactide was initiated from the hydroxyl groups available at the CNW surface to yield CNW-g-PLA nanohybrids. PLA-based nanocomposites were prepared by melt blending to ensure a green concept of the study thereby limiting the use of organic solvents. The influence of PLA-grafted cellulose nanoparticles on the mechanical and thermal properties of the ensuing nanocomposites was deeply investigated. The thermal behavior and mechanical properties of the nanocomposites were determined using differential scanning calorimetry (DSC) and dynamical mechanical and thermal analysis (DMTA), respectively. It was clearly evidenced that the chemical grafting of CNW enhances their compatibility with the polymeric matrix and thus improves the final properties of the nanocomposites. Large modification of the crystalline properties such as the crystallization half-time was evidenced according to the nature of the PLA matrix and the content of nanofillers.

New process of chemical grafting of cellulose nanoparticles with a long chain isocyanate.

Cellulose nanocrystals (or whiskers) and microfibrillated cellulose (MFC) were successfully obtained from sisal fibers and modified with n-octadecyl isocyanate (C(18)H(37)NCO) using two different methods with one innovation that consists of an in situ solvent exchange procedure. The surface chemical modification was characterized by elemental analysis, as well as FTIR and XPS spectroscopies. The crystalline structure of both unmodified and modified nanoparticles was investigated through X-ray diffraction measurements. It was shown that the efficiency of the chemical modification is strongly dependent on the nature of the nanoparticle with explanation linked to specific area, ability of peeling, and solvent dispersion. The surface chemical modification with n-octadecyl isocyanate allows dispersion of the nanoparticles in organic solvents and may allow processing of nanocomposite films from a casting/evaporation technique for a broad range of polymeric matrices.

Thermal and mechanical properties of bio-nanocomposites reinforced by Luffa cylindrica cellulose nanocrystals.

Cellulose nanocrystals have been prepared by acid hydrolysis of Luffa cylindrica fibers. The acid-resistant residue consisted of rod-like nanoparticles with an average length an diameter around 242 and 5.2nm, respectively (aspect ratio around 46). These cellulose nanocrystals have been used as a reinforcing phase for the processing of bio-nanocomposites using polycaprolactone (PCL) as matrix. To promote interfacial filler/matrix interactions the surface of cellulose nanocrystals was chemically modified with n-octadecyl isocyanate (C(18)H(37)NCO). Evidence of the grafting was supported by infrared spectroscopy and elemental analysis. X-ray diffraction analysis was used to confirm the integrity of cellulose nanocrystals after chemical modification. Both unmodified and chemically modified nanocrystals were used to prepare nanocomposites. The thermal properties of these materials were determined from differential scanning calorimetry and their mechanical behavior was evaluated in both the linear and non-linear range.

TEMPO-Oxidized Nanofibrillated Cellulose as a High Density Carrier for Bioactive Molecules

Controlled and efficient immobilization of specific biomolecules is a key technology to introduce new, favorable functions to materials suitable for biomedical applications. Here, we describe an innovative and efficient, two-step methodology for the stable immobilization of various biomolecules, including small peptides and enzymes onto TEMPO oxidized nanofibrillated cellulose (TO-NFC). The introduction of carboxylate groups to NFC by TEMPO oxidation provided a high surface density of negative charges able to drive the adsorption of biomolecules and take part in covalent cross-linking reactions with 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDAC) and glutaraldehyde (Ga) chemistry. Up to 0.27 μmol of different biomolecules per mg of TO-NFC could be reversibly immobilized by electrostatic interaction. An additional chemical cross-linking step prevented desorption of more than 80% of these molecules. Using the cysteine-protease papain as model, a highly active papain-TO-NFC conjugate was achieved. Once papain was immobilized, 40% of the initial enzymatic activity was retained, with an increase in kcat from 213 to >700 s(-1) for the covalently immobilized enzymes. The methodology presented in this work expands the range of application for TO-NFC in the biomedical field by enabling well-defined hybrid biomaterials with a high density of functionalization.

Synthesis of new bis(acyl)phosphane oxide photoinitiators for the surface functionalization of cellulose nanocrystals

A new synthesis of bis(acyl)phosphane oxide (BAPO) photoinitiators was developed which can be used to functionalize cellulose nanocrystal surfaces for polymer grafting. Hybrid materials with excellent graft yields can be rapidly obtained under mild and acid-free conditions.

Effect of Surface Charge on Surface-Initiated Atom Transfer Radical Polymerization from Cellulose Nanocrystals in Aqueous Media

Cellulose nanocrystals (CNCs) with different charge densities were utilized to examine the role of electrostatic interactions on surface-initiated atom transfer radical polymerization (SI-ATRP) in aqueous media. To this end, growth of hydrophilic uncharged poly(N,N-dimethylacrylamide) (PDMAM) brushes was monitored by electrophoresis, (1)H NMR spectroscopy, and dynamic light scattering (DLS). Molecular weight and polydispersity of PDMAM brushes was determined by GPC analysis of hydrolytically cleaved polymers. Initiator and polymer brush grafting densities, and thus, initiator efficiencies were derived from elemental analysis. Higher initiator efficiency of polymer brush growth was observed for CNCs with higher anionic surface sulfate half-ester group density, but at the expense of high polydispersity caused by inefficient deactivation. PDMAM grafts with number-average molecular weights up to 530 kDa and polydispersity indices <1.5 were obtained under highly diluted monomer concentrations. The role of surface chemistry on the growth of neutral polymer brushes from CNCs in water is emphasized and a model of the interfacial region at the onset of polymerization is proposed. The results presented here could have implications for other substrates that present surface charges and for the assumption that the kinetics of Cu-mediated SI-CRP are analogous to those conducted in solution.

Isocyanate-treated cellulose pulp and its effect on the alkali resistance and performance of fiber cement composites

Abstract The impact of grafted surface of cellulose fibers on the mechanical and physical properties of fiber-cement composites (FCC) has been investigated. The grafting was performed with n-octadecyl isocyanate [i.e., with an aliphatic isocyanate (AI)], with the intention to protect the cellulose fiber from alkali degradation in the cement matrix. The chemical changes, observed by contact angle measurements and X-ray photoelectron spectroscopy, showed a higher hydrophobic character of AI-treated fibers. The strength of FCC was tested during 28 days of curing treatment. The extracted AI-treated fibers contributed to higher specific energy and final specific deflection after accelerated aging cycles in comparison with the reference composites reinforced with untreated fibers. The higher values of limit of proportionality and modulus of elasticity for composites with AI-treated fibers are an evidence of the densification of the fiber-matrix transition zone. The modulus of rupture values were higher for composites with AI-treated and Soxhlet-extracted fibers after 200 soak and dry aging cycles. In comparison with the reference, AI-treated fibers decreased the water absorption and the apparent porosity of the FCC. The modification of fibers could be a new strategy to improve the performance and stability of cement products reinforced with natural fibers.

Polyethylene cellulose nanofibrils nanocomposites

This paper investigates the use of an aqueous dispersion of polyethylene copolymer with a relatively high content of acrylic acid as a compatibilizer and as an alternative medium to obtain polyethylene CNF nanocomposites. The CNF content was varied from 1 to 90wt% and the appearance, optical, thermal, mechanical and rheological properties, as well the morphology of the films were evaluated. The PE/CNF films are transparent up to 20wt% of NFC indicating a good dispersion of CNF, but a poor distribution, with PE-rich and CNF-rich regions observed by SEM. Improved mechanical properties were achieved, with a 100% and 15,900% increase in the Youngs modulus with 1wt% and 90wt% NFC, respectively. The rheological behavior indicated good melt processability. According to these results, aqueous polyolefin dispersions seem to be a promising, easy and relatively fast route for obtaining cellulose/polyolefins nanocomposites with low to high contents of cellulose nanofibrils.

Dynamics of Cellulose Nanocrystal Alignment during 3D Printing

The alignment of anisotropic particles during ink deposition directly affects the microstructure and properties of materials manufactured by extrusion-based 3D printing. Although particle alignment in diluted suspensions is well described by analytical and numerical models, the dynamics of particle orientation in the highly concentrated inks typically used for printing via direct ink writing (DIW) remains poorly understood. Using cellulose nanocrystals (CNCs) as model building blocks of increasing technological relevance, we study the dynamics of particle alignment under the shear stresses applied to concentrated inks during DIW. With the help of in situ polarization rheology, we find that the time period needed for particle alignment scales inversely with the applied shear rate and directly with the particle concentration. Such dependences can be quantitatively described by a simple scaling relation and qualitatively interpreted in terms of steric and hydrodynamic interactions between particles at high shear rates and particle concentrations. Our understanding of the alignment dynamics is then utilized to estimate the effect of shear stresses on the orientation of particles during the printing process. Finally, proof-of-concept experiments show that the combination of shear and extensional flow in 3D printing nozzles of different geometries provides an effective means to tune the orientation of CNCs from fully aligned to core-shell architectures. These findings offer powerful quantitative guidelines for the digital manufacturing of composite materials with programmed particle orientations and properties.