Showing papers by "Orlando J. Rojas published in 2010"

Cellulose nanocrystals: chemistry, self-assembly, and applications.

Abstract: Cellulose constitutes the most abundant renewable polymer resource available today. As a chemical raw material, it is generally well-known that it has been used in the form of fibers or derivatives for nearly 150 years for a wide spectrum of products and materials in daily life. What has not been known until relatively recently is that when cellulose fibers are subjected to acid hydrolysis, the fibers yield defect-free, rod-like crystalline residues. Cellulose nanocrystals (CNs) have garnered in the materials community a tremendous level of attention that does not appear to be relenting. These biopolymeric assemblies warrant such attention not only because of their unsurpassed quintessential physical and chemical properties (as will become evident in the review) but also because of their inherent renewability and sustainability in addition to their abundance. They have been the subject of a wide array of research efforts as reinforcing agents in nanocomposites due to their low cost, availability, renewability, light weight, nanoscale dimension, and unique morphology. Indeed, CNs are the fundamental constitutive polymeric motifs of macroscopic cellulosic-based fibers whose sheer volume dwarfs any known natural or synthetic biomaterial. Biopolymers such as cellulose and lignin and † North Carolina State University. ‡ Helsinki University of Technology. Dr. Youssef Habibi is a research assistant professor at the Department of Forest Biomaterials at North Carolina State University. He received his Ph.D. in 2004 in organic chemistry from Joseph Fourier University (Grenoble, France) jointly with CERMAV (Centre de Recherche sur les Macromolecules Vegetales) and Cadi Ayyad University (Marrakesh, Morocco). During his Ph.D., he worked on the structural characterization of cell wall polysaccharides and also performed surface chemical modification, mainly TEMPO-mediated oxidation, of crystalline polysaccharides, as well as their nanocrystals. Prior to joining NCSU, he worked as assistant professor at the French Engineering School of Paper, Printing and Biomaterials (PAGORA, Grenoble Institute of Technology, France) on the development of biodegradable nanocomposites based on nanocrystalline polysaccharides. He also spent two years as postdoctoral fellow at the French Institute for Agricultural Research, INRA, where he developed new nanostructured thin films based on cellulose nanowiskers. Dr. Habibi’s research interests include the sustainable production of materials from biomass, development of high performance nanocomposites from lignocellulosic materials, biomass conversion technologies, and the application of novel analytical tools in biomass research. Chem. Rev. 2010, 110, 3479–3500 3479

The effect of chemical composition on microfibrillar cellulose films from wood pulps: water interactions and physical properties for packaging applications

Abstract: The interactions with water and the physical properties of microfibrillated celluloses (MFCs) and associated films generated from wood pulps of different yields (containing extractives, lignin, and hemicelluloses) have been investigated. MFCs were produced by combining mechanical refining and a high pressure treatment using a homogenizer. The produced MFCs were characterized by morphology analysis, water retention, hard-to-remove water content, and specific surface area. Regardless of chemical composition, processing to convert macrofibrils to microfibrils resulted in a decrease in water adsorption and water vapor transmission rate, both important properties for food packaging applications. After homogenization, MFCs with high lignin content had a higher water vapor transmission rate, even with a higher initial contact angle, hypothesized to be due to large hydrophobic pores in the film. A small amount of paraffin wax, less than 10%, reduced the WVTR to a similar value as low density polyethylene. Hard-to-remove water content correlated with specific surface area up to approximately 50 m2/g, but not with water retention value. The drying rate of the MFCs increased with the specific surface area. Hornified fibers from recycled paper also have the potential to be used as starting materials for MFC production as the physical and optical properties of the films were similar to the films from virgin fibers. In summary, the utilization of lignin containing MFCs resulted in unique properties and should reduce MFC production costs by reducing wood, chemical, and energy requirements.

Development of Langmuir−Schaeffer Cellulose Nanocrystal Monolayers and Their Interfacial Behaviors

Abstract: Model cellulose surfaces based on cellulose nanocrystals (CNs) were prepared by the Langmuir−Schaeffer technique. Cellulose nanocrystals were obtained by acid hydrolysis of different natural fibers, producing rodlike nanoparticles with differences in charge density, aspect ratio, and crystallinity. Dioctadecyldimethylammonium bromide (DODA-Br) cationic surfactant was used to create CN−DODA complexes that allowed transfer of the CNs from the air/liquid interface in an aqueous suspension to hydrophobic solid substrates. Langmuir−Schaeffer horizontal deposition at various surface pressures was employed to carry out such particle transfer that resulted in CN monolayers coating the substrate. The morphology and chemical composition of the CN films were characterized by using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Also, their swelling behavior and stability after treatment with aqueous and alkaline solutions were studied using quartz crystal microgravimetry (QCM). Overall, it ...

Adsorption of a nonionic symmetric triblock copolymer on surfaces with different hydrophobicity

Abstract: This study investigates the adsorption of a symmetric triblock nonionic polymer comprising ethylene oxide (EO) and propylene oxide (PO) blocks (Pluronic P-105, EO(37)PO(56)EO(37)) on a range of substrates including hydrophobic, i.e., polypropylene (PP), poly(ethylene terephthalate) (PET), nylon, and graphite, and hydrophilic, i.e., cellulose and silica. The adsorption process and the structure of the hydrated adsorbed layers are followed by quartz crystal microgravimetry (QCM), surface plasmon resonance (SPR), and atomic force microscopy. The unhydrated surfaces are characterized by ellipsometry and contact angle techniques. The adsorption kinetics and the extent of adsorption are determined by monitoring the changes in resonance frequency and refractive index of sensors coated with ultrathin films of the various substrates. Langmuirian-type adsorption kinetics is observed in all cases studied. The amount of adsorbed Pluronic on hydrophobic polymer surfaces (PP, PET, and nylon) exceeds that on the hydrophilic cellulose. The hydrophobic (graphite) mineral surface adsorbs relatively low polymer mass, typical of a monolayer, while micellar structures are observed on the hydrophilic silica surface. The amount of water coupled to the adsorbed polymer layers is quantified by combining data from QCM, and SPR are found to increase with increasing polarity of the substrate. On the basis of contact angle data, the nonhydrated adsorbed structures produce modest increases in hydrophilicity of all the substrates investigated. Overall, insights are provided into the structure and stability of both hydrated and nonhydrated adsorbed triblock copolymer.

Effect of charge asymmetry on adsorption and phase separation of polyampholytes on silica and cellulose surfaces.

Abstract: The relation between the properties of polyampholytes in aqueous solution and their adsorption behaviors on silica and cellulose surfaces was investigated. Four polyampholytes carrying different charge densities but with the same nominal ratio of positive to negative segments and two structurally similar polyelectrolytes (a polyacid and a polybase) were investigated by using quartz crystal microgravimetry using silica-coated and cellulose-coated quartz resonators. Time-resolved mass and rigidity (or viscoelasticity) of the adsorbed layer was determined from the shifts in frequency (∆f) and energy dissipation (∆D) of the respective resonator. Therefore, elucidation of the dynamics and extent of adsorption, as well as the conformational changes of the adsorbed macromolecules, were possible. The charge properties of the solid surface played a crucial role in the adsorption of the studied polyampholytes, which was explained by the capability of the surface to polarize the polyampholyte at the interface. Under the same experimental conditions, the polyampholytes had a higher nominal charge density phase-separated near the interface, producing a soft, dissipative, and loosely bound layer. In the case of cellulose substrates, where adsorption was limited, electrostatic and polarization effects were concluded to be less significant.

Monitoring Cellulase Protein Adsorption and Recovery Using SDS-PAGE

Abstract: Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) was employed to study the sorption behaviors of cellulases on microcrystalline celluloses and hardwood pulp. The adsorption and recovery of cellulases from Aspergillus niger and Trichoderma reesei were investigated at 25 °C. Cellulase recovery was conducted by rinsing adsorbed enzymes with sodium acetate buffer, Milli-Q water, and sodium hydroxide solution. The initial, equilibrium, and recovered enzymes were analyzed using SDS-PAGE gels. Gels were scanned and analyzed using ImagePro software. The molecular weights of cellulase proteins were determined using a protein marker having seven known proteins. The cellulase system from Trichoderma reesei had a higher adsorption on all substrates studied than the cellulase system from Aspergillius niger, and higher pH favored desorption from the substrates studied. Experimental results also demonstrated that adsorption and desorption amounts determined by SDS-PAGE were proportional to protein concentrations in their crude mixtures.

Surface and Friction Behavior of a Silicone Surfactant Adsorbed on Model Textiles Substrates

Abstract: This study reports on interactions of an amphiphilic block copolymer of polyalkylene oxide-modified poly(dimethylsiloxane) with thin films of polypropylene (PP), polyethylene terephthalate (PET), and nylon, as well as with reference hydrophilic silica surfaces. The dynamics of adsorption, adsorbed mass, and viscoelasticity of the adsorbed layer are quantified by using a quartz crystal microbalance, while boundary layer lubrication behaviors are studied by using lateral force microscopy. Driven by hydrophobic interactions, the silicone surfactant adsorbs following a Langmuir isotherm and forms strongly adsorbed layers on the polymer surfaces with an areal mass directly related to the hydrophobicity of the substrate. The self-assembled silicone surfactant molecules improve significantly wettability and lower friction. The results reported herein will broaden our understanding of lubrication phenomena in textile and fiber processing applications.

Aspects of Raw Materials and Processing Conditions on the Production and Utilization of Microfibrillated Cellulose

Abstract: Microfibrillated Celluloses (MFCs) are generally considered to be fibrils with diameters in the range of 10-100 nm liberated from larger plant-based cellulose fibers. MFCs have garnered much attention for the use in composites, coatings, and films because of high specific surface areas, renewability, and unique mechanical properties. Many of the recent studies of MFC generated from wood pulp have focused on fully bleached chemical pulps; however, these materials must be further modified to be incorporated in hydrophobic matrices for composite reinforcement. The production of MFCs containing hydrophobic lignin may reduce the need for surface modifications and generate MFCs with new properties for use in other applications. To investigate, wood pulps of different chemical compositions were used to produce MFCs to determine the effect of lignin on microfibril and film properties.

Polyampholytes as Paper Dry Strength Agents

Abstract: The results of our work showed that the balance between the charge densities of the surface and the polymer structures is an important factor to be considered when using this polymer. The highest paper strength value, measured by tensile index, was found at the polyampholyte’s isoelectric point (ca. pH of 7.3) when the charge of the fiber surface was negative and the polymer structure charge was symmetric. This observation agrees with our dynamic light scattering results which, demonstrated that at the isoelectric point there was a maximum in association among polyampholyte molecules, leading to a maximum in size of molecular aggregates. When adsorbed on an electrically charged surface, the maximum amount of adsorbed polymer, measured by the shift in resonance frequency in a quartz crystal microgravimetric balance, was observed for the same isoelectric point. Better results for paper dry strength were found when the fiber surface and the polymer structures were oppositely charged or at the isoelectric point of the polymer. Less effective addition strategies were found in the case when the fiber surfaces and the polymer structures had same sign of charge.

Cellulose nanocrystals as reinforcement of poly(vinyl alcohol) nanocomposites

Abstract: The development of micro/nanofibers has attracted significant interest in the last few decades due to the unique properties they endow, such as their very high surface area-to-volume ratio. This characteristic along with the remarkable suitability for surface functionalization and superior mechanical performance makes possible their use in a wide range of applications in medical, pharmaceutical, filtration, and catalysis fields, among others. 2 A versatile and inexpensive method for the production of micro and nanofibers is the electrospinning technique, based on the whipping of polymer solutions under electrostatic forces. Various polymers, including polyolefins, polyamides, polyesters, polyurethanes, polypeptides, and polysaccharides, have successfully been electrospun into microand nanofiber mats. Poly vinyl alcohol (PVA), a commonly used polymer obtained by controlled hydrolysis of poly vinyl acetate (PVAc), can be used to produce fibers via electrospinning.