Ronald Sabo

A comparative study of cellulose nanofibrils disintegrated via multiple processing approaches

Various cellulose nanofibrils (CNFs) created by refining and microfluidization, in combination with enzymatic or 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidized pretreatment were compared. The morphological properties, degree of polymerization, and crystallinity for the obtained nanofibrils, as well as physical and mechanical properties of the corresponding films were evaluated. Compared to refining, intense microfluidization contributed greater separation of nanofibril bundles, which led to an enhancement of mechanical strength and transparency for the resultant film. The selected enzymatic pre-treatments produced shortened fibers due to preferential hydrolysis of amorphous cellulose and, in combination with mechanical treatments, resulted in short and stiff cellulose nanocrystal (CNC)-like materials. Despite films from these CNC-like fibrils having inferior tensile strength, their tensile modulus and transparency were significantly improved compared to CNFs prepared without pre-treatment. The unique fiber morphology and high crystallinity potentially offer a green and ecologically friendly alternative for the preparation of CNCs and CNFs as part of an integrated biorefinery approach.

Integrated production of nano-fibrillated cellulose and cellulosic biofuel (ethanol) by enzymatic fractionation of wood fibers

This study demonstrates the feasibility of integrating the production of nano-fibrillated cellulose (NFC), a potentially highly valuable biomaterial, with sugar/biofuel (ethanol) from wood fibers. Commercial cellulase enzymes were used to fractionate the less recalcitrant amorphous cellulose from a bleached Kraft eucalyptus pulp, resulting in a highly crystalline and recalcitrant cellulose (RC). The RC is difficult to hydrolyze to sugars but very suitable for producing biobased nanomaterials through mechanical homogenization. A range of fractionation yields of RC from 10–70% can be achieved by varying fractionation duration and enzyme dosage. The crystallinity of the RC was found to be as much as 24% higher than that of the original bleached pulp. The cellulase fractionation process facilitated mechanical homogenization by significantly reducing the degree of polymerization (DP) to about 400 and the length of the fibers to about 200 μm. The hydrolyzed sugars were found to be easily converted to ethanol through yeast fermentation with excellent efficiency of 92%. Films made from nano-fibrillated cellulose were found to be optically transparent, with opacity as low as 12%. The NFC films were mechanically strong and stiff, with tensile strengths and moduli approximately 10 and 6 times higher than film made from fibers that had not been nano-fibrillated.

Polyvinyl Alcohol-Cellulose Nanofibrils-Graphene Oxide Hybrid Organic Aerogels

Hybrid organic aerogels consisting of polyvinyl alcohol (PVA), cellulose nanofibrils (CNFs), and graphene oxide nanosheets (GONSs) were prepared using an environmentally friendly freeze-drying process. The material properties of these fabricated aerogels were measured and analyzed using various characterization techniques including compression testing, scanning electron microscopy, thermogravimetric (TGA) analysis, Brunauer-Emmet-Teller (BET) surface area analysis, and contact angle measurements. These environmentally friendly, biobased hybrid organic aerogels exhibited a series of desirable properties including a high specific compressive strength and compressive failure strain, ultralow density and thermal conductivity, good thermal stability, and moisture resistance, making them potentially useful for a broad range of applications including thermal insulation.

Nanofibrillated cellulose (NFC) reinforced polyvinyl alcohol (PVOH) nanocomposites: properties, solubility of carbon dioxide, and foaming

Polyvinyl alcohol (PVOH) and its nanofibrillated cellulose (NFC) reinforced nanocomposites were produced and foamed and its properties—such as the dynamic mechanical properties, crystallization behavior, and solubility of carbon dioxide (CO2)—were evaluated. PVOH was mixed with an NFC fiber suspension in water followed by casting. Transmission electron microscopy (TEM) images, as well as the optical transparency of the films, revealed that the NFC fibers dispersed well in the resulting PVOH/NFC nanocomposites. Adding NFC increased the tensile modulus of the PVOH/NFC nanocomposites nearly threefold. Differential scanning calorimetry (DSC) analysis showed that the NFC served as a nucleating agent, promoting the early onset of crystallization. However, high NFC content also led to greater thermal degradation of the PVOH matrix. PVOH/NFC nanocomposites were sensitive to moisture content and dynamic mechanical analysis (DMA) tests showed that, at room temperature, the storage modulus increased with decreasing moisture content. The solubility of CO2 in the PVOH/NFC nanocomposites depended on their moisture content and decreased with the addition of NFC. Moreover, the desorption diffusivity increased as more NFC was added. Finally, the foaming behavior of the PVOH/NFC nanocomposites was studied using CO2 and/or water as the physical foaming agent(s) in a batch foaming process. Only samples with a high moisture content were able to foam with CO2. Furthermore, the PVOH/NFC nanocomposites exhibited finer and more anisotropic cell morphologies than the neat PVOH films. In the absence of moisture, no foaming was observed in the CO2-saturated neat PVOH or PVOH/NFC nanocomposite samples.

Development of the metrology and imaging of cellulose nanocrystals

The development of metrology for nanoparticles is a significant challenge. Cellulose nanocrystals (CNCs) are one group of nanoparticles that have high potential economic value but present substantial challenges to the development of the measurement science. Even the largest trees owe their strength to this newly appreciated class of nanomaterials. Cellulose is the worlds most abundant natural, renewable, biodegradable polymer. Cellulose occurs as whisker-like microfibrils that are biosynthesized and deposited in plant material in a continuous fashion. The nanocrystals are isolated by hydrolyzing away the amorphous segments leaving the acid resistant crystalline fragments. Therefore, the basic raw material for new nanomaterial products already abounds in nature and is available to be utilized in an array of future materials. However, commercialization requires the development of efficient manufacturing processes and nanometrology to monitor quality. This paper discusses some of the instrumentation, metrology and standards issues associated with the ramping up for production and use of CNCs.

Microwave flexible transistors on cellulose nanofibrillated fiber substrates

In this paper, we demonstrate microwave flexible thin-film transistors (TFTs) on biodegradable substrates towards potential green portable devices. The combination of cellulose nanofibrillated fiber (CNF) substrate, which is a biobased and biodegradable platform, with transferrable single crystalline Si nanomembrane (Si NM), enables the realization of truly biodegradable, flexible, and high performance devices. Double-gate flexible Si NM TFTs built on a CNF substrate have shown an electron mobility of 160 cm2/V·s and fT and fmax of 4.9 GHz and 10.6 GHz, respectively. This demonstration proves the microwave frequency capability and, considering todays wide spread use of wireless devices, thus indicates the much wider utility of CNF substrates than that has been demonstrated before. The demonstration may also pave the way toward portable green devices that would generate less persistent waste and save more valuable resources.

Polyvinyl alcohol (PVA)–cellulose nanofibril (CNF)–multiwalled carbon nanotube (MWCNT) hybrid organic aerogels with superior mechanical properties

Polyvinyl alcohol (PVA)–cellulose nanofibril (CNF)–multiwalled carbon nanotube (MWCNT) hybrid organic aerogels were prepared using an environmentally friendly freeze-drying process with renewable materials. The material properties of these “green” hybrid aerogels were characterized extensively using various techniques. It was found that adding a small amount of CNFs and MWCNTs increased the mechanical properties of the PVA aerogels drastically. The mechanical properties of the hybrid aerogels showed an exponential dependency on the relative aerogel densities. These low-density hybrid aerogels also exhibited very low thermal conductivities and high surface areas, thereby making them potentially useful for many applications including thermal insulation and structural components.

Spatially Resolved Characterization of Cellulose Nanocrystal- Polypropylene Composite by Confocal Raman Microscopy

Raman spectroscopy was used to analyze cellulose nanocrystal (CNC)–polypropylene (PP) composites and to investigate the spatial distribution of CNCs in extruded composite filaments. Three composites were made from two forms of nanocellulose (CNCs from wood pulp and the nano-scale fraction of microcrystalline cellulose) and two of the three composites investigated used maleated PP as a coupling agent. Raman maps, based on cellulose and PP bands at 1098 and 1460 cm−1, respectively, obtained at 1 μm spatial resolution showed that the CNCs were aggregated to various degrees in the PP matrix. Of the three composites analyzed, two showed clear existence of phase-separated regions: Raman images with strong PP and absent/weak cellulose or vice versa. For the third composite, the situation was slightly improved but a clear transition interface between the PP-abundant and CNC-abundant regions was observed, indicating that the CNC remained poorly dispersed. The spectroscopic approach to investigating spatial distribution of the composite components was helpful in evaluating CNC dispersion in the composite at the microscopic level, which helped explain the relatively modest reinforcement of PP by the CNCs.

Resin impregnation of cellulose nanofibril films facilitated by water swelling

Flexible composite films were produced by impregnating aqueous phenol formaldehyde (PF) resin into water-swollen cellulose nanofibril (CNF) films. CNF films were prepared using a pressurized filtration method in combination with freeze drying. The freeze-dried films were swollen with water then impregnated with PF resin by soaking in aqueous resin solutions of varying concentrations. Small amounts of PF slightly enhanced the tensile properties of CNF films. The formulation with the best mechanical properties was CNF/PF films with 8 wt % resin exhibiting tensile stress and toughness of 248 MPa and 26 MJ/m3, respectively. Resin concentrations higher than about 8 % resulted in composites with decreased tensile properties as compared to neat CNF films. The wet strength of the composite films was significantly higher than that of the neat CNF films. The resulting composites showed greater resistance to moisture absorption accompanied by reduced thickness swelling when soaked in water as compared to neat CNF films. The composites also showed decreased oxygen permeability at low humidity compared to neat films, but the composites did not show improved barrier properties at high humidity.

Self-assembled optically transparent cellulose nanofibril films: effect of nanofibril morphology and drying procedure

Cellulose nanofibril (CNF) films currently provide great opportunity in many applications with advantages of excellent mechanical strength, high light transmittance, and good barrier properties. However, processes for preparing CNFs are typically tedious and vary, along with their properties. Here, five preparation methods using various combinations of filtration, freeze-drying, and casting are applied to produce CNF films, and their major properties are compared. Three different types of CNFs having a range of fiber diameter and aspect ratio were examined using each of these five preparation methods. Because of limited hydrogen bonds and nanofibril arrangement, the freeze-dried CNF films displayed reduced mechanical strength and light transmittance compared to the other methods, although freeze-drying was relatively fast. Some effects of film production methods on measured crystallinity were also observed with freeze-dried samples having lower crystallinity than films similarly produced by filtration and drying. Free-standing CNF films produced by casting at room temperature required long times and mold growth was sometimes observed, but cast films made from 2,2,6,6-tetramethylpiperidine-1-oxyl radical-oxidized CNFs had the highest light transmittance of any samples. Filtration of CNF suspensions followed by air- or oven-drying produced films with minimal defects, high mechanical strength, and good light transmittance with relatively little effort. Therefore, this filtration procedure is recommended for producing CNF films.