Alexander Bismarck

Carbon nanotube-based hierarchical composites: a review

The introduction of carbon nanotubes (CNTs) into conventional fibre-reinforced polymer composites creates a hierarchical reinforcement structure and can significantly improve composite performance. This paper reviews the progress to date towards the creation of fibre reinforced (hierarchical) nanocomposites and assesses the potential for a new generation of advanced multifunctional materials. Two alternative strategies for forming CNT-based hierarchical composites are contrasted, the dispersion of CNTs into the composite matrix and their direct attachment onto the primary fibre surface. The implications of each approach for composite processing and performance are discussed, along with a summary of the measured improvements in the mechanical, electrical and thermal properties of the resulting hierarchical composites.

Structure, morphology and thermal characteristics of banana nano fibers obtained by steam explosion

In this work, cellulose nanofibers were extracted from banana fibers via a steam explosion technique. The chemical composition, morphology and thermal properties of the nanofibers were characterized to investigate their suitability for use in bio-based composite material applications. Chemical characterization of the banana fibers confirmed that the cellulose content was increased from 64% to 95% due to the application of alkali and acid treatments. Assessment of fiber chemical composition before and after chemical treatment showed evidence for the removal of non-cellulosic constituents such as hemicelluloses and lignin that occurred during steam explosion, bleaching and acid treatments. Surface morphological studies using SEM and AFM revealed that there was a reduction in fiber diameter during steam explosion followed by acid treatments. Percentage yield and aspect ratio of the nanofiber obtained by this technique is found to be very high in comparison with other conventional methods. TGA and DSC results showed that the developed nanofibers exhibit enhanced thermal properties over the untreated fibers.

High Internal Phase Emulsions Stabilized Solely by Functionalized Silica Particles

High Internal Phase Emulsions (HIPEs) are important for a wide range of applications in the food, cosmetic, pharmaceutical and petroleum industries. If the continuous phase is polymerizable, HIPEs can be used as templates for the synthesis of highly porous polymers with potential applications as low weight structures or scaffolds in tissue engineering. HIPEs are characterized by a minimum internal phase volume ratio of 0.74 but Lissant first defined this minimum as 0.7. HIPEs consisting of a continuous organic phase and an internal aqueous phase (w/o emulsion), are commonly stabilized by large amounts of surfactants. Particle-stabilized emulsions also known as Pickering-emulsions have recently attracted much interest. Unlike surfactants, particles irreversibly adsorb at the interface of emulsions due to their high energy of attachment which makes them good emulsifiers. The ability of particles to adsorb at the interface between the two phases is primarily dependent on the wettability of the particles. Hydrophilic particles such as metal oxides tend to stabilize o/w emulsion while hydrophobic particles such as carbon tend to stabilize w/o emulsions. Nevertheless, it is possible to modify the wettability of particles by adsorbing surfactant molecules onto the particle surfaces or by silanation. All reports on particle-stabilized emulsions deal with emulsions having internal phase levels elow 70 vol.-%. Kralchevsky et al. developed a thermodynamic model, which predicts that

Removal of oxidation debris from multi-walled carbon nanotubes

Conventional liquid phase oxidation of multiwall carbon nanotubes (MWCNTs) using concentrated acids generates contaminating debris that should be removed using aqueous base before further reaction.

Highly Permeable Macroporous Polymers Synthesized from Pickering Medium and High Internal Phase Emulsion Templates

Various applications require macroporous materials with high permeability and a signifi cant compressive strength. For instance, the oil servicing industry is interested in utilizing a liquid medium that can be placed within the annulus between the oil bearing natural formation and a screen wrapped perforated pipe, which turns into a macroporous permeable and mechanically stable solid during a curing step. [ 1 ] The minimum requirements for the solid macroporous material are a permeability of 1 D (10 − 12 m 2 ) and a compressive strength ≥ 3.5 MPa. This challenge could be addressed by employing high internal phase emulsions (HIPE), whose continuous phase consists of monomers, as a template to produce macroporous polymers, commonly known as poly(merized)HIPEs, [ 2 ] with a well defi ned controllable pore structure. However, conventional polyHIPEs synthesized from surfactant stabilized water-in-oil (w/o) HIPEs have poor mechanical properties [ 3 , 4 ] and low permeabilities [ 5 ]

Surface Modification of Natural Fibers Using Bacteria: Depositing Bacterial Cellulose onto Natural Fibers To Create Hierarchical Fiber Reinforced Nanocomposites

Triggered biodegradable composites made entirely from renewable resources are urgently sought after to improve material recyclability or be able to divert materials from waste streams. Many biobased polymers and natural fibers usually display poor interfacial adhesion when combined in a composite material. Here we propose a way to modify the surfaces of natural fibers by utilizing bacteria ( Acetobacter xylinum) to deposit nanosized bacterial cellulose around natural fibers, which enhances their adhesion to renewable polymers. This paper describes the process of modifying large quantities of natural fibers with bacterial cellulose through their use as substrates for bacteria during fermentation. The modified fibers were characterized by scanning electron microscopy, single fiber tensile tests, X-ray photoelectron spectroscopy, and inverse gas chromatography to determine their surface and mechanical properties. The practical adhesion between the modified fibers and the renewable polymers cellulose acetate butyrate and poly(L-lactic acid) was quantified using the single fiber pullout test.

More Than Meets the Eye in Bacterial Cellulose: Biosynthesis, Bioprocessing, and Applications in Advanced Fiber Composites

Bacterial cellulose (BC) nanofibers are one of the stiffest organic materials produced by nature. It consists of pure cellulose without the impurities that are commonly found in plant-based cellulose. This review discusses the metabolic pathways of cellulose-producing bacteria and the genetic pathways of Acetobacter xylinum. The fermentative production of BC and the bioprocess parameters for the cultivation of bacteria are also discussed. The influence of the composition of the culture medium, pH, temperature, and oxygen content on the morphology and yield of BC are reviewed. In addition, the progress made to date on the genetic modification of bacteria to increase the yield of BC and the large-scale production of BC using various bioreactors, namely static and agitated cultures, stirred tank, airlift, aerosol, rotary, and membrane reactors, is reviewed. The challenges in commercial scale production of BC are thoroughly discussed and the efficiency of various bioreactors is compared. In terms of the application of BC, particular emphasis is placed on the utilization of BC in advanced fiber composites to manufacture the next generation truly green, sustainable and renewable hierarchical composites.

High Performance Cellulose Nanocomposites: Comparing the Reinforcing Ability of Bacterial Cellulose and Nanofibrillated Cellulose

This work investigates the surface and bulk properties of nanofibrillated cellulose (NFC) and bacterial cellulose (BC), as well as their reinforcing ability in polymer nanocomposites. BC possesses higher critical surface tension of 57 mN m(-1) compared to NFC (41 mN m(-1)). The thermal degradation temperature in both nitrogen and air atmosphere of BC was also found to be higher than that of NFC. These results are in good agreement with the higher crystallinity of BC as determined by XRD, measured to be 71% for BC as compared to NFC of 41%. Nanocellulose papers were prepared from BC and NFC. Both papers possessed similar tensile moduli and strengths of 12 GPa and 110 MPa, respectively. Nanocomposites were manufactured by impregnating the nanocellulose paper with an epoxy resin using vacuum assisted resin infusion. The cellulose reinforced epoxy nanocomposites had a stiffness and strength of approximately ∼8 GPa and ∼100 MPa at an equivalent fiber volume fraction of 60 vol.-%. In terms of the reinforcing ability of NFC and BC in a polymer matrix, no significant difference between NFC and BC was observed.

Surface characterization of natural fibers; surface properties and the water up-take behavior of modified sisal and coir fibers

The influence of fiber surface modifications like dewaxing, alkali treatment and methyl methacrylate grafting on the thermal and electrokinetic properties of coir (coconut) and sisal fibers has been investigated. Additionally scanning electron microscopy was performed to follow changes in the fiber surface morphology. Electrokinetic properties were measured using the streaming potential method. The measured time dependence of the ζ-potential offers the possibility to characterize the water up-take, i.e. the swelling behavior of natural fibers. The investigated natural fibers, as expected, contain dissociable surface functional groups as verified by measuring the pH-dependence of the ζ-potential. The influence of fiber surface modifications on the ζ-potential was compared to the influence of fiber surface modifications on measured (tensile and flexural strength) mechanical biocomposite properties. The ζ-potential measurement is a very efficient technique to investigate the various changes effected by different surface modifications, which are necessary to improve the compatibility of such natural fibers and polymer matrices for making eco-friendly and low cost composite materials.

High-Porosity Macroporous Polymers Sythesized from Titania-Particle-Stabilized Medium and High Internal Phase Emulsions

Particle-stabilized high internal phase emulsions have been used to synthesize tough and very high porosity macroporus polymers with a closed-cell pore structure. In this study, we show that Pickering water-in-oil emulsion templates with up to an 85 vol % internal phase can be stabilized by only 1 wt % of titania particles with their surfaces suitably modified by the adsorption of 3.5 +/- 0.5 wt % oleic acid. The pore structure and mechanical properties of the resulting macroporous polymers were tailored by altering the internal phase volume ratio of the emulsion template and the titania particle concentration used to stabilize the emulsion templates. The pore size and pore size distributions increase with increasing internal phase volume of the emulsion template as well as decreasing titania particle concentration used to stabilize the emulsion template. The mechanical properties, namely, Youngs modulus and the crush strength of the macroporous polymers, increased with decreasing porosity and increasing foam density. The toughest macroporous polymer had the lowest porosity but also the smallest pore size and narrowest pore size distribution.