All issues / Volume 13 (2019) / Issue 2 (February)
This is an editorial article. It has no abstract.
Keratin, a fibrous protein, that is available from a variety of animal sources as a constituent of hair, nails, horns, hoofs, wool and feathers, has applications in pharmaceutics, cosmetics and as a fertilizer. Like many naturally-derived biomaterials, the intrinsic biological activity and biocompatibility of keratin render this polymer a potential candidate for applications in medicine, and for the fabrication of scaffolds for tissue engineering. While several sources of keratin can be considered, the bioactivity of the keratins obtained can be quite different. In this study we discuss the processing and characterization of keratin from camel hair and goat cashmere. Specifically, the camel hair and cashmere were dissolved in an ionic liquid (1-butyl-3-methylimidazolium chloride), and the characteristics of the soluble and insoluble keratin were evaluated. The structure and properties of the raw material, soluble, and insoluble keratin were studied. Compared to the starting material, the soluble keratin showed chemical changes viz. decrease of cysteine, and minor structural changes. Preliminary in vitro biological properties performed by a lactate dehydrogenase (LDH) assay and scratch test showed good bioactivity in keratin from both sources. In particular, cell migration was observed to be faster when cells were cultured in the presence of soluble keratin extracted from camel hair and cashmere.
Highly tough and biobased epoxidized soybean oil (ESO)/poly(lactic acid) (PLA) blends were formulated through in-situ formation of tannic acid-crosslinked ESO (TA-c-ESO) oligomer as a dispersed phase with PLA matrix by dynamic vulcanization technique. To repair the sacrificial strength of the blends and endow the composites with electrical conductivity, different concentrations of CNTs, i.e., 0.5 to 10 wt%, were incorporated into the TA-c-ESO/PLA blends to prepare CNT/TAc-ESO/PLA nanocomposites. The added CNTs selectively localized within the PLA matrix, leading to a reduced size of TAc-ESO phase. The synergistic effects of CNTs and TA-c-ESO phase on the tensile properties, thermal stabilities, crystallization characteristics, and electrical properties of the nanocomposites were fully investigated along with the understanding of CNT reinforcing and TA-c-ESO toughening mechanism. The formation of TA-c-ESO phase significantly increased the fracture elongation and tensile toughness of the blends, while the incorporation of CNTs resulted in increased tensile strength, tensile modulus, and storage modulus of the nanocomposites; the combination of this two effects contributed to the obtained ternary composites with balanced mechanical properties and favorable electrical conductivity.
The influence of temperature on the formation of high molecular weight poly(lactic acid) (PLA) stereocomplex was studied by evaluation of the precipitates from dioxane solutions of PLA enantiomers (PLLA and PDLA). The racemic mixtures were characterized by Gel Permeation Chromatography, Infrared Spectroscopy, Differential Scanning Calorimetry, Scanning Electronic Microscopy, Wide-Angle X-ray Scattering and Vicat Softening Temperature. Precipitation was carried out under different solution temperatures, keeping constant the mixing ratio (XD), the molecular weight, the optical purity of both PLA enantiomers and the stirring rate. It was found that the precipitates contained only pure stereocomplex crystallites (racemic crystallites), without observing crystal phase separation between both homocrystals. The kinetics of the insoluble phase formation could be adjusted with the Avrami model, classically used for polymer crystallization in molten state. It was observed that the maximum PLA stereocomplex production rate was at about 40 °C. However, more thermally stable racemic crystallites were formed at high solution temperatures. It was found that all the precipitates were sphere-like at 10 g・dl–1 at the solution temperature of 25, 40, 60 and 80 °C.
Chemical-induced grafting processes can be performed either a grafting-to method, where preformed polymer chains are grafted on the surface previously activated, or alternatively, the polymer chains can be grown from the surface via a grafting-from method. The last one can be considered a bottom-up approach in which polymers are generated directly on the surface starting from their precursors. As a preliminary process the substrate is functionalized with specific groups that can initiate a polymerization reaction. While the polymerization can be initiated by a thermal initiator or directly by radicals formed by high energy treatment of the surface, light induced triggering of the grafting reaction has recently found increasing interest. With this review, by placing emphasis on the initiating system, we aim to show the significant feasibility of photografting-from method to properly functionalize any type of surface.
The influence of stabilized fibre structure and skin-core formation induced by rapid thermal stabilization of polyacrylonitrile (PAN) on the tensile properties of carbon fibres was investigated. Three sets of samples were prepared by stabilizing PAN fibres under three temperature profiles using a continuous carbon fibre processing line. Initially, the chemical structure and density variations in stabilized fibres were examined with respect to process conditions using Fourier Transform Infrared Spectroscopy (FTIR) and density column methods. Interestingly, while the cyclization and dehydrogenation indices are similar for all the stabilized fibres irrespective of temperature profiles used, the densities of these fibres varied from 1.34 to 1.366 g/cc. Micro-Raman studies showed the existence of structural heterogeneity in the fibres from low temperature (LT) carbonization (I(D)/I(G) ratio of core was ~5.6% higher than the skin) that eventually reduced with high temperature (HT) carbonization because of uniform sp3 to sp2 hybridization of carbons. However, modulus mapping revealed heterogeneous storage modulus distribution in the HT carbon fibre cross-section from Trial-2 (storage modulus of core was ~23 GPa less than the skin). Interestingly, this heterogeneity did not show a significant effect on the bulk properties of carbon fibres suggesting skin-core formation is an effect rather than a defect.
Natural fibers, as replacement of engineered fibers, have been one of the most researched topics over the past years. This is due to their inherent properties, such as biodegradability, renewability and their abundant availability when compared to synthetic fibers. Synthetic fibers derived from finite resources (fossil fuels) and are thus, affected mainly by volatility oil prices and their accumulation in the environment and/or landfill sites as main drawbacks their mechanical properties and thermal properties surpass that of natural fibers. A combination of these fibers/fillers, as reinforcement of various polymeric materials, offers new opportunities to produce multifunctional materials and structures for advanced applications. This article intends to cover recent developments from 2013-up to date on hybrid composites, based on natural fibers with other fillers. Hybrid composites preparation and characterization towards their applicability in advanced applications and the current challenges are also presented.
The morphology, crystallization behavior and mechanical properties of PLA, PHBV and their blends and nanocomposites with TiO2 as filler were investigated. An uncommon morphological change was observed with increasing PLA content in the blends, and this complicated the isothermal crystallization kinetics analyses of the different samples. It was, for example, observed that PLA, which do not normally crystallize during the cooling of the sample, showed isothermal crystallization for certain blend compositions. TiO2 was found to be mainly present in the PLA phase, which was also confirmed through broadband dielectric analysis. The blend composition, as well as the presence of TiO2 nanoparticles, had an influence on the cold crystallization of the PLA. The tensile properties changed with blend and nanocomposite composition, and these changes could to a certain extent be related to the respective morphologies. Very little could be said about differences between the melting behavior of the different samples, because the PLA and PHBV melted at almost exactly the same temperature.