All issues / Volume 8 (2014) / Issue 1 (January)
This is an editorial article. It has no abstract.
Blends were prepared from poly(lactic acid) (PLA) and three thermoplastics, polystyrene (PS), polycarbonate (PC) and poly(methyl methacrylate) (PMMA). Rheological and mechanical properties, structure and component interactions were determined by various methods. The results showed that the structure and properties of the blends cover a relatively wide range. All three blends have heterogeneous structure, but the size of the dispersed particles differs by an order of magnitude indicating dissimilar interactions for the corresponding pairs. Properties change accordingly, the blend containing the smallest dispersed particles has the largest tensile strength, while PLA/PS blends with the coarsest structure have the smallest. The latter blends are also very brittle. Component interactions were estimated by four different methods, the determination of the size of the dispersed particles, the calculation of the Flory-Huggins interaction parameter from solvent absorption, from solubility parameters, and by the quantitative evaluation of the composition dependence of tensile strength. All approaches led to the same result indicating strong interaction for the PLA/PMMA pair and weak for PLA and PS. A general correlation was established between interactions and the mechanical properties of the blends.
This work demonstrates a simple method to develop highly conducting high impact polystyrene (HIPS)/multiwall carbon nanotube (MWCNT) nanocomposites through selective dispersion of MWCNT in HIPS matrix. The method involves in-situ polymerization of polybutadiene containing styrene monomer in the presence of HIPS beads and MWCNT. A significantly lower percolation threshold value than ever reported for HIPS/MWCNT systems was obtained using unmodified unaligned MWCNTs. With the increase in HIPS bead content (at a particular MWCNT loading) in the HIPS/MWCNT nanocomposites, the conductivity value was increased, suggesting that the presence of HIPS beads acted as excluded volume that decreased the percolation threshold. An increase in electrical conductivity from 1.91•10–7 to 1.15•10–5 S/cm was evident, when the HIPS bead content was increased from 30 to 60 wt% at a constant loading of MWCNT (i.e. 0.6 wt%). The morphological investigation of the HIPS/MWCNT nanocomposites revealed that, the MWCNTs were selectively dispersed in the in-situ polymerized HIPS region, outside the HIPS beads, which resulted in the lowering of the percolation threshold to a lower value of 0.54 wt%. The morphology and electrical properties of the nanocomposites have been discussed in detail in the manuscript.
Modification of poly(benzimidazole) (PBI) by N-alkylation leads to polymers capable of undergoing microphase separation. Polymers with different amounts of C18 alkyl chains have been prepared. The polymers were analyzed by spectroscopy, thermal analysis, electron microscopy and X-ray scattering. The impact of the amount of alkyl chains on the observed microphase separation was analyzed. Membranes prepared from the polymers do show microphase separation, as evidenced by scattering experiments. While no clear morphology could be derived for the domains in the native state, evidence for the formation of lamellar morphologies upon doping with phosphoric acid is provided. Finally, the proton conductivity of alkyl-modified PBI is compared with that of pure PBI, showing that the introduction of alkyl side chains does not result in significant conductivity changes.
Poly(dimethyl siloxane) (PDMS) films were modified using poly(2-hydroxyethyl methacrylate) (PHEMA) by sequential method of interpenetrating polymer networks (IPNs). Collagen (type I) was also linked onto the modified films via methyl sulfonyl chloride. Collagen reacted with the activated groups to obtain covalently linked protein layers. The surface properties of samples were characterised by attenuated total reflectance Fourier transform infrared spectroscopy (ATRFTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and water contact angle measurements. Dynamic Mechanical Thermoanalysis (DMTA) and tensile testing were used to study the physical and mechanical properties of the IPNs. Grafting of collagen on the surface was confirmed using ATR-FTIR and XPS. The results also showed that the hydrophilicity of modified samples increased as the content of PHEMA increased. The biocompatibility of the PDMS surface was evaluated by culture of fibroblast cells (L929). The collagen-linked surfaces showed significant cell adhesion and growth in comparison with unmodified PDMS samples.
Biodegradable poly(ester amide)s (PEAs) were synthesized from lactic acid and 11-aminoundecanoic acid via melt polycondensation. Molecular weights, chemical structures and thermal properties of the poly(ester amide)s were characterized in terms of gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (1H NMR), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), respectively. The PEAs have low molecular weights and display a lower cold crystallization temperature as well as smaller crystallinity by comparison with the pure poly(lactic acid) (PLA). The incorporation of the 11-aminoundecanoic acid into the PLA chain not only improved the thermal stability but changed the decomposition process.
A novel method for comparing structural health of different types of brittle epoxy nanocomposites filled with carbon nanostructured fillers is presented. Epoxy – 0.2 vol% carbon black (CB) and epoxy – 0.2 vol% carbon nanotube (CNT) nanocomposite bars were prepared by calendering and thermal curing. Nanocomposite bars were subjected to Vickers diamond indentation to produce sub-surface damage. Electrical conductivities were analysed by 4-point method to estimate the structural damage caused by indentation. For comprehensive comparison, fracture toughness and percolation threshold were analysed as well. Because of the systematically induced indentation damage, a sharp decrease of 89% was observed in the electrical conductivity of epoxy – CNT nanocomposite as compared to 25% in the electrical conductivity of epoxy – CB nanocomposite. CNTs impart superior damage sensing capability in brittle nanocomposite structures, in comparison to CB, due to their high aspect ratio (fibrous nature) and high electrical conductivity.
The effect of polyacrylonitrile nanofibrous interlaminar layers on the impact properties of unidirectional and woven carbon fabric (CF)-reinforced epoxy (EP) matrix composites was investigated. The nanofibers were produced directly on the surface of carbon fabrics by a needleless electrospinning method, and composites were then prepared by vacuum-assisted impregnation. Interlaminar shear stress tests, three-point bending, Charpy-impact and instrumented falling weight tests were carried out. The fracture surfaces were analyzed by scanning electron microscopy. Due to the nano-sized reinforcements, the interlaminar shear strength of the woven and unidirectional fiber-reinforced composites was enhanced by 7 and 11%, respectively. In the case of the falling weight impact tests carried out on woven reinforced composites, the nanofibers increased the absorbed energy to maximum force by 64% compared to that measured for the neat composite. The Charpy impact tests indicated that the nanofiber interleaves also led to a significant increase in the initiation and total break energies. Based on the results, it can be concluded that the presence of nanofibers can effectively increase the impact properties of composites without compromising their in-plane properties because the thickness of the composites was not altered by the presence of interleaves. The improvement of the impact properties can be explained by the good load distribution behavior of the nanofibers.