All issues / Volume 11 (2017) / Issue 7 (July)
This is an editorial article. It has no abstract.
We introduced a general concept to create smart, (multi)functional interphases in polymer composites with layered reinforcements, making use of 3D printing. The concept can be adapted for both thermoplastic and thermoset matrix-based composites with either thermoplastic- or thermoset-enriched interphases. We showed feasibility using an example of a composite containing a thermoset matrix/thermoplastic interphase. Carbon fiber unidirectional reinforcing layers were patterned with poly(ε-caprolactone) (PCL) through 3D printing, then infiltrated with an amine-cured epoxy (EP). The corresponding composites were subjected to static and dynamic flexure tests. The PCL-rich interphase markedly improved the ductility in static tests without deteriorating the flexural properties. Its effect was marginal in Charpy impact tests, which can be explained with effects of specimen and PCL pattern sizes. The PCL-rich interphase ensured self-healing when triggered by heat treatment above the melting temperature of PCL.
Plasticized poly(lactic acid) (PLA) samples with embedded Cu-doped ZnO powder functionalized with Ag nanoparticles composites were prepared by melt blending processing technique. The effect of ZnO:Cu/Ag nanoparticles on the properties of plasticized PLA was investigated in terms of structural (by Attenuated total reflectance – Fourier Transform Infrared – ATR-FT-IR, X–Ray Diffraction analysis – XRD) and morphological (using Scanning Electron Microscopy – SEM and Transmission Electron Microscopy – TEM examination) modifications, thermal (Differential Scanning Calorimetry – DSC), mechanical (transmittance, tensile strength), barrier antimicrobial properties and also in respect to migration of Cu, Zn an Ag nanoparticles into food simulants. It was found that the increase of nanoparticle content (from 0.5 to 1.5 wt%) of PLA formulations leads to the increase of the degree of crystallinity of PLA. The overall migration of all samples into three food simulants was below 10 mg·dm–2, accepted value according to EU Regulation No 10/2011 for plastic materials and articles intended to come into contact with food. Based on the obtained results it can conclude that the optimum composition is PLA/ZnO:Cu/Ag 0.5, this bionanocomposite offering suitable mechanical and thermal properties, good barrier properties to ultraviolet light, water vapour, oxygen and carbon dioxide, antibacterial activity and low migration amount of nanoparticles into food simulants.
Filler particles are not homogeneously distributed in real composites consisting of polymer matrices and rigid spherical particles. In the presence of a crack, high multiaxial stresses develop in the material which are a function of the distance from the crack tip. This variation effects the stress concentration in the periphery of the inhomogeneously distributed particles. For a certain stress level closer particles are able to debond from the matrix while particle pairs with larger centre to centre distances may still be bonded to the matrix. Based on a two particle model, the debonding and matrix yielding energies were calculated. Subsequent integration over the local composite deformation results in the fracture toughness of such composites. Randomly distributed particles provide a lower fracture toughness in modelling as compared to inhomogeneoulsy distributed particles in the composite.
In this work the effect of multiple reprocessing was studied on molecular structure, morphology and properties of poly(lactic acid)/hydrotalcites (PLA/HT) nanocomposites compared to neat PLA. In addition, the influence of two different kinds of HT – organically modified (OM-HT) and unmodified (U-HT) – was evaluated. Thermo-mechanical degradation was induced by means of five subsequent extrusion cycles. The performance of the recycled materials was investigated by mechanical and rheological tests, differential scanning calorimetry (DSC), intrinsic viscosity measurements and SEM observation. The results indicated that the best morphology was achieved in the systems incorporating OM-HT. On increasing the extrusion reprocessing cycles, the properties showed behavior due to two opposite effects: i) chain scission due to thermo-mechanical degradation and ii) filler dispersion effect resulting from multiple processing. In particular, at low reprocessing cycles, both tensile and rheological properties seem to be mainly affected by HT dispersion, especially when OM-HT was added. After five reprocessing cycles, on the contrary, chain scission, i.e. thermo-mechanical degradation, dominated. As regards the effect of the presence of organic modifier in HT, the results indicated that this variable apparently did not affect the macroscopic performance of the nanocomposites, especially at high reprocessing cycles.
High-performance mixed-matrix membranes with altered interfacial and surface chemistry through benign reinforcement of functionalized carbon nanotubes of different configurations
S. Kar, A. Pal, A. K. Debnath, B. Bhushan, A. K. Sahu, R. C. Bindal
Vol. 11., No.7., Pages 565-580, 2017
Vol. 11., No.7., Pages 565-580, 2017
Nanomaterials potentially minimize the inherent trade-off between productivity and selectivity of membranebased ultrafiltration (UF) process. A comparative study on the reinforcement effect of pristine carbon nanotubes (CNTs) of three different configurations, viz. single-walled (SWNT), double-walled (DWNT) and multi-walled (MWNT), and their carboxylated counterparts, onto a polysulfone (Psf) host matrix of mixed-matrix UF membranes is illustrated herein. The varying structural features of carboxylated CNTs, probed by XPS analysis, underpin the enrichment of CNTs with oxygen rich functionalities following the trend of MWNT > DWNT > SWNT. The membranes with enhanced hydrophilicity and altered electrokinetics substantiate the efficacy of facilitated reinforcement of functionalized CNTs over the pristine ones. Variations in surface topography and mechanical feature of the membranes elucidate that carboxylation influences the interfacial chemistry by enhancing the dispersion stability of MWNTs more profoundly than its configurational counterparts like SWNTs and DWNTs, and concurrently alters its distribution within the membranous matrix. The enhanced ultrafiltration performances, as achieved by twofold enhancement in solvent fluxes without compromise in the solute rejection capabilities (~89–90% toward PEG, Mw: 35 kDa), confirm the potential of carboxylated CNTs in leading to development of high-performance mixed-matrix membranes.
Graphene-polymer nanocomposite films show good piezoresistive behaviour and it is reported that the sensitivity increases either with the increased sheet resistance or decreased number density of the graphene fillers. A little is known about this behaviour near the percolation region. In this study, graphene nanoplatelet (GNP)/poly (methyl methacrylate) (PMMA) flexible films are fabricated via solution casting process at varying weight percent of GNP. Electrical and piezoresistive behaviour of these films is studied as a function of GNP concentration. Piezoresistive strain sensitivity of the films is measured by affixing the film to an aluminium specimen which is subjected to monotonic uniaxial tensile load. The change in resistance of the film with strain is monitored using a four probe. An electrical percolation threshold at 3 weight percent of GNP is observed. We report non-monotonic piezoresistive behaviour of these films as a function GNP concentration. We observe an increase in gauge factor (GF) with unstrained resistance of the films up to a critical resistance corresponding to percolation threshold. Beyond this limit the GF decreases with unstrained resistance.
The optical and rheological properties of aqueous solutions of block copolymer composed of low molecular weight poly(N-isopropylacrylamide)-b-polystyrene are studied as a function of temperature. From light scattering measurements the block copolymer solution is found to form micelles at very low concentrations and the critical micellar concentration is identified as 0.005 wt%. Apart from the concentration dependence, a unique temperature dependent micelle formation is noted at 34 °C. Further, temperature dependent refractive index measurements shows that the refractive index increases with temperature (beyond the lower critical solution temperature, 31.6 °C of the polymer), and is attributed to the stable rearrangement of the proximal hydrophobic isopropyl-polystyrene chains in the collapsed polymer so as to overcome the steric hindrance effects offered by the hydrophobic chains. In the polymer concentrations investigated for rheological studies, the solution flows, yet manifested solid like behavior with G' > G" with the modulus being frequency dependent and the magnitude of G' two-fold higher than G" implying a weak gel state. Weak gel states are in general noted at high temperatures in most of the polymer systems, contrary to this, in our studies weak gel state is observed at lower temperature. Further, a transition from weak gel to sol state is observed at slightly elevated temperatures. The reason for the existence of weak gel state below the lower critical solution temperature is due to the micellar entanglements of poly(N-isopropylacrylamide)-b-polystyrene with one another and whereas above the lower critical solution temperature disentanglement of the micelles makes the system behave like a viscoelastic liquid.