School of Applied Mathematical and Physical Sciences, Section of Mechanics, National Technical University of Athens, 5 Heroes of Polytechnion, GR–15773, Athens, Greece
Abstract: The outstanding improvement in physical properties of cyanate esters (CE)s compared to competitor resins such as epoxies has attracted appreciable attention recently. Cyanate esters undergo thermal polycyclotrimerization to give polycyanurates (PCN)s. However, like most thermosetting resins, CEs main drawback is brittleness. To overcome this shortage, CEs can be toughened by the introduction of polytetramethylene glycol (PTMG), a hydroxyl-terminated polyether. However, PTMG has detrimental impact on modulus. To simultaneously enhance ductility and stiffness of CE, we added both PTMG and organoclay (montmorillonite, MMT). A series of PCN/PTMG/MMT nanocomposites with a constant PTMG weight ratio was prepared, and the resulting nano-phase morphology, i.e, degree of filler dispersion and distribution in the composite and thermo-mechanical performance were examined using scanning electron microscopy (SEM), dynamic mechanical analysis (DMA) and stress–strain measurements. Properties such as the glass transition temperature, Young’s modulus, tensile strength and elongation at break were determined. It was found that below 2 wt% MMT content, MMT nanoparticles were homogeneously distributed into the matrix, suggesting a lower agglomeration degree for these materials. In the glassy state, the substantial increment of storage modulus revealed the great stiffening effect of MMT due to its high modulus. Modification by PTMG led to a 233% improvement of elongation at break, compared with the neat PCN. Nanocomposites exhibited consistently higher Young’s modulus than PCN/PTMG over the entire organoclay content examined, with the 2 wt% material displaying the most pronounced increase. This optimized behaviour at 2 wt% has also been attested by the experimental techniques applied.
Abstract: The advantages of cyanate esters (CEs) versus competitor systems such as epoxies and polyimides, as well as the great reinforcing potential of organoclays properly dispersed into a polymeric matrix, have been examined in a series of polycyanurate (PCN)/montmorillonite (MMT) nanocomposites prepared under appropriate polymerization conditions. The curing schedule applied resulted in gradual propagation of polymerization. Through this procedure, the intragallery curing rate becomes comparable to the extragallery one, allowing intercalation before gelation. Systems with clay loadings from 1 to 3% per weight were synthesized and their morphology and mechanical properties were studied by means of scanning electron microscopy (SEM), atomic force microscopy (AFM), wide angle X–ray scattering (WAXS), dynamic mechanical analysis (DMA) and tensile tests. Microscopy investigations revealed better dispersion for the 3 wt% system compared to smaller concentrations, where aggregation and in some cases agglomeration were the conspicuous features. Roughness and area analyses revealed more homogeneous dispersion for this nanocomposite. Topology and 3D–phase images further suggested considerable reduction of the average particle diameters. WAXS analysis showed that the interlayer spacing of nanocomposites was increased compared to pristine MMT, indicating the formation of intercalated structures. On the other hand, tensile strength and elongation at break values displayed abrupt diminution with MMT addition, while Young’s modulus exhibited a slight but systematic increment with MMT content. The decreasing glass transition tendency observed for small clay loadings was reversed in the case of 3 wt%, while secondary transitions were practically unaffected by the presence of MMT.
Abstract: A large number of works deal with material synthesis and characterization of polymer nanocomposites due to their improved thermo–mechanical properties, but the fundamental mechanisms for mechanical property enhancement are not yet completely defined. In particular, a special class, that of polymer/organoclay nanocomposites has been observed to exhibit an impressive improvement in different types of properties, physical and chemical ones. In the present work, a model is presented and applied to formulate the elastoplastic response of epoxy/clay nanocomposites, experimentally tested elsewhere. The model based on Mori–Tanaka theory, for the estimation of the elastic stiffness tensor for composite materials, is combined with the self–consistent model of Budiansky and Wu, valid for crystal plasticity. Then the macroscopic plastic response of the heterogeneous material is linked with the microstructural parameters, i.e. the plastic behaviour of the effective particle. The model was proved to successfully describe the tensile response of the epoxy/clay nanocomposites with varying clay weight fraction.
Abstract: A series of polystyrene (PS)/SiO2 nanocomposites were prepared. Silica nanoparticles with an average diameter of 16 nm were used, and treated with dimethyldichlorosilane, while their weight fraction varied from 4 up to 10%. The viscoelastic–thermomechanical properties of the nanocomposites and their interrelation with the material’s structure were studied with various experimental techniques. Scanning electron microscopy, differential scanning calorimetry, dynamic mechanical analysis, and tensile testing at three different temperatures were applied. The stress–strain curves at 85ºC, where the material’s viscoplastic response is manifested, were simulated through a plasticity model, developed in previous works. The 4% weight fraction was found to be the optimum one for the enhancement of the thermomechanical properties.
Abstract: The enhancement in mechanical properties of nanocomposites in terms of Young's modulus has been modeled with a number of micromechanical models. Application of the `effective particle' approach to models previously developed for the calculation of the stiffness of nanocomposites such as the Chen–Cheng and Odegard models, improves the agreement between theory and experiment only to a modest extent, because in these models only one kind of inhomogeneity is considered. On the contrary, Taya and Chou model initially proposed for conventional composites with two separate kinds of inclusions was proven to effectively describe the nanocomposite behaviour. The two types of inhomogeneities were considered to be the exfoliated layers and the intercalated particles, respectively. Four nanocomposite systems were examined, prepared and characterized elsewhere, and parameters required for the analysis such as the number of silicate layers in a stack, the length of the platelets and interlayer spacing were estimated from quantitative digital image analysis performed on transmission electron microscopy photomicrographs and wide angle X-ray scattering studies. The present approach reveals a method for quantification of both the degree of exfoliation as well as degree of reinforcement through micromechanical modeling, which is often formidable with available characterization techniques.
Abstract: The current work focuses on the testing of a novel material used as an adhesive film in Composite Patch Repair (CPR). A series of Differential Scanning Calorimetry (DSC) results along with various curing cycles not only led to the optimum material composition but also demonstrated the compatibility to the composite pre-impregnated patches. This in turn was subjected to mechanical testing including shear strength measurements. The substrate was chosen to be 2017 T4 aluminium alloy which is customarily used in the aerospace industry, taking into account that CPR is a technique mainly applied in this field. The subsequent surface preparation of the specimens was investigated for the specified context resulting to the selection of the Ferric Sulphate Sulphuric acid etching process. Finally, a series of specimens representing actual skin repairs were created and subjected to cyclic loading, specifying the suitability of the novel material, compared to commercially available materials.
Abstract: A special class of nanocomposites, that of polymer/clay ones, has been observed to exhibit an impressive improvement in different types of properties, physical and thermo–mechanical. It must be underlined that the concept of matrix and filler attains now a different meaning as known in conventional particulate composites. This is due to the hierarchical nanometer lengthscale morphology of the particle structure. In the present work, a model is presented and applied to formulate the elastoplastic response of epoxy/clay nanocomposites, experimentally tested elsewhere. The model based on Mori–Tanaka theory, for the estimation of the elastic stiffness tensor for composite materials, is combined with the self–consistent model, valid for crystal plasticity. Then the macroscopic plastic response of the heterogeneous material is linked with the microstructural parameters, i.e. the plastic behaviour of the effective particle. The model was proved to successfully describe the tensile response of the epoxy/clay nanocomposites with varying clay weight fraction.
