editorial corner

In rubber investigation, many strain energy models have been proposed for the hyperelastic behavior. One of earlier models, that of Gent-Thomas (1958), based on theoretical and experimental considerations, has been forsaken in the rubber modeling literature, because of difficulties in establishing its domain of validity and then evaluating the relative mechanical parameters. This note presents how to validate a partial model that is suitable just in a portion of the complete experimental domain: moderate, transition and stiffening deformations. Illustrations of domain validation for the incomplete phenomenological Gent-Thomas model and a simulation with the constrained chain model of Flory-Erman are exposed and their numerical characterisation from experimental data is also revealed.

Non-isothermal crystallization and subsequent melting of three grades of ethylene-vinyl acetate copolymer were investigated by differential scanning calorimetry (DSC) technique. The DSC crystallization curves show that vinyl acetate (VAc) content has the same effect on the onset, peak and final crystallization temperatures, and these copolymers have almost the same spacing of thermal windows under identical crystallization condition. Subsequent melting DSC results suggest that EVA14 (14 wt% VAc) has the narrowest distribution of lamellar thickness and the most perfect crystals. Though the instantaneous nucleation was preferred, non-isothermal crystallization kinetics shows that EVA14 could form tridimensional crystallites, whereas EVA18 (18 wt% VAc) and EVA28 (28 wt% VAc) are prone to crystallize two-dimensionally, as a result of more noncrystallizable VAc co-monomers introduced in the crystallizable ethylene segments. The growth rate falls on the following sequence: EVA14>EVA18>EVA28. Moreover, the kinetic crystallizability G also well characterizes the variation of the non-isothermal crystallization of these EVA materials, in the view of structural impediment caused by the VAc content.

Structural, thermal and electrical behavior of polymer-clay nanocomposite electrolytes consisting of polymer (polyethylene oxide (PEO)) and NaI as salt with different concentrations of organically modified Na⁺ montmorillonite (DMMT) filler have been investigated. The formation of nanocomposites and changes in the structural properties of the materials were investigated by X-ray diffraction (XRD) analysis. Complex impedance analysis shows the existence of bulk and material-electrode interface properties of the composites. The relative dielectric constant (ε_r) decreases with increase in frequency in the low frequency region whereas frequency independent behavior is observed in the high frequency region. The electrical modulus representation shows a loss feature in the imaginary component. The relaxation associated with this feature shows a stretched exponential decay. Studies of frequency dependence of dielectric and modulus formalism suggest that the ionic and polymer segmental motion are strongly coupled manifeasting as peak in the modulus (M″) spectra with no corresponding feature in dielectric spectra. The frequency dependence of ac (alternating current) conductivity obeys Jonscher power law feature in the high frequency region, where as the low frequency dispersion indicating the presence of electrode polarization effect in the materials.

The functional copolymers, having a combination of rigid/flexible linkages and an ability of complex-formation with interlayered surface of organo-silicate, and their nanocomposites have been synthesized by interlamellar complex-radical (co)terpolymerization of intercalated monomer complexes of maleic anhydride (MA) and itaconic acid (IA) with dimethyl dodecylamine surface modified montmorillonite (organo-MMT) (MA…DMDA-MMT and IA…DMDA-MMT) n-butyl methacrylate (BMA) and/or BMA/styrene monomer mixtures. The results of nanocomposite structure–composition– property relationship studies indicate that interlamellar complex-formation between anhydride/acid units and surface alkyl amine and rigid/flexible linkage balance in polymer chains are important factors providing the effective intercalation/ exfoliation of the polymer chains into the silicate galleries, the formation of nanostructural hybrids with higher thermal stability, dynamic mechanical behaviour and well dispersed morphology.

Nanocomposites based on polyamide 6/polypropylene (PA6/PP = 70/30) blend containing organophilic montmorillonite (OMMT) and maleated polypropylene (PP-g-MA) as compatibilizer were prepared by melt compounding followed by injection molding. Modification of montmorillonite (MMT) with dodecyalmine was successfully performed. The morphological and mechanical properties of nanocomposites were investigated by using x-ray diffraction (XRD), transmission electron microscopy (TEM), tensile, flexural, and impact tests. The thermal stability of nanocomposites was characterized by using thermogravimetric analysis (TGA) and heat distortion temperature (HDT). XRD and TEM results indicated that the intercalated structure was obtained for PA6/PP/MMT composite, a mixture of intercalated and exfoliated structures for PA6/PP/OMMT nanocomposite, and exfoliated structure for PP-g-MA compatibilized PA6/PP/OMMT nancomposite. Thermal stability and HDT of PA6/PP matrix were improved by the addition of both MMT and OMMT. The introduction of PP-g-MA into the PA6/PP/OMMT nanocomposite enhanced the properties such as stiffness, strength, ductility, impact strength, and HDT. This was attributed to the compatibilizing effect of PP-g-MA which improved interfacial adhesion between OMMT with PA6/PP matrix and also promoted the degree of exfoliation of silicate layers in the PA6/PP matrix.

The effect of residual stress due to the curing process on damage evolution in unidirectional (UD) fibre-reinforced polymer-matrix composites under longitudinal and transverse loading has been investigated using a three-dimensional micromechanical representative volume element (RVE) model with a hexagonal packing geometry and the finite element method. Residual stress has been determined by considering two contributions: volume shrinkage of matrix resin from the crosslink polymerization during isothermal curing and thermal contraction of both resin and fibre as a result of cooling from the curing temperature to room temperature. To examine the effect of residual stress on failure, a study based on different failure criteria and a stiffness degradation technique has been used for damage analysis of the RVE subjected to mechanical loading after curing for a range of fibre volume fractions. Predicted damage initiation and evolution are clearly influenced by the presence of residual stress.

In this study, potassium diperiodatocuprate (Cu³⁺) was selected as an initiator to prepare poly(methyl methacrylate)/organo-montmorillonite composites (OMMT-g-PMMA) by in situ graft copolymerization. Three synthetic parameters were systematically evaluated as a function of the temperature, the concentration of initiator, pH and the ratio of MMA to OMMT. It was found that Cu³⁺ was a highly efficient initiator for the preparation of OMMT-g-PMMA i.e., monomer conversion and grafting efficiency were as higher as 95%. The X-ray diffraction measurement showed the intercalation of PMMA chains into OMMT layers on base of an increasing basal spacing after polymerization. FTIR analysis also suggested that the PMMA chains were effectively grafted onto OMMT substrate. The enhanced thermal stabilities of OMMT-g-PMMA composites were confirmed by the thermal gravimetric analysis (TGA). Finally, a single-electron-transfer mechanism was proposed to illustrate the formation of radicals and the preparation process of OMMT-g-PMMA composites. Cu³⁺ can be used as an effective and practical initiator in preparing the organic/inorganic composite due to its high grafting efficiency and the milder reaction condition.