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Abstract: The study described here is in sequel to our studies on the mechanics of microscale deformation in thermoplastic materials where a perspective of deformation processes occurring over different strain–strain rate regimes was obtained. The specific aim here is to enhance our fundamental understanding from microscale to nanoscale deformation in polymeric material systems containing dispersion of nanoparticles, considering the growing use of polymeric materials as functional materials. A Berkovich nanoindenter was used to induce surface damage using a load of 0.5–1mN and scratch velocity of 1μm/s. The secondary objective is to investigate the commonality in surface deformation behavior between nano- and microscale deformation to reinforce the underlying fundamental principles governing surface deformation. Combining electron microscopy with nanoscale surface deformation experiments enabled us to extend the understanding of microscale deformation to nanoscale deformation. In the intercalated polypropylene-nanoclay system, the susceptibility to surface deformation was significantly reduced because of the shift of von Mises stress from the surface to the sub-surface region, with consequent reduction in the maximum tensile stress induced by the nanoindenter. This conclusion was consistent with electron microscopy observations and theoretical analysis. [Copyright &y& Elsevier]

Abstract: In recent years, the focus of our research has been to obtain insights on microscale deformation mechanisms in thermoplastic materials reinforced with micrometer-sized particles. A perspective of deformation processes occurring over different strain–strain rate regimes was obtained [Dasari, A., Misra, R.D.K., 2004. Acta Mater. 52, 1683; Nathani, H., Dasari, A., Misra, R.D.K., 2004. Acta Mater. 52, 3217; Surampudi, N.L., Pesacreta, T.C., Misra, R.D.K., 2007. Mater. Sci. Eng. A 456, 218]. The objective here is to advance our basic understanding to nanoscale deformation in polymeric systems containing dispersion of nanoparticles, considering the growing use of polymeric materials as functional materials. A Berkovich nanoindenter was used to induce surface damage using a load of 0.5–1mN and scratch velocity of 1μm/s. An accompanying objective is to investigate the commonality in surface deformation behavior between nano- and microscale deformation to reinforce the underlying fundamental principles governing surface deformation. An understanding of mechanics of surface deformation is accomplished via electron microscopy analysis of scratch damage at and beneath the surface in conjunction with physical and mechanical properties. The electron microscopy analysis suggests that the understanding of microscale surface deformation can be extended to nanoscale surface deformation. In the intercalated high density polyethylene–nanoclay system, the susceptibility to scratch deformation is significantly reduced because of the shift of von Mises stress from the surface to the sub-surface region, with consequent reduction in the maximum tensile stress induced by the scratch. This conclusion is derived from electron microscopy and supported by the theoretical analysis. To the best of our understanding this is the first electron microscopy study that combines micro- and nanoscale deformation response during scratching of polymer nanocomposites in terms of processes that occur at and beneath the surface. [Copyright &y& Elsevier]

Abstract: We describe here phase and morphological changes that are induced when polyethylene-containing dispersion of nanoclay is isothermally crystallized in the pressure range of 0.1–150MPa. To delineate and separate the effects of applied crystallization pressure from nanoparticle effects, a relative comparison is made between neat polyethylene and polyethylene containing dispersion of nanoclay under similar experimental conditions. X-ray diffraction studies support the evidence of monoclinic phase at high crystallization pressure in addition to commonly observed orthorhombic phase of polyethylene. The nucleation of monoclinic phase is encouraged by nanoclay even at low crystallization pressures. Furthermore, orientation effects were observed in the nanocomposite system. The equilibrium melting point is insignificantly influenced on the addition of nanoclay and weakly refines the structure, such that the crystallization pressure has no obvious effect. The weak thermodynamic interaction between nanoclay and polyethylene compared to polypropylene–nanoclay system is supported by the small shift in glass transition temperature and no changes in the modification of absorption bands of polyethylene in Fourier transform infrared spectrum. [Copyright &y& Elsevier]

Abstract: The objective of the study is to understand the nanoscale near-surface deformation response of two polymer nanocomposite systems with significant differences in ductility during nanoscratching with a Berkovich indenter using a load of 1mN and a scratch velocity of 1μms−1. An accompanying objective is to investigate the commonality in surface deformation behavior between nano- and microscale deformation to reinforce the underlying fundamental principles governing surface deformation. An understanding of surface deformation response is accomplished through determination of physical and mechanical properties, structural characterization and electron microscopy analysis of surface deformation tracks and residual plastically deformed structures. The study suggests for the first time that the understanding derived from microscale surface deformation studies can be extended to nanoscale surface deformation. The microscale response in a polypropylene-based system is characterized by periodic multiple ripple-type deformation tracks that form via a mechanism identical to the periodic single-ripple-type tracks during nanoscale deformation. Similarly, in a less ductile polyethylene-based system, the periodic parabolic tracks and ironing mode of deformation during microscale deformation tend to be significantly reduced in intensity, with ironing being the primary deformation mechanism at the nanoscale. The surface deformation topography suggests that both micro- and nanoscale response is material specific. Additionally, the study suggests that reinforcement of polymers with nanoclay is a viable route to significantly decrease the susceptibility of polymeric materials to micro- and nanoscale deformation and can be discussed in terms of physical and mechanical properties of materials notably percentage crystallinity and elastic recovery. [Copyright &y& Elsevier]

Abstract: We report here for the first time the changes in spherulitic structure and phase formation that occur in polypropylene in the presence of nanoclay on crystallization at high pressures. To delineate the effects of pressure and nanoclay on the crystallization behavior, polypropylene with and without nanoclay were studied under identical experimental conditions. The two important findings are (i) the spherulite size and morphology of polypropylene in the absence and presence of nanoclay is a function of the crystallization pressure and (ii) at constant crystallization pressure, the presence of nanoclay reduced the spherulite size from 155μm to 19μm at 0.1MPa. Another important observation was the evidence of γ-phase in polypropylene with nanoclay at pressures between ∼0.1MPa and 59MPa. In neat polypropylene, increasing crystallization pressure changed the deformation process from vein-type crazing to fibrillation, while in polypropylene-containing 4wt.% clay, an increased tendency towards fibrillated fracture with increase in crystallization pressure was observed. At the present time, the observations imply the following possibilities for future considerations: (a) nanoclay changes the equilibrium state of the polymer chain or its conformation and (b) nanoclay provides a surface that is favorable for possible epitaxial growth of γ-phase because the lattice mismatch is <10%. The observed nanoparticle and pressure-induced effects, namely dramatic reduction in spherulite size and γ-phase appears to be beneficial for impact strength with consequent change in the micromechanism of plastic deformation. [Copyright &y& Elsevier]

Abstract: The influence of two concentrations of clay nanoparticles on the nonisothermal crystallization behavior of the intercalated polypropylene–clay nanocomposites is investigated here. It is observed that the crystallization peak temperature (T p) of PP–clay nanocomposites is marginally higher than neat PP at various cooling rates. Furthermore, the half-time for crystallization (t 0.5) decreased with increase in clay content, implying the nucleating role of clay nanoparticles. The nonisothermal crystallization data is analyzed using Avrami, Ozawa and Mo and coworkers methods. The validity of kinetic models on the nonisothermal crystallization process of PP–clay nanocomposites is discussed. The approach developed by Mo and coworkers successfully describes the nonisothermal crystallization behavior of PP and PP–clay nanocomposites. The activation energy for nonisothermal crystallization of pure PP and PP–clay nanocomposites based on Kissinger method is evaluated. [Copyright &y& Elsevier]

Abstract: The objective of the study described here is to investigate the determining influence of impact modifier on the microstructure and physico-chemical and mechanical properties of polypropylene crystallized at elevated pressures. The impact modified polypropylene referred as thermoplastic olefin (TPO) is characterized by a small spherulite size in the range of ∼1–8μm and the effect is retained when nanoclay is introduced into TPO. The impact strength of TPO and TPO–nanoclay system though similar, but is significantly higher than the polypropylene system, a behavior attributed to refinement of spherulite size. The study demonstrates that the nanoclay in TPO retains the impact strength of TPO and controls the dispersion of elastomer. [Copyright &y& Elsevier]

Abstract: The near surface deformation during scratching of neat polypropylene (PP) and polymer nanocomposites (PP–clay) at loads of 0.2–6kgf is described in terms of characteristics of scratch morphology and scratch deformation parameters. In neat PP, the application of higher loads introduces severe ironing deformation, disrupting the periodic parabolic nature of scratch tracks such that the material plastically flows along the ironing zone. Under identical test conditions, the severity of surface deformation in PP–clay nanocomposites, which leads to stress whitening is significantly reduced. At lower loads, the periodic nature of tracks is retained together with ironing, while at higher loads, ploughing occurred. The resistance to scratch deformation is discussed in terms of modulus, elastic recovery, and scratch hardness. [Copyright &y& Elsevier]