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The dispersion of carbon nanotubes (CNTs) in polypropylene (PP) introduces a significant change in the structure and phase evolution during crystallisation at ambient and elevated pressures. The differences between the behaviours of PP and PP containing CNTs are attributed to CNT/polymer interface driven nucleation, which is also responsible for the significant reduction in spherulite size and is associated with the change in equilibrium melting point and percentage crystallinity. This opens the gateway for the use of polymer CNT composites for functional devices, including biomedical and electronic devices. A study of the impact of 1 wt-%CNT additions on the microstructure evolution of PP during pressure induced crystallisation is reported. It is demonstrated that the dispersion of CNTs produces a significant difference in the macromolecular structure and phase evolution during crystallisation at ambient and elevated pressures. The change in equilibrium melting point with pressure is higher for PP containing CNT, which is indicative of the chemical interaction between polymer and CNT. It is found that CNT additions promote the formation of γ-phase at low crystallisation pressure of 0·1-25 MPa, in contrast to the neat PP where α-crystals nucleate at low to moderate pressures (about 0·1-60 MPa) and the γ-phase nucleates at crystallisation pressures exceeding ∼60 MPa. The dependence of equilibrium melting temperature on crystallisation pressure and spherulite size delineates two distinct regimes, characterised predominantly by α- and γ-phases respectively, for both PP and PP-CNT systems. [ABSTRACT FROM AUTHOR]

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: Phyllosilicates including mica offer one of the easiest methods to obtain atomically flat surface of macroscopic size by cleavage. Thus, thin films of polypropylene crystallized at different pressure and as a function of polypropylene concentration were investigated. The objective was to probe the nucleation and chain organization in thin films on basal plane surface from the view point of relating the nucleation and growth process to the intergallery space between the clay layers. The crystal morphology was altered from leaf-like structure at high undercooling to fully developed spherulites at low undercooling. Furthermore, the lamellae structure change from well-oriented to cross-hatched with increase in the crystallization pressure was indicative of epitaxial growth between polymer matrix and mica surface. The possible epitaxial mechanism is discussed here. The study provides a method to explore the macro- and microstructure of polymer and epitaxy between polymer matrix and inorganic particle surface. The good interfacial bonding favored by epitaxial crystallization is beneficial for toughness. [Copyright &y& Elsevier]

Abstract: Significant differences in the microstructure occur when the polypropylene matrix dispersed with 4wt.% and 8wt.% nanoclay. In neat polypropylene, α-crystals nucleate in the low to moderate crystallization pressure range of (∼0.1–60MPa), while γ-phase nucleates at crystallization pressure of ∼60–200MPa. In contrast to neat polypropylene, the presence of nanoclay in the polymer matrix, promotes the formation of γ-phase at ambient crystallization pressure of 0.1MPa. Nanoclay and pressure-induced crystallization governs phase evolution and microstructure of polypropylene and is associated with change in the thermodynamic and physical properties such as equilibrium melting point and glass transition temperature. This change in equilibrium melting point and glass transition temperature points toward thermodynamic interaction between polymer-nanoparticle and nanoparticle interface-driven microstructural evolution. The dependence of equilibrium melting point on crystallization pressure and spherulite size on undercooling delineates two distinct regimes which are ascribed to dominant α- and γ-phases of polypropylene. However, the effect of nanoparticles is less dramatic when nanoclay content is increased from 4wt.% to 8wt.% implying that the effect maximizes at ∼4wt.% nanoclay. [ABSTRACT FROM AUTHOR]

Abstract: The objective of this study is to describe the evolution of structure and phases during pressure-induced crystallization of polymers containing dispersion of nanoparticles, in the pressure range of 0.1–200MPa. The model material for nanoparticles is nanoclay and the model polymer is polypropylene, which can potentially form several crystalline phases. While the phase selection in polypropylene is dictated by pressure and temperature, however, the introduction of nanoparticles alters the nucleation and growth of phases via nanoparticle interface driven evolution. To delineate and separate the effects of applied crystallization pressure from nanoparticle effects, a relative comparison is made between neat polypropylene and polypropylene containing dispersion of nanoclay under similar experimental conditions. The significant finding is that nanoclay interacts with the host polypropylene in a manner such that it alters the structural morphology of α- and γ-crystals of polypropylene. Furthermore, nanoclay promotes the formation of γ-phase at ambient pressure suggesting its role as structure and morphology director in the stabilization of the less accessible γ-phase, and with the possibility of epitaxial growth that enhances toughness. The equilibrium melting point measurements point to thermodynamic interaction between nanoclay and polypropylene, which is supported by the change in glass transition temperature. Thus, the two components, nanoclay and pressure, together provide a unique opportunity to tune hierarchical structures and phase evolution, which has significant implication on physico-chemical and mechanical properties. [Copyright &y& Elsevier]

Abstract: We describe here phase and morphological changes that are induced when polypropylene containing dispersion of nanoclay is isothermally crystallized in the pressure range of 0.1–200MPa. To delineate and separate the effects of applied crystallization pressure from nanoparticle effects, a relative comparison is made between neat polypropylene and polypropylene containing dispersion of nanoclay under similar experimental conditions. The significant finding is that nanoparticles of clay interact with the host polypropylene in a manner such that they alter the structural morphology of α- and γ-crystals of polypropylene presumably via nanoparticle interface driven evolution. Interestingly, nanoclay promotes the formation of γ-phase at low pressure, which is expected to have significant bearing on physico-chemical and mechanical properties. The equilibrium melting point measurements point to thermodynamic interaction between nanoclay and polypropylene, which is supported by the change in glass transition temperature and modification of absorption bands of polypropylene between 995 and 973cm−1 corresponding to tertiary methyl skeletal in Fourier transform infrared spectrum. [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 micromechanism of plastic deformation during impact loading of polypropylene–clay nanocomposites is examined and compared with the unreinforced polypropylene under identical conditions of processing to underscore the determining role of clay. The addition of clay to polypropylene increases the impact strength in the temperature range of 0 to +70°C. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and transmission electron microscopy (TEM), wide-angle X-ray diffraction (WAXD) and scanning electron microscopy (SEM) techniques provided an understanding of the micromechanism of plastic deformation in terms of the response of the polymer matrix, nucleating capability of the reinforcement, crystal structure, percentage crystallinity, lamellae thickness, and particle–matrix interface. The enhancement of toughness on reinforcement of polypropylene with nanoclay is associated with change in the primary mechanism of plastic deformation from crazing and vein-type in neat polypropylene to microvoid-coalescence-fibrillation process in the nanocomposite. [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]