Rakesh K. Gupta

Model for Anomalous Moisture Diffusion through a Polymer-Clay Nanocomposite

Experimental data are reported on moisture diffusion and the elastoplastic response of an intercalated nanocomposite with vinyl ester resin matrix and montmorillonite clay filler at room temperature. Observations in diffusion tests showed that water transport in the neat resin is Fickian, whereas it becomes anomalous (non-Fickian) with the growth of the clay content. This transition is attributed to immobilization of penetrant molecules on the surfaces of hydrophilic clay layers. Observations in uniaxial tensile tests demonstrate that the response of vinyl ester resin is strongly elastoplastic, whereas an increase in the clay content results in a severe decrease of plastic strains observed as a noticeable reduction in the curvatures of the stress-strain diagrams. This is explained by slowing down the molecular mobility in the host matrix driven by confinement of chains in galleries between platelets. Constitutive equations are developed for moisture diffusion through and the elastoplastic behavior of a nanocomposite. Adjustable parameters in these relations are found by fitting the experimental data. Fair agreement is demonstrated between the observations and the results of numerical simulation. A striking similarity is revealed among changes in diffusivity, ultimate water uptake, and the rate of plastic flow with an increased clay content.

Environmental Aging of Fiber-Reinforced Polymer-Wrapped Concrete Cylinders

The use of fiber-reinforced polymer (FRP) composites is becoming increasingly common in construction, both in new construction, as in the use of reinforcing bars employed to reinforce concrete bridge decks, and in the repair of deteriorated structures, as in reinforcing concrete columns by wrapping with FRP layers. An environmental aging test protocol for evaluation of environmental durability of concrete wrapped with FRP composites is proposed. The wrap material was made of E-glass and aramid-woven fabric impregnated in an epoxy resin matrix. The effect of temperature, moisture, pH level, and freezing-and-thawing cycles on the mechanical properties of FRP-wrapped concrete was investigated. Aged FRP-concrete hybrid cylinders were tested in compression after 1,000 hr, 3,000 hr, and 8,000 hr of environmental exposure. FRP composite coupons were exposed to the same conditions and tested in tension to identify the degradation mechanisms. Hygrothermal swelling of the FRP wrap was observed for the differing aging conditions and led to a reduction in concrete confinement. The main cause of environmental damage in the hybrid system was the combined effect of moisture and elevated temperature on the tensile strength of E-glass fibers. Residual hygrothermal strains in the FRP wrap and reduction in tensile strength of the E-glass fibers controlled the changes in the stress-strain response of the concrete-FRP hybrid system caused by environmental aging. The following mechanical indicators are proposed to characterize environmental durability of FRP-wrapped concrete: critical stress (onset of unstable cracking propagation) of concrete-FRP cylinders, retained compressive strength of concrete FRP cylinders, and retained tensile strength of FRP coupons. The rate at which the retained compressive strength varied with time under the influence of the various environmental conditions was studied.

Non-linear viscoelasticity and viscoplasticity of isotactic polypropylene

Abstract Observations are reported in tensile tests with constant cross-head speeds (ranging from 5 to 200 mm/min), relaxation tests (at strains from 0.02 to 0.08), creep tests (at stresses from 15.0 to 25.0 MPa) and recovery tests (after straining up to the maximal strains ranging from 0.04 to 0.12 and subsequent retraction) on isotactic polypropylene at room temperature. A constitutive model is derived for the time- and rate-dependent responses of a semicrystalline polymer at isothermal deformation with small strains. A polymer is treated as an equivalent heterogeneous network of chains bridged by temporary junctions (entanglements, physical cross-links and lamellar blocks). The network is thought of as an ensemble of meso-regions linked with each other. The viscoelastic behavior of the ensemble reflects thermally-induced rearrangement of strands (separation of active strands from temporary junctions and merging of dangling strands with the network). To describe the viscoplastic response, the entire plastic deformation is split into the sum of two components: one of them is associated with sliding of junctions in the non-affine network of chains, while the other accounts for coarse slip and fragmentation of lamellar blocks. Stress–strain relations and kinetic equations for the plastic strains are developed by using the laws of thermodynamics. The constitutive equations involve five material constants that are found by fitting the observations. Fair agreement is demonstrated between the experimental data and the results of numerical simulation.

ELASTIC EFFECTS IN FLOW OF FLUIDS THROUGH SINUOUS TUBES

A comprehensive experimental study of the pressure drop vs flow rate relationship was carried out using a variety of Newtonian as well as non‐Newtonian liquids in circular cross‐section tubes having a radius that varied sinusoidally with axial distance. The goal of this work was to verify or refute the existence of viscoelastic effects in this Lagrangian unsteady flow field under conditions of low Reynolds numbers and large Weissenberg numbers. Highly elastic and rheologically well‐characterized polyacrylamide solutions having zero‐shear viscosities as large as 140 Pa s were utilized for this purpose; some of the fluids were shear thinning while others were not. Clear elastic effects were observed at high enough Weissenberg numbers.

Effect of Size and Surface Charge of Gold Nanoparticles on their Skin Permeability: A Molecular Dynamics Study

Molecular level understanding of permeation of nanoparticles through human skin establishes the basis for development of novel transdermal drug delivery systems and design and formulation of cosmetics. Recent experiments suggest that surface coated nano-sized gold nanoparticles (AuNPs) can penetrate the rat and human skin. However, the mechanisms by which these AuNPs penetrate are not well understood. In this study, we have carried out coarse grained molecular dynamics simulations to explore the permeation of dodecanethiol coated neutral hydrophobic AuNPs of different sizes (2–5 nm) and surface charges (cationic and anionic) through the model skin lipid membrane. The results indicate that the neutral hydrophobic AuNPs disrupted the bilayer and entered in it with in ~200 ns, while charged AuNPs were adsorbed on the bilayer headgroup. The permeation free energy calculation revealed that at the head group of the bilayer, a very small barrier existed for neutral hydrophobic AuNP while a free energy minimum was observed for charged AuNPs. The permeability was maximum for neutral 2 nm gold nanoparticle (AuNP) and minimum for 3 nm cationic AuNP. The obtained results are aligned with recent experimental findings. This study would be helpful in designing customized nanoparticles for cosmetic and transdermal drug delivery application.

Ultrasound-assisted surface-modification of wood particulates for improved wood/plastic composites

The combined effects of alkali and ultrasound treatment of wood flour on the mechanical properties of polypropylene-based wood/plastic composites (WPCs) were examined. FT-IR measurements confirmed that the alkali treatment removed both hemicellulose and lignin from the wood, and there was an increase in the number of hydroxyl groups on the cellulose surface. This process was promoted by ultrasound treatment. Mechanical testing of injection-molded WPC samples revealed that alkali treatment improved both composite strength and modulus when polypropylene grafted with maleic acid was used as a coupling agent. The strength increase is due to improved adhesion between the fiber and matrix, while improved modulus is due to the removal of lignin and hemicellulose that are not as stiff as cellulose. Polarized optical microscopy showed the presence of well-defined polymer crystals on the surface of the modified wood, and this is also responsible for the improved mechanical properties. It is conclusively demonstrated that the combination of chemical treatment of wood and ultrasound assistance is more effective in improving the mechanical properties of the composites than the use of chemical treatment alone.

Moisture Absorption Behavior of Wood/Plastic Composites Made with Ultrasound-Assisted Alkali-Treated Wood Particulates

The combined effects of alkali and ultrasound treatment of wood flour on the moisture absorption behavior of polypropylene-based wood/plastic composites (WPCs) were examined. FT-IR measurements confirmed that the alkali treatment removed both hemicellulose and lignin from the wood, and there was an increase in the number of hydroxyl groups on the cellulose surface. This process was promoted by ultrasound treatment. Particle size analysis of treated wood fibers revealed that alkali treatment reduced the particle size slightly, and the particle size was further reduced by the assistance of ultrasound. Moisture absorption measurements revealed the interplay between the presence of relative amounts of cellulose, hemicellulose and lignin in the wood and the amount of coupling agent added. It was found that appropriate alkali treatment and an optimum amount of added coupling agent could simultaneously improve composite mechanical properties and reduce both the rate and the maximum amount of moisture absorbed. Further improvements require preferential removal of hemicellulose from the wood and conditions that promote covalent bonding between the coupling agent and the wood reinforcement.

Interfacial adhesion of cellulose fiber and natural fiber filled polypropylene compounds and their effects on rheological and mechanical properties

Rayon fiber (RN) and pine wood fiber (PW) filled polypropylene (PP) compounds, PP/RN (90/05 and 75/25 wt%) and PP/PW (90/05, 75/25 and 50/50 wt%), are investigated for their interfacial adhesion, rheological properties, morphology, nucleation and mechanical properties. The interfacial adhesion of the RN-filled PP compounds is better than that of the PW ones. As the concentration of the RN and the PW particles is increased, the dynamic viscosity, the crystallization temperature, and the tensile modulus are increased; however, the tensile strain is decreased. The viscosity of the RN-filled compounds is higher than that of the PW ones at the same loadings. Significant differences are found in the elongation yield test. As the concentration of the particles is increased, the elongation yield stress of the RN compounds is increased. Elongation yield stress of the PW compounds is decreased and more spherulites are locally developed on the RN surface than the PW surface. The interfacial adhesion of the RN surface with PP is better than that of the PW surface. The elimination of extractives on the PW surface improves the mechanical property of the PW/PP compounds; however, it reduces processability of the PW/PP compounds.

The viscoelastic and viscoplastic behavior of polymer composites: polycarbonate reinforced with short glass fibers

Abstract Observations are reported on polycarbonate filled with various amounts of short glass fibers (i) in tensile tests with a constant strain rate and (ii) in oscillatory torsion tests at room temperature. Constitutive equations are derived for the viscoelastic and viscoplastic responses of a polymer composite. The composite is treated as an equivalent network of chains bridged by junctions (entanglements and glass fibers). The network is thought of as an ensemble of meso-regions with arbitrary shapes and sizes. With reference to the theory of cooperative relaxation, the viscoelastic response is attributed to rearrangement of meso-domains that occurs at random times when the regions are thermally activated. The viscoplastic behavior is associated with displacements of meso-regions with respect to each other. The rate of sliding of meso-domains is proportional to the rate of macro-deformation. Constitutive equations for isothermal deformation of a polymer composite are derived by using the laws of thermodynamics. The stress–strain relations are determined by five adjustable parameters that are found by fitting the experimental data. The effect of the filler content on the material parameters is studied in detail.

Effect of the coupling agent on the properties of PNC-based wood/plastic composites

The increase in modulus of polypropylene (PP) upon addition of montmorillonite makes PP-based polymer nanocomposites (PNCs) ideal matrix materials for wood/plastic composites (WPCs) for decking, railing, roofing and siding applications, among others. In this work, the influence of maleated PP (that is used both as a coupling agent for WPCs and as a compatibilizer for the PNC) on increasing the modulus of extruded and injection molded PP-based WPCs is investigated. Also examined are the effects of the sequence of compounding steps and the use of different processing conditions. The improvement in mechanical properties is explained with the help of available theories and is also related to the composite morphology with the help of scanning and transmission electron micrographs.