Several studies show that the dynamics of solvent molecules around a solute slows down in a nanos... more Several studies show that the dynamics of solvent molecules around a solute slows down in a nanoscale confined geometry compared to the bulk condition. Here we perform numerical simulations to investigate the microscopic mechanisms of such slowing down. We show a substantial slowing down of solvation dynamics around a solute in strong solvophilic confinements due to suppression of fluid diffusion in the presence of solvophilic walls, along with restricted solvent dynamics due to geometrical constraints. The solvation in strong solvophobic confinements becomes slower than the same in the bulk as well, but not as significantly as in the solvophilic case. This is due to the competition between restriction in solvent dynamics and faster in-plane solvent diffusion. We place our findings in perspective of various solvation dominated chemical processes in nanoconfined geometry.
Several earlier studies have shown signatures of crossover in various static and dynamics propert... more Several earlier studies have shown signatures of crossover in various static and dynamics properties of a confined fluid when the confining dimension decreases to about a nanometer. The density fluctuations govern the majority of such properties of a fluid. Here, we illustrate the crossover in density fluctuation in a confined fluid, to provide a generic understanding of confinement-induced crossover of fluid properties, using computer simulations. The crossover can be understood as a manifestation of changes in the long-wavelength behavior of fluctuation in density due to geometrical constraints. We further show that the confining potential significantly affects the crossover behavior.
Journal of the Mechanics and Physics of Solids, 2011
A continuum model of plasticity, Phenomenological Mesoscopic Field Dislocation Mechanics (PMFDM),... more A continuum model of plasticity, Phenomenological Mesoscopic Field Dislocation Mechanics (PMFDM), is used to study the effect of surface passivation, grain orientation, grain boundary constraints and film thickness on the mechanical response of multicrystalline thin films. The numerical experiments presented in this paper show that a surface passivation layer on thin films introduces thickness dependence of the mechanical response. However, the effect of passivation decreases in films with impenetrable grain boundaries. The orientation of individual grains of the multicrystal also has a significant effect on the mechanical response. Our results are in qualitative agreement with experimental observations. A primary contribution of this work is the implementation of a jump condition that enables the modeling of important limits of grain boundary constraints to plastic flow, independent of ad-hoc constitutive assumptions and interface conditions.
Reinforcing fillers are added to elastomeric compounds to improve and adjust several mechanical, ... more Reinforcing fillers are added to elastomeric compounds to improve and adjust several mechanical, dynamical, tribological, etc. properties with respect to different applications, i.e. for automotive tires, or technical rubber goods. Carbon black and precipitated silica are widely used as rubber reinforcing fillers; however, some new classes of nanosized substances like organophilic modified clay or carbon nanotubes are presently intensive studied as possible future filler systems in combination with carbon black or silica.An important parameter for the dispersibility and compatibility of the filler in the polymer matrix of rubber compounds is the surface energy and surface polarity of the solid filler particles. Therefore, we systematically measured and compared the dynamic contact angles of a collection of different filler types (carbon blacks, silica, carbon nanotubes and organoclays) using the Wilhelmy method, whereby the particles were fixed as a thin layer at a double-sided adhesive tape. From the contact angle values the polar and disperse part of the surface energies of the filler particles were calculated by fitting Fowkes formula. For an estimation of the compatibility of the fillers with different types of rubber polymers we additionally analyzed the surface energy and polarity of the gum (unfilled) elastomers. From the evaluated surface energies and polarities, thermodynamic predictors for the dispersibility (enthalpy of immersion), the adhesion between filler particles and polymer matrix in the nanocomposite, and for the flocculation behaviour of the particles in a rubber matrix (difference in the works of adhesion) were derived. These thermodynamic predictors improve considerably the compounding process of novel rubber nanocomposites with respect to target-oriented adjustment of rubber properties.
The most important factor, which indicates the improvement of properties in rubber by nanoclay, i... more The most important factor, which indicates the improvement of properties in rubber by nanoclay, is the distribution in the rubber matrix. The simple mixing of rubber and nanoclay will not exfoliate the filler in the rubbery matrix. Hence, some sort of compatibilizer like epoxidized natural rubber (ENR) can be utilized safely in exfoliating the nanoclay in the matrix polymer. Epoxidized natural rubber and organically modified nanoclay composites (EC) were prepared by solution mixing. The nanoclay employed in this study was Cloisite 20A. The obtained nanocomposites were incorporated in nitrile butadiene rubber (NBR) with sulphur as a curing agent. The morphological study showed the intercalation of nanoclay in ENR and further incorporation of EC in NBR matrix leads to exfoliation of nanoclay. Curing study demonstrated faster scorch time, cure time and increase in maximum torque for the EC incorporated NBR compounds compared to pure NBR. Dynamic mechanical thermal analysis showed increase in storage modulus and lesser damping characteristics for the compounds containing EC loading in NBR matrix. In addition, these particular compounds showed enhancement in overall mechanical properties, higher swelling resistance in oil and solvent and decrease in compression set due to higher reinforcing efficiency of nanoclay in the NBR matrix.
The present paper describes the mechanical reclaiming of ground rubber tire (GRT) by tetra methyl... more The present paper describes the mechanical reclaiming of ground rubber tire (GRT) by tetra methyl thiuram disulfide (TMTD), a multi-functional reclaiming agent. The versatility of the proposed agent is that it acts as a reclaiming agent during reclaiming and as a curing agent during revulcanization of the reclaimed sample. Reclaiming of GRT was carried out on an open two roll mixing mill at various time intervals and different concentrations of the reclaiming agent (TMTD). The degree of reclaiming was evaluated by measurement of the gel content, inherent viscosity of sol rubber, Mooney viscosity of the reclaimed rubber, crosslink density, swelling ratio and molecular weight between two crosslink bonds as a function of milling time. Also, the influence of the gel content on crosslink density at various time intervals on the open two roll mixing mill was determined. A unique correlation between gel fraction and crosslink density obtained at various time intervals and concentrations of reclaiming agent indicated that an optimization of the concentration of TMTD and milling time has a positive influence on improving the efficiency of reclaiming. The reclaiming conditions have been optimized in view of the mechanical properties of the revulcanized GRT and the aging resistance properties of the revulcanized reclaim. The influence of the concentration of sulfur on the mechanical properties in the revulcanized reclaim was also studied. Scanning electron microscopy (SEM) studies further indicate the coherency and homogeneity in the revulcanized reclaim rubber when reclamation is carried out by optimum concentration of TMTD after maximum time of reclaiming.
The effect of two chemically opposite type of nanofillers, namely montmorillonite (MMT) and layer... more The effect of two chemically opposite type of nanofillers, namely montmorillonite (MMT) and layered double hydroxide (LDH), both in pristine and organically modified forms (OLDH and OMMT) on the mechanical properties and crystallization behavior of chloroprene rubber (CR) vulcanizates have been investigated. While the addition of very small amount of pristine clays shows a significant improvement of the physical properties of the CR based nanocomposites, the organic modification improves the reinforcing efficiency of MMT more than that of LDH. MMT is found to enhance the crystallization tendency of CR chains, whereas LDH has a role to resist it as observed from the dynamic mechanical analysis (DMA). The DMA results also show that the presence small amount of nanoclay enhances the storage modulus of CR based vulcanizates, which becomes more pronounced in case of organically modified clays, especially with OMMT. Finally, we report the 'house of cards' structure of exfoliated platelets of montmorillonite in the CR matrix which are produced by mechanical shear mixing.
This paper focuses on the influence of ionic liquid on carbon nanotube based elastomeric composit... more This paper focuses on the influence of ionic liquid on carbon nanotube based elastomeric composites. Multi-walled carbon nanotubes (MWCNTs) are modified using an ionic liquid at room temperature, 1-butyl 3-methyl imidazolium bis (trifluoromethylsulphonyl) imide (BMI) and modified MWCNTs exhibit physical (cation-p/p-p) interaction with BMI. The polychloroprene rubber (CR) composites are prepared using unmodified and BMI modified MWCNTs. The presence of BMI not only increases the alternating current (AC) electrical conductivity and polarisability of the composites but also improves the state of dispersion of the tubes as observed from dielectric spectroscopy and transmission electron microscopy respectively. In addition to the hydrodynamic reinforcement, the formation of improved filler-filler networks is reflected in the dynamic storage modulus (E 0 ) for modified MWCNTs/ CR composites in amplitude sweep measurement upon increasing the proportion of BMI. Hardness and mechanical properties are also studied for the composites as a function of BMI.
Polychloroprene (CR) composites were prepared using unmodified and ionic liquid modified carbon n... more Polychloroprene (CR) composites were prepared using unmodified and ionic liquid modified carbon nanotubes (CNTs) respectively. The composites with ionic liquid modified CNTs (M-CNTs) were found to exhibit high electrical conductivity, improved tensile modulus and thermal stability as opposed to the composites with unmodified CNTs (R-CNTs) [1,2]. The current article deals with the accelerated thermal aging of the aforementioned two different composites at various temperatures and time periods in aerobic conditions. Post-aging results are better for M-CNTs/CR composites: Enhanced retention of mechanical property (tensile strength) and reduced surface defects (as revealed by scanning electron microscopy pictures) were observed. It is inferred that ionic liquid (IL) acts as an anti-oxidant protecting the composites from surface oxidation. Fourier transform infrared (FTIR) spectroscopic studies confirmed the presence of hydroxyl and carbonyl regions in the aged samples; however, the intensity of the regions was least for the composites with M-CNTs. Surprisingly, the electrical conductivity of M-CNTs/CR composites increased upon aging and therefore, these composites can be used for applications where conductivity at elevated temperatures is required.
Several studies show that the dynamics of solvent molecules around a solute slows down in a nanos... more Several studies show that the dynamics of solvent molecules around a solute slows down in a nanoscale confined geometry compared to the bulk condition. Here we perform numerical simulations to investigate the microscopic mechanisms of such slowing down. We show a substantial slowing down of solvation dynamics around a solute in strong solvophilic confinements due to suppression of fluid diffusion in the presence of solvophilic walls, along with restricted solvent dynamics due to geometrical constraints. The solvation in strong solvophobic confinements becomes slower than the same in the bulk as well, but not as significantly as in the solvophilic case. This is due to the competition between restriction in solvent dynamics and faster in-plane solvent diffusion. We place our findings in perspective of various solvation dominated chemical processes in nanoconfined geometry.
Several earlier studies have shown signatures of crossover in various static and dynamics propert... more Several earlier studies have shown signatures of crossover in various static and dynamics properties of a confined fluid when the confining dimension decreases to about a nanometer. The density fluctuations govern the majority of such properties of a fluid. Here, we illustrate the crossover in density fluctuation in a confined fluid, to provide a generic understanding of confinement-induced crossover of fluid properties, using computer simulations. The crossover can be understood as a manifestation of changes in the long-wavelength behavior of fluctuation in density due to geometrical constraints. We further show that the confining potential significantly affects the crossover behavior.
Journal of the Mechanics and Physics of Solids, 2011
A continuum model of plasticity, Phenomenological Mesoscopic Field Dislocation Mechanics (PMFDM),... more A continuum model of plasticity, Phenomenological Mesoscopic Field Dislocation Mechanics (PMFDM), is used to study the effect of surface passivation, grain orientation, grain boundary constraints and film thickness on the mechanical response of multicrystalline thin films. The numerical experiments presented in this paper show that a surface passivation layer on thin films introduces thickness dependence of the mechanical response. However, the effect of passivation decreases in films with impenetrable grain boundaries. The orientation of individual grains of the multicrystal also has a significant effect on the mechanical response. Our results are in qualitative agreement with experimental observations. A primary contribution of this work is the implementation of a jump condition that enables the modeling of important limits of grain boundary constraints to plastic flow, independent of ad-hoc constitutive assumptions and interface conditions.
Reinforcing fillers are added to elastomeric compounds to improve and adjust several mechanical, ... more Reinforcing fillers are added to elastomeric compounds to improve and adjust several mechanical, dynamical, tribological, etc. properties with respect to different applications, i.e. for automotive tires, or technical rubber goods. Carbon black and precipitated silica are widely used as rubber reinforcing fillers; however, some new classes of nanosized substances like organophilic modified clay or carbon nanotubes are presently intensive studied as possible future filler systems in combination with carbon black or silica.An important parameter for the dispersibility and compatibility of the filler in the polymer matrix of rubber compounds is the surface energy and surface polarity of the solid filler particles. Therefore, we systematically measured and compared the dynamic contact angles of a collection of different filler types (carbon blacks, silica, carbon nanotubes and organoclays) using the Wilhelmy method, whereby the particles were fixed as a thin layer at a double-sided adhesive tape. From the contact angle values the polar and disperse part of the surface energies of the filler particles were calculated by fitting Fowkes formula. For an estimation of the compatibility of the fillers with different types of rubber polymers we additionally analyzed the surface energy and polarity of the gum (unfilled) elastomers. From the evaluated surface energies and polarities, thermodynamic predictors for the dispersibility (enthalpy of immersion), the adhesion between filler particles and polymer matrix in the nanocomposite, and for the flocculation behaviour of the particles in a rubber matrix (difference in the works of adhesion) were derived. These thermodynamic predictors improve considerably the compounding process of novel rubber nanocomposites with respect to target-oriented adjustment of rubber properties.
The most important factor, which indicates the improvement of properties in rubber by nanoclay, i... more The most important factor, which indicates the improvement of properties in rubber by nanoclay, is the distribution in the rubber matrix. The simple mixing of rubber and nanoclay will not exfoliate the filler in the rubbery matrix. Hence, some sort of compatibilizer like epoxidized natural rubber (ENR) can be utilized safely in exfoliating the nanoclay in the matrix polymer. Epoxidized natural rubber and organically modified nanoclay composites (EC) were prepared by solution mixing. The nanoclay employed in this study was Cloisite 20A. The obtained nanocomposites were incorporated in nitrile butadiene rubber (NBR) with sulphur as a curing agent. The morphological study showed the intercalation of nanoclay in ENR and further incorporation of EC in NBR matrix leads to exfoliation of nanoclay. Curing study demonstrated faster scorch time, cure time and increase in maximum torque for the EC incorporated NBR compounds compared to pure NBR. Dynamic mechanical thermal analysis showed increase in storage modulus and lesser damping characteristics for the compounds containing EC loading in NBR matrix. In addition, these particular compounds showed enhancement in overall mechanical properties, higher swelling resistance in oil and solvent and decrease in compression set due to higher reinforcing efficiency of nanoclay in the NBR matrix.
The present paper describes the mechanical reclaiming of ground rubber tire (GRT) by tetra methyl... more The present paper describes the mechanical reclaiming of ground rubber tire (GRT) by tetra methyl thiuram disulfide (TMTD), a multi-functional reclaiming agent. The versatility of the proposed agent is that it acts as a reclaiming agent during reclaiming and as a curing agent during revulcanization of the reclaimed sample. Reclaiming of GRT was carried out on an open two roll mixing mill at various time intervals and different concentrations of the reclaiming agent (TMTD). The degree of reclaiming was evaluated by measurement of the gel content, inherent viscosity of sol rubber, Mooney viscosity of the reclaimed rubber, crosslink density, swelling ratio and molecular weight between two crosslink bonds as a function of milling time. Also, the influence of the gel content on crosslink density at various time intervals on the open two roll mixing mill was determined. A unique correlation between gel fraction and crosslink density obtained at various time intervals and concentrations of reclaiming agent indicated that an optimization of the concentration of TMTD and milling time has a positive influence on improving the efficiency of reclaiming. The reclaiming conditions have been optimized in view of the mechanical properties of the revulcanized GRT and the aging resistance properties of the revulcanized reclaim. The influence of the concentration of sulfur on the mechanical properties in the revulcanized reclaim was also studied. Scanning electron microscopy (SEM) studies further indicate the coherency and homogeneity in the revulcanized reclaim rubber when reclamation is carried out by optimum concentration of TMTD after maximum time of reclaiming.
The effect of two chemically opposite type of nanofillers, namely montmorillonite (MMT) and layer... more The effect of two chemically opposite type of nanofillers, namely montmorillonite (MMT) and layered double hydroxide (LDH), both in pristine and organically modified forms (OLDH and OMMT) on the mechanical properties and crystallization behavior of chloroprene rubber (CR) vulcanizates have been investigated. While the addition of very small amount of pristine clays shows a significant improvement of the physical properties of the CR based nanocomposites, the organic modification improves the reinforcing efficiency of MMT more than that of LDH. MMT is found to enhance the crystallization tendency of CR chains, whereas LDH has a role to resist it as observed from the dynamic mechanical analysis (DMA). The DMA results also show that the presence small amount of nanoclay enhances the storage modulus of CR based vulcanizates, which becomes more pronounced in case of organically modified clays, especially with OMMT. Finally, we report the 'house of cards' structure of exfoliated platelets of montmorillonite in the CR matrix which are produced by mechanical shear mixing.
This paper focuses on the influence of ionic liquid on carbon nanotube based elastomeric composit... more This paper focuses on the influence of ionic liquid on carbon nanotube based elastomeric composites. Multi-walled carbon nanotubes (MWCNTs) are modified using an ionic liquid at room temperature, 1-butyl 3-methyl imidazolium bis (trifluoromethylsulphonyl) imide (BMI) and modified MWCNTs exhibit physical (cation-p/p-p) interaction with BMI. The polychloroprene rubber (CR) composites are prepared using unmodified and BMI modified MWCNTs. The presence of BMI not only increases the alternating current (AC) electrical conductivity and polarisability of the composites but also improves the state of dispersion of the tubes as observed from dielectric spectroscopy and transmission electron microscopy respectively. In addition to the hydrodynamic reinforcement, the formation of improved filler-filler networks is reflected in the dynamic storage modulus (E 0 ) for modified MWCNTs/ CR composites in amplitude sweep measurement upon increasing the proportion of BMI. Hardness and mechanical properties are also studied for the composites as a function of BMI.
Polychloroprene (CR) composites were prepared using unmodified and ionic liquid modified carbon n... more Polychloroprene (CR) composites were prepared using unmodified and ionic liquid modified carbon nanotubes (CNTs) respectively. The composites with ionic liquid modified CNTs (M-CNTs) were found to exhibit high electrical conductivity, improved tensile modulus and thermal stability as opposed to the composites with unmodified CNTs (R-CNTs) [1,2]. The current article deals with the accelerated thermal aging of the aforementioned two different composites at various temperatures and time periods in aerobic conditions. Post-aging results are better for M-CNTs/CR composites: Enhanced retention of mechanical property (tensile strength) and reduced surface defects (as revealed by scanning electron microscopy pictures) were observed. It is inferred that ionic liquid (IL) acts as an anti-oxidant protecting the composites from surface oxidation. Fourier transform infrared (FTIR) spectroscopic studies confirmed the presence of hydroxyl and carbonyl regions in the aged samples; however, the intensity of the regions was least for the composites with M-CNTs. Surprisingly, the electrical conductivity of M-CNTs/CR composites increased upon aging and therefore, these composites can be used for applications where conductivity at elevated temperatures is required.
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