Ashok Kumar Dasmahapatra | Indian Institute of Technology Guwahati (original) (raw)

Papers by Ashok Kumar Dasmahapatra

Research paper thumbnail of Phase separation and crystallization in double crystalline symmetric binary polymer blends

Journal of Polymer Research, 2016

Research paper thumbnail of Coil-Globule Transition of Polymer -- Tricriticality of Theta

Research paper thumbnail of Effect of copolymer microstructure on single chain collapse

We present dynamic Monte Carlo simulations of the collapse of copolymers containing sticky comono... more We present dynamic Monte Carlo simulations of the collapse of copolymers containing sticky comonomers, c. There is a qualitative difference in the transition depending on c content. For c content >˜ 50%, copolymer collapse is qualitatively similar to that observed for homopolymers, when rescaled to account for comonomer solvophobicity. However, collapse of copolymers with c < ˜50% is qualitatively steeper

Research paper thumbnail of Effect of additive particles on the crystallization of homopolymers

The effect of additive particles on polymer crystallization has been investigated using lattice d... more The effect of additive particles on polymer crystallization has been investigated using lattice dynamic Monte Carlo simulation. Additives are compatible with the polymer matrix (viz. there is an attractive ``sticky'' interaction between additives and monomers) and, additive particles have the same size as a monomer. Polymer crystallization is strongly influenced by both additive fraction, x and the additive-monomer interaction strength,

Research paper thumbnail of Collapse Transition of Branched Polymers in Dilute Solutions: Telechelic Star vs. H-Polymer

Macromolecular Symposia, 2015

Research paper thumbnail of Experimental studies and PNN prediction on flow pattern of viscous oil-water flow through circular horizontal pipe

Industrial & Engineering Chemistry Research

Research paper thumbnail of Correlation based Pressure Drop Prediction of visous oil-water two-phase flow through Horizontal Pipeline

Research paper thumbnail of Effect of an uphill elbow on interfacial wave encountered during the simultaneous flow of a pair of immiscible liquids through an undulated pipeline in its peak configuration

Research paper thumbnail of Coarse-grain molecular dynamics study of fullerene transport across a cell membrane

The Journal of chemical physics, Jan 14, 2015

The study of the ability of drug molecules to enter cells through the membrane is of vital import... more The study of the ability of drug molecules to enter cells through the membrane is of vital importance in the field of drug delivery. In cases where the transport of the drug molecules through the membrane is not easily accomplishable, other carrier molecules are used. Spherical fullerene molecules have been postulated as potential carriers of highly hydrophilic drugs across the plasma membrane. Here, we report the coarse-grain molecular dynamics study of the translocation of C60 fullerene and its derivatives across a cell membrane modeled as a 1,2-distearoyl-sn-glycero-3-phosphocholine bilayer. Simulation results indicate that pristine fullerene molecules enter the bilayer quickly and reside within it. The addition of polar functionalized groups makes the fullerenes less likely to reside within the bilayer but increases their residence time in bulk water. Addition of polar functional groups to one half of the fullerene surface, in effect creating a Janus particle, offers the most pr...

Research paper thumbnail of An appraisal of viscous oil–water two-phase flow through an undulated pipeline in peak configuration

Experimental Thermal and Fluid Science, 2015

ABSTRACT Experimental results on flow patterns and pressure drop characteristics of viscous oil-w... more ABSTRACT Experimental results on flow patterns and pressure drop characteristics of viscous oil-water flow through undulated pipeline in peak configuration have been studied in this work Undulated pipeline consists of two inclined sections (uphill and downhill) connected between two horizontal pipes (upstream and downstream) at peak configuration. Experiments have been conducted over a wide range of superficial velocities of oil (USO = 0.015 m/s to 1.3 m/s) and water (USW = 0.1 m/s to 1.2 m/s). Seven different flow patterns (viz., plug, slug, wavy stratified, stratified mixed, wispy annular, dispersion of oil in water and dispersion of water in oil) are identified by visual, imaging and electrical conductance probe techniques at all the four sections. Our results shows that present undulation (5°) has a marginal effect on the flow behavior of viscous oil-water mixture and the range of fluid velocity differs for a particular flow pattern. The pressure gradient characteristics across the peak experiences relatively more pressure gradient as compared to upstream and downstream at USW ⩾ 0.6 m/s, which is attributed to the combined effect of undulation (viz., peak configuration) along the pipeline, flow pattern, and effective viscosity and density of the fluid mixture.

Research paper thumbnail of Graphene based multifunctional superbots

Carbon, 2015

A versatile graphene coated glass microswimmer displayed directed motions under the influence of ... more A versatile graphene coated glass microswimmer displayed directed motions under the influence of applied electric field, chemical potential gradient and external magnetic field.

Research paper thumbnail of Correlations for Prediction of Pressure Gradient of Liquid-Liquid Flow Through a Circular Horizontal Pipe

Journal of Fluids Engineering, 2014

We report a detailed investigation on the measurement and prediction of pressure gradient charact... more We report a detailed investigation on the measurement and prediction of pressure gradient characteristics of moderately viscous lubricating oil-water flow through a horizontal pipe of 0.025 m internal diameter. Experiments are carried out over a wide range of phase velocities of both oil (U SO ¼ 0.015-1.25 m/s) and water (U SW ¼ 0.1-1.1 m/s). Experimental pressure gradients yield significant errors when fitted to the existing correlations, which are largely used for gas-liquid flow. To predict pressure gradient characteristics for liquid-liquid flow, the existing correlations need to be modified. We propose two correlations, based on the Lockhart-Martinelli's approach (by modifying the correlation between the Lockhart-Martinelli parameter and a two-phase multiplier suitable for the present system) and dimensionless analysis, following the Buckingham's Pi-theorem. We observe significant improvement in the prediction of pressure gradient. The correlation based on the dimensionless analysis predicts better with an average absolute error of 17.9%, in comparison with the modified Lockhart-Martinelli correlation, which yields an average error of 22%, covering all the flow patterns. The present analysis shows better prediction as compared to two-fluid model Zhang et al. (2012, "Modeling High-Viscosity Oil/Water

Research paper thumbnail of Experimental Studies and Probabilistic Neural Network Prediction on Flow Pattern of Viscous Oil–Water Flow through a Circular Horizontal Pipe

Industrial & Engineering Chemistry Research, 2013

ABSTRACT We report detailed analysis on the flow patterns of moderately viscous oil–water two-pha... more ABSTRACT We report detailed analysis on the flow patterns of moderately viscous oil–water two-phase flow through a circular horizontal pipe with an internal diameter of 0.025 m. Lubricating oil and water with viscosity and density ratio of 107 and 0.889, respectively, have been selected as system fluids with interfacial tension 0.024 N/m. We have applied visual and imaging techniques to identify different flow patterns (viz., plug flow, slug flow, wavy stratified flow, stratified mixed flow, dispersion of oil in water, and dispersion of water in oil flow) for a wide range of superficial velocities of oil (USO = 0.015 to 1.25m/s) and water (USW = 0.1 to 1.1 m/s). The present map has also been correlated with the prediction by probabilistic neural network (PNN) along with six other flow pattern maps from the literature, having the wide variation of system properties to establish the PNN as a predictive tool for flow pattern map. For the construction of PNN, phase superficial velocities, conduit diameter, pipe inclination, viscosity, interfacial tension, and density are used as governing parameters of the flow patterns.

Research paper thumbnail of Effect of block asymmetry on the crystallization of double crystalline diblock copolymers

Monte Carlo simulation on the crystallization of double crystalline diblock copolymer unravels an... more Monte Carlo simulation on the crystallization of double crystalline diblock copolymer unravels an intrinsic relationship between block asymmetry and crystallization behaviour. We model crystalline A-B diblock copolymer, wherein the melting temperature of A-block is higher than that of the B-block. We explore the composition dependent crystallization behaviour by varying the relative block length with weak and strong segregation strength between the blocks. In weak segregation limit, we observe that with increasing the composition of B-block, its crystallization temperature increases accompanying with higher crystallinity. In contrast, A-block crystallizes at a relatively low temperature along with the formation of thicker and larger crystallites with the increase in B-block composition. We attribute this non-intuitive crystallization trend to the dilution effect imposed by B-block. When the composition of the B-block is high enough, it acts like a “solvent” during the crystallization of A-block. A-block segments are more mobile and hence less facile to crystallize, resulting depression in crystallization temperature with the formation of thicker crystals. At strong segregation limit, crystallization and morphological development are governed by the confinement effect, rather than block asymmetry. Isothermal crystallization reveals that the crystallization follows a homogeneous nucleation mechanism with the formation of two-dimensional crystals. Two-step, compared to one-step isothermal crystallization leads to the formation of thicker crystals of A-block due to the dilution effect of the B-block.

Research paper thumbnail of Crystallization of double crystalline symmetric diblock copolymers

We report dynamic Monte Carlo simulation results on the crystallization of double crystalline sym... more We report dynamic Monte Carlo simulation results on the crystallization of double crystalline symmetric A-B diblock copolymer, wherein the melting temperature of A-block is higher than B-block. Crystallization of A-block precedes the crystallization of B-block upon cooling from a homogeneous melt. The morphological development is controlled by the interplay between crystallization and microphase separation. With increasing segregation strength, we observe a gradual decrease in crystallinity accompanying with smaller and thinner crystals. During crystallization, A-block crystallizes first and creates confinement for the crystallization of B-block. Thus, crystallization of B-block slows down influencing the overall crystal morphology. At higher segregation strength, due to the repulsive interaction between blocks, block junction is stretched out, which is reflected in the increased value of mean square radius of gyration. As a result, a large number of smaller size crystals form with less crystallinity. The onset of microphase separation shifts towards higher temperature with increasing segregation strength. Isothermal crystallization reveals that the transition pathways strongly depend on segregation strength. The value of Avrami index shows the formation of two dimensional lamellar crystals of both the blocks. Two-step (sequential), compared to one-step (coincident) isothermal crystallization, produces higher crystallinity in A-block, however, the crystallinity of B-block is almost identical in both the cases.

Research paper thumbnail of Conformational transition of H-shaped branched polymers

We report dynamic Monte Carlo simulation on conformational transition of H-shaped branched polyme... more We report dynamic Monte Carlo simulation on conformational transition of H-shaped branched polymers by varying main chain (backbone) and side chain (branch) length. H-shaped polymers in comparison with equivalent linear polymers exhibit a depression of theta temperature accompanying with smaller chain dimensions. We observed that the effect of branches on backbone dimension is more pronounced than the reverse, and is attributed to the conformational heterogeneity prevails within the molecule. With an increase in branch length, backbone is slightly stretched out in the coil and globule state. However, in the pre-collapsed (cf. crumpled globule) state, backbone size decreases with the increase of branch length. We attribute this non-monotonic behavior as the interplay between excluded volume interaction and intra-chain bead-bead attractive interaction during collapse transition. Structural analysis reveals that the inherent conformational heterogeneity promotes the formation of a collapsed structure with segregated backbone and branch units (resembles to “sandwich” or “Janus” morphology) rather an evenly distributed structure consisting of all the units. The shape of the collapsed globule becomes more spherical with increasing either backbone or branch length.

Research paper thumbnail of Conformational transition of telechelic star polymers

We report dynamic Monte Carlo (DMC) simulation study of solution behavior of multi-arm telechelic... more We report dynamic Monte Carlo (DMC) simulation study of solution behavior of multi-arm telechelic star polymers, wherein the number of branching (f) increases with decreasing arm length (n); thus keeping the total number of monomer (N) in the molecule around a constant value. On deteriorating solvent quality, the conformational change occurs from an open to a compact globule structure. The terminal functional groups form aggregate as the solution is cooled progressively and the aggregation phenomenon drives the collapse transition. The transition temperature follows a non-monotonic trend with the functionality, in comparison with an equivalent series of star homopolymers. The non-monotonic behavior of telechelic star polymer is attributed to the interplay between enthalpic gain due to end-monomer aggregation and entropic loss due to loop formation. Structural analysis reveals that, aggregation of end group yields a structure resembles to “watermelons” (WM). Simulation results for the systems with varying N (keeping f or n constant and varying n or f) shows the similar mechanism to the constant N system. Further, we discuss results on highly branched telechelic star polymer with shorter arm length, where, we observe the formation of collapsed structures with single and double aggregates in isothermal and non-isothermal cooling respectively.

Research paper thumbnail of Pathway to copolymer collapse in dilute solution: Uniform versus random distribution of comonomers

Chemical Physics, 2007

Monte Carlo simulations show that copolymers with uniformly (or periodically) distributed sticky ... more Monte Carlo simulations show that copolymers with uniformly (or periodically) distributed sticky comonomers collapse &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;cooperatively,&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; abruptly forming a compact intermediate comprising a monomer shell surrounding a core of the aggregated comonomers. In comparison, random copolymers collapse through a relatively less-compact intermediate comprising a comonomer core surrounded by a fluffy monomer shell that densifies over a wide temperature range. This difference between the collapse pathways for random and uniform copolymers persists to higher chain lengths, where uniform copolymers tend to form multiple comonomer cores. In this paper, we describe the formation of such an intermediate state, and the subsequent collapse, by recognizing that these arise from the expected balance between comonomer aggregation enthalpy and loop formation entropy dictated by the chain microstructure.

Research paper thumbnail of Collapse Transition in Random Copolymer Solutions

Macromolecules, 2006

We present dynamic Monte Carlo lattice simulations of the coil to globule collapse of single chai... more We present dynamic Monte Carlo lattice simulations of the coil to globule collapse of single chains of a copolymer comprising monomer units, m and c, wherein there is a net attractive interaction between c-units. As the copolymer is cooled, the solvent quality becomes poorer, and the size of the chain decreases, driven by the net m-m and c-c attractions. The strong c-c attraction increases the overall solvophobicity of the chain relative to a homopolymer and, therefore, copolymers collapse more abruptly and at a higher effective temperature relative to homopolymers. We compare copolymers with homopolymers by rescaling collapse data to the same θ values to account for the effect of overall solvophobicity. This comparison shows that the behavior of copolymers and the corresponding homopolymers is identical as the chain size reduces from high temperatures to the θ value. Beyond θ, copolymers with c-content < ∼50% collapse more abruptly than either homopolymer, after accounting for the difference in overall solvophobicity. Collapse of copolymers containing higher c-content is dominated entirely by the c-c attractions, and these chains behave qualitatively like homopolymers with a higher effective solvophobicity. Analysis of the chain structure during collapse provides a structural reason for the qualitative change in copolymer collapse at low c-content. When such copolymers are cooled below θ, the c-units rapidly aggregate to form an isotropic, compact core surrounded by an anisotropic solvated shell of m-units. The shell densifies as the copolymer is further cooled, but remains anisotropic for the finite chain sizes investigated.

Research paper thumbnail of Polymer crystallization in the presence of ``sticky'' additives

Chemical Physics, 2009

The effect of ``sticky'' additives (viz., those that have attractive interactions with the polyme... more The effect of ``sticky'' additives (viz., those that have attractive interactions with the polymer) on polymer crystallization, has been investigated by dynamic Monte Carlo (DMC) simulations. Additive-polymer attractive interactions result in a slowing down of the polymer chain diffusivity in the melt state. Our results show that with increasing additive stickiness, polymer crystallinity decreases monotonically, and thinner crystallites form, viz., crystallization is inhibited by the presence of sticky additives. Unusually, the observed ``specific heat'' peak at the phase transition shows nonmonotonic behavior with additive stickiness, and exhibits a maximum for intermediate values of additive stickiness. While the origins of this unexpected behavior are not clear, we show that it correlates with a large interchange between crystalline and amorphous states of the monomers, in the vicinity of the additives. At this intermediate additive stickiness, we also find that crystallization follows a qualitatively different route-crystallinity shows a non-Avrami-like evolution, unlike the case at low or high additive stickiness.

Research paper thumbnail of Phase separation and crystallization in double crystalline symmetric binary polymer blends

Journal of Polymer Research, 2016

Research paper thumbnail of Coil-Globule Transition of Polymer -- Tricriticality of Theta

Research paper thumbnail of Effect of copolymer microstructure on single chain collapse

We present dynamic Monte Carlo simulations of the collapse of copolymers containing sticky comono... more We present dynamic Monte Carlo simulations of the collapse of copolymers containing sticky comonomers, c. There is a qualitative difference in the transition depending on c content. For c content >˜ 50%, copolymer collapse is qualitatively similar to that observed for homopolymers, when rescaled to account for comonomer solvophobicity. However, collapse of copolymers with c < ˜50% is qualitatively steeper

Research paper thumbnail of Effect of additive particles on the crystallization of homopolymers

The effect of additive particles on polymer crystallization has been investigated using lattice d... more The effect of additive particles on polymer crystallization has been investigated using lattice dynamic Monte Carlo simulation. Additives are compatible with the polymer matrix (viz. there is an attractive ``sticky'' interaction between additives and monomers) and, additive particles have the same size as a monomer. Polymer crystallization is strongly influenced by both additive fraction, x and the additive-monomer interaction strength,

Research paper thumbnail of Collapse Transition of Branched Polymers in Dilute Solutions: Telechelic Star vs. H-Polymer

Macromolecular Symposia, 2015

Research paper thumbnail of Experimental studies and PNN prediction on flow pattern of viscous oil-water flow through circular horizontal pipe

Industrial & Engineering Chemistry Research

Research paper thumbnail of Correlation based Pressure Drop Prediction of visous oil-water two-phase flow through Horizontal Pipeline

Research paper thumbnail of Effect of an uphill elbow on interfacial wave encountered during the simultaneous flow of a pair of immiscible liquids through an undulated pipeline in its peak configuration

Research paper thumbnail of Coarse-grain molecular dynamics study of fullerene transport across a cell membrane

The Journal of chemical physics, Jan 14, 2015

The study of the ability of drug molecules to enter cells through the membrane is of vital import... more The study of the ability of drug molecules to enter cells through the membrane is of vital importance in the field of drug delivery. In cases where the transport of the drug molecules through the membrane is not easily accomplishable, other carrier molecules are used. Spherical fullerene molecules have been postulated as potential carriers of highly hydrophilic drugs across the plasma membrane. Here, we report the coarse-grain molecular dynamics study of the translocation of C60 fullerene and its derivatives across a cell membrane modeled as a 1,2-distearoyl-sn-glycero-3-phosphocholine bilayer. Simulation results indicate that pristine fullerene molecules enter the bilayer quickly and reside within it. The addition of polar functionalized groups makes the fullerenes less likely to reside within the bilayer but increases their residence time in bulk water. Addition of polar functional groups to one half of the fullerene surface, in effect creating a Janus particle, offers the most pr...

Research paper thumbnail of An appraisal of viscous oil–water two-phase flow through an undulated pipeline in peak configuration

Experimental Thermal and Fluid Science, 2015

ABSTRACT Experimental results on flow patterns and pressure drop characteristics of viscous oil-w... more ABSTRACT Experimental results on flow patterns and pressure drop characteristics of viscous oil-water flow through undulated pipeline in peak configuration have been studied in this work Undulated pipeline consists of two inclined sections (uphill and downhill) connected between two horizontal pipes (upstream and downstream) at peak configuration. Experiments have been conducted over a wide range of superficial velocities of oil (USO = 0.015 m/s to 1.3 m/s) and water (USW = 0.1 m/s to 1.2 m/s). Seven different flow patterns (viz., plug, slug, wavy stratified, stratified mixed, wispy annular, dispersion of oil in water and dispersion of water in oil) are identified by visual, imaging and electrical conductance probe techniques at all the four sections. Our results shows that present undulation (5°) has a marginal effect on the flow behavior of viscous oil-water mixture and the range of fluid velocity differs for a particular flow pattern. The pressure gradient characteristics across the peak experiences relatively more pressure gradient as compared to upstream and downstream at USW ⩾ 0.6 m/s, which is attributed to the combined effect of undulation (viz., peak configuration) along the pipeline, flow pattern, and effective viscosity and density of the fluid mixture.

Research paper thumbnail of Graphene based multifunctional superbots

Carbon, 2015

A versatile graphene coated glass microswimmer displayed directed motions under the influence of ... more A versatile graphene coated glass microswimmer displayed directed motions under the influence of applied electric field, chemical potential gradient and external magnetic field.

Research paper thumbnail of Correlations for Prediction of Pressure Gradient of Liquid-Liquid Flow Through a Circular Horizontal Pipe

Journal of Fluids Engineering, 2014

We report a detailed investigation on the measurement and prediction of pressure gradient charact... more We report a detailed investigation on the measurement and prediction of pressure gradient characteristics of moderately viscous lubricating oil-water flow through a horizontal pipe of 0.025 m internal diameter. Experiments are carried out over a wide range of phase velocities of both oil (U SO ¼ 0.015-1.25 m/s) and water (U SW ¼ 0.1-1.1 m/s). Experimental pressure gradients yield significant errors when fitted to the existing correlations, which are largely used for gas-liquid flow. To predict pressure gradient characteristics for liquid-liquid flow, the existing correlations need to be modified. We propose two correlations, based on the Lockhart-Martinelli's approach (by modifying the correlation between the Lockhart-Martinelli parameter and a two-phase multiplier suitable for the present system) and dimensionless analysis, following the Buckingham's Pi-theorem. We observe significant improvement in the prediction of pressure gradient. The correlation based on the dimensionless analysis predicts better with an average absolute error of 17.9%, in comparison with the modified Lockhart-Martinelli correlation, which yields an average error of 22%, covering all the flow patterns. The present analysis shows better prediction as compared to two-fluid model Zhang et al. (2012, "Modeling High-Viscosity Oil/Water

Research paper thumbnail of Experimental Studies and Probabilistic Neural Network Prediction on Flow Pattern of Viscous Oil–Water Flow through a Circular Horizontal Pipe

Industrial & Engineering Chemistry Research, 2013

ABSTRACT We report detailed analysis on the flow patterns of moderately viscous oil–water two-pha... more ABSTRACT We report detailed analysis on the flow patterns of moderately viscous oil–water two-phase flow through a circular horizontal pipe with an internal diameter of 0.025 m. Lubricating oil and water with viscosity and density ratio of 107 and 0.889, respectively, have been selected as system fluids with interfacial tension 0.024 N/m. We have applied visual and imaging techniques to identify different flow patterns (viz., plug flow, slug flow, wavy stratified flow, stratified mixed flow, dispersion of oil in water, and dispersion of water in oil flow) for a wide range of superficial velocities of oil (USO = 0.015 to 1.25m/s) and water (USW = 0.1 to 1.1 m/s). The present map has also been correlated with the prediction by probabilistic neural network (PNN) along with six other flow pattern maps from the literature, having the wide variation of system properties to establish the PNN as a predictive tool for flow pattern map. For the construction of PNN, phase superficial velocities, conduit diameter, pipe inclination, viscosity, interfacial tension, and density are used as governing parameters of the flow patterns.

Research paper thumbnail of Effect of block asymmetry on the crystallization of double crystalline diblock copolymers

Monte Carlo simulation on the crystallization of double crystalline diblock copolymer unravels an... more Monte Carlo simulation on the crystallization of double crystalline diblock copolymer unravels an intrinsic relationship between block asymmetry and crystallization behaviour. We model crystalline A-B diblock copolymer, wherein the melting temperature of A-block is higher than that of the B-block. We explore the composition dependent crystallization behaviour by varying the relative block length with weak and strong segregation strength between the blocks. In weak segregation limit, we observe that with increasing the composition of B-block, its crystallization temperature increases accompanying with higher crystallinity. In contrast, A-block crystallizes at a relatively low temperature along with the formation of thicker and larger crystallites with the increase in B-block composition. We attribute this non-intuitive crystallization trend to the dilution effect imposed by B-block. When the composition of the B-block is high enough, it acts like a “solvent” during the crystallization of A-block. A-block segments are more mobile and hence less facile to crystallize, resulting depression in crystallization temperature with the formation of thicker crystals. At strong segregation limit, crystallization and morphological development are governed by the confinement effect, rather than block asymmetry. Isothermal crystallization reveals that the crystallization follows a homogeneous nucleation mechanism with the formation of two-dimensional crystals. Two-step, compared to one-step isothermal crystallization leads to the formation of thicker crystals of A-block due to the dilution effect of the B-block.

Research paper thumbnail of Crystallization of double crystalline symmetric diblock copolymers

We report dynamic Monte Carlo simulation results on the crystallization of double crystalline sym... more We report dynamic Monte Carlo simulation results on the crystallization of double crystalline symmetric A-B diblock copolymer, wherein the melting temperature of A-block is higher than B-block. Crystallization of A-block precedes the crystallization of B-block upon cooling from a homogeneous melt. The morphological development is controlled by the interplay between crystallization and microphase separation. With increasing segregation strength, we observe a gradual decrease in crystallinity accompanying with smaller and thinner crystals. During crystallization, A-block crystallizes first and creates confinement for the crystallization of B-block. Thus, crystallization of B-block slows down influencing the overall crystal morphology. At higher segregation strength, due to the repulsive interaction between blocks, block junction is stretched out, which is reflected in the increased value of mean square radius of gyration. As a result, a large number of smaller size crystals form with less crystallinity. The onset of microphase separation shifts towards higher temperature with increasing segregation strength. Isothermal crystallization reveals that the transition pathways strongly depend on segregation strength. The value of Avrami index shows the formation of two dimensional lamellar crystals of both the blocks. Two-step (sequential), compared to one-step (coincident) isothermal crystallization, produces higher crystallinity in A-block, however, the crystallinity of B-block is almost identical in both the cases.

Research paper thumbnail of Conformational transition of H-shaped branched polymers

We report dynamic Monte Carlo simulation on conformational transition of H-shaped branched polyme... more We report dynamic Monte Carlo simulation on conformational transition of H-shaped branched polymers by varying main chain (backbone) and side chain (branch) length. H-shaped polymers in comparison with equivalent linear polymers exhibit a depression of theta temperature accompanying with smaller chain dimensions. We observed that the effect of branches on backbone dimension is more pronounced than the reverse, and is attributed to the conformational heterogeneity prevails within the molecule. With an increase in branch length, backbone is slightly stretched out in the coil and globule state. However, in the pre-collapsed (cf. crumpled globule) state, backbone size decreases with the increase of branch length. We attribute this non-monotonic behavior as the interplay between excluded volume interaction and intra-chain bead-bead attractive interaction during collapse transition. Structural analysis reveals that the inherent conformational heterogeneity promotes the formation of a collapsed structure with segregated backbone and branch units (resembles to “sandwich” or “Janus” morphology) rather an evenly distributed structure consisting of all the units. The shape of the collapsed globule becomes more spherical with increasing either backbone or branch length.

Research paper thumbnail of Conformational transition of telechelic star polymers

We report dynamic Monte Carlo (DMC) simulation study of solution behavior of multi-arm telechelic... more We report dynamic Monte Carlo (DMC) simulation study of solution behavior of multi-arm telechelic star polymers, wherein the number of branching (f) increases with decreasing arm length (n); thus keeping the total number of monomer (N) in the molecule around a constant value. On deteriorating solvent quality, the conformational change occurs from an open to a compact globule structure. The terminal functional groups form aggregate as the solution is cooled progressively and the aggregation phenomenon drives the collapse transition. The transition temperature follows a non-monotonic trend with the functionality, in comparison with an equivalent series of star homopolymers. The non-monotonic behavior of telechelic star polymer is attributed to the interplay between enthalpic gain due to end-monomer aggregation and entropic loss due to loop formation. Structural analysis reveals that, aggregation of end group yields a structure resembles to “watermelons” (WM). Simulation results for the systems with varying N (keeping f or n constant and varying n or f) shows the similar mechanism to the constant N system. Further, we discuss results on highly branched telechelic star polymer with shorter arm length, where, we observe the formation of collapsed structures with single and double aggregates in isothermal and non-isothermal cooling respectively.

Research paper thumbnail of Pathway to copolymer collapse in dilute solution: Uniform versus random distribution of comonomers

Chemical Physics, 2007

Monte Carlo simulations show that copolymers with uniformly (or periodically) distributed sticky ... more Monte Carlo simulations show that copolymers with uniformly (or periodically) distributed sticky comonomers collapse &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;cooperatively,&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; abruptly forming a compact intermediate comprising a monomer shell surrounding a core of the aggregated comonomers. In comparison, random copolymers collapse through a relatively less-compact intermediate comprising a comonomer core surrounded by a fluffy monomer shell that densifies over a wide temperature range. This difference between the collapse pathways for random and uniform copolymers persists to higher chain lengths, where uniform copolymers tend to form multiple comonomer cores. In this paper, we describe the formation of such an intermediate state, and the subsequent collapse, by recognizing that these arise from the expected balance between comonomer aggregation enthalpy and loop formation entropy dictated by the chain microstructure.

Research paper thumbnail of Collapse Transition in Random Copolymer Solutions

Macromolecules, 2006

We present dynamic Monte Carlo lattice simulations of the coil to globule collapse of single chai... more We present dynamic Monte Carlo lattice simulations of the coil to globule collapse of single chains of a copolymer comprising monomer units, m and c, wherein there is a net attractive interaction between c-units. As the copolymer is cooled, the solvent quality becomes poorer, and the size of the chain decreases, driven by the net m-m and c-c attractions. The strong c-c attraction increases the overall solvophobicity of the chain relative to a homopolymer and, therefore, copolymers collapse more abruptly and at a higher effective temperature relative to homopolymers. We compare copolymers with homopolymers by rescaling collapse data to the same θ values to account for the effect of overall solvophobicity. This comparison shows that the behavior of copolymers and the corresponding homopolymers is identical as the chain size reduces from high temperatures to the θ value. Beyond θ, copolymers with c-content < ∼50% collapse more abruptly than either homopolymer, after accounting for the difference in overall solvophobicity. Collapse of copolymers containing higher c-content is dominated entirely by the c-c attractions, and these chains behave qualitatively like homopolymers with a higher effective solvophobicity. Analysis of the chain structure during collapse provides a structural reason for the qualitative change in copolymer collapse at low c-content. When such copolymers are cooled below θ, the c-units rapidly aggregate to form an isotropic, compact core surrounded by an anisotropic solvated shell of m-units. The shell densifies as the copolymer is further cooled, but remains anisotropic for the finite chain sizes investigated.

Research paper thumbnail of Polymer crystallization in the presence of ``sticky'' additives

Chemical Physics, 2009

The effect of ``sticky'' additives (viz., those that have attractive interactions with the polyme... more The effect of ``sticky'' additives (viz., those that have attractive interactions with the polymer) on polymer crystallization, has been investigated by dynamic Monte Carlo (DMC) simulations. Additive-polymer attractive interactions result in a slowing down of the polymer chain diffusivity in the melt state. Our results show that with increasing additive stickiness, polymer crystallinity decreases monotonically, and thinner crystallites form, viz., crystallization is inhibited by the presence of sticky additives. Unusually, the observed ``specific heat'' peak at the phase transition shows nonmonotonic behavior with additive stickiness, and exhibits a maximum for intermediate values of additive stickiness. While the origins of this unexpected behavior are not clear, we show that it correlates with a large interchange between crystalline and amorphous states of the monomers, in the vicinity of the additives. At this intermediate additive stickiness, we also find that crystallization follows a qualitatively different route-crystallinity shows a non-Avrami-like evolution, unlike the case at low or high additive stickiness.