arXiv subjects

Debashish Mukherji

Publications and source records attributed to Debashish Mukherji.

At least 19 recordsLinked to original sources

arXiv2026

Computing finite--temperature elastic constants with noise cancellation

Elastic constants are central material properties, frequently reported in experimental and theoretical studies. While their computation is straightforward in the absence of thermal fluctuations, finite--temperature methods often suffer from poor signal--to--noise ratios or the presence of strong anharmonic effects. Here, we show how to compute elastic constants in thermal ordered and disordered systems by generalizing a noise--cancellation method originally developed for piezoelectric coupling coefficients. A slight strain is applied to an equilibrated solid. Simulations of both the strained and unstrained (or oppositely strained) reference systems are performed using identical thermostatting schemes. As demonstrated theoretically and with generic one--dimensional models, this allows stress differences to be evaluated and elastic constants to be determined with much reduced thermal noise. We then apply this approach across a diverse set of systems, spanning crystalline argon, ordered silicon as well as amorphous silicon, poly(methyl methacrylate), and cellulose derivatives.

cond-mat.mtrl-sci↗

arXiv2025

Thermal conductivity of commodity polymers under high pressures

Understanding the thermal conductivity of polymers under high-pressure conditions is essential for a range of applications, from aerospace and deep-sea engineering to common lubricants. However, the complex relationship between pressure, $P$, the thermal transport coefficient, $κ$, and polymer architecture poses substantial challenges to both experimental and theoretical investigations. In this work, we study the pressur-dependent thermal transport properties of a widely used commodity polymer -- poly(methyl methacrylate) (PMMA) -- using a combination of all-atom molecular dynamics simulations and semi-analytical approaches. While we report both classical and quantum-corrected estimates of $κ$, the latter approach reveals that as the pressure increases from 1 atm to 10 GPa, $κ$ rises by up to a factor of four -- from 0.21 W m$^{-1}$ K$^{-1}$ to 0.80 W m$^{-1}$ K$^{-1}$. To better understand the mechanisms behind this increase, we disentangle the contributions from bonded and nonbonded monomer interactions. Our analysis shows that nonbonded energy-transfer rates increase by a factor of six over the pressure range, while bonded interactions show a more modest increase -- about a factor of three. This observation further consolidates the fact that the nonbonded interactions play the dominant role in dictating the microscopic heat flow in polymers. These individual energy-transfer rates are also incorporated into a simplified heat diffusion model to predict $κ$. The results obtained from different approaches show internal consistency and align well with available experimental data. Additionally, some data for polylactic acid (PLA) are presented.

cond-mat.soft↗

arXiv2024

Thermal conductivity of polymers: A simple matter where complexity matters

Thermal conductivity coefficient $κ$ measures the ability of a material to conduct a heat current. In particular, $κ$ is an important property that often dictates the usefulness of a material over a wide range of environmental conditions. For example, while a low $κ$ is desirable for the thermoelectric applications, a large $κ$ is needed when a material is used under the high temperature conditions. These materials range from common crystals to commodity amorphous polymers. The latter is of particular importance because of their use in designing light weight high performance functional materials. In this context, however, one of the major limitations of the amorphous polymers is their low $κ$, reaching a maximum value of about 0.4 W/Km that is 2--3 orders of magnitude smaller than the standard crystals. Moreover, when energy is predominantly transferred through the bonded connections, $κ\ge 100$ W/Km. Recently, extensive efforts have been devoted to attain a tunability in $κ$ via macromolecular engineering. In this work, an overview of the recent results on the $κ$ behavior in polymers and polymeric solids is presented. In particular, computational and theoretical results are discussed within the context of complimentary experiments. Future directions are also highlighted.

cond-mat.soft↗

arXiv2024

Smart polymer solution and thermal conductivity: How important is an exact polymer conformation?

Heat management in devices is a key to their efficiency and longevity. Here, thermal switches (TS) are of great importance because of their ability to transition between different thermal conductivity $κ$ states. While traditional TS are bulky and slow, recent experiments have suggested "smart" responsive (bio--inspired) polymers as their fast alternatives. One example is poly(N--isopropylacrylamide) (PNIPAM) in water, where $κ$ drops suddenly around a temperature $T_{\ell} \simeq 305$ K when a PNIPAM undergoes a coil--to--globule transition. At a first glance, this may suggest that the change in polymer conformation has a direct influence on TS. However, it may be presumptuous to trivially "only" link conformations with TS, especially because many complex microscopic details control macroscopic conformational transition. Motivated by this, we study TS in "smart" polymers using generic simulations. As the test cases, we investigate two different modes of polymer collapse using external stimuli, i.e., changing $T$ and cosolvent mole fraction $x_{\rm c}$. Collapse upon increasing $T$ shows a direct correlation between the conformation and $κ$ switching, while no correlation is observed in the latter case. These results suggest that the (co--)solvent--monomer interactions play a greater important role than the exact conformation in dictating TS. While some results are compared with the available experiments, possible future directions are also highlighted.

cond-mat.soft↗

arXiv2024

Tuning the thermal conductivity of silicon nanowires by surface passivation

Using large scale molecular dynamics simulations, we study the thermal conductivity of bare and surface passivated silicon nanowires (SiNWs). For the cross-sectional widths $w \le 2$ nm, SiNWs become unstable because of the surface amorphosization and also due to the evaporation of a certain fraction of Si atoms. The observed surface (in-)stability is related to a large excess energy $Δ$ of the surface Si atoms with respect to the bulk Si, resulting from the surface atoms being less coordinated and having dangling bonds.We first propose a practically relevant method that uses $Δ$ as a guiding tool to passivate these dangling bonds with hydrogen or oxygen, stabilizing the SiNWs. These passivated SiNWs are used to calculate the thermal conductivity coefficient $κ$.While the expected trend of $κ\propto w$ is observed for all SiNWs, surface passivation provides an added flexibility of tuning $κ$ with the surface coverage concentration $c$ of passivated atoms.Indeed, with respect to the bulk $κ$, passivation of SiNW reduces $κ$ by 75-80\% for $c \to 50\%$ and recovers again by 50\% for the fully passivated samples. Analyzing the phonon band structures via spectral energy density, we discuss separate contributions from the surface and the core to $κ$. Our results also reveal that surface passivation increases SiNW stiffness, contributing to the tunability in $κ$.

cond-mat.mes-hall↗

arXiv2024

Computing the thermal transport coefficient of neutral amorphous polymers using exact vibrational density of states: Comparison with experiments

Thermal transport coefficient $κ$ is an important property that often dictates broad applications of a polymeric material, while at the same time its computation remains challenging. In particular, classical simulations overestimate $κ$ than the experimentally measured $κ^{\rm exp}$ and thus hinder their meaningful comparison. This is even when very careful simulations are performed using the most accurate empirical potentials. A key reason for such a discrepancy is because polymers have quantum--mechanical, nuclear degrees--of--freedom whose contribution to the heat balance is non--trivial. In this work, two semi--analytical approaches are considered to accurately compute $κ$ by using the exact vibrational density of states $g(ν)$. The first approach is based within the framework of the minimum thermal conductivity model, while the second uses computed quantum heat capacity to scale $κ$. Computed $κ$ of a set of commodity polymers compares quantitatively with $κ^{\rm exp}$.

cond-mat.soft↗

arXiv2024

Thermal conductivity of bottle-brush polymers

Using molecular dynamics (MD) simulations of a generic model, we investigate heat propagation in bottle--brush polymers (BBP). An architecture is referred to as a BBP when a linear (backbone) polymer is grafted with the side chains of different length $N_{\rm s}$ and grafting density $ρ_{\rm g}$, which control the bending stiffness of a backbone. A BBP is of particular interest due to two competing mechanics: increased backbone stiffness, via $N_{\rm s}$ and $ρ_{\rm g}$, increases the thermal transport coefficient $κ$, while the presence of side chains provides additional pathways for heat leakage. We show how a delicate competition between these two effects controls $κ$. These results reveal that going from a weakly grafting ($ρ_{\rm g} < 1$) to a highly grafting ($ρ_{\rm g} \ge 1$) regime, $κ$ changes non--monotonically that is independent of $N_{\rm s}$. The effect of side chain mass on $κ$ and heat flow in the BBP melts are also discussed.

cond-mat.soft↗

arXiv2022

Stabilizing $α-$helicity of polypeptide in aqueous urea: Dipole orientation or hydrogen bonding?

Urea denatures proteins due to its strong tendency to dehydrate the first solvation shell via urea-residue preferential binding. However, even after extensive experimental and computational investigations, the influence of urea on the stability of secondary structures remains elusive. For example, contrary to the common understanding, experimental studies have indicated that specific polypeptides, such as poly-alanine or alanine-rich systems, may even show an improved tendency to form secondary structures in aqueous urea. We investigate this seemingly counter-intuitive behaviour using over 15$μ$s long all-atom simulations. These results show how a delicate balance between the localized dipole orientations and hydrogen bonding dictates polypeptide solvation in aqueous urea. Our work establishes a structure-property relationship that highlights the importance of microscopic dipole-dipole orientations/interactions for the operational understanding of macroscopic protein solvation.

cond-mat.soft↗

arXiv2022

Simple generic picture of toughness in solid polymer blends

Toughness $\mathcal{T}$ of a brittle polymeric solid can be enhanced by blending another compatible and ductile polymer. While this common wisdom is generally valid, a generic picture is lacking that connects the atomistic details to the macroscopic non-linear mechanics. Using all-atom and complementary generic simulations we show how a delicate balance between the side group contact density of the brittle polymers $ρ_{\rm c}$ and its dilution upon adding a second component controls $\mathcal{T}$. A broad range of systems follows a universal trend in $\mathcal{T}$ with ${\rm d}ρ_{\rm c}/{\rm d}\varepsilon$, where $\varepsilon$ is the tensile strain. The simulation data is consistent with a simple model based on the parallel spring analogy.

cond-mat.soft↗

arXiv2022

Computational Indentation in Highly Cross-linked Polymer Networks

Indentation is a common experimental technique to study the mechanics of polymeric materials. The main advantage of using indentation is because this provides a direct correlation between the microstructure and the small-scale mechanical response, which is otherwise difficult within the standard tensile testing. Here, majority of studies have investigated hydrogels, microgels and/or elastomers. However, a lesser investigated system is the indentation in highly cross-linked polymer (HCP) networks, where the complex network structure plays a key role in dictating their physical properties. In this work, we investigate the structure-property relationship in HCP networks using the computational indentation of a generic model. We establish a correlation between the local bond breaking, the network rearrangement, and the small-scale mechanics. The results are compared with the elastic-plastic deformation model. HCPs harden upon indentation.

cond-mat.soft↗

arXiv2022

Comparison of all atom and united atom models for thermal transport calculations of amorphous polyethylene

Polymer simulations routinely employ models with different molecular resolutions. United atom (UA) models are one such example, where groups of certain atoms in a molecule are clustered into superatoms. Although their computational simplicity makes them particularly attractive for studying a wide range of polymer properties, the missing degrees of freedom in UA models can impact certain properties that are intimately linked to localized vibrations, such as the heat capacity and the thermal transport coefficient $κ$. In contrast, the numerically exhausting all atom (AA) models produce results that better match experimental data. In this work, we systematically investigate and compare $κ$ obtained from an AA and a UA models for an amorphous polyethylene system. The results indicate that the UA description may not be a suitable model for evaluating thermal transport, since it underestimates $κ$ in comparison to an AA description and the experimental value. The coarse-graining leads to the softer interactions and its presence is highlighted in a weaker mechanical response from the UA model, thus also underestimates $κ$. We further consolidate our findings by extracting the bonded and the nonbonded contributions to $κ$ within the framework of the single chain energy transfer model.

cond-mat.soft↗

arXiv2021

Correlating thermodynamics, morphology, mechanics and thermal transport in PMMA-PLA blends

Thermodynamics controls structure, function, stability and morphology of polymer blends. However, obtaining the precise information about their mixing thermodynamics is a challenging task, especially when dealing with complex macromolecules. This is partially because of a delicate balance between the local concentration/composition fluctuations and the monomer level (multi-body) interactions. In this context, the Kirkwood-Buff (KB) theory serves as a useful tool that connects the local pairwise fluid structure to the mixing thermodynamics. Using larger scale molecular dynamics simulations, within the framework of KB theory, we investigate a set of technologically relevant poly(methyl methacrylate)-poly(lactic acid) (PMMA-PLA) blends with the aim to elucidate the underlying microscopic picture of their phase behavior. Consistent with these experiments, we emphasize the importance of properly accounting for the entropic contribution, to the mixing Gibbs free energy change $Δ{\mathcal G}_{\rm mix}$, that controls the phase morphology. We further show how the relative microscopic interaction details and the molecular level structures between different mixing species can control the non-linear mechanics and ductility. As a direct consequence, we provide a correlation that links thermodynamics, phase behavior, mechanics, and thus also thermal transport in polymer blends. Therefore, this study provides a guiding principle for the design of light weight functional materials with extraordinary physical properties.

cond-mat.soft↗

arXiv2021

Comparing simulated specific heat of liquid polymers and oligomers to experiments

The specific heat is a central property of condensed matter systems including polymers and oligomers in their condensed phases. Yet, predictions of this quantity from molecular simulations and successful comparisons to experimental data are scarce if existing at all. One reason for this may be that the internal energy and thus the specific heat cannot be coarse-grained so that they defy their rigorous computation with united-atom models. Moreover, many modes in a polymer barely contribute to the specific heat because of their quantum mechanical nature. Here, we demonstrate that an analysis of the mass-weighted velocity autocorrelation function allows specific heat predictions to be corrected for quantum effects so that agreement with experimental data is on par with predictions of other routinely computed quantities. We outline how to construct corrections for both all-atom and united-atom descriptions of chain molecules. Corrections computed for eleven hydrocarbon oligomers and commodity polymers deviate by less than $k_\textrm{B}/10$ within a subset of nine molecules. Our results may benefit the prediction of heat conductivity.

cond-mat.soft↗

arXiv2020

Tuning thermal transport in highly cross-linked polymers by bond induced void engineering

Tuning the heat flow is fundamentally important for the design of advanced functional materials. Here, polymers are of particular importance because they provide different pathways for the energy transfer. More specifically, the heat flow between two covalently bonded monomers is over 100 times faster than between the two non-bonded monomers interacting via van der Waals (vdW) forces. Therefore, the delicate balance between these two contributions often provide a guiding tool for the tunability in thermal transport coefficient k of the polymeric materials. Traditionally most studies have investigated k in the linear polymeric materials, the recent interests have also been directed towards the highly cross-linked polymers (HCP). In this work, using the generic molecular dynamics simulations we investigate the factors effecting k of HCP. We emphasize on the importance of the cross-linking bond types and its influence on the network microstructure with a goal to provide a guiding principle for the tunability in k. While these simulation results are discussed in the context of the available experimental data, we also make predictions.

cond-mat.soft↗

arXiv2020

Reduced thermal conductivity in molecular forests

Heat propagation in quasi-one dimensional materials (Q1DMs) often appears paradoxical. While an isolated Q1DM, such as a nanowire, carbon nanotube, or polymer, can exhibit a high thermal conductivity \k{appa}, forests of the same materials show a reduction in \k{appa}. Here, the complex structures of these assemblies have hindered the emergence of a clear molecular picture of this intriguing phenomenon. We combine multiscale (coarse-grained) simulation with the concepts known from polymer physics and thermal transport to unveil a generic (microscopic) picture of \k{appa} reduction in molecular forests. We show that a delicate balance between the bond orientations, the persistence length of the Q1DM and the flexural vibrations govern the knock-down of \k{appa}.

cond-mat.mes-hall↗

arXiv2020

Why Do Elastin-Like Polypeptides Possibly Have Different Solvation Behaviors in Water-Ethanol and Water-Urea Mixtures?

The solvent quality determines the collapsed or the expanded state of a polymer. For example, a polymer dissolved in a poor solvent collapses, whereas in a good solvent it opens up. While this standard understanding is generally valid, there are examples when a polymer collapses even in a mixture of two good solvents. This phenomenon, commonly known as co-non-solvency, is usually associated with smart polymers. Moreover, recent experiments have shown that the elastin-like polypeptides (ELPs) show co-non-solvency behavior in aqueous-ethanol mixtures. In this study, we investigate the phase behavior of ELPs in aqueous binary mixtures using molecular dynamics simulations of all-atom and complementary explicit solvent generic models. The model is parameterized by mapping the solvation free energy obtained from the all-atom simulations onto the generic interaction parameters. For this purpose, we derive segment based generic parameters for four different peptides, namely proline (P), valine (V), glycine (G) and alanine (A). Here we compare the conformational behavior of two ELP sequences, namely VPGGG and VPGVG, in aqueous-ethanol and -urea mixtures. Consistent with recent experiments, we find that ELPs show co-non-solvency in aqueous-ethanol mixtures. Ethanol molecules have preferential binding with all ELP residues and thus driving the coil-to-globule transition. On the contrary, ELP conformations show weak variation in aqueous-urea mixtures. Our simulations suggest that the glycine residues dictate the overall behavior of ELPs in aqueous-urea, where urea molecules have a rather weak preferential binding with glycine, i.e., less than kT. While the validation of the latter findings will require more detailed experimental investigation, the results presented here may provide a new twist to the present understanding of cosolvent interactions with peptides and proteins.

cond-mat.soft↗

arXiv2019

Elasticity and thermal transport of commodity plastics

Applications of commodity polymers are often hindered by their low thermal conductivity. In these systems, going from the standard polymers dictated by weak van der Waals interactions to biocompatible hydrogen bonded smart polymers, the thermal transport coefficient k varies between 0.1 - 0.4 W/Km. Combining all-atom molecular dynamics simulations with some experiments, we study thermal transport and its link to the elastic response of commodity plastics. We find that there exists a maximum attainable stiffness (or sound wave velocity), thus providing an upper bound of k for these solid polymers. The specific chemical structure and the glass transition temperature play no role in controlling k, especially when the microscopic interactions are hydrogen bonding based. Our results are consistent with the minimum thermal conductivity model and existing experiments. The effect of polymer stretching on k is also discussed.

cond-mat.soft↗

arXiv2019

Tuning morphology and thermal transport of asymmetric smart polymer blends by macromolecular engineering

A grand challenge in designing polymeric materials is to tune their properties by macromolecular engineering. In this context, one of the drawbacks that often limits broader applications under high temperature conditions is their poor thermal conductivity $κ$. Using molecular dynamics simulations, we establish a structure-property relationship in hydrogen bonded polymer blends for possible improvement of $κ$. For this purpose, we investigate two experimentally relevant hydrogen bonded systems -- one system consists of short poly({N}-acryloyl piperidine) (PAP) blended with longer chains of poly(acrylic acid) (PAA) and the second system is a mixture of PAA and short poly(acrylamide) (PAM) chains. Simulation results show that PAA-PAP blends are at the onset of phase separation over the full range of PAP monomer mole fraction $ϕ_{PAP}$, which intensifies even more for $ϕ_{PAP} > 0.3$. While PAA and PAP interact with preferential hydrogen bonding, phase separation is triggered by the dominant van der Waals attraction between the hydrophobic side groups of PAP. However, if PAP is replaced with PAM, which has a similar chemical structure as PAP without the hydrophobic side group, PAA-PAM blends show much improved solubility. Better solubility is due to the preferential hydrogen bonding between PAA and PAM. As a result, PAM oligomers act as cross-linking bridges between PAA chains resulting in a three dimensional highly cross-linked network. While $κ$ for PAA-PAP blends remain almost invariant with $ϕ_{PAP}$, PAA-PAM systems show improved $κ$ with increasing PAM concentration and also with respect to PAA-PAP blends. Consistent with the theoretical prediction for the thermal transport of amorphous polymers, we show that $κ$ is proportional to the materials stiffness, i.e., the bulk modulus K and sound velocity v of PAA-PAM blends.

cond-mat.soft↗