BITS Faculty Publications

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Subject: search.filters.subject.Molecular Dynamics Simulations

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    Effect of Substrate on Structural Phase Transition in a Conducting Polymer during Ion Injection and Water Intake: A View from a Computational Microscope
    (ACS, 2020-12) Ghosh, Sarbani
    Conducting polymers operating in aqueous electrolyte represent mixed electron-ion conductors, where the ion injection and water intake can lead to structural and morphological changes that can strongly affect the material morphology and device performance. In the present paper, using molecular dynamics simulations, we provide an atomistic understanding of the structural phase transitions during electrochemical oxidation and ion injection in a conjugated polymer with glycolated side chains recently reported by Bischak et al. [J. Am. Chem. Soc., 2020, 142, 7434], where the polymer switched between two structurally distinct phases corresponding to different oxidation levels. To outline the structural changes, we calculated the polymer film morphology and X-ray diffraction patterns at different oxidation levels. We demonstrated that the observed phase transition arises due to interplay between several factors, including the effect of the substrate leading to the preferential edge-on arrangement of the chains and formation of lamellas; unzipping of the interdigitated polymer chains during oxidation and ion intake; and changes in the morphology when π–π stacking is absent at low oxidation level and forms at the high oxidation level facilitating the electron mobility and enabling the oxidation of the polymer film. Our calculations quantitatively reproduce the experimental data, which outlines the predictive power of the molecular modeling of the polymer systems that can be utilized for the design of materials and devices with improved performance.
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    Improving morphology of P3HT:PCBM bulk heterojunction solar cells with anisotropic shaped silica nanoparticles
    (Elsevier, 2023) Ghosh, Sarbani; Garg, Mohit
    Using coarse-grained molecular dynamics simulations we study blends of Poly(3-hexylthiophene-2,5-diyl) (P3HT), [6,6]-Phenyl-C61-butyric acid methyl ester (PCBM) and Silica nanoparticle (SiNP) to understand the effect of adding SiNP on morphology of P3HT:PCBM in Bulk heterojunction (BHJ) solar cells. We use an approximately 3 nm anisotropic shaped SiNP and predicted the morphology of BHJ upon its incorporation. The SiNP arrange themselves into anisotropic structures depending on the concentration of P3HT, PCBM and SiNP respectively creating a network like morphology. PCBM molecules utilize the surface energy of SiNP and gather at its surface forming a morphology which is beneficial for device efficiency. Our results suggest that an optimum weight fraction of all the three components leads to higher surface area of contact, optimum domain size and high percolation of domains throughout the system. The effective control of all the morphological parameters help in improving the charge generation, extraction and transport to electrodes, thereby improving the performance of BHJ solar cells.
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    Ion Diffusion through Nanocellulose Membranes: Molecular Dynamics Study
    (ACS, 2021-12) Garg, Mohit
    One of the most promising applications of nanocellulose is for membranes for energy storage devices including supercapacitors, batteries, and fuel cells. Several recent studies reported the fabrication of cellulose-based membranes where ionic conductivity was confined to channels. So far, theoretical understanding of the effect of the nanoconfinement and surface charged groups on the diffusion coefficient of ions in cellulose nanochannels is missing. In the present study, we perform atomistic molecular dynamics simulations to provide this theoretical understanding and unravel mechanisms affecting the ionic diffusion in nanochannels. We demonstrate that the diffusion coefficient of ions in cellulose nanochannels is reduced in comparison to its bulk value. The change of the diffusion coefficient depends on the density of charged surface groups in nanochannels and the channel height, and it is primarily caused by the Coulomb interaction between the ions and the surface. We believe that our results reveal an important structure/property relationship in cellulose nanochannels, and they show that accounting for the dependence of the diffusion coefficient on the charge of the surface groups and channel height can be important for the Nernst–Plank–Poisson modeling of the ion conductivity in nanomembranes as well as for accurate fitting the experimental data to extract the material parameters.
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    Structural relaxation in amorphous materials under cyclic tension-compression loading
    (Elsevier, 2020-07) Jana, Pritam Kumar
    The process of structural relaxation in disordered solids subjected to repeated tension-compression loading is studied using molecular dynamics simulations. The binary glass is prepared by rapid cooling well below the glass transition temperature and then periodically strained at constant volume. We find that the amorphous system is relocated to progressively lower potential energy states during hundreds of cycles, and the energy levels become deeper upon approaching critical strain amplitude from below. The decrease in potential energy is associated with collective nonaffine rearrangements of atoms, and their rescaled probability distribution becomes independent of the cycle number at sufficiently large time intervals. It is also shown that yielding during startup shear deformation occurs at larger values of the stress overshoot in samples that were cyclically loaded at higher strain amplitudes. These results might be useful for mechanical processing of amorphous alloys in order to reduce their energy and increase chemical resistivity and resistance to crystallization.
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    Dynamics and Rheology of Polymer Melts via Hierarchical Atomistic, Coarse-Grained, and Slip-Spring Simulations
    (ACS, 2021-02) Jana, Pritam Kumar
    A hierarchical (triple scale) simulation methodology is presented for the prediction of the dynamical and rheological properties of high molecular-weight entangled polymer melts. The methodology consists of atomistic, moderately coarse-grained (mCG), and highly coarse-grained slip-spring (SLSP) simulations. At the mCG level, a few chemically bonded atoms are lumped into one coarse-grained bead. At this level, the chemical identity of the underlying atomistic system and the interchain topological constraints (entanglements) are preserved. The mCG interaction potentials are derived by matching local structural distributions of the mCG model to those of the atomistic model through iterative Boltzmann inversion. For matching mCG and atomistic dynamics, the mCG time is scaled by a time scaling factor, which compensates for the lower monomeric friction coefficient of the mCG model than that of the atomistic one. At the SLSP level, multiple Kuhn segments of a polymer chain are represented by one coarse-grained bead. The very soft nonbonded interactions between beads do not prevent chain crossing and, hence, can not capture entanglements. The topological constraints are represented by slip-springs, restricting the lateral motion of polymer chains. A compensating pair potential is used in the SLSP model to keep the static macromolecular properties unaltered upon the introduction of slip-springs. The static and kinetic parameters of the SLSP model are determined based on the lower-level simulation models. Particularly, matching the orientational autocorrelation of the end-to-end vector, we determine the number of slip-springs and calibrate the timescale of the SLSP model. As a test case, the hierarchical methodology is applied to cis-1,4-polybutadiene (cPB) at 413 K. Dynamical single-chain and linear viscoelastic properties of cPB melts are calculated for a broad range of molecular weights, ranging from unentangled to well-entangled chains. The calculations are compared, and found in good agreement, with experimental data from the literature.