Debashish Mukherji

Polymer collapse in miscible good solvents is a generic phenomenon driven by preferential adsorption

Water and alcohol, such as methanol or ethanol, are miscible and, individually, good solvents for poly(N-isopropylacrylamide) (PNIPAm), but this polymer precipitates in water–alcohol mixtures. The intriguing behaviour of solvent mixtures that cannot dissolve a given polymer or a given protein, while the same macromolecule dissolves well in each of the cosolvents, is called cononsolvency. It is a widespread phenomenon, relevant for many formulation steps in the physicochemical and pharmaceutical industry, that is usually explained by invoking specific chemical details of the mixtures: as such, it has so far eluded any generic explanation. Here, by using a combination of simulations and theory, we present a simple and universal treatment that requires only the preferential interaction of one of the cosolvents with the polymer. The results show striking quantitative agreement with experiments and chemically specific simulations, opening a new perspective towards an operational understanding of macromolecular solubility.

Kirkwood–Buff Coarse-Grained Force Fields for Aqueous Solutions

We present an approach to systematically coarse-grain liquid mixtures using the fluctuation solution theory of Kirkwood and Buff in conjunction with the iterative Boltzmann inversion method. The approach preserves both the liquid structure at pair level and the dependence of solvation free energies on solvent composition within a unified coarse-graining framework. To test the robustness of our approach, we simulated urea-water and benzene-water systems at different concentrations. For urea-water, three different coarse-grained potentials were developed at different urea concentrations, in order to extend the simulations of urea-water mixtures up to 8 molar urea concentration. In spite of their inherent state point dependence, we find that the single-site models for urea and water are transferable in concentration windows of approximately 2 M. We discuss the development and application of these solvent models in coarse-grained biomolecular simulations.

Kirkwood-Buff Analysis of Liquid Mixtures in an Open Boundary Simulation

Using the adaptive resolution (AdResS) molecular dynamics scheme, we present a new approach to calculate the thermodynamic properties of liquid mixtures in an open boundary simulation. As a test case, we simulate methanol-water mixtures. We show that Kirkwood-Buff integrals (KBI), which directly connect global thermodynamic properties to microscopic molecular distributions, can be efficiently calculated over a wide range of methanol mole fractions by choosing only a very small (∼3% of total simulation domain) open boundary explicit (all atom) region and a surrounding coarse-grained reservoir that takes care of correct particle fluctuations.

Co-non-solvency: Mean-field polymer theory does not describe polymer collapse transition in a mixture of two competing good solvents

Smart polymers are a modern class of polymeric materials that often exhibit unpredictable behavior in mixtures of solvents. One such phenomenon is co-non-solvency. Co-non-solvency occurs when two (perfectly) miscible and competing good solvents, for a given polymer, are mixed together. As a result, the same polymer collapses into a compact globule within intermediate mixing ratios. More interestingly, polymer collapses when the solvent quality remains good and even gets increasingly better by the addition of the better cosolvent. This is a puzzling phenomenon that is driven by strong local concentration fluctuations. Because of the discrete particle based nature of the interactions, Flory-Huggins type mean field arguments become unsuitable. In this work, we extend the analysis of the co-non-solvency effect presented earlier [D. Mukherji et al., Nat. Commun. 5, 4882 (2014)]. We explain why co-non-solvency is a generic phenomenon, which can only be understood by the thermodynamic treatment of the competitive displacement of (co)solvent components. This competition can result in a polymer collapse upon improvement of the solvent quality. Specific chemical details are not required to understand these complex conformational transitions. Therefore, a broad range of polymers are expected to exhibit similar reentrant coil-globule-coil transitions in competing good solvents.

Effects of stereochemistry and copolymerization on the LCST of PNIPAm

Poly(N-isopropylacrylamide) (PNIPAm) is a smart polymer that presents a lower critical transition temperature (LCST) of 305 K. Interestingly, this transition point falls within the range of the human body temperature, making PNIPAm a highly suitable candidate for bio-medical applications. However, it is sometimes desirable to have a rather flexible tuning of the LCST of these polymers to further increase their range of applications. In this work, we use all-atom molecular dynamics simulations to study the LCST of PNIPAm-based (co-)polymers. We study different molecular architectures where the polymer sequences are tuned either by modifying its stereochemistry or by the co-polymerization of PNIPAm with acrylamide (Am) units. Our analysis connects global polymer conformations with the microscopic intermolecular interactions. These findings suggest that the collapse of a PNIPAm chain upon heating is dependent on the hydration structure around the monomers, which is strongly dependent on the tacticity and the presence of more hydrophilic acrylamide monomers. Our results are found to be in good agreement with the existing experimental data.

C–IBI: Targeting cumulative coordination within an iterative protocol to derive coarse-grained models of (multi-component) complex fluids

We present a coarse-graining strategy that we test for aqueous mixtures. The method uses pair-wise cumulative coordination as a target function within an iterative Boltzmann inversion (IBI) like protocol. We name this method coordination iterative Boltzmann inversion (C-IBI). While the underlying coarse-grained model is still structure based and, thus, preserves pair-wise solution structure, our method also reproduces solvation thermodynamics of binary and/or ternary mixtures. Additionally, we observe much faster convergence within C-IBI compared to IBI. To validate the robustness, we apply C-IBI to study test cases of solvation thermodynamics of aqueous urea and a triglycine solvation in aqueous urea.

Anomalous ductility in thermoset/thermoplastic polymer alloys

Mechanical properties of highly cross-linked polymer (HCP) networks, e.g., thermosets, can be significantly modified by adding linear polymer chains, e.g., thermoplastics. In this work, we study thermoset/thermoplastic polymer alloys by means of large scale molecular dynamics simulations (MD) of a coarse-grained model. We focus here on the effect of the linear chain fraction, Gammal, on the mechanical properties of HCP network for a fixed chain length. Our MD simulations show that the ductility (measured by the strain-to-fracture) of an alloy decreases with increasing Gammal up to a threshold fraction, Gammal*, beyond which it increases with Gammal. We find that for GammalGammal* adhesive failure occurs. We suggest that the possible origin of this unexpected non-monotonic behavior is due to a competition between (a) growth of microvoids which stores mechanical energy and is compromised as Gammal increases, and (b) reduction of cross-linker density with increasing Gammal.

Sequence transferable coarse-grained model of amphiphilic copolymers

Polymer properties are inherently multi-scale in nature, where delicate local interaction details play a key role in describing their global conformational behavior. In this context, deriving coarse-grained (CG) multi-scale models for polymeric liquids is a non-trivial task. Further complexities arise when dealing with copolymer systems with varying microscopic sequences, especially when they are of amphiphilic nature. In this work, we derive a segment-based generic CG model for amphiphilic copolymers consisting of repeat units of hydrophobic (methylene) and hydrophilic (ethylene oxide) monomers. The system is a simulation analogue of polyacetal copolymers [S. Samanta et al., Macromolecules 49, 1858 (2016)]. The CG model is found to be transferable over a wide range of copolymer sequences and also to be consistent with existing experimental data.

Unexpected crossover dynamics of single polymer in a corrugated tube

We present molecular dynamics study of a generic (coarse-grained) model for single-polymer diffusion confined in a corrugated cylinder. For a narrow tube, i.e., diameter of the cylinder δ < 2.3, the axial diffusion coefficient D(∣∣) scales as D(∣∣) ∝ N(-3∕2), with chain length N, up to N ≈ 100 and then crosses over to Rouse scaling for the larger N values. The N(-3∕2) scaling is due to the large fluctuation of the polymer chain along its fully stretched equilibrium conformation. The stronger scaling, namely N(-3∕2), is not observed for an atomistically smooth tube and/or for a cylinder with larger diameter.

Polyacrylamide “revisited”: UCST-type reversible thermoresponsive properties in aqueous alcoholic solutions

Combining experiments and all-atom molecular dynamics simulations, we study the conformational behavior of polyacrylamide (PAM) in aqueous alcohol mixtures over a wide range of temperatures. This study shows that even when the microscopic interaction is dictated by hydrogen bonding, unlike its counterparts that present a lower critical solution temperature (LCST), PAM shows a counterintuitive tunable upper critical solution temperature (UCST)-type phase transition in water/alcohol mixtures that was not reported before. The phase transition temperature was found to be tunable between 4 and 60 °C by the type and concentration of alcohol in the mixture as well as by the solution concentration and molecular weight of the polymer. In addition, molecular dynamics simulations confirmed a UCST-like behaviour of the PAM in aqueous alcoholic solutions. Additionally, it was observed that the PAM is more swollen in pure alcohol solutions than in 80% alcoholic solutions due to θ-like behaviour. Additionally, in the globular state, the size of the aggregates was found to increase with increasing solvent hydrophobicity and polymer concentration of the solutions. Above its phase transition temperature, PAM might be present as individual polymer chains in the coil state (≤10 nm). As PAM is a widespread polymer in many biomedical applications (gel electrophoresis, etc.), this finding could be of high relevance for many more practical applications in high performance pharmaceuticals and/or sensors.