Polymer Conformation Research Articles

Solvation-free energy of uncharged and charged water-soluble synthetic polymer using adaptive Poisson-Boltzmann solver: poly(acrylic acid)

ABSTRACT Implicit solvent based on the Poisson-Boltzmann model of electrostatics provides a faster approach to the calculation of solvation and conformations of polymers compared to explicit solvents in molecular dynamics simulations. While PB-based methods have been used extensively for biological macromolecules, here we present the use of the Adaptive Poisson-Boltzmann Solver (APBS) approach for the solvation of synthetic polymer in water. Stereo-specific (isotactic) poly(acrylic acid) of different chain lengths (20–55 for uncharged and 15–50 for charged) are sampled from atomistic molecular dynamic simulations in explicit water followed by APBS in implicit water. The hydration energy of uncharged and charged polymers varies linearly with the number of units (N). The variation of total solvation energy for SASA (Solvent Accessible Surface Area) and SAV (Solvent Accessible Volume) shows a crossover from one scaling to the other. While such a transition is known for small hydrophobic solutes, we report this for a dilute solution of polymer at uncharged and fully charged conditions and in implicit solvent.

A discretized representation for Monte Carlo simulation of deformed semiflexible chains.

In this study, we present a novel orientation discretization approach based on the rhombic triacontahedron for Monte Carlo simulations of semiflexible polymer chains, aiming at enhancing structural analysis through rheo-small-angle scattering (rheo-SAS). Our approach provides a more accurate representation of the geometric features of semiflexible chains under deformation, surpassing the capabilities of traditional lattice structures. Validation against the Kratky-Porod chain system demonstrated superior consistency, underscoring its potential to significantly improve the precision of uncovering geometric details from rheo-SAS data. This approach opens new avenues for investigating the conformations of semiflexible polymers and mechanically induced phase transitions in more complex polymer structures, offering deeper insights into their behavior under various conditions.

Conformation and topology of cyclical star polymers.

We study the conformation and topological properties of cyclical star polymers with f ring arms, each made of n beads. We find that the conformational properties of unlinked cyclical star polymers are compatible with those of linear star polymers with 2f arms made of n/2 beads each. This compatibility vanishes when the topology of the star, measured as the degree of linking between arms, changes. In fact, when links are allowed, we notice that the gyration radius decreases as a function of the absolute linking number |Lk| of the arms, regardless of the protocol that is employed to introduce said links. Furthermore, the internal structure of the macromolecules, as highlighted by the radial density function, changes qualitatively for large values of |Lk|.

Preserving large-scale features in simulations of elastic turbulence

Simulations of elastic turbulence, the chaotic flow of highly elastic and inertialess polymer solutions, are plagued by numerical difficulties: the chaotically advected polymer conformation tensor develops extremely large gradients and can lose its positive-definiteness, which triggers numerical instabilities. While efforts to tackle these issues have produced a plethora of specialized techniques – tensor decompositions, artificial diffusion, and shock-capturing advection schemes – we still lack an unambiguous route to accurate and efficient simulations. In this work, we show that even when a simulation is numerically stable, maintaining positive-definiteness and displaying the expected chaotic fluctuations, it can still suffer from errors significant enough to distort the large-scale dynamics and flow structures. We focus on two-dimensional simulations of the Oldroyd-B and FENE-P equations, driven by a large-scale cellular body forcing. We first compare two positivity-preserving decompositions of the conformation tensor: symmetric square root (SSR) and Cholesky with a logarithmic transformation (Cholesky-log). While both simulations yield chaotic flows, only the latter preserves the pattern of the forcing, i.e. its fluctuating vortical cells remain ordered in a lattice. In contrast, the SSR simulation exhibits distorted vortical cells that shrink, expand and reorient constantly. To identify the accurate simulation, we appeal to a hitherto overlooked mathematical bound on the determinant of the conformation tensor, which unequivocally rejects the SSR simulation. Importantly, the accuracy of the Cholesky-log simulation is shown to arise from the logarithmic transformation. We also consider local artificial diffusion, a potential low-cost alternative to high-order advection schemes. Unfortunately, the artificially enhanced diffusive smearing of polymer stress in regions of intense stretching substantially modifies the global dynamics. We then show how the spurious large-scale motions, identified here, contaminate predictions of scalar mixing. Finally, we discuss the effects of spatial resolution, which controls the steepness of gradients in a non-diffusive simulation.

Recovered graphene-hydrogel nanocomposites for multi-modal human motion recognition via optimized triboelectrification and machine learning

Hydrogels have extensive applications in portable, flexible, wearable, and self-powered electronic devices based on triboelectric nanogenerators (TENGs). An important issue with hydrogels is their tendency to dehydrate over time, which leads to a decline in both ionic conductivity and mechanical flexibility. Furthermore, the current techniques used to produce these hydrogels mostly rely on the freeze–thaw process, which has limited ability to modify the polymer conformation. Herein, a novel water-assisted recovered hydrogel is proposed using a simple strategy to prepare high-performance hydrogel-based TENGs by optimizing the cross-linking and crystalline domains. Synthesis of the electrostatic electrode in the TENG involved the incorporation of polyethylene oxide (PEO) into a polyvinyl alcohol (PVA) hydrogel network via cross-linking. Graphene nanoplatelets (GNP) were added to precisely tune the electrical conductivity. GNP constructs the backbone structures in the hydrogel and enhances the charge transport capacity. Electrical conductivity is changed by the GNP concentration and thus, electrical output of the hydrogel can be facilely controlled. The water reabsorption increased density and crystallinity of the cross-linking and allowed the hydrogel to show superior performance compared to the original one. The 7th recovery hydrogel produced around 594 V, 40 μA, and 32 nC. The 7th recovery hydrogel had exceptional endurance, with the capacity to withstand over 16,000 cycles of contact separation. Moreover, it could be stretched up to 541 % of its original length and improved by almost twice as much as that without the recovery process. By combining multi-modal graphene-based TENG sensors with machine learning, a state-of-the-art behavioral monitoring system was created that could reliably detect tapping fingers with an average accuracy rate of 95 %. The findings of this research will pave the way for new approaches to the development of autonomous motion sensors and flexible renewable energy sources.

Off-Lattice Markov Chain Monte Carlo Simulations of Mechanically Driven Polymers.

We develop off-lattice simulations of semiflexible polymer chains subjected to applied mechanical forces by using Markov Chain Monte Carlo. Our approach models the polymer as a chain of fixed length bonds, with configurations updated through adaptive nonlocal Monte Carlo moves. This proposed method enables precise calculation of a polymer's response to a wide range of mechanical forces, which traditional on-lattice models cannot achieve. Our approach has shown excellent agreement with theoretical predictions of persistence length and end-to-end distance in quiescent states as well as stretching distances under tension. Moreover, our model eliminates the orientational bias present in on-lattice models, which significantly impacts calculations such as the scattering function, a crucial technique for revealing the polymer conformation.

Chain Conformation of Polymers Densely Grafted onto Plate-Shaped Particles

Chain Conformation of Polymers Densely Grafted onto Plate-Shaped Particles

Rheological Behavior of Clay Tailings in the Presence of Divalent Cations and Sodium Polyacrylate: Insights from Molecular Dynamics Simulations.

This study analyzes the behavior of sodium polyacrylate (NaPA) as a rheological modifier for clay-based tailings. Special emphasis is placed on the impact of calcium and magnesium ions in industrial water, which are analyzed through rheograms, zeta potential measurements, and molecular dynamics simulations. The results are interpreted as electrostatic interactions, steric phenomena, and cation solvation. This interpretation integrates experimental studies with microscopic analyses, employing molecular dynamics simulations to elucidate the underlying mechanisms. In all cases, a decrease in the yield stress of synthetic slurries is observed as the dosing of NaPA increases due to greater repulsion between tailings particles through an increase in electrostatic repulsion and larger steric forces that hinder agglomeration. However, efficiency is reduced in the presence of divalent cations as zeta potential measurements suggest a reduction in the electrical charges of the particles and the polymer, making its application more challenging. The differences obtained in the presence of calcium compared to magnesium are explained in terms of the solvation of these ions and their impact on the polymer conformation in solution and adsorption on the mineral surfaces. This explanation is reinforced by molecular dynamics studies, which indicate that polymer adsorption on minerals depends on the type of mineral and type of ion. Particularly for quartz, the highest adsorption of NaPA occurs in the presence of calcium, whereas for a kaolinite surface, the highest polymer adsorption is obtained in the presence of magnesium. The competitive effect of these phenomena leads to the rheological behavior of the tailings being dominated by the effects originating in the clay.

Role of Sterilization on In Situ Gel-Forming Polymer Stability

In recent years, stimulus-sensitive drug delivery systems have been developed for parenteral administration as a depot system. In situ systems incorporate smart polymers that undergo a phase transition at the site of administration. All parenteral and ocular dosage forms must meet sterility requirements. Careful selection of the sterilization method is required for any type of stimuli-sensitive system. Current sterilization methods are capable of altering the conformation of polymers or APIs to a certain extent, ultimately causing the loss of pharmacological and technological properties of the drug. Unfortunately, the issues of risk assessment and resolution regarding the sterilization of stimuli-sensitive systems, along with ways to stabilize such compositions, are insufficiently described in the scientific literature to date. This review provides recommendations and approaches, formulated on the basis of published experimental data, that allow the effective management of risks arising during the development of in situ systems requiring sterility.

Reverse Nonequilibrium Molecular Dynamics Simulations of a Melt of Kremer-Grest Type Model under Fast Shear.

Although the reverse nonequilibrium molecular dynamics (RNEMD) simulation method has been widely employed, the range of applicability is yet to be discussed. In this study, for the first time, we systematically examine the method against an unentangled melt of the Kremer-Grest type chain. The simulation results indicate that as the shear rate increases, the temperature and density become inhomogeneous. However, the average viscosity remains consistent with the results obtained using the SLLOD method under homogeneous temperature and density. We also confirm that the temperature-density inhomogeneity does not significantly affect polymer conformation.

A Gram-Charlier Analysis of Scattering to Describe Nonideal Polymer Conformations.

Theories of interpreting polymer physics and rheology at the molecular level from experiments, including small-angle scattering, typically rely on the assumption that polymer chains possess a Gaussian configuration distribution. This assumption frequently fails to describe features of real polymer molecules both at equilibrium (when polymers have nonlinear topology or heterogeneous chemistry) and out of equilibrium (when they are subjected to nonlinear deformations). To better describe non-Gaussian polymer conformation distributions, we propose a moments analysis based on the Gram-Charlier expansion as a natural framework for describing structure and scattering from non-Gaussian polymers. The expansion describes the conformation distribution in terms of cumulants (equivalent to moments of the distribution) of the underlying segment density distribution function, providing low-dimensional descriptors that can be inferred directly from measured scattering in a way that is agnostic to a polymer's topology, chemistry, or state of deformation. We use this framework to show that cumulants can be used to "fingerprint" non-Gaussian conformation distributions of polymers either at equilibrium (applied to sequence-defined heteropolymers) or out of equilibrium (applied to polymers experiencing nonlinear deformation due to flow). We anticipate that this new analysis method will provide a general framework for examining nonideal polymer configurations and the properties that arise from them.

Flow dichroism of DNA can be quantitatively predicted via coarse-grained molecular simulations

We demonstrate the use of multiscale polymer modeling to quantitatively predict DNA linear dichroism (LD) in shear flow. LD is the difference in absorption of light polarized along two perpendicular axes and has long been applied to study biopolymer structure and drug-biopolymer interactions. As LD is orientation dependent, the sample must be aligned in order to measure a signal. Shear flow via a Couette cell can generate the required orientation; however, it is challenging to separate the LD due to changes in polymer conformation from specific interactions, e.g., drug-biopolymer. In this study, we have applied a combination of Brownian dynamics and equilibrium Monte Carlo simulations to accurately predict polymer alignment, and hence flow LD, at modest computational cost. As the optical and conformational contributions to the LD can be explicitly separated, our findings allow for enhanced quantitative interpretation of LD spectra through the use of an in silico model to capture conformational changes. Our model requires no fitting and only five input parameters: the DNA contour length, persistence length, optical factor, solvent quality, and relaxation time, all of which have been well characterized in prior literature. The method is sufficiently general to apply to a wide range of biopolymers beyond DNA, and our findings could help guide the search for new pharmaceutical drug targets via flow LD.

Competitive effects of anion mobility and viscous forces on the interactions of hyaluronic acid and alginic acid with hofmeister salts

The specific ion effect takes into account the hydration, mobility, and kosmotropic/chaotropic effects. However, another important aspect to consider is the solution viscosity, which influences ion mobility. A limited number of publications have considered the impact of specific ion effects on polyelectrolytes. However, none of the studies have considered the viscous effect of polyelectrolytes. This study aims to investigate the impact of kosmotropic and chaotropic anions on negatively charged alginic acid and hyaluronic acid using dynamic light scattering, rheology, and molecular dynamic simulations. Different concentration regimes and the change in the chain conformations in these regimes, along with the viscosity scaling and impact of the salts on the polyelectrolytes, have been discussed. An interesting finding regarding the solution viscosity was reported. The solution viscosity leads to the reversal of the diffusivity and viscosity trend of the polymer with kosmotropes and chaotropes. A threshold is also identified above which viscous forces dominate. Specific ion effects dominate below the threshold. It was concluded that the specific ion effect shows a competitive nature with the viscous forces in the case of a high-viscosity polyelectrolyte solution, which becomes an important aspect to consider for unveiling such interactions. Prior knowledge of the conformations of polymers can be used for designing drug delivery/nutrient delivery systems and for understanding phase separation behavior. Considering the viscosity of polymer solution becomes an important factor which, in turn, impacts the anion's mobility and ultimately influences the polymer conformations.

Smart Polymer Solution and Thermal Conductivity: How Important Is an Exact Polymer Conformation?

Smart Polymer Solution and Thermal Conductivity: How Important Is an Exact Polymer Conformation?

Bottlebrush Midblocks Promote Colloidal Bridging of Telechelic Polymers.

Telechelic polymers are effective rheological modifiers that bridge between associative constituents to form elastic networks. The performance of linear telechelic chains, however, is controlled by entropic forces and thus suffers from an upper limit on bridge formation. This work overcomes this limitation by utilizing telechelic triblock copolymers containing bottlebrush midblocks. By comparing the rheological properties of emulsions linked by telechelic bottlebrush polymers to those containing linear chains, we determined that telechelic polymers with bottlebrush midblocks form elastic networks more efficiently. These enhanced rheological properties arise from the high stiffness of the bottlebrush midblocks, which offsets the entropic stretching penalty for bridge formation, enabling them to more readily form networks. This molecular-level control over polymer conformation in complex fluids opens avenues for designing highly elastic networks with minimal polymeric additives.

Two-Regime Conformation of Grafted Polymer on Nanoparticle Determines Symmetry of Nanoparticle Self-Assembly.

One of the key design factors that regulate the properties of grafted nanoparticles (GNPs) and their self-assembly is the conformation of the grafted polymer. On the curved surface of the GNP core, the conformation of the polymer chain is not uniform in the radial direction. The segment is a non-Gaussian chain in the concentrated polymer brush (CPB) regime near the interface between GNP core and grafted polymer, while it is less constrained in the semidilute polymer brush (SDPB) regime near the surface of GNP. Here, the property of polymer conformation showing crossover behavior at the CPB/SDPB threshold through the coarse-grain molecular dynamics simulation of nanoparticles with explicit grafted chains is explored. Moreover, the self-assembly structure depends on the effective softness, which is defined as a function of the threshold of two regimes estimated from the conformation of the polymer.

Universal Time and Length Scales of Polar Active Polymer Melts.

We present an in-depth multiscale analysis of the conformations and dynamics of polar active polymers, comparing very dilute and very dense conditions. We unveil characteristic length and time scales, common to both dilute and dense systems, that recapitulate the conformational and dynamical properties of these active polymers upon varying both the polymer size and the strength of the activity. Specifically, we find that a correlation (or looping) length characterizes the polymer conformations and the monomer dynamics. Instead, the dynamics of the center of mass can be fully characterized by the end-to-end mean-square distance and by the associated relaxation time. As such, we show that the dynamics of individual chains in melts of polar active polymers is not controlled by entanglements, but only by the strength of the self-propulsion.

Entropy optimization of a FENE-P viscoelastic model: a numerically guided comprehensive analysis

The influence of polymers on entropy generation processes is substantial, particularly in the fields of fluid dynamics and rheology. The FENE-P (Finitely Extensible Nonlinear Elastic-Peterlin) model describes the polymer’s dynamics as a result of the interaction between the stretching caused by the velocity gradient and the elastic force that restores the polymer to its equilibrium position. Models such as FENE-P aid in understanding and predicting polymer flow behaviour allowing for the reduction of entropy generation by optimizing system designs. A continuum approach is employed to express the heat flux vector and polymer confirmation tensor of the model. The study investigates the complex relationship between polymer conformation, flow dynamics, and heat transfer taking into account the thermophoresis (Soret effect) and mass diffusion-thermal diffusion coupling (Dufour effect) phenomena to optimize processes by reducing entropy. This study illuminates polymer’s significance in entropy minimization, improving engineering design methodologies and applications in materials science, chemical engineering, and fluid dynamics. As result, the presence of polymers leads to a substantial decrease in the total entropy of the system. This understanding provides opportunities for enhancing heat transfer systems, thereby facilitating the development of more efficient and sustainable technology.

General Condition for Polymer Cononsolvency in Binary Mixed Solvents.

Starting from a generic model based on the thermodynamics of mixing and abstracted from the chemistry and microscopic details of solution components, three consistent and complementary computational approaches are deployed to investigate the general condition for polymer cononsolvency in binary mixed solvents at the zeroth order. The study reveals χPS - χPC + χSC as the underlying universal parameter that regulates cononsolvency, where χαβ is the immiscibility parameter between the α- and β-component. Two disparate cononsolvency regimes are identified for χPS - χPC + χSC < 0 and χPS - χPC + χSC > 2, respectively, based on the behavior of the second osmotic virial coefficient at varying solvent mixture composition x C. The predicted condition is verified using self-consistent field calculations by directly examining the polymer conformational transition as a function of x C. It is further shown that in the regime χPS - χPC + χSC < 0, the reentrant polymer conformation transition is driven by maximizing the solvent-cosolvent contact, but in the regime χPS - χPC + χSC > 2, it is driven by promoting polymer and cosolvent contact. In-between the two regimes when neither effect is dominant, a monotonic response of polymer conformation to x C is observed. Effects of the mean-field approximation on the predicted condition are evaluated by comparing the mean-field calculations with computer simulations. It shows that the fluctuation effects lead to a higher threshold value of χPS - χPC + χSC in the second regime, where local enrichment of cosolvent in polymer proximity plays a critical role.

Versatile fabrication of carbon nanotube yarn composites by in-situ interfacial polymerization of polyetherimide

Manufacturing of high-performance carbon nanotube (CNT)-based polymer composites has long been complicated by the difficulty of infiltrating the nanoporous network presented by continuous CNT materials; this can yield composites with heterogeneities that drastically worsen their mechanical properties. We show the synthesis of CNT-polyetherimide (PEI) composite yarns via in-situ interfacial polymerization (ISIP), which side-steps slow, viscous polymer transport by infiltrating and reacting monomer species in-situ via a rapid and scalable process. We demonstrate the ISIP technique on two distinct, industrially-produced CNT yarns, and identify processing parameters that achieve conformal polymer coatings, yielding statistically-significant increases to linear density-specific tensile properties. Using ISIP on pre-densified yarns results in composites with specific stiffness and tenacity of 142 N/tex and 2.2 N/tex, respectively. When ISIP is applied to lightly-processed, porous CNT yarn, the specific stiffness and tenacity reach up to 66 N/tex and 0.7 N/tex. The role of interfacial effects, particularly from amorphous carbon, on the composite properties is also explored. Finally, we demonstrate a prototype roll-to-roll ISIP apparatus which can process arbitrary lengths of yarn for continuous composite production.