Transient crosslinking controls the condensate formation pathway within chromatin networks.

The network structure of densely packed chromatin within the nucleus of eukaryotic cells acts in concert with nonequilibrium processes. Using statistical physics simulations, we explore the control provided by transient crosslinking of the chromatin network by structural-maintenance-of-chromosome (SMC) proteins over (i) the physical properties of the chromatin network and (ii) condensate formation of embedded molecular species. We find that the density and lifetime of transient SMC crosslinks regulate structural relaxation modes and tune the sol-vs-gel state of the chromatin network, which imparts control over the kinetic pathway to condensate formation. Specifically, lower density, shorter-lived crosslinks induce sollike networks and a droplet-fusion pathway, whereas higher density, longer-lived crosslinks induce gellike networks and an Ostwald-ripening pathway.

Depletion Strategies for Crystallized Layers of Two-Dimensional Nanosheets to Enhance Lithium-Ion Conductivity in Polymer Nanocomposites.

The assembly of long-range aligned structures of two-dimensional nanosheets (2DNSs) in polymer nanocomposites (PNCs) is in urgent need for the design of nanoelectronics and lightweight energy-storage materials of high conductivity for electricity or heat. These 2DNS are thin and exhibit thermal fluctuations, leading to an intricate interplay with polymers in which entropic effects can be exploited to facilitate a range of different assemblies. In molecular dynamics simulations of experimentally studied 2DNSs, we show that the layer-forming crystallization of 2DNSs is programmable by regulating the strengths and ranges of polymer-induced entropic depletion attractions between pairs of 2DNSs, as well as between single 2DNSs and a substrate surface, by exclusively tuning the temperature and size of the 2DNS. Enhancing the temperature supports the 2DNS-substrate depletion rather than crystallization of 2DNSs in the bulk, leading to crystallized layers of 2DNSs on the substrate surfaces. On the other hand, the interaction range of the 2DNS-2DNS depletion attraction extends further than the 2DNS-substrate attraction whenever the 2DNS size is well above the correlation length of the polymers, which results in a nonmonotonic dependence of the crystallization layer on the 2DNS size. It is demonstrated that the depletion-tuned crystallization layers of 2DNSs contribute to a conductive channel in which individual lithium ions (Li ions) migrate efficiently through the PNCs. This work provides statistical and dynamical insights into the balance between the 2DNS-2DNS and 2DNS-substrate depletion interactions in polymer-2DNS composites and highlights the possibilities to exploit depletion strategies in order to engineer crystallization processes of 2DNSs and thus to control electrical conductivity.

Rheological Role of Stiff Nanorings on Concurrently Strengthening and Toughening Polymer Nanocomposites.

Nanorings, which are increasingly uncovered in natural systems and synthesized in man-made materials, exhibit dynamics distinct from those known for linear chains. We show in this study that, when immersed in a polymer melt matrix, segments of a stiff nanoring (SNR) have more facilitated subdiffusion, i.e., with a larger scaling exponent in the mean squared displacement, than those belonging to one flexible counterpart, while the whole SNR is more suppressed by its surroundings. It is revealed that adding SNRs contributes to achieving the long-anticipated rheological objective of sol- and gel-like characteristics at high and low shearing frequencies, respectively. This study suggests the promising prospect of exploiting SNRs to concurrently strengthen and toughen target polymer nanocomposites.

Screening confinement of entanglements: Role of a self-propelling end inducing ballistic chain reptation.

Synthetic and natural nanomaterials with self-propelling mechanisms continue to be explored to boost chain mobility beyond normal reptation in the crowded environments of entangled chains. Here we employ scaling theory and numerical simulations to demonstrate that activating one chain end of a singular or isolated chain boosts entanglement-constrained chain reptation from the one-dimensional diffusive mobility as described by the de Gennes-Edwards-Doi model to ballistic motion along the entanglement tube contour. The active chain is effectively screened from the constraint of entanglements on length scales exceeding the tube size.

Fabrication and Elastic Properties of TiO2 Nanohelix Arrays through a Pressure-Induced Hydrothermal Method.

TiO2 nanohelices (NHs) have attracted extensive attention owing to their high aspect ratio, excellent flexibility, elasticity, and optical properties, which endow promising performances in a vast range of vital fields, such as optics, electronics, and micro/nanodevices. However, preparing rigid TiO2 nanowires (TiO2 NWs) into spatially anisotropic helical structures remains a challenge. Here, a pressure-induced hydrothermal strategy was designed to assemble individual TiO2 NWs into a DNA-like helical structure, in which a Teflon block was placed in an autoclave liner to regulate system pressure and simulate a cell-rich environment. The synthesized TiO2 NHs of 50 nm in diameter and 5-7 mm in length approximately were intertwined into nanohelix bundles (TiO2 NHBs) with a diameter of 20 µm and then assembled into vertical TiO2 nanohelix arrays (NHAs). Theoretical calculations further confirmed that straight TiO2 NWs prefer to convert into helical conformations with minimal entropy (S) and free energy (F) for continuous growth in a confined space. The excellent elastic properties exhibit great potential for applications in flexible devices or buffer materials.

Chain stiffness boosts active nanoparticle transport in polymer networks.

Recent advances in technologies such as nanomanufacturing and nanorobotics have opened new pathways for the design of active nanoparticles (NPs) capable of penetrating biolayers for biomedical applications, e.g., for drug delivery. The coupling and feedback between active NP motility (with large stochastic increments relative to passive NPs) and the induced nonequilibrium deformation and relaxation responses of the polymer network, spanning scales from the NP to the local structure of the network, remain to be clarified. Using molecular dynamics simulations, combined with a Rouse mode analysis of network chains and position and velocity autocorrelation functions of the NPs, we demonstrate that the mobility of active NPs within cross-linked, concentrated polymer networks is a monotonically increasing function of chain stiffness, contrary to passive NPs, for which chain stiffness suppresses mobility. In flexible networks, active NPs exhibit a behavior similar to passive NPs, with a boost in mobility proportional to the self-propulsion force. These results are suggestive of design strategies for active NP penetration of stiff biopolymer matrices.

Molecular Dynamics Simulations to Explore the Structure and Rheological Properties of Normal and Hyperconcentrated Airway Mucus.

We develop the first molecular dynamics model of airway mucus based on the detailed physical properties and chemical structure of the predominant gel-forming mucin MUC5B. Our airway mucus model leverages the LAMMPS open-source code [https://lammps.sandia.gov], based on the statistical physics of polymers, from single molecules to networks. On top of the LAMMPS platform, the chemical structure of MUC5B is used to superimpose proximity-based, non-covalent, transient interactions within and between the specific domains of MUC5B polymers. We explore feasible ranges of hydrophobic and electrostatic interaction strengths between MUC5B domains with 9 nanometer spatial and 1 nanosecond temporal resolution. Our goal here is to propose and test a mechanistic hypothesis for a striking clinical observation with respect to airway mucus: a 10-fold increase in non-swellable, dense structures called flakes during progression of cystic fibrosis disease. Among the myriad possible effects that might promote self-organization of MUC5B networks into flake structures, we hypothesize and confirm that the clinically confirmed increase in mucin concentration, from 1.5 to 5 mg/mL, alone is sufficient to drive the structure changes observed with scanning electron microscopy images from experimental samples. We post-process the LAMMPS simulated datasets at 1.5 and 5 mg/mL, both to image the structure transition and compare with scanning electron micrographs and to show that the 3.33-fold increase in concentration induces closer proximity of interacting electrostatic and hydrophobic domains, thereby amplifying the proximity-based strength of the interactions.

Field-triggered vertical positional transition of a microparticle suspended in a nematic liquid crystal cell.

In this paper, based on the numerical calculation of total energy utilizing the Green's function method, we investigate how a field-triggered vertical positional transition of a microparticle suspended in a nematic liquid crystal cell is influenced by the direction of the applied field, surface anchoring feature, and nematic's dielectric properties. The new equilibrium position of the translational movement is decided via a competition between the buoyant force and the effective force built on the microparticle by the elastic energy gradient along the vertical direction. The threshold value of external field depends on thickness L and Frank elastic constant K and slightly on the microparticle size and density, in a Fréedericksz-like manner, but by a factor. For a nematic liquid crystal cell with planar surface alignment, a bistable equilibrium structure for the transition is found when the direction of the applied electric field is (a) perpendicular to the two plates of the cell with positive molecular dielectric anisotropy or (b) parallel to the two plates and the anchoring direction of the cell with negative molecular dielectric anisotropy. When the electric field applied is parallel to both plates and perpendicular to the anchoring direction, the microparticle suspended in the nematic liquid crystal tends to be trapped in the midplane, regardless of the sign of the molecular dielectric anisotropy. Such a phenomenon also occurs for negative molecular dielectric anisotropy if the external field is applied perpendicular to the two plates. Explicit formulas proposed for the critical electric field agree extremely well with the numerical calculation.

Mechanical Strength Management of Polymer Composites through Tuning Transient Networks.

The addition of transient networks to polymer composites marks a new direction toward the design of novel materials, with numerous biomedical and industrial applications. The network structure connected by transient cross-links (CLs) relaxes as time evolves, which results in the stretching release of polymer strands between transient CLs during strain. Using molecular dynamics simulations, we measure directly the stress-strain curves of double polymer networks (DPNs), containing both transient and permanent components, at different strain rates. Lifetime and density of transient CLs control the relaxation spectrum of transient networks and determine the mechanical properties of DPNs. A Rouse mode analysis reveals that at high strain rates the mechanical strength of DPNs is defined jointly by the cross-linking structures of permanent and transient networks. At low strain rates, the cross-linking structure of transient network relaxes, leaving the permanent component of the network as a sole contributor to the mechanical strength of DPNs. The transient network is shown to facilitate a dissipation of energy at higher strain rates and prevents a rupture of the network, while the permanent network preserves the structural integrity of the composite at low strain rates. This study provides computational and theoretical foundations for designing polymer composites with desirable mechanical strength and toughness by means of tuning transient networks.

Nanoparticle Loading of Unentangled Polymers Induces Entanglement-Like Relaxation Modes and a Broad Sol-Gel Transition.

We combine molecular dynamics simulations, imaging and data analysis, and the Green-Kubo summation formula for the relaxation modulus G(t) to elicit the structure and rheology of unentangled polymer-nanoparticle composites distinguished by small NPs and strong NP-monomer attraction, εNPM ≫ kBT. A reptation-like plateau emerges in G(t) beyond a terminal relaxation time scale as the volume fraction, cNP, of NPs increases, coincident with a structure transition. A condensed phase of NP-aggregates forms, tightly interlaced with thin sheets of polymer chains, the remaining phase consisting of free chains void of NPs. Rouse mode analyses are applied to the two individual phases, revealing that long-wavelength Rouse modes in the aggregate phase are the source of reptation-like relaxation. Imaging reveals chain motion confined within the thin sheets between NPs and exhibits a 2D analogue of classical reptation. In the NP-free phase, Rouse modes relax indistinguishable from a neat polymer melt. The Fourier transform of G(t) reveals a sol-gel transition across a broad frequency spectrum, tuned by cNP and εNPM above critical thresholds, below which all structure and rheological transitions vanish.

Rheological Tuning of Entangled Polymer Networks by Transient Cross-links.

The remarkable functionalities of transiently cross-linked, biopolymer networks are increasingly becoming translated into synthetic materials for biomedical and materials science applications. Various computational and theoretical models, representing different transient cross-linking mechanisms, have been proposed to mimic biological and synthetic polymer networks, and to interpret experimental measurements of rheological, transport, and self-repair properties. Herein, we employ molecular dynamics simulations of a baseline entangled polymer melt coupled with parametrized affinities for binding and unbinding of transient cross-links (CLs). From these assumptions alone, we determine the emergent CL mean density and fluctuations, and the induced rheology, across the 2-parameter space of binding and unbinding affinities for a moderately long chain, entangled the polymer melt. For sufficiently weak (short-lived) CLs, nonmonotonicity emerges with respect to the affinity to form CLs: the stress relaxation, viscous, and elastic moduli all shift above the baseline if CLs form rapidly, reverse below the baseline as CLs form slowly, and reverse again, recovering the baseline as CLs form very slowly. For sufficiently strong (long-lived) CLs and sufficiently fast CL formation, a dramatic rise emerges in the viscous and elastic moduli at all frequencies, more prominently in the elastic moduli at medium to high frequencies, inducing a sol-gel transition. These results are placed in context with the experimental and theoretical literature on transient polymer networks.

Tuning Adsorption Duration To Control the Diffusion of a Nanoparticle in Adsorbing Polymers.

Controlling the nanoparticle (NP) diffusion in polymers is a prerequisite to obtain polymer nanocomposites (PNCs) with desired dynamical and rheological properties and to achieve targeted delivery of nanomedicine in biological systems. Here we determine the suppression mechanism of direct NP-polymer attraction to hamper the NP mobility in adsorbing polymers and then quantify the dependence of the effective viscosity ηeff felt by the NP on the adsorption duration τads of polymers on the NP using scaling theory analysis and molecular dynamics simulations. We propose and confirm that participation of adsorbed chains in the NP motion break up at time intervals beyond τads due to the rearrangement of polymer segments at the NP surface, which accounts for the onset of Fickian NP diffusion on a time scale of t ≈ τads. We develop a power law, ηeff ∼ (τads)ν, where ν is the scaling exponent of the dependence of polymer coil size on the chain length, which leads to a theoretical basis for the design of PNCs and nanomedicine with desired applications through tuning the polymer adsorption duration.

A theoretical study of dispersion-to-aggregation of nanoparticles in adsorbing polymers using molecular dynamics simulations.

The properties of polymer-nanoparticle (NP) mixtures significantly depend on the dispersion of the NPs. Using molecular dynamics simulations, we demonstrate that, in the presence of polymer-NP attraction, the dispersion of NPs in semidilute and concentrated polymers can be stabilized by increasing the polymer concentration. A lower polymer concentration facilitates the aggregation of NPs bridged by polymer chains, as well as a further increase of the polymer-NP attraction. Evaluating the binding of NPs through shared polymer segments in an adsorption blob, we derive a linear relationship between the polymer concentration and the polymer-NP attraction at the phase boundary between dispersed and aggregated NPs. Our theoretical findings are directly relevant for understanding and controlling many self-assembly processes that use either dispersion or aggregation of NPs to yield the desired materials.

Thin polymer-layer decorated, structure adjustable crystals of nanoparticles.

Flattened polymer chain decorated crystals of nanoparticles (NPs) are observed for polymer-NP mixtures confined between two parallel substrates. In order to minimize the entropy loss, polymer chains instead of NPs aggregate at the substrate surfaces when the number of NPs is high enough to have the conformation of chains significantly disturbed. Increasing NP concentration to be much higher than that of polymer chains leads to an ordered arrangement of NPs in the central region, which are sandwiched between two thin layers of polymer chains. A scaling model regarding polymer chains consisting of packed correlation blobs is provided to clarify the physics mechanism behind the formation of thin polymer layer and the crystallization of NPs. The order structure of the crystallized NPs is shown to be switchable through an adjustment of the bulk concentrations of polymer chains and NPs.

Polymer-induced inverse-temperature crystallization of nanoparticles on a substrate.

Using molecular dynamics simulations, we study the properties of liquid state polymer-nanoparticle composites confined between two parallel substrates, with an attractive polymer-substrate interaction. Polymers are in the semidilute regime at concentrations far above the overlap point, and nanoparticles are in good solvent and without enthalpic attraction to the substrates. An increase of temperature then triggers the crystallization of nanoparticles on one of the two substrate surfaces-a surprising phenomenon, which is explained in terms of scaling theory, such as through competing effects of adsorption-and correlation blobs. Moreover, we show that the first, closely packed layer of nanoparticles on the substrate increases the depletion attraction of additional nanoparticles from the bulk, thereby enhancing and stabilizing the formation of a crystalline phase on the substrate. Within the time frame accessible to our numerical simulations, the crystallization of nanoparticles was irreversible; that is, their crystalline phase, once created, remained undamaged after a decrease of the temperature. Our study leads to a class of thermoreactive nanomaterials, in which the transition between a homogeneous state with dissolved nanoparticles and a surface-crystallized state is triggered by a temperature jump.

Polymer-induced entropic depletion potential.

We study the effective interactions between nanoparticles immersed in an athermal polymer solution using Molecular dynamics. The directly measured polymer-induced depletion forces are well described with a scaling model in which the attraction between particles is caused by the depletion of concentration blobs and thus independent of the length of the polymer chains. We find strong evidence for a repulsive barrier which arises when the distance between the particles is of the order of the correlation length of the solution and which can be interpreted as a packing effect of concentration blobs. Interestingly, the scaling picture can be extended into the regime in which higher virial coefficients of the polymer solution become relevant. We derive a universal relation between the attraction force at the particle contact, f(0), and the osmotic pressure Π as f(0)∼Π(2/3), demonstrating its validity over a wide range of concentrations of the polymer solution.