Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session M02: From Statistical Physics to High Performance Materials IFocus
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Sponsoring Units: GSNP DSOFT DPOLY Chair: Shengfeng Cheng, Virginia Tech Room: Room 125 |
Wednesday, March 8, 2023 8:00AM - 8:36AM |
M02.00001: Entanglement Topology of Ring-Linear Polymer Blends Invited Speaker: Gary S Grest The topological constraints of no free ends force non-concatenated ring polymers in a melt to form compact loopy globular conformations. The closed-loop structure of ring polymers also enables them to be threaded by linear polymers in ring/linear blends. This threading of the rings by linear chains leads to several interesting phenomena including a non-monotonic increase in the shear viscosity as a function of ring fraction and transition in the stress relaxation from a power law decay for large ring fractions to a plateau as the fraction of rings decreases. In this talk, I will present results from equilibrium and non-equilibrium molecular dynamics simulations of the equilibrium dynamics, entanglement topology, and nonlinear extensional rheology of ring/linear polymer blends, systematically varying the fraction of rings and the ratio of ring and linear chain lengths. The relaxation time of the rings is found to dependent strongly on the linear chain length as the rings cannot relax until the linear chains threading them reptate away. Primitive path analysis is used to visualize and quantify the structure of the composite ring-linear entanglement network and degree of threading. Under uniaxial extensional flow, there is an initial stress overshoot, which is due to overstretching of the rings, followed by a recoil of the rings as they unthread from the linear chain. The relaxation times to re-thread the linear chains into the rings after cessation of flow is followed and related to the equilibrium relaxation times for the ring and linear chains for varying linear chain length. |
Wednesday, March 8, 2023 8:36AM - 8:48AM |
M02.00002: Chemical Features of Convected Constraint Release in Polymer Melts Peter D Olmsted, Benjamin E Dolata, Marco Aurelio Galvani Cunha, Mark O Robbins The concept of entanglements, as proposed by de Gennes and quantified by Doi and Edwards, pervades studies of polymer melt dynamics. Entanglements provide mechanical strength once a polymer has solidified, and give rise to the non-Newtonian effects such as die swell, spurt, and normal stresses that often plague polymer processing. The fused filament fabrication method of additive manufacturing, in particular, can produce weak parts because of poorly entangled interfaces between printed filaments. It is now known from simulations that the degree of entanglement can change dramatically under strong flows. I will discuss some of the observed phenomenology from simulations, the circumstantial evidence from experiments; and new theories to describe these effects quantitatively. Here, we compare predictions of a recently developed constitutive equation for disentanglement to united-atom polyethylene and Kremer-Grest molecular dynamics simulations in steady-state shear and extensional flow. Quantitative agreement is obtained in measurements of re-entanglement following cessation steady-state shear flow, which confirm the model prediction of re-entanglement on the Rouse time. We find that entanglement kinetics are independent of molecular weight and thus the number of entanglements, but depend the number of Kuhn segments per entanglement strand Ne. Hence, melts with stiffer molecules will disentangle faster than those with more flexible chains. We interpret this in terms of the number of retraction events required to effectively remove an entanglement. |
Wednesday, March 8, 2023 8:48AM - 9:00AM |
M02.00003: Effect of Compatibilizers on Polymer Blend Interfaces Shoumik Saha, Dilip Gersappe Polymer blends demonstrate improved rheological and mechanical properties but are difficult to mix due to high energy interfaces between the components. This results in significant mechanical weakness but can be mediated by using compatibilizers. Despite the large number of studies on the effect of copolymer compatibilizers, the effect of adding sheet-like compatibilizers (for e.g., nanoclay) has not been investigated computationally. Experiments have shown that sheet-like fillers are effective, but the exact mechanisms are unknown. Here, we use coarse-grained molecular dynamics simulations to determine the effect of diblock copolymers and sheet-like compatibilizers at polymer blend interfaces. Our results indicate that sheet fillers that have equal affinity to either polymer in a binary blend, can produce a larger reduction of interfacial tension when compared to diblock copolymers at equal volume fractions. However, the localization of sheet fillers at the interface can be a possible limiting factor. We attempted to bracket sheet filler behavior by either allowing them to localize at the interface or migrate to the bulk polymer phase. We also showed that sheet fillers reduce slip, thus providing for improved stress transfer across the interface, leading to a stronger blend. |
Wednesday, March 8, 2023 9:00AM - 9:12AM |
M02.00004: Force-Driven Active Dynamics of Thin Nanorods in Unentangled Polymer Melts Siteng Zhang, Jiuling Wang, Ting Ge Recent advances in the functional material and biomedical applications of nanorods call for a fundamental understanding of the active motion of nanorods in a viscoelastic medium. Molecular dynamics simulations are performed to investigate a model system consisting of force-driven active thin nanorods in a melt of unentangled polymers. The activeness of a thin nanorod arises from a constant external force applied uniformly along the rod. The simulations demonstrate that the active force overcomes the randomness of the diffusive motion and results in a ballistic motion along the direction of the applied force at long timescales. The friction coefficient for the ballistic motion decreases as the active force increases. The origin of the reduction is the high terminal speed that allows the nanorod to renew its local environment faster than the relaxation time of melt chains. A scaling theory is developed to quantify the dependence of the friction coefficient on the strength of the active force. On the scaling level, the long-time trajectory of a force-driven active nanorod piercing through unentangled polymers may be described as a stretched array of "active blobs", where the short-time random-walk trajectory within an active blob is unperturbed by the active force. |
Wednesday, March 8, 2023 9:12AM - 9:48AM |
M02.00005: Flow-Driven Dynamic Heterogeneity in Elongating Associative Polymer Networks Invited Speaker: Thomas O'Connor Associating polymers form dynamic networks of reversible bonds that can rearrange their network topology. These transient networks can form mechanically durable materials that remain - in principle - reprocessable and recyclable. However, the nonequilibrium dynamics of associative networks are challenging to predict and control, making practical reprocessing difficult. The constant rearrangement of dynamic bonds in the associating network produces a complex coupling between the relaxation modes of individual chains that carry stress and the surrounding self-assembled network that mediates strain from the macroscopic to the molecular scale. Understanding these dynamics requires models that can capture the feedback between dynamic bonding and polymer relaxation in nonequilibrium conditions. Here we apply a new reactive bead-spring model for associating polymers with coordinated dynamic bonds and study their dynamics during extensional deformation. We model the rate-dependent response of associative networks formed from linear polymers with binary associative groups for a wide range of associative bond strengths. We observe that the coupling between chain and network relaxation drives a strong heterogeneity in chain elongation during deformation, producing broad distributions of chain stretch at all strain rates. This broad nonlinear response cannot be described by average order parameters used in established molecular theories of polymer dynamics, motivating the need for new physical models for associative polymers. |
Wednesday, March 8, 2023 9:48AM - 10:00AM |
M02.00006: Uniaxial elongational flow of entangled, associating linear polymer melts Rosita Sivaraj, Supun S Mohottalalage, Dvora Perahia, Thomas O'Connor, Gary S Grest The response of polymers to uniaxial elongational flow affects the structure of polymers which is critical for their processing. The more structured the polymer, the more complex their response becomes. Here we probe the effects of uniaxial elongational flow on entangled, linear polymer melts. The polymers are depicted by a bead spring model with 5% randomly incorporated interacting associating beads, as the interaction strengths varying from 1kBT to 10kBT, using molecular dynamics simulations. Chains of length 100 to 800 beads/chain are studied, covering the range from weakly to highly entangled chains. We find that cluster size increases with increasing interaction between the associating beads. Under flow, these clusters continuously break and reform as the chains stretch. As the sticker strength increases, the distribution of end-to-end distances becomes heterogeneous. Surprisingly, for polymers baring strong associating group even at high extension rates, the clusters do not fully break up and only a fraction of chains is fully stretched. Results from constant pressure and constant volume simulations are compared. |
Wednesday, March 8, 2023 10:00AM - 10:12AM |
M02.00007: Molecular Dynamics Simulation of Diffusiophoretic Motion of Nanoparticles Binghan Liu, Gary S Grest, Shengfeng Cheng Diffusiophoresis refers to the spontaneous motion of a particle or a polymer chain in a solution induced by a concentration gradient of a different type of solutes. Diffusiophoresis is suggested to be responsible for the counterintuitive stratification phenomena recently found in colloidal and polymer solutions undergoing rapid drying. To understand the underlying physics, large scale molecular dynamics simulations are performed to investigate the diffusiophoretic motion of particles in a colloidal suspension with the solvent modeled explicitly as a Lennard-Jones liquid. A concentration gradient of the suspended nanoparticles is induced by using a molecular pump. After the concentration gradient is stabilized, a large nanoparticle is added to the system and its motion along and normal to the gradient direction is recorded. The results indicate that diffusiophoretic motion indeed emerges when the magnitude of the concentration gradient and the size contrast between the solute and diffusiophoretic particles are above certain thresholds. The simulation results are used to test the existing models and theories of diffusiophoresis. By elucidating the physics of diffusiophoresis, the results will help reveal the fundamental mechanism underlying the stratification phenomena in suspensions of colloidal particles with polydisperse size distributions. |
Wednesday, March 8, 2023 10:12AM - 10:24AM |
M02.00008: Evaporation of Sessile Droplets: A Molecular Dynamics Study Yisheng Huang, Shengfeng Cheng The evaporation process of a sessile droplet plays a ubiquitous role in a wide range of natural phenomena and industrial procedures. Theories and models exist to predict the local evaporation flux as a function of the azimuthal angle. Although plenty of experimental and computational studies have been conducted on drop evaporation, data on the local evaporation flux with a good spatiotemporal resolution are still of great value and can be used to test the existing theories. Much of the challenges lies in the fact that evaporation is a transient process, making it hard to achieve good data statistics, and it is also difficult to measure the evaporation flux with a sufficiently fine spatial resolution. In this work we report molecular dynamics simulations of the evaporation process of a droplet adsorbed on a solid substrate with the contact line pinned by a chemical heterogeneity. We have designed a setup where the evaporating droplet is kept in a steady state by putting the escaping flux back to the center of the droplet. This setup allows us to average the local evaporation flux over a long time, thus obtaining data with a good accuracy and high spatial resolution. We have compared our simulation results to the predictions of various theories and models. In particular, our data show that for a wetted drop, the local evaporation flux increases from the apex of the drop to the contact line, in agreement with a model of drop evaporation based on vapor diffusion. |
Wednesday, March 8, 2023 10:24AM - 10:36AM |
M02.00009: Using scale-dependent elasticity to probe and influence the distribution of localization lengths in the amorphous solid state Boli Zhou, Rafael S Hipolito, Paul M Goldbart The equilibrium amorphous solid state – formed, for example, by randomly crosslinking a macromolecular fluid – is characterized by a universal distribution of localization lengths. This distribution obeys a scaling form near to the continuous transition out of the amorphous solid state: it has a single peak at a length-scale that diverges at the transition along with the width of the distribution. Naturally, in the long wavelength limit the elastic rigidity of this state does not depend on the distribution of localization lengths. However, the elastic behavior at progressively shorter length-scales becomes increasingly insensitive to the more weakly localized particles, and thus the scale-dependence of the elastic modulus provides a probe of the universal localization-length distribution. We show, in particular, how the response to short length-scale elastic deformations sheds light on the asymptotics of the distribution at short localization length-scales. In addition, we explore the extent to which elastic deformations at various length-scales can induce a rearrangement of statistical weight in the localization-length distribution. |
Wednesday, March 8, 2023 10:36AM - 10:48AM |
M02.00010: Characterization of local-perturbation-induced non-affine displacement fields in amorphous solids Jinpeng Fan, Evan Willmarth, Weiwei Jin, Amit Datye, Udo D Schwarz, Mark D Shattuck, Corey S O'Hern Amorphous solids exhibit complex non-affine displacement fields in response to applied stress. However, the structural origins of the non-affine displacements in amorphous solids are difficult to identify due to the lack of long-range structural order. In previous studies, we employed Delaunay triangularization to characterize the non-affine displacement fields in two dimensional binary Lennard-Jones (LJ) solids undergoing athermal, quasistatic simple shear (AQS). We showed that local pure shear of single triangles can give rise to quadrupolar displacement fields, though in most cases there were significant contributions from other types of defects. To further characterize the displacement fields that arise from single triangle perturbations, we decompose the displacement fields into an orthogonal basis of monopole, dipole, quadrupole, and vortex contributions. We find that only 20% of the displacement fields can be accurately recovered using this set of basis functions. Thus, pure shear deformations of Delaunay triangles give rise to displacement fields that are not a superposition of monopoles, dipoles, quadrupoles, and vortices. In contrast, pure shear triangle perturbations to hexagonal or disordered spring networks without pre-stress give rise to displacement fields that mimic Eshelby inclusions. |
Wednesday, March 8, 2023 10:48AM - 11:00AM |
M02.00011: Understanding non-affine displacement fields of amorphous solids Evan Willmarth, Jinpeng Fan, Weiwei Jin, Mark D Shattuck, Corey S O'Hern Numerous previous studies have shown that athermal quasi-static shear of amorphous solids induces non-affine displacement fields, both during the continuous segments of stress versus strain and during stress drops, which exhibit quadrupolar behavior. However, these non-affine displacement fields cannot be represented exactly by the displacement fields induced by a linear superposition of Eshelby inclusions. We take a stepwise approach to understanding the differences in the nonaffine displacement fields of amorphous solids and those obtained from Eshelby inclusions. We first apply a pure shear perturbation to a single triangle within a hexagonal lattice in two dimensions and show that the resulting displacement field matches that from an Eshelby inclusion. We then perform similar perturbations to positionally disordered, but unstressed spring networks with uniform spring constants and find that the displacement fields also match those from a single Eshelby inclusion with similar values of the root-mean-square error. We show that the displacement fields of spring networks can differ from those induced by Eshelby inclusions when we consider spring networks with non-uniform spring constants and residual stress. An improved understanding of when quadrupolar structures are induced will allow us to better characterize shear transformation zones, and non-affine displacement fields more generally, that occur during deformation of amorphous solids. |
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