Session CC05: V: Computational Advances in Polymer Physics

Entropic Augmentation of Coarse-Grained Potentials for Resolving Dynamics Using the Iterative Boltzmann Inversion (IBI) Method

Recently, we studied an approach for coarse-grained (CG) molecular dynamics (MD) in which we used Iterative Boltzmann Inversion (IBI) in combination with Langevin dynamics to parametrize force-fields for polymers melts [J. Chem. Phys. 154, 084114 (2021); J. Phys. Chem. B, 127 (31), (2023)]. Here, we study the potential dynamics of this model to determine more precisely the effect of Langevin friction and use that as a basis for model improvement. Dynamic characterization of the intramolecular and pair potentials shows that here Langevin friction modifies only the pair dynamics in this formulation. Based on the hypothesis that the intramolecular potentials need similar entropic augmentation as Langevin provides for pair, we use simplification of GLE models for dynamic bond friction to derive friction-based models for dynamic correction of the bonded potentials. Pedagogically, the results show the virtual bonds in the CG space function as damped anharmonic oscillators where the enthalpic terms resolve the structural distributions and the frictional terms resolve dynamics. Indirect tests of the model are used to show that intramolecular forces are also an important factor in tuning the dynamics of IBI based CG models.   

Accessibility of Ring-closing depolymerization for polycarbonates with multiple theory validation

We present an accelerated protocol to qualitatively screen polycarbonate monomers for viability of depolymerization in solvents of varying polarities. The largest barrier in the exploration of reaction kinetics is the necessary conformational analysis to identify lowest energy paths in depolymerization, which can depend on the solvent. Tight-binding DFT allows for an accurate quantification of depolymerization barriers as compared to DFT, with an increase in speed by up to 3 orders of magnitude. By a detailed featurization of over 30 compounds we verified that depolymerization barriers are not linearly correlated to the dielectric constant of the solvent. Volume of the molecules is explicitly not correlated with the depolymerization barrier; the solvent-molecule interactions and chemistry of the molecules is extremely influential due to the over coordinated oxygens in the transition state. Though computation of the solvent-solute interaction parameter is necessary for future predictive work in kinetic barriers, even alongside the 1800 Mordred features, conformational analysis is still necessary to determine the lowest kinetic barrier across solvents. We present multiple examples of the of lowest-barrier polycarbonates in acetonitrile, tetrahydrofuran, and toluene to highlight the large opportunity in this field for development in this multi-dimensional optimization problem. With the current available simulation and predictive models, our high-throughput protocol can evaluate the ideal solvent for 6-member polycarbonate depolymerization. From this work, further investigation into solvent-molecule interactions for quick computing features will lead towards the goal of data-driven methods for polycarbonate discovery and optimization.   

Thermal Stiffening in Polymer Nanocomposites with Dynamically Heterogenous Interfaces

There is extensive research on polymer nanocomposite systems showing that fillers can improve the mechanical properties of polymer melts through hydrodynamic interactions between nanoparticles and polymer chains, distortion of shear fields, decreased matrix chain mobility, nanoparticle jamming, and percolation of a transient immobile high-density polymer network assisted by nanofillers. In the current study, dynamically asymmetric composite blends and polymer-grafted-nanoparticle polymer composites (PGNPCs) with molecular weights above the entanglement length were studied via Molecular Dynamics simulations. Both blends and  PGNPCs were made up of two polymer chains having a large glass transition temperature (T_g) difference − grafted chains having the higher T_g. Simulation results showed vastly different temperature responses among the neat (Low-T_g polymer), blends, and PGNPCs. The entangled PGNPCs showed enhanced reinforcement, when compared to the neat polymer matrix at graft concentrations far lower than that of the blends. Additionally, a thermomechanical analysis showed a unique thermal stiffening behavior in PGNPCs (even at high temperatures) that was not observed in other systems. Two mechanisms were identified for the observed reinforcement in PGNPCs: (i) dynamic coupling of High-T_g grafts and Low-T_g matrix chains at high temperatures, (ii) increased number of graft/matrix entanglements with increasing interfacial thickness.   

Multibody Interaction Effects in the Phase Behaviors of Molten Diblock Copolymers

In this study, we investigate the necessity of effective multibody interactions in properly understanding the phase separation of molten diblock copolymers with disparity in self cohesion and association between constituent blocks. Using a Landau approach based on a molecular equation of state, the location of critical points is determined, where the multibody interactions play an important role. It is also discussed by using the companion self-consistent field theory that at what processing window the ordinary and inverted mesophase morphologies are exhibited.   

Multilayered ordered arrays self-assembled from nanoparticle mixtures via salt dialysis

Biology has inspired many bottom-up strategies for synthesizing materials via the self-assembly of nanostructures for applications in drug delivery, catalysis, and sensing, among others. Using coarse-grained molecular dynamics simulations and experiments, we study the self-assembly of mixtures of up to 4 different types of virus-like particles (VLPs) derived from bacteriophage P22, which can be genetically engineered to express different surface charges, over a broad range of salt concentration (~0.01 - 1 M). Through different combinations of two- , three-, and four-component mixtures of VLPs, we demonstrate that salt dialysis can be used as an approach to engineer multilayered ordered arrays in the presence of oppositely-charged linker macromolecules, where each layer is composed of a single type of VLP. VLP-VLP pair correlation functions are extracted to characterize the structure of the core-shell ordered arrays, emphasizing the differences between the aggregates resulting from rapid quenching versus dialysis of the VLP solutions. In solutions with multiple VLP types, we observe that the presence of higher charged variants increases the ionic strength threshold of the smaller charged variant to assemble into ordered structures. Linker-VLP correlation functions are extracted to explain this effect.   

Critical scaling of shear modulus for particle-filled soft elastomers in the jamming limit

Soft composite solids are composed of stiff particles dispersed within a soft polymeric matrix. They form the basis of many multi-phase materials in nature and industries, including biological tissues and field-responsive soft devices. While the mechanics of dilute composites has been well-understood by the Eshelby framework, the governing mechanical principles of dense soft composites remain unclear. Dense soft composites often display novel nonlinear behaviors that are determined by the complex multiscale interactions within soft composites. In this work, we focus on revealing the role of granular jamming in the strain-stiffening behaviors of soft composites. For composites with different particle volume fractions and matrix shear moduli, we systematically characterized their shear responses to varying uni-axial compressive strains. We uncovered the underlying connections between composite mechanics and the jamming rheology of granular suspensions. In particular, we demonstrate a critical scaling collapse of the composite modulus near the shear-jamming transitions of granular systems. In addition, we constructed a phenomenological model that quantitatively predicts the strain-stiffening of dense soft composites.   

Expansion Kinetics of Single Flexible Polymers upon Release from a Circular Cavity in Three- and Two-Dimensional Spaces

In this study, we investigate the behavior of single polymers when they are instantaneously released from a spherical confinement in a d-dimensional space. We develop a compelling theory to explain the intricate kinetics of the expansion process, which can be divided into two main stages.  During the first stage, the chain undergoes a rapid expansion while maintaining its structure as a sphere. In the second stage, the expansion process slows down significantly, and the chain assumes a coil-like conformation. The kinetics of the expansion are derived by using Onsager's variational principle. Molecular dynamics simulations are then conducted to examine the theory in both quasi-2D and three-dimensional spaces. The average time evolution of the chain size displays a featured sigmoidal variation on a logarithmic scale, characterized by two times and associated exponents that represent the fast and slow dynamics, respectively. Through a direct analysis of the kinetic equations, we discover two important and unique universal behaviors for the two expansion stages. We also investigate the scaling properties of the characteristic times and exponents under different confinement conditions. The results confirm the validity of our theory, demonstrating its robustness in describing the kinetics of polymer expansion in both two- and three-dimensional spaces.   

Toughening of Soft Materials by Low-hysteretic Dissipations

Toughening of soft materials has been long attributed to hysteretic dissipations such as Mullins effect and viscoelasticity, which still suffer from fatigue fracture under multiple cycles of mechanical loads due to the shakedown of hysteretic dissipations. Burgeoning interests are being focused on toughening of soft materials by low-hysteretic dissipations, enabling highly elastic, resilient, and fatigue-resistant properties under cyclic mechanical loads. This talk will discuss one low-hysteretic dissipation mechanism, strain-induced crystallization (SIC), a molecular phenomenon prevalently strengthening and toughening in elastomers. The reported SIC in common elastomers made of randomly crosslinked polymer networks, such as natural rubber, is typically below 20%. We recently discovered a new class of ideal-network elastomers made of end-linked ideal polymer networks, which can achieve a SIC of up to 50%. The ideal-network elastomers exhibit a high work to rupture of 6.3-24.6 MJ m^-3 and fracture energy of 4.2-4.5 kJ m^-2, while maintaining low bulk hysteresis of 0.05. We further investigate the impact of dangling-chain defects on SIC in such ideal-network elastomers, which show two competing effects: (i) the effective increase of polymer chain length and stretchability due to the presence of defects, and (ii) the nonuniform deformation and local stress concentration due to the random distribution of defects that leads to reduced stretchability and SIC. This work will not only elucidate the low-hysteresis toughening mechanism adopted in load-bearing biological tissues but also facilitate healthcare and sustainability challenges associated with robustness and longevity of soft materials.