Session E04: Polymers Under Dynamic Environmental Conditions

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Exposing materials to ionizing radiation is a reliable means of sterilization and is a common approach taken in accelerated aging experiments. Initial atomistic-level radiation damage in chemically reactive materials such as silicones is thought to induce a sequence of network-altering events that lead to undesirable macroscale degradation, including permanent set and mechanical failure. We develop a multiscale approach based on semiempirical quantum molecular dynamics (QMD) to predict and analyze primary radiation damage in polydimethylsiloxane (PDMS). Large ensembles of QMD simulations are used to predict the initial reaction cascades that follow from primary knock-on atom radiation events. A graph-based analysis is developed to automatically identify changes to the backbone structure and quantify mechanically relevant network alterations including formation of junction points and chain scissions. Distinguishing characteristics of radiation coupling to different parts of the PDMS backbone are explored.   

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We use all-atom molecular dynamics simulations to characterize the response of polystyrene (PS) and polyvinyl nitrate (PVN) to shock loading. MD simulations on PVN using the reactive force field ReaxFF predict volume expanding exothermic chemistry above a threshold shock strength and NO₂ dissociation as the main initiation mechanism. These simulations enable a direct comparison against ultrafast spectroscopy experiments on laser-driven shocked samples. We find that a widely-used ReaxFF parameterization is in excellent agreement with experiments in both the threshold strength for fast chemistry and the associated timescales. Non-reactive simulations on PS characterize the effect of shock loading on chain relation dynamics and non-elastic deformation.   

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3D printed components typically show poorer mechanical strength and thereby increased potential for material damage during fabrication and use. Here, we incorporate self-healing properties towards extending the lifetimes of 3D printed polymeric objects. Inspired by biological self-healing, in which a damage event triggers an autonomic healing response, microcapsules containing healing agents are embedded within the 3D printed composites during fabrication. During a damage event these microcapsules rupture, release the healing agent, and heal the surrounding material by polymerization, entanglement, and/or cross-linking. Microcapsules containing healing agents are either incorporated into the host polymer matrix or are coated onto polymer filaments to create 3D printed objects. Microcapsule distribution within composites is visualized using X-ray Nano-CT imaging. Self-healing properties are evaluated via examination of the healing efficiency and mechanical strength of the 3D printed objects. Overall, these are promising approaches to the inclusion of self-healing behavior in 3D printed composites.   

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We studied the effect of chain extender structure and composition on the thermomechanical properties of liquid crystal elastomers (LCE). The molecular stiffness of the thiol chain extender and its relative molar ratio to acrylate-based host mesogens determine the thermomechanical strains, transition temperatures and the mechanical work-content. Higher concentrations of flexible extenders first magnify the thermomechanical sensitivity, but a continued increase leads to weaker actuation. This study leads to a composite material platform that achieves a peak specific work of ~2 J/kg using ~115 mW of electrical power. Composites of LCE with eGaIn liquid metal (LM) are prepared, which act both as heaters and actuators. The thermomechanically active electrodes actuate by Joule heat and efficiently couple with the neat LCE to which they are bound. This system harnesses the nascent responsiveness of the LCE using collaborative electrodes. Specific work generated by the composite actuators increases with load until ~260x their weight. These ideas are extended to tri-layered actuators, where LCE films with orthogonal molecular orientations sandwich LCE-LM composite heaters, harnessing torsional actuation modes.   

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Knots are intriguing topological objects and have been the focus of considerable study in the context of polymer physics. The effect of knot formation on diffusion of DNA confined in nanochannels in particular remains an open question. Two competing factors are expected to affect the change in friction of the confined DNA due to the presence of knots. Under the de Gennes regime in nanochannel confinement, the DNA is a non-draining object whose friction is proportional to its size, thus the formation of knots decreases the friction. The counteracting effect is the increased friction between the DNA and channel surface. Here, we present experimental data on the diffusion of T4 DNA before and after knot formation via a combination of a nanofluidic “knot factory” device for knot generation and fluorescence microscopy for DNA observation. We will discuss the measured diffusivity of unknotted and knotted T4 DNA molecules which will address this question, which competing effects dominates the knotted DNA diffusion in nanochannels.   

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Semiflexible polymers are ubiquitous in biological systems, e.g., in the form of double-stranded DNA or as building blocks of the cytoskeleton, and they also play an important role for various materials due to their ability to form liquid-crystalline order. We found that binary mixtures of semiflexible polymers with the same chain length but different persistence lengths separate into two coexisting different nematic phases when the osmotic pressure of the lyotropic solution is varied. Molecular Dynamics simulations and Density Functional Theory predict phase diagrams either with a triple point, where the isotropic phase coexists with two nematic phases, or a critical point of unmixing within the nematic mixture. The difference in locally preferred bond angles between the constituents drives this unmixing without any attractive interactions between monomers.   

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The delivery of a polymer chain from the chamber of origin to the destination through a nano-scale pore (nanopore) is called polymer translocation. This process has a crucial role in many biological processes, for example, virus injection and drug delivery, and it has been studied for more than three decades. However, before the chain can travel through the pore, it must first find the entrance of the nanopore and insert. This process is called polymer capture and has a significant impact on the conformation of the translocation. It has been observed that during the capture process, the polymer can form folded shapes called hairpins. From our molecular dynamics-lattice Boltzmann simulations, we observed that the presence of hydrodynamic flow not only facilitates the finding process but also can result in the extension of the chain and unravelling of the hairpins. This phenomenon, which we call the pulley effect, stems from the different velocity of different strands of the folded chain.   

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We have studied the dynamics of a homogeneously charged flexible polymer in an environment where enzymes and counterions affect the dynamics of a polymer. An enzyme, in its bound state, exerts an electrostatic force on a monomer, and eventually, it gets unbound due to thermal fluctuations. This binding and unbinding process generates active fluctuations to the dynamics of the polymer. We have analytically investigated how these active fluctuations affect the coupled dynamics of the polymer and counterions. It turns out that these active fluctuations enhance the effective diffusivity of the polymer. The derived closed-form expression for diffusivity is pertinent to accurate interpretation of light scattering data on multi-component systems with binding-unbinding equilibria. The present charged macromolecular system defines a new class of active matter.   

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Shape memory polymers (SMPs) are a class of smart materials that have been extensively used in a variety of biomedical applications. Typically, the shape changes in SMPs are through phase transformation that is triggered by an external stimulus. SMPs that use a non-invasive trigger, like magnetic field, are therefore desirable in biocompatible stents, implants, etc. Here, we simulate the shape memory behavior of binary polymer blends with magnetic nanoparticle (NP) fillers and investigate the role of magnetic field on polymer-polymer-NP compatibility, which inturn affects its shape memory performance. Our approach harnesses Flory-Huggins theory of mixing for polymer interactions, combined with free energy formulation of spherical NPs. In particular, we construct the phase diagram of the SMP blends and determine the optimized SMP characteristics as a function of NP concentration and magnetic field strength. We demonstrate that the shape fixity and recovery ratios can be tailored for the existing SMP blends by tuning the composition of NPs. Our findings can be used to design and predict the characteristics of SMP blends to be used as biomechanical devices.   

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Many biopolymers exhibit reversible conformational transitions within the chain, which affect their bending stiffness and their response to a stretching force (e.g. ds-DNA with denatured bubbles or bundles with reversible cross-links). The simplest theoretical model which captures what such systems have in common is a freely jointed chain (FJC) with reversible hinges. Each hinge can be open, as in the usual FJC or closed, forcing the linked segments to align. This model was analysed in the Helmholtz ensemble by Glatting et al. (Colloid Polym Sci, 1995). In this talk, we analyse it in the Gibbs ensemble. Remarkably, the reversible FJC exhibits ensemble inequivalence. A mean field treatment suggests a continuous phase transition to a fully closed state at a certain force, but the generating function method ("necklace model") shows that there is no phase transition. However, there is a crossover between two states with clearly different response. At the low force (linear response) regime, the reversible FJC has higher tensile compliance than its usual counterpart. On the contrary, at the strong force regime, the tensile compliance of the reversible FJC is much lower than that of the usual FJC.   

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Electrospray deposition (ESD) provides a versatile means of fabricating polymer thin films. The morphology and properties of such films depend on kinetic processes that occur during the deposition. Such deposition occurs in a dynamic environment by sequential buildup of polymer chains that relax and diffuse after delivery to a heated substrate from charged droplets of initially dilute solution. We examine the deposition kinetics by observing the evolution of surface coverage with time during ESD of a homopolymer and a block copolymer. The kinetics are modeled using a 1^st order adsorption equation with a term, S, that incorporates the effect of droplet spreading over time. The kinetics depend strongly on molar mass, M, with S ~ 1/M². We interpret S in terms of an effective polymer diffusivity, and find that the temperature dependence is well-described using the WLF equation with an unmodified T_g. These results indicate that the mobility of the polymer is important in determining the coverage kinetics and are consistent with a picture in which substrate coverage evolves by the delivery of material to the substrate from the reservoirs of already deposited droplets as a rate limiting step, alongside ongoing arrival of new material from the ESD feed. These results are corroborated by a stochastic random deposition simulation that further elucidates the relevance of diffusion in describing the coverage kinetics of films formed by ESD.