Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session A04: Dynamic Polymer Networks IFocus Live
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Sponsoring Units: DPOLY DSOFT Chair: Svetlana Sukhishvili, Texas A&M University |
Monday, March 15, 2021 8:00AM - 8:12AM Live |
A04.00001: Phase-Separation, Gelation, and Dynamics of Associative Polymers Scott Danielsen, Michael Rubinstein An equilibrium theory for reversible network formation in two-component solutions of associative polymers is presented to account for the phase behavior and 'sticky' dynamics due to hydrogen bonding, metal–ligand, electrostatic, or other pairwise associative interactions. We consider polymers of types A and B with many associating groups per chain and consider only A–B association between the groups. A simple analytical expression for the free energy is derived and is shown to be consistent with the classical Flory–Stockmayer gelation theory. It is shown that association and formation of a reversible network is always accompanied by a tendency for phase separation, even at good solvent conditions, a significant difference from self-associative polymers. Homogeneous networks are most easily stabilized near stoichiometric conditions between A and B associative groups, resulting in a sol–gel–sol transition as the overall composition is altered. Chemical incompatability between the A and B polymers drives a competition between attractively and repulsively-driven phase separation, leading to microphase formation and eutectic behavior. The associative interactions slow molecular relaxations leading to a more general description of the sticky rouse and reptation models. |
Monday, March 15, 2021 8:12AM - 8:24AM Live |
A04.00002: Experimental test of the bond lifetime renormalization model in telechelic associating polymers SIRUI GE, Martin Tress, Subarna Samanta, Kunyue Xing, Peng-Fei Cao, Tomonori Saito, Alexei Sokolov Associating polymers with transient (dynamic) bonds demonstrate unique viscoelastic properties in comparison to the conventional polymers. The mechanisms controlling dynamics of associating polymers is rather complex and still not well understood. Transient bonds introduce additional relaxation times into the system. Recent discoveries of strong difference between transient bond lifetimes and the mechanical relaxation times in associating polymers challenged the classical theories. The bond lifetime renormalization model offers an interpretation of this effect. In order to provide detailed test of this model, we study telechelic associating polymers with different molecular weights and strong variations in strength of associations of the end groups. Using oscillatory shear rheology and dielectric spectroscopy, we were able to measure mechanical relaxations and characteristic relaxation times of bond dissociations. Detailed analysis demonstrates reasonable agreement of the experimental data with the predictions of the bond lifetime renormalization model on a semi-quantitative level. Surprisingly, our results suggest a kind of entangled dynamics even for telechelic associating polymers with molecular weight significantly smaller than the entanglement molecular weight. |
Monday, March 15, 2021 8:24AM - 8:36AM Live |
A04.00003: New model for physical bond breaking in associating copolymer liquids Ashesh Ghosh, Kenneth Schweizer We combine ideas from polymer and glass physics to construct a new model for the mean bond breaking time scale of attractive sticker groups in associating copolymer liquids that form transient networks. The activated bond breaking event is argued to be a two-step process. The first step defines the primary alpha relaxation and involves the release of non-sticker dynamic caging constraints which can be strongly modified by long-lived physical crosslinks, for which a microscopic theory has been recently developed (Macromolecules 53, 4366 (2020)). The slower bond breaking process involves attractive stickers surmounting an association free energy barrier subject to a local frictional resistance which can be strongly affected by relaxation-diffusion decoupling induced by locally heterogeneous dynamics. The latter is strongly polymer chemistry specific, and related to the fragility of the alpha process. The ideas embedded in the model produce a consistent and good description of the bond breaking timescale for diverse polymer chemistries and architectures as a function of both temperature and fraction of sticky groups. Chemically sensible values for association free energies are deduced. In strong contrast, the existing phenomenological models are shown to incur qualitative failures. |
Monday, March 15, 2021 8:36AM - 8:48AM Live |
A04.00004: Mechanical Reinforcement by Interfacial Layer in Phase-separating Associating Telechelic Polymers Subarna Samanta, SIRUI GE, Martin Tress, Bingrui Li, Kunyue Xing, Peng-Fei Cao, Alexei Sokolov Associating polymers capable of forming reversible bonds exhibit macroscopic properties (e.g. unique viscoelastic behavior, self-healing capability etc.), which are governed by their microstructure. Here, we investigate the structure and dynamics of short telechelic PDMS polymers with H-bonding end groups. Small angle X-ray scattering measurements indicate phase separation (clustering of functional end groups) due to the polarity difference between polymer backbone and sticky end groups. The presence of the clusters significantly alters the viscoelastic behavior of these materials, resulting in extremely long rubbery plateau. Using dielectric spectroscopy and an approach developed earlier for polymer nanocomposite, we estimate the thickness of an interfacial polymer layer around these clusters. The dynamics of this interfacial layer is almost an order of magnitude slower than segmental dynamics of bulk-like polymer. Using the interfacial layer model (ILM) analysis, we find that shear modulus of this interfacial polymer layer is ~100 MPa. Our analysis reveals that phase-separating telechelic associating polymers are quite similar to polymer nanocomposites, and interfacial layer plays a critical role in their mechanical and viscoelastic properties. |
Monday, March 15, 2021 8:48AM - 9:00AM Live |
A04.00005: Temperature Response of Endo/Exo Crosslinking in Diels-Alder Polymer Networks Qing Zhou, Zhen Sang, Kartik Kumar Rajagopalan, Frank Gardea, Svetlana Sukhishvili We will discuss thermodynamic and kinetic aspects of endo vs. exo isomeric attachments in Diels-Alder (DA) polymer networks. Endo and exo isomer have different dissociation temperatures and the ratio between them plays a crucial role in harnessing DA network’s thermomechanical behaviors. Unlike in solution, where the ratio between endo and exo can be easily adjusted by reaction temperature, the DA reaction in a polymeric matrix is carried out during the cooling process in which the equilibrium of DA reaction sweeps from dissociation to association. DSC analysis showed that reaction temperature had limited impact on the endo/exo ratio in the final crosslinked networks. Instead, the crosslinking density significantly affected the endo/exo ratio, increasing the number of endo attachments. In addition, activation energies, enthalpy, and entropy of the DA reaction were estimated for different DA polymer networks. The results suggest the steric restrictions at the crosslink points and the entropy of crosslinkers play a major role in controlling the thermal response of DA polymers. For instance, when a more flexible crosslinker was used, the dissociation temperature increased from 115 oC to 128 oC. These findings enable rational tailoring of thermomechanical properties of DA networks. |
Monday, March 15, 2021 9:00AM - 9:12AM Live |
A04.00006: Dialkylamino Disulfide Chemistry: A Simple, Robust Catalyst-Free Chemistry for Dynamic Covalent Polymer Networks Exhibiting Full Cross-Link Density Recovery after Recycling and Excellent Creep Resistance at Elevated Temperature. Mohammed Bin Rusayyis, John Torkelson Conventionally cross-linked polymer networks known as thermosets contain permanent cross-links which prevent their recyclability, leading to major environmental issues. To overcome this problem, dynamic covalent polymer networks (DCPNs) containing dynamic covalent bonds have been explored over the past two decades. Because of their dynamic nature, DCPNs are capable of undergoing reversible or exchange reactions rendering them reprocessable, offering a sustainable alternative to thermosets. While many dynamic chemistries have been explored and employed in the synthesis of DCPNs, most of the very few reported DCPNs that exhibit full cross-link density recovery after recycling were prepared via step growth polymerization, and no study has reported full cross-link density of DCPNs prepared exclusively from vinyl monomers. Here, we present a simple one-step method to design a bifunctional bis(dialkylamino) disulfide cross-linker that can be used in the synthesis of DCPNs from any monomers or polymers that can undergo free radical polymerization without need for external catalyst. We demonstrate the robustness and reprocessability of the networks, including the full recovery of cross-link density after multiple recycling steps and excellent creep resistance at elevated temperatures. |
Monday, March 15, 2021 9:12AM - 9:24AM Live |
A04.00007: Filament Nucleation Tunes Mechanical Memory in Active Polymer Networks Vikrant Yadav, Deb Sankar Banerjee, Alan Tabatabai, David R Kovar, Taeyoon Kim, Shiladitya Banerjee, Michael Murrell Incorporating growth into contemporary material functionality presents a grand challenge in materials design. The F-actin cytoskeleton is an active polymer network which serves as the scaffolding for eukaryotic cells, growing and remodeling in order to determine changes in cell shape. Nucleated from the membrane, filaments polymerize and grow into a dense network whose dynamics of assembly and disassembly, or ‘turnover’, coordinates both fluidity and rigidity. Here, we vary the extent of F-actin nucleation from a membrane surface in a biomimetic model of the cytoskeleton. We find that nucleation of F-actin mediates the accumulation and dissipation of polymerization-induced F-actin bending energy. At high and low nucleation, bending energies are low and easily relaxed yielding an isotropic material. However, at an intermediate critical nucleation, stresses are not relaxed by turnover and the internal energy accumulates 100-fold. In this case, high filament curvatures template further assembly of F-actin, driving the formation and stabilization of vortex-like topological defects. Thus, nucleation coordinates mechanical and chemical timescales to encode shape memory into active materials. |
Monday, March 15, 2021 9:24AM - 9:36AM Live |
A04.00008: Mechanically adaptive conducting polymers Yue (Jessica) Wang We report the development of mechanically adaptive conducting polymers—a new class of materials with toughness that dynamically scales with the rate of deformation. For example, as the rate of stretching increased from 2.5 to 10000 %/min, the ultimate elongation of this material becomes 7 times higher and the toughness increases by over 100 folds. This unusual rate property is achieved by creating dynamic bonds of different strengths within the polymer microstructure. The stronger dynamic bonds maintain material integrity, whereas the clusters of weaker dynamic bonds serve as a sacrificial network and dissociate at a rate that directly scales with the loading rate. In-situ small and wide angle X-ray scattering (SAXS and WAXS, respectively) studies reveal that when the rate of loading exceeds a critical strain rate, deformation through the sacrificial network becomes more favorable than the classical viscoelastic pathway, which led the polymer to defy viscoelasticity and exhibit mechanical adaptiveness. This mechanism can potentially lead to soft thermoplastic materials that are more durable under high impact conditions, increasing the reliability and lifetime of the resulting devices. |
Monday, March 15, 2021 9:36AM - 9:48AM Live |
A04.00009: Mesoscale modeling of controlled degradation and erosion in hydrogel films Vaibhav Palkar, Chandan Kumar Choudhury, Olga Kuksenok Controlled degradation in hydrogel materials finds numerous applications from controlled drug release to sequential release of encapsulated cells for tissue engineering. While several chemical pathways can be used to introduce controlled degradation in hydrogels, the physical processes that occur during degradation are not well understood. Better understanding of the physics behind the polymer degradation process is essential to further develop applications. Herein, we present a Dissipative Particle Dynamics (DPD) based simulation approach for modeling controlled degradation and erosion in hydrogel films. Our recently developed framework allows us to model controlled degradation in films with first order kinetics for the degradation reaction. We track the evolution of the weight average degree of polymerization and occurrence of network disintegration (reverse gelation) in our simulations. We focus on the crossover between the surface and bulk erosion in hydrogel films. Our simulations help us to analyze impact of parameters such as crosslink density, system size and stoichiometric ratio of degradable bonds. |
Monday, March 15, 2021 9:48AM - 10:00AM Live |
A04.00010: Supramolecular Cross-linked Hydrogels: Similarities and Differences with Chemically Cross-linked Networks Sandrine Laquerbe, Julien Es Sayed, Cédric Lorthioir, Christophe Meyer, Tetsuharu Narita, Guylaine Ducouret, Patrick Perrin, Nicolas Sanson Over the past decades, hydrogels have been widely studied due to their numerous applications especially in medical sciences. Depending on the application, the network of water-soluble polymer chains can be chemically or physically cross-linked by using permanent or transient bonds respectively. This diversity of cross-linking junctions offers a great versatility to synthesized hydrogels. In this scope, we developed a supramolecular cross-linker based on coordination chemistry that can be integrated in a stimuli-responsive gel through a one-pot synthesis. Supramolecular gels show great similarities with the structure of a chemically cross-linked network build with the same monomer but exhibit a different swelling behavior and an ability to undergo a gel-to-sol transition when an external stimulus is applied. This stimuli-responsiveness gives rise to the controlled tuning of the overall cross-link density and so of the network macroscopic properties. It also gives interesting insights on the accessible range of properties of dual gels that can be synthesized with both chemical and supramolecular cross-linkers. |
Monday, March 15, 2021 10:00AM - 10:12AM Live |
A04.00011: Dynamic Covalent Networks As Polymer Electrolyte Adhesives Ryo Kato, Priyadarshini Mirmira, Stuart Rowan, Shrayesh Patel Managing interfacial contact and charge transfer are inherent challenges when using solid electrolytes for all-solid-state electrochemical systems. Here, crosslinked polymer electrolytes containing structurally dynamic disulfide bonds have been synthesized to investigate their combined ion transport and adhesive properties. Dynamic network polymers of varying cross-link densities are synthesized via thiol oxidation. At optimal loading of LiTFSI salt, the ionic conductivity (σ) at 90 ° C is ca. 10−4 S/cm. Notably, in comparison to the equivalent nondynamic network, the dynamic network shows a positive deviation in σ above 90 °C, which suggests the enhancement of ion transport occurs from the difference in structural relaxation due to dissociation of disulfide bonds. Lap shear adhesion and conductivity tests on ITO substrates reveal the dynamic network exhibits a higher adhesive shear strength of 0.2 MPa (vs 0.03 MPa for the nondynamic network) and higher σ after the application of external stimulus (UV light or heat). The adhesive strength and σ are stable over multiple debonding/rebonding cycles and, thus, demonstrating the utility of these materials as polymer electrolyte adhesives. |
Monday, March 15, 2021 10:12AM - 10:24AM Live |
A04.00012: Melt-Recyclable Shape-Memory Elastomers Containing Bisurea Segments Daniel Krajovic, Mitchell Anthamatten Shape-memory (SM) elastomers can undergo triggered actuation from metastable deformed states to permanent shapes, offering diverse applications. Covalently crosslinked, semicrystalline SM networks are capable of storing large amounts of elastic energy (>3 MJ/m3) with full recovery; however, these materials cannot be melt-processed, limiting their use across various device geometries. Here, we demonstrate that replacing covalent crosslinks with bisurea H-bond interactions can enable fully melt-recyclable SM elastomers with little performance loss. Two high molecular weight poly(ε-caprolactones) with main-chain hydrogen bonding groups have been synthesized and show excellent strain fixation and shape recovery before and after shredding, melt-pressing, and annealing. Dynamic mechanical analysis reveals a stiffness plateau that persists to temperatures over 100°C above the shape-triggering temperature. Stress relaxation studies are combined with X-ray scattering and calorimetry data to explain viscoelastic behavior and evaluate the role of phase segregation. Relaxation kinetics are consistent with chain disentanglement as the primary relaxation mode, and bisurea linkages slow the reptation timescale by over three orders of magnitude. |
Monday, March 15, 2021 10:24AM - 11:00AM Live |
A04.00013: Toward understandig the (scale-depending) dynamics in non-covalent polymer networks Invited Speaker: Annette Schmidt The dynamic, time-dependent properties of non-covalent polymer networks are of crucial impact in many areas of soft matter science, including living tissue and responsive matter. The talk focuses on the structure-property relationships in dynamic polymer networs as investigated on two model systems: |
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