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 J04: Dynamic Polymer Networks IIFocus Live
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Sponsoring Units: DPOLY DSOFT Chair: Christopher Evans, University of Illinois at Urbana-Champaign |
Tuesday, March 16, 2021 3:00PM - 3:12PM Live |
J04.00001: Molecular Control over Vitrimer-like Mechanics – Tuneable Dynamic Motifs based on the Hammett Equation in Polyimine Materials Sybren Schoustra, Han Zuilhof, Joshua Dijksman, Maarten Smulders We report the quantitative control over macroscopic dynamic material properties using the Hammett equation in dynamic (imine-based) polymer networks. Via this established physical-organic principle, operating on the molecular level, one can fine-tune and control dynamic material properties on the macroscopic level, by systematic variation of dynamic (covalent) bond dynamics through selection of the appropriate substituent of the aromatic imine building blocks. |
Tuesday, March 16, 2021 3:12PM - 3:24PM Live |
J04.00002: Rheology of Vitrimers: A Hybrid Molecular Dynamics-Monte Carlo Simulation Study Alessandro Perego, Fardin Khabaz Thermoset polymers are classified amongst the most challenging materials to recycle due to the introduction of covalent intermolecular crosslinks that aims to increase strength and stiffness compared to their thermoplastic counterparts. A new class of polymers called vitrimers presents a promising route to achieve recyclability in thermosets by implementing dynamic covalent bonds within the system. In this work, coarse-grained molecular dynamics in conjunction with a Monte Carlo method, will be used as a simulation methodology to model the bond exchanges in these networks. This hybrid technique can detect the glass transition temperature and the topology freezing temperature from both the volumetric and rheological data. The limited frequency range obtained from the rheology simulations will be extended from three to ten orders of magnitude with the aid of the time-temperature superposition principle and will allow capturing the terminal regime of the moduli at low frequencies characteristic of these systems. The temperature dependence of the zero-shear viscosity of the model will be determined and will be linked with the relaxation time of the network. This simulation methodology provides a theoretical basis for the rational design of the thermophysical properties of vitrimers. |
Tuesday, March 16, 2021 3:24PM - 3:36PM Live |
J04.00003: Polybenzoxazine vitrimer from cardanol based on exchangeable disulfide bonds: reprocessing, recycling, and self-healing and reversible adhesion Acerina Trejo Machin, Laura Puchot, Pierre Verge Polybenzoxazines (PBZ) are a relatively new class of thermosets with outstanding properties and exceptional molecular design flexibility making possible the use of renewable resources for their synthesis.1,2 However, similarly to other thermosets such as phenolic and epoxy resins they cannot be recycled and reprocessed. To address this issue, this work details how exchangeable disulfide bonds can be introduced in the chemical structure of a bio-based PBZ network. To this aim, 4-aminophenyl disulfide was reacted with cardanol, a bio-based phenolic compound, to produce the first vitrimer based on PBZ.3 Thanks to the disulfide bond, the novel PBZ was recycled, reshaped, and reprocessed. Interestingly, the material exhibits a fast stress relaxation (18 s. at 120 °C) with low activation energy (64.5 kJ/mol). One of the most amazing features of the material is its reversible and self-healable adhesive properties, showing a straightforward reparation and re-use under mild conditions. This presentation will describe the synthesis and the properties of this new vitrimer. |
Tuesday, March 16, 2021 3:36PM - 3:48PM Live |
J04.00004: Unentangled Vitrimer Melts: Generalized Rouse Theory Reveals Impact of Cross-link and Backbone Chemistry on Linear Viscoelasticity RALM RICARTE, Sachin Shanbhag Vitrimers are polymer networks that have covalent cross-links which preserve network connectivity but permit topology fluctuations. We employ a generalized inhomogeneous Rouse model (IHR) to study the rheology of unentangled vitrimer melts. We observe that vitrimers with uniform and random cross-link distributions exhibit larger viscosities and relaxation times than gradient and blocky types. Polydimethylsiloxane vitrimer (which has a flexible backbone) shows an Arrhenius temperature dependence for viscosity, while polystyrene and poly(methyl methacrylate) vitrimers (which have rigid backbones) are only Arrhenius at high temperatures. During stress relaxation, the short time dynamics represent monomer friction, while the long time dynamics encompass a combination of network strand relaxation and cross-link exchange. Because of the different temperature dependences of the processes, time-temperature superposition fails. The effective rheological activation energy can be estimated a priori from the cross-link exchange activation energy and the backbone Williams-Landel-Ferry parameters. Based on these findings, we discuss the utility of the IHR for understanding vitrimer rheology, and best practices for characterizing the viscosity and relaxation time. |
Tuesday, March 16, 2021 3:48PM - 4:00PM Live |
J04.00005: Viscoelastic properties of PDMS vitrimers over a 200C window Laura Porath, Christopher Evans We have prepared vitrimers of poly(dimethyl siloxane) with tunable crosslink density of dynamic boronic ester bonds to investigate the viscoelastic properties of dynamic networks with extremely low Tg. Both frequency sweeps and stress relaxation tests show the anticipated Arrhenius behavior of relaxation time with inverse temperature. Frequency domain data show an increase in relaxation times with increased crosslink density at a fixed temperature. Time-temperature superposition of the frequency sweeps demonstrates that the flow regime is thermorheologically simple, while the modulus of the plateau regime increases with increasing temperature, consistent with a preserved network topology. As stress relaxation times decrease, the rubbery plateau modulus increases, which indicates decoupling dynamics from mechanics. Relaxation times deduced from both frequency sweeps and stress relaxation data were compared, and all give the same activation energy within error. For data taken below 40°C, stress relaxation tests show the appearance of a second Arrhenius regime with lower activation energy, which emphasizes the importance of taking measurements over a broad window. |
Tuesday, March 16, 2021 4:00PM - 4:12PM Live |
J04.00006: Control of Crystallinity and Deformation in Polymer Networks Using Thiol-Thioester Dynamic Chemistry Alexa Kuenstler, Alina Martinez, Christopher Bowman Covalent adaptable networks (CANs) leverage exchangeable bonds to enable reconfigurability of cross-linked polymer networks. While several CANs systems have been developed containing a myriad of materials properties and capabilities ranging from soft dynamic cell scaffolds to rigid shape memory devices, new platforms are needed to exploit bond exchange to control material behavior. Towards this goal, we incorporated exchangeable thioester bonds into semi-crystalline thiol-ene networks to form tough and strong thermoplastics that can be dynamically reconfigured in the presence of catalytic nucleophiles via thiol-thioester exchange. By tailoring network composition and catalyst strength, we explore the interplay between bond exchange and crystallinity and how these processes can be used to program anisotropic crystallization under applied stress. Furthermore, we use these insights to construct shape-memory materials by leveraging directed crystallization and melting to drive controlled deformation. Finally, we show that the incorporation of photolatent catalysts affords spatiotemporal control over exchange and crystallization dynamics and enables hands-free manipulation and actuation. |
Tuesday, March 16, 2021 4:12PM - 4:24PM Live |
J04.00007: Instant-healing double-network polydimethylsiloxane with low stiffness and high toughness Chao Chen, Huafeng Fei, James J Watkins, Alfred Crosby The double-network mechanism is a key advance that has led to soft and tough materials. Solvent-swollen double-network materials containing a dynamic network have exhibited additional fast healing functionality. However, achieving similar performance in solvent-free materials is a challenge. This work develops a tough and soft double-network polydimethylsiloxane (DN-PDMS) material that instantly heals in seconds. The DN-PDMS comprises a bottlebrush network and a dynamic borate PDMS. The DN-PDMS with an equal weight of each network demonstrates high fracture toughness of 3.8kJ/m2, high failure strain of 15, and flaw-insensitivity to a centimeter-long notch. When the material is cut and brought back in contact, 40% of the initial toughness and 50% of the initial failure strain are recovered in ten seconds at room temperature. The dynamic mechanical analysis demonstrates fast kinetics of physical interactions in the timescale of seconds, enabling fast healing. We anticipate that this development paves a path to designing soft materials that can bear repeated mechanical loads. |
Tuesday, March 16, 2021 4:24PM - 4:36PM Live |
J04.00008: Linking Molecular Behavior to Macroscopic Properties in Ideal Dynamic Covalent Networks Bruno Marco Dufort, Ramon Iten, Mark W Tibbitt Dynamic covalent networks (DCvNs) are increasingly used as advanced materials with applications ranging from recyclable thermosets to self-healing hydrogels. However, the relationship between the chemistry at the junctions of DCvNs and their macroscopic properties is poorly understood. Here, we present a framework to predict how complex network behavior in DCvNs emerges from the chemical landscape of the dynamic chemistry at the junction [1]. Ideal boronic ester-based hydrogels were used as model DCvNs. We systematically explored the effect of the chemical environment on network behavior, as the complex properties of boronic ester-based DCvNs are regulated by pH and temperature [2]. We developed physical models that describe how viscoelasticity is linked to the molecular behavior of the dynamic junction, quantified via fluorescence and NMR spectroscopy and DFT calculations. Additionally, shear rheometry was used to quantify the kinetics and thermodynamics of network rearrangements, enabling a mechanistic understanding of the junction. These findings, grounded in molecular principles, advance our understanding and rational design of dynamic polymer networks. |
Tuesday, March 16, 2021 4:36PM - 4:48PM Live |
J04.00009: Tailoring the Rheology of Dynamic Covalent Composites with Particle Size and Bond Lifetime Grayson Jackson, Joseph M Dennis, Neil Dolinski, Stuart Rowan, Heinrich Jaeger The mechanical properties of dynamic polymer networks are known to depend on the time-average crosslink density, bond lifetime, and donor/acceptor stoichiometric balance. However, it is unclear how these factors map to composites where particles dispersed in a polymer melt can form dynamic bonds at the filler-matrix interface. Here, we use these dynamic covalent composites comprising thiol-coated silica particles and ditopic benzalcyanoacetamide-based Michael acceptors to understand how their steady and oscillatory shear response depends on particle size. While larger particles form flowable composites which exhibit negative thixotropy, decreasing particle size suppresses shear hysteresis and leads to solid-like samples due to an increased dynamic crosslink density. Utilizing dynamic bond lifetimes measured for small molecule analogs, we also explore how the electron-donating/withdrawing nature of the thia-Michael acceptor influences stress relaxation in these dynamic composite networks. We anticipate these insights will help integrate dynamic covalent chemistries and polymer composites into stress-responsive smart materials. |
Tuesday, March 16, 2021 4:48PM - 5:00PM Live |
J04.00010: Stress relaxation in tunable hybrid (chemical+physical) hydrogels Chiara Raffaelli, Wouter Ellenbroek Hydrogels are great for biomedical applications because they are easily made biocompatible and, like the tissues they need to interact with, consist largely of water. Tight control over their mechanical properties promises great improvement for their usability in these applications, but a detailed understanding of how properties follow from molecular architecture is lacking. Here, we present a computational study of a highly designable class of gels: 4-arm star polymers that are held together by a combination of chemical (permanent) and physical (reversible) crosslinks. In these materials, the permanent bonds provide long-term integrity while the reversible bonds provide switchability of mechanical properties. Using coarse-grained molecular dynamics simulations, we determine at which ratio of permanent to reversible crosslinks we can achieve a gel that is tunable between fluid and solid behavior, and analyze the effect of the reversible crosslinks on the long-term stress relaxation properties of the resulting materials. |
Tuesday, March 16, 2021 5:00PM - 5:12PM Live |
J04.00011: Proline isomerization regulates the phase behavior of elastin-like polypeptides in water Yani Zhao, Kurt Kremer Responsiveness of proteins and polymers in aqueous solutions plays an important role in biomedical applications and in designing the advanced functional materials. A well-known class of synthetic intrinsically disordered proteins (IDPs) is that of elastin-like polypeptides (ELPs), which exhibits a lower critical solution temperature (LCST) behavior in pure water. Here, we study and compare the influence of the cis/trans proline isomerization on the phase behavior of ELPs in dilute aqueous solution. Our results reveal that cis isomers play an important role in tuning the phase behavior of ELPs by hindering the peptide-water hydrogen bonding while promoting intramolecular interactions. In particular, an ELP with all proline residues in cis state remains collapsed without any noticeable LCST-like behavior as temperature increases, while showing a first-order-like LCST transition if all proline residues are in trans state. |
Tuesday, March 16, 2021 5:12PM - 5:24PM Live |
J04.00012: Entropy-controlled cross-linking in linker-mediated vitrimers Xiuyang XIA, Qun-li Lei, Juan Yang, Massimo Pica Ciamarra, Ran Ni Recently developed linker-mediated vitrimers based on metathesis of dioxaborolanes with various commercially available polymers have show good both processability and outstanding performance, suggesting new ways of processing cross-linked polymers in industry, of which the design principle remains unknown [Science 356, 62–65 (2017)]. Here we formulate a theoretical framework to elucidate the phase behavior of the linker-mediated vitrimers, in which entropy plays a governing role. We find that, with increasing the linker concentration, vitrimers undergo a reentrant gel–sol transition, which explains a recent experiment [Macromolecules 53, 1180–1190 (2020)]. More intriguingly, at the low temperature limit, the linker concentration still determines the cross-linking degree of the vitrimers, which originates from the competition between the conformational entropy of polymers and the translational entropy of linkers. Our theoretical predictions offer guidelines in understanding and controlling the properties of this newly developed vitrimer system. |
Tuesday, March 16, 2021 5:24PM - 5:36PM Live |
J04.00013: Effects of salt addition on viscoelasticity in conductive dynamic networks Brian Jing, Patricia Mata, Christopher Evans We have investigated a series of dynamic networks made from precise ethylene glycols (2, 3 or 4 repeat units), boric acid, and added Li salt. The resultant boronic ester crosslinks are typically associative dynamic covalent bonds, but can coordinate with anions in the presence of salt which alters the topology and rheology of the networks. We studied a range of linker lengths and Li:ethylene oxide (Li:EO) ratios to understand the effect of these parameters on conductivity and viscoelasticity. The storage modulus decreases from 10 to 0.5 MPa at room temperature with added salt, contrary to linear PEO, attributed to boron-anion interactions which decrease the number of elastically active boron centers and lead to a non-Arrhenius temperature dependence of stress relaxation times. The crossover of G’ and G” drops from 100 C (at 1 Hz) in a neutral system to 15 C with added salt. Conductivities up to 3 x 10-4 S/cm were measured at 90 C and go through a maximum which is attributed to the competition of added salt and increasing Tg. By tuning the linker length of the networks from 2 to 3 to 4 repeat units, a systematic increase in conductivity and decrease in modulus is observed due to mesh size and Tg effects. |
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