Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session L70: Vitrimers and Associative NetworksFocus
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Sponsoring Units: DPOLY DSOFT Chair: Christopher Evans, University of Illinois at Urbana-Champaign Room: 208 |
Wednesday, March 4, 2020 8:00AM - 8:12AM |
L70.00001: Microscopic Theory of the Role of Strong Attractions on the Local Dynamics and Elasticity of Associating Copolymer Liquids Ashesh Ghosh, Kenneth Schweizer We construct a microscopic liquid state theory for how attractions between sticky groups regularly co-polymerized in a chain backbone affect local structure and dynamics of unentangled polymer liquids. Based on the bare attractive interaction and single-chain structure as input, PRISM integral equation theory is combined with activated dynamics approaches that capture caging and physical bond formation to study emergent high frequency elasticity and local relaxation processes. The dynamic free energies and corresponding sticker and non-sticker barrier hopping timescales that define the coupled bond breakage and cage escape processes are predicted within a 2-step dynamical scenario that applies in the strong attraction regime. The first step involves non-sticker hopping (alpha process) which is perturbed due to physical bonds between sticky segments that act as pinning constraints. After the non-sticker hops, the dynamical constraints and friction experienced by the sticky groups are renormalized and activated bond-breaking defines the second step. We present representative results for structure and dynamics as a function of sticker fraction, strength and range of the attraction, density, and temperature. Connections between dynamics and equilibrium properties are identified. |
Wednesday, March 4, 2020 8:12AM - 8:24AM |
L70.00002: LINEAR VISCOELASTICITY AND FLOW OF SELF-ASSEMBLED VITRIMERS: THE CASE OF A POLYETHYLENE/DIOXABOROLANE SYSTEM Ralm Ricarte, François Tournilhac, Michel Cloître, Ludwik Leibler We investigated the linear viscoelasticity of a polyethylene (PE) vitrimer that has cross-linkable dioxaborolane maleimide grafts. Strong interactions between the PE backbone and grafts cause the molten vitrimer to macro- and microphase separate into hierarchical structures. Small-amplitude oscillatory shear, stress relaxation, and creep measurements were used for characterization. Graft functionalized PE (which does not contain cross-linker, but still self-assembles) had a terminal relaxation time that was two orders of magnitude larger than neat PE. When cross-linker was added to form the vitrimer, the material exhibited a higher melt strength but did not achieve steady-state flow within 8 hr. The soluble graft-poor portion of PE vitrimer had similar flow behavior as neat PE, but its flow activation energy was twice as large. Conversely, the insoluble graft-rich fraction behaved as a viscoelastic solid that relaxed very little over 8 hr. Blend experiments suggested the interface between the graft-rich and graft-poor phases of PE vitrimer also influenced relaxation. These findings indicate that self-assembly/associative cross-linking interplay greatly impacts the rheology and, consequently, processability of vitrimers. |
Wednesday, March 4, 2020 8:24AM - 8:36AM |
L70.00003: Effect of salt on viscoelasticity and conductivity in vitrimers and dynamic networks Brian Jing, Christopher Evans Polymer networks with associative dynamic bonds (vitrimers) have been investigated as recyclable and self-healing materials. We have developed vitrimers made from polyethylene glycol, dynamic boronic ester crosslinks, and added Li salt that function as Li-ion conductors. These networks were studied over a range of Li:ethylene oxide (Li:EO) ratios to understand the effect of salt concentration on conductivity and viscoelasticity. The storage modulus decreases from 10 to 0.5 MPa at room temperature with added salt, attributed to boron-anion interactions which decrease the number of elastically active boron centers (supported by 11B NMR analysis). The temperature dependent moduli also show a crossover of G’ and G” which occurs at ~100 °C (at 1 Hz) in a neutral system and drops by 85 °C with added salt. Conductivities up to 3 x 10-4 S/cm were measured (solvent-free) at 90 °C, and go through a maximum which is attributed to the competition of added salt and increasing Tg. The networks can also be dissolved in water back to monomer and recover conductivity after damage due to the dynamic bonds. Finally, we discuss how these vitrimers transition to dissociative networks with added salt due to anion-boron interactions which lead to non-Arrhenius stress relaxation times. |
Wednesday, March 4, 2020 8:36AM - 8:48AM |
L70.00004: Understanding the Self-healing of Reversible Polymer Networks through Molecular Dynamic Simulation Zhiqiang Shen, HUILIN YE, Ying Li Hydrogels with reversible crosslinking motifs exhibit extraordinary short healing time and high healing strength. However, the understanding about the self-healing behavior of reversible polymer network is limited. Especially, the key problem about relation between the microscopic evolution of polymer networks and the macroscopic stress-strain behaviors during the self-healing process remains largely elusive. In this work, a molecular dynamics / Monte Carlo hybrid coarse-grained model is developed to investigate the relaxation and self-healing behaviors of polymer networks with reversible crosslinking. We systamatically investigate the shelf-healing process of the polymer network with a fracture. It is found that the polymer chains at the fracture interface can diffusive through the interfacial gap and form new crosslinking bonds, resulting the recovery of polymer network mechanical strength. We found that the healing time is both determined by the dwelltime of the dynmaic bond and relaxation of the polymer chains. Our simulations provide a direct relation between dynamic bond and the self-healing process, which might provide an insight for the design of smart self-healing polymers. |
Wednesday, March 4, 2020 8:48AM - 9:00AM |
L70.00005: Role of Dynamic Bonds on Crystallization in Polyethylene Vitrimers Bhaskar Soman, Christopher Evans Crystallization of linear polyethylene (PE) has been extensively investigated for decades. Here, we study the effects of dynamic bonds on the crystallization in PE vitrimers by preparing networks with exactly 8, 10, and 12 carbons (C8-C12) between dynamic boronic ester crosslink points. The C12 networks crystallize immediately upon cooling, while C8 networks only show evidence of crystallization after waiting multiple weeks at 20 °C. The C10 networks show intermediate initial crystallization. Samples were aged at room temperature for 30 days, and in all cases, monotonic increase in melting temperature (Tm) is observed which is attributed to dynamic bonds allowing local rearrangements of network strands in the amorphous regions or at the crystal-amorphous interface. A control sample of linear PE shows instant crystallization and no substantial long term Tm evolution. Initial melting temperatures are analyzed in the Hoffman-Weeks framework, and reasonably predict the long-time Tm of orthorhombic crystal. Upon aging, a transition in the morphology is observed via wide-angle x-ray scattering and the time required for the transition tracks with dynamic bond density. Finally, we demonstrate that step annealing can result in PE networks with Tm approaching 100 °C. |
Wednesday, March 4, 2020 9:00AM - 9:12AM |
L70.00006: Tuning vitrimer mechanics with prepolymer and crosslinker structure Julia Kalow Dynamic covalent polymer networks in which the topology is reconfigured through associative exchange reactions are known as vitrimers. These materials hold promise as repairable and recyclable thermosets and elastomers. A hallmark of vitrimers is the Arrenhius relationship between viscocity and temperature; the activation energy for stress relaxation is often used to describe the energetics of the reconfiguration process. However, the mechanistic interpretation of this activation energy and its relationship to the associative exchange event are often unclear. We combine physical organic approaches and mechanical characterization to correlate the activation energy for stress relaxation to molecular reactivity and macromolecular structure. This study is enabled by the use of crosslinks that exchange via a catalyst-free conjugate addition/elimination pathway. |
Wednesday, March 4, 2020 9:12AM - 9:48AM |
L70.00007: Catalyst and Architecture Effects in Polyester Dynamic Covalent Materials Invited Speaker: Christopher Bates Polyester networks that include dynamic covalent bonds have attracted significant attention as materials with tunable plasticity. This talk will discuss the role of catalyst strength and building block architecture on dynamic covalent polyester exchange kinetics and mechanical properties. To do so, we introduce a versatile synthetic platform that provides good control over network connectivity through formulation and precursor design. The fundamental insights described herein expand the utility of crosslinked polyesters by broadening the range of accessible material properties. |
Wednesday, March 4, 2020 9:48AM - 10:00AM |
L70.00008: Recycling of Poly(thiourethane) Thermosets Enabled by Thiourethane Bonds Sijia Huang, Maciej Podgorski, Xun Han, Christopher Bowman Recycling of polyurethanes is an inviable process due to the harsh reprocessing conditions and high risk of side reactions. Constructing polymer networks incorporating covalent dynamic bonds becomes an attractive strategy in the design of recyclable materials. Here, we report findings on the dynamic nature of thiourethanes, and their application as a new class of recyclable analogs of urethane materials. A series of small molecule experiments was initially conducted to determine equilibrium constant and reverse reaction kinetic constant for the thiourethane reaction. Furthermore, incorporating those thiourethane moieties into a cross-linked network resulted in thermoset materials that can be facilely depolymerized to liquid oligomers. The resultant oligomers can be recrosslinked back to thiourethanes without any loss of performance nor properties (peak stress of 30 MPa with Young’s modulus of 1 GPa). Moreover, the increase in value that thiourethane network can undergo when upcycled to the thiol oligomer enables a potential pathway to build materials with properties that exceed its pristine material. Overall, this reprocessing strategy may advance many global sustainability goals, and is applicable to other low-cost commodity materials. |
Wednesday, March 4, 2020 10:00AM - 10:12AM |
L70.00009: Rheology and Rupture of Partial Vitrimer Shengqiang Cai Polymer network with dynamic covalent bonds, also known as vitrimer, exhibit interesting mechanical behaviors which are very different from conventional polymers. Because of the bond exchanging reactions, vitrimers have shown great potential as excellent recyclable polymers with self-healing capability. Our recent experiments have shown that the mechanical properties of a vitrimer such as fracture toughness and stress relaxation can be greatly tuned by selectively mixing the polymer chains with dynamic covalent bonds and routine covalent bonds (referred as partial vitrimer), while the self-healing capability of the polymer can be still maintained. To quantitatively understand the stress relaxation of the partial vitrimer, we developed a simple rheological model for it with certain combinations of spring and dashpot. We have also developed a simple energy argument to explain the rupture phenomenon of vitrimer under different mechanical loading conditions. |
Wednesday, March 4, 2020 10:12AM - 10:24AM |
L70.00010: Investigation of viscoelastic behavior over wide temperature range in PDMS vitrimers Laura Porath, Christopher Evans We have prepared vitrimers of poly(dimethyl siloxane) containing dynamic boronic ester bonds to investigate the viscoelastic properties of dynamic networks with extremely low Tg. The stress relaxation times and frequency dependent behavior are probed over a > 200 °C window and show the anticipated Arrhenius behavior at temperatures ranging from 180 to 40°C. The relaxation times here are independent of network strand molecular weight, which varies from 550-11,000 g/mol. We consider this temperature range to be a reaction-limited regime far above Tg, where bond exchange kinetics dominate macroscopic relaxation. Below 40°C, non-Arrhenius behavior emerges, and the apparent activation energy decreases. Time-temperature-superposition of frequency sweeps shows that the flow regime is thermorheologically simple, while the modulus of the plateau regime increases with increasing temperature, which is consistent with a preserved network architecture. As stress relaxation times speed up by orders of magnitude, the rubbery plateau modulus increases, which leads to a strategy for decoupling dynamics from mechanics. Our studies point to the importance of measuring vitrimer properties over a broad temperature window to understand the full range of response in this class of dynamic network. |
Wednesday, March 4, 2020 10:24AM - 10:36AM |
L70.00011: Intrinsically reprocessable, self-healing elastomers Liheng Cai Conventional elastomers formed by solely permanent, chemical crosslinks are not reprocessable, which causes environmental burden to the society. By contrast, networks crosslinked by solely reversible, physical bonds are reprocessable and self-healable. The physical bonds, however, are much weaker than chemical crosslinks, limiting the strength of the resulted polymer networks. Here, we propose a concept that exploits the self-assembly of block copolymers integrating strong physical associations and weak reversible hydrogen bonds to create an intrinsically reprocessable, self-healing elastomer. We find that the reversible bonds promote the formation of ordered nanostructures, which, in turn, result in unique macroscopic mechanical properties. Our results provide insights on the development of intrinsically reprocessable, self-healing elastomers. |
Wednesday, March 4, 2020 10:36AM - 10:48AM |
L70.00012: Melt Recyclable Shape Memory Elastomers through Main Chain Association 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 or recycled. Here, we demonstrate that replacing covalent crosslinks with hydrogen bond interactions can enable fully melt recyclable SM elastomer with little performance loss. Two high molecular weight poly(caprolactone)s with interchain hydrogen bonding groups have been synthesized and show excellent strain fixation and shape recovery both before and after shredding, melt-pressing, and reannealing. Dynamic mechanical analysis reveals a stiffness plateau that persists to temperatures above the shape-triggering temperature. Stress relaxation studies suggest chain disentanglement is the primary relaxation mode. Disentanglement is sluggish just above the trigger temperature and is much faster at higher temperatures where plastic flow can occur. The role of the polymers’ hard segment associative strength and a first-cycle training effect are under continued investigation. |
Wednesday, March 4, 2020 10:48AM - 11:00AM |
L70.00013: Leveraging the Stability of Ionic Liquids in Processing Polyampholytes David Delgado, Jian Ping Gong, Kenneth R Shull Double network gels composed of polyampholytes have been shown to have both tough and strong properties due to both inter and intra electrostatic bonding. Upon deformation, the weaker bonds break and dissipate energy allowing the gel to be load bearing despite retaining up to 90% water. While these gels are suited for bulk applications such as the knee or achilles heel, it remains a challenge to fabricate thin films that can coat non-planer surfaces such as the hip joint. Traditional polyelectrolyte processing has used salt to screen bonds and move across the polyelectrolyte coacervate continuum. Upon evaporation of water, however, this salt can recrystallize and disrupt film morphology. In this work, an ionic liquid composed of choline chloride and urea is used to dissolve and rapidly deposit double network gels. Moreover, in contrast to salt processing, the ionic liquid is stable at ambient conditions. Finally, rheological studies and phase diagrams are presented to understand the interplay between ionic liquid, polyampholyte, and water. |
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