An official website of the United States government
Abstract Materials with tunable modulus, viscosity, and complex viscoelastic spectra are crucial in applications such as self-healing, additive manufacturing, and energy damping. It is still challenging to predictively design polymer networks with hierarchical relaxation processes, as many competing factors affect dynamics. Here, networks with both pendant and telechelic architecture are synthesized with mixed orthogonal dynamic bonds to understand how the network connectivity and bond exchange mechanisms govern the overall relaxation spectrum. A hydrogen-bonding group and a vitrimeric dynamic crosslinker are combined into the same network, and multimodal relaxation is observed in both pendant and telechelic networks. This is in stark contrast to similar networks where two dynamic bonds share the same exchange mechanism. With the incorporation of orthogonal dynamic bonds, the mixed network also demonstrates excellent damping and improved mechanical properties. In addition, two relaxation processes arise when only hydrogen-bond exchange is present, and both modes are retained in the mixed dynamic networks. This work provides molecular insights for the predictive design of hierarchical dynamics in soft materials.
more »
« less
Vinylogous Urea—Urethane Vitrimers: Accelerating and Inhibiting Network Dynamics through Hydrogen Bonding
Abstract Vinylogous urethane (VUO) based polymer networks are widely used as catalyst‐free vitrimers that show rapid covalent bond exchange at elevated temperatures. In solution, vinylogous ureas (VUN) undergo much faster bond exchange than VUOand are highly dynamic at room temperature. However, this difference in reactivity is not observed in their respective dynamic polymer networks, as VUOand VUNvitrimers prepared herein with very similar macromolecular architectures show comparable stress relaxation and creep behavior. However, by using mixtures of VUOand VUNlinkages within the same network, the dynamic reactions can be accelerated by an order of magnitude. The results can be rationalized by the effect of intermolecular hydrogen bonding, which is absent in VUOvitrimers, but is very pronounced for vinylogous urea moieties. At low concentrations of VUN, these hydrogen bonds act as catalysts for covalent bond exchange, while at high concentration, they provide a pervasive vinylogous urea ‐ urethane (VU) network of strong non‐covalent interactions, giving rise to phase separation and inhibiting polymer chain dynamics. This offers a straightforward design principle for dynamic polymer materials, showing at the same time the possible additive and synergistic effects of supramolecular and dynamic covalent polymer networks.
more »
« less
- PAR ID:
- 10487378
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Angewandte Chemie International Edition
- Volume:
- 63
- Issue:
- 9
- ISSN:
- 1433-7851
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Vitrimers, dynamic polymer networks with topology conserving exchange reactions, have emerged as a promising platform for sustainable and reprocessable materials. While prior work has documented how dynamic bonds impact stress relaxation and viscosity, their role on crystallization has not been systematically explored. Precise ethylene vitrimers with 8, 10, or 12 methylene units between boronic ester junctions were investigated to understand the impact of bond exchange on crystallization kinetics and morphology. Compared to linear polyethylene which has been heavily investigated for decades, a long induction period for crystallization is seen in the vitrimers ultimately taking weeks in the densest networks. An increase in melting temperatures ( T m ) of 25–30 K is observed with isothermal crystallization over 30 days. Both C 10 and C 12 networks initially form hexagonal crystals, while the C 10 network transforms to orthorhombic over the 30 day window as observed with wide angle X-ray scattering (WAXS) and optical microscopy (OM). After 150 days of isothermal crystallization, the three linker lengths led to double diamond (C 8 ), orthorhombic (C 10 ), and hexagonal (C 12 ) crystals indicating the importance of precision on final morphology. Control experiments on a precise, permanent network implicate dynamic bonds as the cause of long-time rearrangements of the crystals, which is critical to understand for applications of semi-crystalline vitrimers. The dynamic bonds also allow the networks to dissolve in water and alcohol-based solvents to monomers, followed by repolymerization while preserving the mechanical properties and melting temperatures.more » « less
-
Abstract A liquid crystalline elastomer (LCE) network consisting of dynamic covalent bonds (DCBs) is referred as a LCE vitrimer. The mesogen alignment and the network topology can be reprogrammed locally in the LCE vitrimer by activating the bond exchange reactions using an external stimulus. After removal of the external stress, a new network is formed and the reprogrammed shape can be fixed, leading to a different set of the physical properties of the LCE vitrimers. Herein, this type of emerging materials is reviewed by a brief introduction of the fundamentals of LCEs, followed by discussions of various DCBs and the design principles for LCE vitrimers. After a presentation of different strategies to improve the stability and reprogrammability of the registered mesogen alignment, approaches to prepare LCE vitrimers with complex shapes and their actuations are discussed. Potential applications such as self‐healing and recycling, mechanochromic effects, and post‐functionalization of nanopores are also reviewed, followed by the conclusion of the remaining challenges and opportunities.more » « less
-
Abstract Sustainable development of new technologies requires materials having advanced physical and chemical properties while maintaining reprocessability and recyclability. Vitrimers are designed for this purpose; however, their dynamic covalent chemistries often have drawbacks or are limited to specialized polymers. Here, fluoride‐catalyzed siloxane exchange is reported as an exceptionally robust chemistry for scalable production of high‐performance vitrimers through industrial processing of commodity polymers such as poly(methyl methacrylate), polyethylene, and polypropylene. The vitrimers show improved resistance to creep, heat, oxidation, and hydrolysis, while maintaining excellent melt flow for processing and recycling. Furthermore, the siloxane exchange between different vitrimers during mechanical blending results in self‐compatibilized blends without any compatibilizers. This offers a general, scalable method for producing sustainable high‐performance vitrimers and a new strategy for recycling mixed plastic wastes.more » « less
-
Abstract The chemically crosslinked network structures make epoxies, the most common thermosets, unable or hard to be recycled, causing environmental problems and economic losses. Epoxy‐based vitrimers, polymer networks deriving from epoxy resins, can be thermally malleable according to bond exchange reactions (BERs), opening the door to recycle epoxy thermosets. Here a series of experiments were carried out to study the effects of processing conditions (such as particle size distributions, temperature, time, and pressure) on recycling of an epoxy‐anhydride vitrimer. Polymer powders from the epoxy‐anhydride vitrimer with different size distributions were prepared and characterized, and the influence of particle size on the mechanical performance of recycled epoxy‐anhydride vitrimers was investigated by dynamic mechanical analysis and uniaxial tensile test. Experimental results demonstrated that finer polymer powders can increase the contacting surfaces of recycled materials and thus result in high quality of recycled materials. In addition, the influences of other treating parameters, such as temperature, time, and pressure, were also discussed in this study. Adjusting these treating parameters can help the design of an optimized reprocessing procedure to meet practical engineering applications.more » « less
