An official website of the United States government
One method to improve the properties of covalent adaptable networks (CANs) is to reinforce them with a fraction of permanent cross‐links without sacrificing their (re)processability. Here, a simple method to synthesize poly(n‐hexyl methacrylate) (PHMA) and poly(n‐lauryl methacrylate) (PLMA) networks containing static dialkyl disulfide cross‐links (utilizing bis(2‐methacryloyl)oxyethyl disulfide, or DSDMA, as a permanent cross‐linker) and dynamic dialkylamino sulfur‐sulfur cross‐links (utilizing BiTEMPS methacrylate as a dissociative dynamic covalent cross‐linker) is presented. The robustness and (re)processability of the CANs are demonstrated, including the full recovery of cross‐link density after recycling. The authors also investigate the effect of static cross‐link content on the stress relaxation responses of the CANs with and without percolated, static cross‐links. As PHMA and PLMA have very different activation energies of their respective cooperative segmental mobilities, it is shown that the dissociative CANs without percolated, static cross‐links have activation energies of stress relaxation that are dominated by the dissociation of BiTEMPS methacrylate cross‐links rather than by the cooperative relaxations of backbone segments, i.e., the alpha relaxation. In CANs with percolated, static cross‐links, the segmental relaxation of side chains, i.e., the beta relaxation, is critical in allowing for large‐scale stress relaxation and governs their activation energies of stress relaxation.
more »
« less
Internal Catalysis in Dynamic Hydrogels with Associative Thioester Cross-Links
Thioesters are an essential functional group in biosynthetic pathways, which has motivated their development as reactive handles in probes and peptide assembly. Thioester exchange is typically accelerated by catalysts or elevated pH. Here, we report the use of bifunctional aromatic thioesters as dynamic covalent cross-links in hydrogels, demonstrating that at physiologic pH in aqueous conditions, transthioesterification facilitates stress relaxation on the time scale of hundreds of seconds. We show that intramolecular hydrogen bonding is responsible for accelerated exchange, evident in both molecular kinetics and macromolecular stress relaxation. Drawing from concepts in the vitrimer literature, this system exemplifies how dynamic cross-links that exchange through an associative mechanism enable tunable stress relaxation without altering stiffness.
more »
« less
- PAR ID:
- 10504975
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- ACS Macro Letters
- ISSN:
- 2161-1653
- Page Range / eLocation ID:
- 621 to 626
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Upcycling plastic waste into reprocessable materials with performance-advantaged properties would contribute to the development of a circular plastics economy. Here, we modify branched polyolefins and postconsumer polyethylene through a versatile C−H functionalization approach using thiosulfonates as a privileged radical group transfer functionality. Cross-linking the functionalized polyolefins with polytopic amines provided dynamically cross-linked polyolefin networks enabled by associative bond exchange of diketoenamine functionality. A combination of resonant soft X-ray scattering and grazing incidence X-ray scattering revealed hierarchical phase morphology in which diketoenamine-rich microdomains phase-separate within amorphous regions between polyolefin crystallites. The combination of dynamic covalent cross-links and microphase separation results in useful and improved mechanical properties, including a ∼4.5-fold increase in toughness, a reduction in creep deformation at temperatures relevant to use, and high-temperature structural stability compared to the parent polyolefin. The dynamic nature of diketoenamine cross-links provides stress relaxation at elevated temperatures, which enabled iterative reprocessing of the dynamic covalent polymer network with little cycle-to-cycle property fade. The ability to convert polyolefin waste into a reprocessable thermoformable material with attractive thermomechanical properties provides additional optionality for upcycling to enable future circularity.more » « less
-
Vitrimers are a class of covalent adaptable networks (CANs) that undergo topology reconfiguration via associative exchange reactions, enabling reprocessing at elevated temperatures. Here, we show that cross-linker reactivity represents an additional design parameter to tune stress relaxation rates in vitrimers. Guided by calculated activation barriers, we prepared a series of cross-linkers with varying reactivity for the conjugate addition—elimination of thiols in a PDMS vitrimer. Surprisingly, despite a wide range of stress relaxation rates, we observe that the flow activation energy of the bulk material is independent of the cross-linker structure. Superposition of storage and loss moduli from frequency sweeps can be performed for different cross-linkers, indicating the same exchange mechanism. We show that we can mix different cross-linkers in a single material in order to further modulate the stress relaxation behavior.more » « less
-
Dynamic covalent cross-links impart hydrogels with viscoelastic and self-healing properties, motivating applications as biomimetic cell scaffolds and injectable materials. The long bond lifetime results in complex rheological behavior including shear thickening. We hypothesized that this behavior applies broadly across dynamic covalent hydrogels and can be engineered through reaction rate constants. Thus, we synthesized multiarm poly(ethylene glycol) (PEG) hydrogels with conjugate addition, boronate ester, or terpyridine-zinc cross-links, which tune bond dissociation kinetics and hydrogel relaxation times over four orders of magnitude. All formulations exhibited shear thickening, with the onset dictated by the relaxation time. Although multiple mechanisms may underlie this behavior, chain stretching is hypothesized to contribute to shear thickening, as the cross-linking concentration remained constant under shear and networks with more defects correlated with increased shear thickening. These molecular and structural drivers of shear thickening apply across dilute dynamic covalent tetra-PEG hydrogels, clarifying their suitability for applications under shear.more » « less
-
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
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1021/acsmacrolett.4c00245
