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.
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Reprocessable Polymer Networks Containing Sulfur‐Based, Percolated Dynamic Covalent Cross‐Links and Percolated or Non‐Percolated, Static Cross‐Links
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.
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- Award ID(s):
- 1912694
- PAR ID:
- 10524257
- Publisher / Repository:
- Wiley-VCH GmbH
- Date Published:
- Journal Name:
- Macromolecular Rapid Communications
- ISSN:
- 1022-1336
- Page Range / eLocation ID:
- 2400303
- Subject(s) / Keyword(s):
- covalent adaptable network cross-link dynamic permanent sustainability vitrimer
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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