Mechanism Dictates Mechanics: A Molecular Substituent Effect in the Macroscopic Fracture of a Covalent Polymer Network
- 2 March 2021
- journal article
- research article
- Published by American Chemical Society (ACS) in Journal of the American Chemical Society
- Vol. 143 (10), 3714-3718
- https://doi.org/10.1021/jacs.1c00265
Abstract
The fracture of rubbery polymer networks involves a series of molecular events, beginning with conformational changes along the polymer backbone and culminating with a chain scission reaction. Here, we report covalent polymer gels in which the macroscopic fracture "reaction" is controlled by mechanophores embedded within mechanically active network strands. We synthesized poly(ethylene glycol) (PEG) gels through the end-linking of azide-terminated tetra-arm PEG (M-n = 5 kDa) with bis-alkyne linkers. Networks were formed under identical conditions, except that the bis-alkyne was varied to include either a cis-diaryl (1) or cis-dialkyl (2) linked cyclobutane mechanophore that acts as a mechanochemical "weak link" through a force-coupled cycloreversion. A control network featuring a bis-alkyne without cyclobutane (3) was also synthesized. The networks show the same linear elasticity (G' = 23-24 kPa, 0.1-100 Hz) and equilibrium mass swelling ratios (Q = 10-11 in tetrahydrofuran), but they exhibit tearing energies that span a factor of 8 (3.4 J, 10.6, and 27.1 J.m(-)(2). for networks with 1, 2, and 3, respectively). The difference in fracture energy is well-aligned with the force-coupled scission kinetics of the mechanophores observed in single-molecule force spectroscopy experiments, implicating local resonance stabilization of a diradical transition state in the cycloreversion of 1 as a key determinant of the relative ease with which its network is torn. The connection between macroscopic fracture and a small-molecule reaction mechanism suggests opportunities for molecular understanding and optimization of polymer network behavior.Funding Information
- Division of Chemistry (CHE-1832256)
- Duke University
This publication has 40 references indexed in Scilit:
- Stress-Responsive Polymers Containing Cyclobutane Core Mechanophores: Reactivity and Mechanistic InsightsJournal of the American Chemical Society, 2013
- Ring opening vs. direct bond scission of the chain in polymeric triazoles under the influence of an external forceChemical Communications, 2012
- Synthesis of well-defined hydrogel networks using Click chemistryChemical Communications, 2006
- Ultrasound-Induced Site-Specific Cleavage of Azo-Functionalized Poly(ethylene glycol)Macromolecules, 2005
- A literature survey on fatigue analysis approaches for rubberInternational Journal of Fatigue, 2002
- A Stepwise Huisgen Cycloaddition Process: Copper(I)-Catalyzed Regioselective “Ligation” of Azides and Terminal AlkynesAngewandte Chemie, 2002
- Single molecule force spectroscopy by AFM indicates helical structure of poly(ethylene-glycol) in waterNew Journal of Physics, 1999
- The theory of resonance in chemistryProceedings of the Royal Society of London. Series A - Mathematical and Physical Sciences, 1977
- The strength of highly elastic materialsProceedings of the Royal Society of London. Series A - Mathematical and Physical Sciences, 1967
- Rupture of rubber. I. Characteristic energy for tearingJournal of Polymer Science, 1953