Structure Activity Relationships in the Fracture of Hybrid Covalent/Metallosupramolecular Organogels
Hybrid polymeric networks constructed using both covalent and reversible cross-links have been shown to be effective in preventing fracture and ultimately failure in polymeric materials. The prevention of failure has been largely attributed to the ability of the reversible cross-links to dissipate energy without breaking the covalent cross-links. The ability to rationally design materials that optimize this strategy would benefit from quantitative and systematic studies of the relationship between the number and strength of reversible interactions and the failure behavior of hybrid networks. This dissertation describe studies of fracture under compression in a family of hybrid networks, in which the timescale of reversible cross-linker dissociation is varied over several orders of magnitude, whereas the covalent components are kept constant.
Polymeric networks were constructed with 4-vinylpyridine. Bimetallic pincer Pd and Pt complexes were inserted into the network, forming reversible metal-ligand bonds that cross-link pyridine residues. The additional reversible cross-links prolong the lifetime of the hybrid networks under compressive strain when compared to their covalent counterparts. The observed failure behavior is dependent on the rate at which the networks are compressed as well as the strength of reversible interaction. Most interestingly, the addition of very dynamic and weak reversible interactions, so weak as to make no measurable contribution to bulk modulus, still leads to enhanced fracture strains. The failure of the covalent component within these hybrid networks was probed directly by incorporating a mechanophore that emits light upon chain scission. It was confirmed that the addition of these dynamic and weak reversible cross-links delays the catastrophic bond scission events associated with failure in the materials.
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