Event Information
Supramolecular Control of Polymer Mechanical Properties
- Abstract:
Metallo-supramolecular networks between bis-Pd(II) and Pt(II) organometallic cross-linkers and poly(4-vinylpyridine) are formed in DMSO. The dynamic mechanical response of these networks is directly related to the molecular dissociation kinetics of the metal-ligand coordination bond, which is responsible for the cross-linking. The consequences of this molecular control on bulk mechanical properties are further explored with detailed rheological experiments. These results indicate that the mechanical response of the networks is governed by how often the cross-links dissociate and not for how long they are dissociated from the network structure. The structure-function relationship between the cross-links and the bulk mechanical response are found to be such that precise control of the mechanical properties is achievable through a mix-and-match approach of cross-linking. The reversible nature of the labile metal-ligand bond is further exploited to obtain dramatic changes in the material properties in response to both chemical and thermal stimuli. The most dramatic response occurs when the network is near its threshold for percolation (gel point).
This metallo-supramolecular platform is expanded to control the mechanical behavior of poly(4-vinylpyridine) brushes grown from a gold surface. Subsequent cross-linking by the same series of Pd(II) cross-links causes changes in the observed mechanical response of the brushes. The dynamics of this interaction are shown to influence the force felt by an AFM tip that is laterally pulled across the brush layer. A cross-linker with a fast dissociation rate is shown to decrease the observed lateral force of the brush layer, while a cross-linker with a slower dissociation rate increases the lateral force felt by an AFM tip.
The metallo-supramolecular motif is incorporated into a fixed polymer elastomer to create a hybrid organogel, whose structure is fixed with covalent cross-links. The material maintains the shape dictated by the covalent structure; however, the supramolecular cross-links contribute to the mechanical response of the material. The dynamics of the cross-links still contribute to the mechanical response of the organogels, and their reversible nature appears to provide a protective mechanism that is akin to rudimentary self-healing.
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