Supramolecular Elastomers Based On Metal-ligand Interaction:Design And Characterization

Supramolecular elastomers(SEs),as novel materials with rubbery elasticity based on their non-covalent crosslinks,are particularly superior in their re-processible ability and self-healing under certain circumstances,which extended their life of service markedly.Recently,much attention has been paid on metal-coordinated SEs.The metal-coordination-based SEs have mostly been reported to use nitrogen-containing aromatic rings as ligands to build the cross-linked networks with transition metals or rare earth metals.According to the valence bond theory of coordination chemistry,the metal ions/atoms in the complex act as electron acceptors,and the ligands act as electron donors(with lone electron pairs),and then the coordination bonds formed by the electron pairing.Therefore,the ligand-metals that can serve as coordinated cross-links are ever-changing,but not limited to nitrogen-containing aromatic ring ligands or transition metals as central atoms/ions.This work aims at constructing SEs using simple organic groups with metals,by designing and synthesizing carboxyl-side-groups modified polydimethylsiloxanes(GCS)as macromolecular ligands,which are subsequently coordinated with Aluminum chloride to afford the SEs(Al-GCS).The coordination structure were characterized using ~1H-NMR,FT-IR and the results indicated diversified coordination structure in system.The protonation of carboxyl groups can be controlled by adjusting the grafting ratio of carboxyl and the feed ratio of COOH/Al(?),which gives the chances of regulating the total content and the ratio between coordination-based crosslinking and hydrogen-bond-based crosslinking.Besides,the Van der Waals forces can be controlled by changing the polysiloxane chain lengths.It has been proved that Al-GCS are amorphous crosslinked elastomers with rubbery elasticity at ambient conditions.The elasticity results from the COO-Al coordination that dominated the mechanic strength of Al-GCS and the hydrogen bonds increased the modulus.Due to the coordination between the thioether(-S-)in the GCS and Al(?),the coordination crosslinking system of Al-GCS is complicated.By comparing the systems of non-thioether-containing carboxy-grafted-polysiloxanes(GC)or non-carboxyl-containing thioether-grafted-polysiloxanes(GBA)coordinating with Al(?),it was found that the S-Al coordination bond is easy to break under external force and heating conditions.Therefore,in the Al-GCS system,S-Al bond coordination should not occur when the carboxyl group is not fully deprotonated.If aluminum isopropoxide,instead of aluminum chloride,is used to coordinate with GCS,the synthesized AlO-GCS has a higher modulus than Al-GCS,which is mainly due to the higher crosslink density of AlO-GCS.