Sliding on slide-ring gels

Recent investigations have pointed to physical entanglements that greatly outnumber chemical crosslinks as key sources of energy dissipation and low friction in hydrogel networks. Slide-ring gels are an emerging class of hydrogels described by their mobile crosslinks, which are formed by rings topologically constrained to slide along linear polymer chains within the network. These materials have enjoyed decades of study by polymer chemists but have been underexplored by the tribology community. In this work, we synthesized a pseudo-rotaxane crosslinker from poly(ethylene glycol) diacrylate (PEG-diacrylate) and α-cyclodextrin-acrylate followed by hydrogel networks by connecting the sliding crosslinks with polyacrylamide chains. The mechanical and tribological properties of slide-ring hydrogels were investigated using a custom-built microtribometer. Slide-ring hydrogels exhibit unique behavior compared to conventional covalently-crosslinked polyacrylamide hydrogels and offer a vast design space for future investigations.

Methods
Microindentations and sliding experiments were conducted with a custom-built linear reciprocating tribometer. Data was analyzed and processed with the following Matlab codes Compiling_Friction_Data_v5 and Tribometer_Indent_Analysis_v3_Wad

NMR spectroscopy was conducted with a Varian VNMRS 600 MHz spectrometer using D2O as a solvent. In all cases, the number of scans was 32, and the relaxation time was 1 s. The data files were processed in MestReNova. Using MestReNova, NMR CSV files were generated for this repository.