Machine prototype to electrochemomechanically characterise poly[ferrocenylsilane] hydrogels and their viscoelastic modelling

The Twente group has been developing redox-driven macromolecular motors by investigating various stimuli-responsive poly[ferrocenylsilanes] (PFSs). PFS can reversibly be oxidized and reduced by external potentials which lead to changes in their mechanical properties and can be used to drive cyclic molecular motors. Studies of their closed mechano-electrochemical cycles were done using electrochemical AFM-based single-molecule force spectroscopy (SMFS). Oxidised chains lengthened compared to neutral ones, which is attributed to the electrostatic repulsion between the oxidized ferrocene centers along the chain. A force of about 140 pN on the AFM cantilever was detected upon redox stimulation, while a single pre-stretched PFS chain was fixed in position z. Single-chain efficiency was found to increase with increasing stretching ratio. Experimentally, a maximum efficiency of 26% was observed. Technologies to study viscoelastic materials adequately are difficult to find, therefore new measurement techniques and devices have been developed and prototyped by the Catania group. The test machine produced is able to mechanically and electrochemically characterise redox responsive hydrogels, with a 0.58% repeatability using Young’s modulus.The mechanical characterization of soft materials is studied by relaxation tests. Our machine works in Control Displacement-mode, applying a static compression displacement to the sample and monitoring the stress.The elastic modulus was found between 1.5 ÷ 3.2 kPa. An experimental protocol was produced to standardize the results for different tests.