Resolving the molecular mechanisms controlling the rheology in a supramolecular telechelic polymer

Supramolecular polymers are important for printing, adhesives, coatings and nano-fabrication, and the ability to tune their properties by control of the supramolecular associations makes them versatile and powerful. We present a detailed investigation of a telechelic supramolecular polymer (UPyPPG) made from poly(propylene glycol) end-functionalised with hydrogen bond associating pyrimidinone groups. This polymer exhibits fascinating dynamics, combining very slow flow with relatively fast segmental and chain dynamics. We demonstrate the origin of these properties at the molecular level through a combination of rheology, AFM, broadband dielectric spectroscopy and X-ray scattering (SAXS/WAXS). Using rheology, we compare the response of UPyPPG to pure PPG, demonstrating a dramatic slow-down in the terminal rheological response, which is linked to the local chain dynamics (normal mode response probed by BDS). We identify the effects to be due to both association-driven chain-extension and to the presence of nano-scale fibre-like aggregates; the molecular structure is revealed through a combination of atomic force microscopy (AFM) and X-ray scattering (SAXS/WAXS). We use high-speed AFM to track the motions of the fibre-like aggregates down to nanometre lateral resolution and time steps of seconds, and a differential dynamic microscopy (DDM) approach is used to quantify the AFM nanoscale relaxation movies, providing a direct link between the nanoscale motions and the bulk rheology.