Opposing Temperature Dependence of the Stretching Response of Single PEG and PNiPAM Polymers

The response of switchable polymer blends and coatings to temperature variation is important for the development of high-performance materials. Although this has been well studied for bulk materials, a proper understanding at the molecular level, in particular for high stretching forces, is still lacking. Here we investigate the molecular details of the temperature-dependent elastic response of two widely used water-soluble polymers, namely, polyethylene glycol (PEG) and poly(N-isopropylacrylamide) (PNiPAM) with a combined approach using atomic force microscopy (AFM) based single molecule force spectroscopy (SMFS) experiments and molecular dynamics (MD) simulations. SMFS became possible by the covalent attachment of long and defined single polymers featuring a functional end group. Most interestingly, varying the temperature produces contrasting effects for PEG and PNiPAM. Surprising as these results might occur at first sight, they can be understood with the help of MD simulations in explicit water. We find that hydration is widely underestimated for the mechanics of macromolecules and that a polymer chain has competing energetic and entropic elastic components. We propose to use the temperature dependence to quantify the energetic behavior for high stretching forces. This fundamental understanding of temperature-dependent single polymer stretching response might lead to innovations like fast switchable polymer blends and coatings with polymer chains that act antagonistically.