Coil-Globule Transition of a Water-Soluble Polymer

The coil-globule transition is a fundamental issue in polymer science and key to the performance of many smart materials. However, an experimental study on the globule structure and real-time dynamics of transition remains a challenge. Using single-molecule magnetic tweezers (MT) and atomic force microscopy (AFM) imaging, the temperature-and solvent-dependent transition of poly(N-isopropylacrylamide) (PNIPAM) single chain, a water-soluble thermoresponsive polymer, is directly observed under an external force. Surprisingly, the globule structure is composed of quantized beads with a basic/ minimum size of -31 repeat units. Our results indicate that upon heating or salt concentration change, the PNIPAM coil first forms a series of nuclei each consisting of -31 repeat units, rather than random sizes. The subsequent transition involves a mergence of adjacent beads. Finally, the beads gradually stack to form a loose spheroidal aggregate, rather than a uniform compact globule. The distinct collapsing rates and mechanical stabilities for different collapsed structures are identified for the first time.

Automated summary

This study directly observed the temperature- and solvent-dependent transition of water-soluble thermoresponsive polymer PNIPAM under external force using single-molecule magnetic tweezers and atomic force microscopy. The research found that upon heating or salt concentration change, the PNIPAM first forms nuclei each consisting of -31 repeat units, rather than random sizes, before undergoing subsequent transitions involving a merger of adjacent beads.