Coil–Globule Transition of a Water-Soluble Polymer
Jianyu Liu, Huazhang Guo, Qingjie Gao, Hongbin Li, Zesheng An, Wenke Zhang
Abstract
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.