Study of the Dynamic Viscoelasticity of Single Poly(N-isopropylacrylamide) Chains Using Atomic Force Microscopy

Abstract

Thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) is of great interest in the fields of medicine and pharmacology because its lower critical solubility temperature (LCST) is close to the physiological temperature. The understanding of the phase transfer mechanism of PNIPAM near the LCST is of great significance for the design, development, and application of its derivatives. In this study, a dynamic single-molecule force spectroscopy (SMFS) approach was used to quantitatively assess the dynamic viscoelasticity of single PNIPAM chains at different temperatures. We found that the relationship between the viscoelasticity coefficient of a single polymer chain in the low elongation region and the number of chain segments below the LCST was in accordance with the prediction of the Kirkwood model. Above the LCST, the PNIPAM chains exhibited different viscoelastic behaviors that were determined by their conformations. Importantly, the characterization of dynamic viscoelasticity allowed us to observe the phase transition behavior of single polymer chains in the low elongation region where they are similar to random coils, which helped us to understand the microscopic mechanism of their temperature response. In particular, above the LCST, the PNIPAM chains no longer underwent purely entropic elastic behavior but instead displayed enthalpic viscoelastic behavior dominated by intramolecular hydrogen bonds.