Poly(N-isopropyl-acrylamide) (PNIPAM) is a paradigm
thermal sensitive polymer, which has a lower critical solution temperature
(LCST) around 32oC in water. Herein by AFM-based single molecule
force spectroscopy (SMFS), we measured the single chain elasticity of PNIPAM
across the LCST in water. Below LCST, the force curves obtained at different
temperatures have no remarkable difference; while above LCST, an unexpected
temperature dependent elasticity is observed, mainly in the middle force
regime. We found that 35oC is a turning point of the variation: from
31 to 35oC, the middle parts of the force curves drop gradually,
whereas from 35 to 40oC, the middle parts rise gradually. A possible
mechanism for the unexpected temperature dependent elasticity is proposed. The
increase of the single PNIPAM chain elasticity upon heating from 35 to 40oC
may cast new light on the design of molecular devices that convert thermal
energy to mechanical work.
