Single polymer chain elongation of poly(N-isopropylacrylamide) and poly(acrylamide) by atomic force microscopy

The nanomechanical properties of poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylamide) (PAAM) in deionized water and 8 M urea aqueous solution were studied by using an atomic force microscopy based technique-single-molecule force spectroscopy (SMFS). Different force-extension traces were obtained in both systems. They could be normalized and superimposed, respectively. The force curves of PNIPAM and PAAM in 8 M urea aqueous solution could be fitted by extended freely jointed chain (FJC) model separately. These facts indicate that single polymer chain stretching was obtained. Moreover, the elasticities of PNIPAM and PAAM in pure water and in 8 M urea aqueous solution were compared. It is worthy to be pointed out that for PAAM in deionized water, a deviation from the extended FJC model at the middle regime (from similar to 50 to 450 pN) in the force curve was found. When 8 M urea aqueous solution was used as buffer solution, the deviation disappeared and the force curve could be fitted with extended FJC model. This result may be related to the fact that PAAM could form certain intramolecular hydrogen bending in water. After the PNIPAM sample was heated to 33 degreesC, above its LCST, more signals were observed in one stretching. This could be attributed to the fact that polymer chains form more contact points with the glass substrate after thermal treatment. In addition, another way just different from the normal one to do the SMFS experiment was also provided.