Folding of a single polymer chain and phase transition

Using an ultra-sensitive differential scanning calorimetry (US-DSC), we have investigated the folding and aggregation behaviors of poly(N-isopropylacrylamide) (PNIPAM) chains in dilute and semidilute solutions. In the heating process, the intrachain folding and interchain aggregation simultaneously occur in the dilute solutions, and the ratio of intrachain folding increases with decreasing concentration. In the semidilute solutions, PNIPAM chains show limited interchain aggregation with elevated temperature, because most of the PNIPAM chains have been collapsed at lower temperature. In an extremely dilute solution, PNIPAM chains undergo a single folding transition in the heating process. By extrapolating heating rate and concentration to zero, we have obtained the phase transition temperature (T S ) and enthalpy change (ΔH S ) of the single chain folding. ΔH S is higher than that for a phase transition involving intrachain collapse and interchain aggregation, indicating that a single chain folding can not be taken to be a macroscopic phase transition. Supported by the National Natural Science Funds for Distinguished Young Scholar (Grant No. 20474060) and Major State Basic Research Development Program of China (Grant No. 2007CB936401)