Can coil-to-globule transition of a single chain be treated as a phase transition?

The effects of the concentration (C) and heating rate on the collapse and association of poly(N-isopropylacrylamide) chains in water have been investigated by use of ultrasensitive differential scanning calorimetry. In the dilute solutions, both the phase transition temperature (Tp) and enthalpy change (DeltaH) increase with the heating rate but decrease with concentration. By extrapolation to zero heating rate and zero concentration, Tp and DeltaH for coil-to-globule transition of a single chain in thermodynamic equilibrium can be obtained. In semidilute solutions, both Tp and DeltaH increase with the heating rate but slightly vary with the concentration. Tp and DeltaH for pure interchain association in equilibrium are obtained by extrapolation to zero heating rate. Our experiments reveal that only intrachain contraction occurs when the concentration is infinitely close to zero. When the concentration is above the overlap concentration (C*), only interchain association exists. In the range 0<C<C*, both intrachain contraction and interchain association coexist.