Energetics of cooperative transitions of N-vinylcaprolactam polymers in aqueous solutions

Phase and conformational behavior of poly(N-vinylcaprolactam) was studied as a function of polymer concentration, concentration of NaCI, and concentration of sodium dodecyl sulfate (SDS). Precise measurements of the partial heat capacity of the polymer were first carried out within the temperature range 10-125 °C. Thermodynamic parameters of the phase separation transition (transition temperature, enthalpy, and heat capacity increment) do not change with variations in polymer concentration. In the presence of salt the transition temperature decreases, the enthalpy increases, and the heat capacity increment does not vary significantly. Beginning with the SDS concentration of 0.005 M, the phase separation is suppressed and the heat capacity peak splits into two peaks. Upon further increase in surfactant concentration the amplitude of the low-temperature peak increases while that of the high-temperature peak vanishes. Phase separation transition in aqueous solutions of N-vinylcaprolactam-methacrylic acid copolymers of different compositions were studied at different polymer concentrations and pH (4-12). The transition parameters do not depend on polymer concentration and pH.