Formation of Colloidally Stable Phase Separated Poly(N-vinylcaprolactam) in Water:  A Study by Dynamic Light Scattering, Microcalorimetry, and Pressure Perturbation Calorimetry

The effect of temperature on aqueous solutions of poly(N-vinylcaprolactam) (PVCL) samples of molecular weights ranging from 21 000 to 1.5 × 106 g mol-1 was monitored by dynamic light scattering (DLS), high-sensitivity microcalorimetry (HS DSC), and pressure perturbation calorimetry (PPC) from 10 to 80 °C. The polymer was soluble in cold water and underwent phase separation at TCP ∼ 31−38 °C, depending on the molecular weight. The phase transition was endothermic, with an enthalpy change of 4.4 ± 0.4 kJ mol-1. Stable particles of average diameters 80 nm (high molecular weight PVCL) and ∼ 180 nm (low molecular weight PVCL) formed above the phase transition temperature. The coefficient of thermal expansion of PVCL in water (αpol), determined by PPC, underwent a sharp decrease at the temperature corresponding to the onset of phase transition, then it passed through a maximum, and decreased continuously with increasing temperature. The stability of the particles formed in PVCL solutions kept at temperatures exceeding TCP suggests that their surfaces possess a hydrophilic character. Results of microcalorimetric measurements carried out with solutions of PVCL in D2O add further strength to this conclusion. The thermodynamic and volumetric changes associated with the phase transition of aqueous PVCL solutions are compared to those of aqueous solutions of poly(N-isopropylacrylamide) (PNIPAM), a polymer that also undergoes a phase transition in water at ∼ 31 °C, and of poly(vinylpyrrolidone) (PVP), a polymer structurally related to PVCL, which is soluble in water at all the temperatures investigated here.