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 T_CP ∼ 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 (α_pol), 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 T_CP 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.