Effect of chain architecture on the phase transition of star and cyclic poly(N-isopropylacrylamide) in water

The heat-induced phase transition of aqueous solutions of Poly(N-isopropylacrylamide) (PNIPAM) in water is examined for a four-arm PNIPAM star (s-PNIPAM), a cyclic PNIPAM (c-PNIPAM), and their linear counterparts (l-PNIPAM) in the case of polymers (1.0 g L−1) of 12,700 g mol−1 < Mn < 14,700 g mol−1. Investigations by turbidity, high-sensitivity differential scanning calorimetry (HS-DSC), and light scattering (LS) indicate that the polymer architecture has a strong effect on the cloud point (Tc: decrease for s-PNIPAM; increase for c-PNIPAM), the phase transition enthalpy change (ΔH decrease for s-PNIPAM and c-PNIPAM), and the hydrodynamic radius of the aggregates formed above Tc (RH: c-PNIPAM < s-PNIPAM < l-PNIPAM). The properties of s-PNIPAM are compared with those of previously reported PNIPAM star polymers (3 to 52 arms). The overall observations are described in terms of the arm molecular weight and the local chain density in the vicinity of the core of the star, by analogy with the model developed for PNIPAM brushes on nanoparticles or planar surfaces. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2059–2068.