Volume phase transition of spherical microgel particles

The volume phase transition of poly(N-isopropylacrylamide) (PNIPAM) spherical microgel particles was studied by static and dynamic laser light scattering (LLS). The results were compared with the coil-to-globule transition of individual long linear PNIPAM chains. The microgel particles have a higher transition temperature, but a less sharp phase transition, in comparison with that of long linear chains. This difference has been attributed to both the short length and the broad length distribution of the subchains inside the microgels. A combination of static and dynamic LLS results revealed that even in a highly collapsed state the microgel particles retained similar to 70% of water and the density of the microgel networks increased from 0.021 g/cm(3) to similar to 0.30 g/cm(3) during the phase transition. The temperature-dependence of the NMR spin-lattice relaxation times T-1 of PNIPAM indicate an association between water and the CH proton on the N-isopropyl group. Our results also showed that the transition was strongly influenced by the presence of surfactant. Addition of anionic surfactant, such as sodium dodecyl sulfate (SDS), promotes the swelling of the particles and shifts the transition to a higher temperature, while the addition of a cationic surfactant, such as dodecyl pyridine bromide (DPB), has less effects on the swelling and phase transition, which has been attributed to the electron-rich amide group in PNIPAM. Moreover, a two-step phase transition was observed for the first time in the presence of SDS. The dynamic LLS results demonstrates that SDS is expelled gradually from the microgel in the first-step volume phase transition.