Internal Motions of both Poly(N-isopropylacrylamide) Linear Chains and Spherical Microgel Particles in Water

Poly(N-isopropylacrylamide) (PNIPAM) linear chains and spherical microgel particles in very dilute aqueous solutions were studied by dynamic laser light scattering (LLS) over a wide range of scattering angles (6-154°). For the linear chains, at x < 1, only one peak related to diffusion was observed in the line-width distribution G(Γ), while for x > 1, we detected a second peak related to internal motions, where x ) (qRg) with q and Rg the scattering vector and the radius of gyration, respectively. The average line width of the second peak can be related to some internal motions predicted in the nondraining beadand-spring model, but other predicted internal motions are not observable. In the swollen microgel particles (Rg ∼ 160 nm), the internal motions were only observed at x g 13 (1/q g 44 nm). At very high x, in both the chains and particles, the two peaks merge into one broad peak and the plot of 〈Γ〉/q3 vs x1/2 approaches two different plateaus, where 〈Γ〉 ) ∫0∞G(Γ)Γ dΓ. Both of the plateaus are lower than predicted in the bead-and-spring model. The plateau of the particles is much lower than that of the chains. Energetically favorable internal motions may not be observable in dynamic LLS, possibly due to the observation length scale (1/q).