INTERNAL MOTIONS OF BOTH POLY(N-ISOPROPYLACRYLAMIDE) LINEAR-CHAINS AND SPHERICAL MICROGEL PARTICLES IN WATER

Poly(N-isopropylacrylamide) (PNIPAM) linear chains and spherical micro gel particles in very dilute aqueous solutions were studied by dynamic laser light scattering (LLS) over a wide range of scattering angles ( 6-154 degrees). For the linear chains, at x < 1, only one peak related to diffusion was observed in the line-width distribution G(Gamma), wh ile for x > 1, we detected a second peak related to internal motions, where x = (qR(g))(2) with q and R(g) the scattering vector and the rad ius of gyration, respectively. The average line width of the second pe ak can be related to some internal motions predicted in the nondrainin g bead-and-spring model, but other predicted internal motions are not observable. In the swollen microgel particles (R(g) similar to 160 nm) , the internal motions were only observed at x greater than or equal t o 13 (1/q greater than or equal to 44 nm). At very high x, in both the chains and particles, the two peaks merge into one broad peak and the plot of [Gamma]/q(3) vs x(1/2) approaches two different plateaus, whe re [Gamma] = f(0)(infinity)G(Gamma)Gamma d Gamma. 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 fa vorable internal motions may not be observable in dynamic LLS, possibl y due to the observation length scale (1/q).