Internal Motions of Linear Chains and Spherical Microgels in Θ and Poor Solvents

Abstract

Internal motions of narrowly poly(N-isopropylacrylamide) (PNIPAM) linear chains and spherical microgels in a very dilute aqueous solution and dispersion were studied under the Θ and poor solvent conditions by dynamic laser light scattering (LLS) over a wide range of scattering angles. As expected, only one narrow peak related to the translational diffusion was observed in the line-width distribution G(Γ) of both linear chains and spherical microgels when x < 1, where x = (qRg)2 with q and Rg the scattering vector and the radius of gyration, respectively. As x increases to approach 1, we start to see a second peak related to fast internal motions of linear chains, whose average line width is related to some of the internal motions (normal modes) predicted in the nondraining bead-and-spring model. However, for swollen microgels, the second peak related to the internal motions only appears when x is much higher (1/q ∼ 50 nm), indicating that thermal energy is not able to excite the entire microgel but only a small portion of the gel network. As the solvent quality changes from good to poor, the shrinking of a linear chain or subchains in the gel network gradually suppresses the internal motions, presumably due to stronger intersegment interaction. Surprisingly, we found that the relative contribution of the second peak makes a turning near the Θ temperature, leading to a new way to estimate the Θ condition of a given polymer solution.