Internal Structures of Thermosensitive Hybrid Microgels Investigated by Means of Small-Angle X-ray Scattering

Abstract

Internal structures of thermosensitive microgels and their hybrid counterparts that contain Au nanoparticles are investigated by means of small-angle X-ray scattering (SAXS). Thermosensitive cationic microgels were synthesized by aqueous free radical precipitation polymerization from N-isopropylacrylamide and 3-(methacrylamino) propyltrimethylammonium chloride used as monomers. Using the microgels as templates, Au nanoparticles were synthesized in situ, using the cationic sites in the microgel to nucleate particle growth. To obtain different types of the hybrid microgels, Au nanoparticles were synthesized in the presence of the microgels by changing the reduction conditions of the precursor ions, such as temperature and species of reducing reagent. The hybrid microgels were characterized mainly by TEM and SAXS. For SAXS investigation, the hybrid microgels were analyzed in the wide q-range of 0.07 nm(-1) < q < 20 nm(-1), where q is the magnitude of the scattering vector. A quantitative description of the scattering intensities, I(q), for the nonhybrid microgels requires a sum of five components having different physical origins. An upturn increase of the forward intensity originates from the interface of microgels, obeying the Porod law, I(q) ∝ q(-4). An additional Guinier term found in q < 0.2 nm(-1) seems to arise from solidlike density fluctuation due to the inhomogeneities of chemical cross-links. The power-law behavior manifested in the low- to intermediate-q range is directly linked with the fractal nature of the swollen (coil) polymer networks and well described by the Ornstein-Zernike equation. Two interference peaks centered at q ≈ 5 nm (-1) and q ≈ 15 nm(-1) are likely to reflect inter- and intrachain correlations of pNIPAm, respectively, which are formally fitted by pseudo-Voigt equations. As for the hybrid microgels, a pronounced new contribution from the Au nanoparticles emerges, which calls for an additional scattering component describing polydisperse spheres having a homogeneous internal electron density distribution. The width of the Gaussian distribution for the radius of the Au nanoparticles evaluated from the SAXS data turns out to be consistent with those obtained with TEM observation.