Highly Hydrophilic Copolymers of N,N-Dimethylacrylamide, Acrylamido-2-methylpropanesulfonic acid, and Ethylenedimethacrylate: Nanoscale Morphology in the Swollen State and Use as Exotemplates for Synthesis of Nanostructured Ferric Oxide

The polymer framework of water-swollen copolymers of N,N-dimethylacrylamide, acrylamido-2-methylpropanesulfonic acid, and ethylenedimethacrylate (nominal cross-linking degrees of 4, 8, and 20 mol %) is composed of highly expanded domains, with “pores” not less than 6 nm in diameter. When the 4 % cross-linked copolymer (DAE 26-4) is swollen with a 10−4 M solution of 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) in water, MeOH, EtOH, or nBuOH, the molecules of the paramagnetic probe rotate rapidly (τ<1000 ps) and as fast as in the bulk liquid in the case of water. The swelling degree of DAE 26-4 is related to the Hansen solubility parameters of a number of liquids, including linear alcohols up to n-octanol. It is also found that the rotational correlation time of TEMPOL in the copolymer swollen by water and the lightest alcohols increases with decreasing specific absorbed volume. Time-domain NMR spectrometry of water-swollen DAE 26-4 shows that sorption of only 14 % of the liquid required for its complete swelling is enough for full hydration of the polymer chains. Accordingly, in fully swollen DAE 26-4 the longitudinal relaxation time of water closely approaches the value of pure water. {13C} CP-MAS NMR on partially and fully water swollen samples of DAE 26-4 shows that swelling increases the mobility of the polymer chains, as clearly indicated by the narrowing of the best-resolved peaks. DAE 26-4 was used as an exotemplate for the synthesis of nanocomposites composed of the polymer and nanostructured Fe2O3 through a series of ion-exchange/precipitation cycles. After the first cycle the nanoparticles are 3–4 nm in diameter, with practically unchanged size after subsequent cycles (up to five). In fact, the nanoparticle size never exceeded the diameter of the largest available pores. This suggests that the polymer framework controls the growth of the nanoparticles according to the template-controlled synthesis scheme. Selected-area electron diffraction, TEM, and high-resolution electron microscopy show that the nanostructured inorganic phase is composed of hematite.