Synthesis and characterization of heptaphenyl polyhedral oligomeric silsesquioxane-capped poly(N-isopropylacrylamide)s

Abstract

In this work, a novel initiator bearing heptaphenyl polyhedral oligomeric silsesquioxane (POSS) was synthesized via the copper-catalyzed Huisgen 1,3-cycloaddition (i.e., click chemistry). With this initiator, the atom transfer radical polymerization (ATRP) of N-isopropylacrylamide (NIPAAm) was carried out to afford the POSS-capped PNIPAAm. The organic–inorganic amphiphiles were characterized by means of nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography (GPC). Atomic force microscopy (AFM) showed that the POSS-capped PNIPAAm amphiphiles in bulk displayed microphase-separated morphologies. In aqueous solutions, the POSS-capped PNIPAAm amphiphiles were self-assembled into micelle-like aggregates as evidenced by dynamic light scattering (DLS) and transmission election microscopy (TEM). It was found that the sizes of the self-organized nanoobjects decreased with increasing the lengths of PNIPAAm chains. By means of UV–vis spectroscopy, the lower critical solution temperature (LCST) behavior of the organic–inorganic amphiphiles in aqueous solution was investigated and the LCSTs of the organic–inorganic amphiphiles decreased with increasing the percentage of POSS termini. It is noted that the self-assembly behavior of the POSS-capped PNIPAAm in aqueous solutions exerted the significant restriction on the macromolecular conformation alteration of PNIPAAm chains while the coil-to-globule collapse occurred.