Synthesis of hollow spherical calcium phosphate nanoparticles using polymeric nanotemplates

Poly(methylmethacrylate)- block -poly(methacrylic acid) (PMMA- b -PMAA) copolymer was synthesized by an atom transfer radical polymerization (ATRP) technique. The block copolymer was employed as a template for the controlled precipitation of calcium phosphate from aqueous solution at different pH values. A Ca 2+ ion selective electrode was used to study the interactions between Ca 2+ ions and the polymer, which indicated a possible weak interaction between Ca 2+ and un-ionized MAA segments at pH∼4.0 in addition to electrostatic interaction between Ca 2+ and ionized MAA segments at higher pH. An interesting structure representing that of a superstructure consisting of hybrid nano-filaments was observed by the transmission electron microscope at pH∼4.0. The filaments originated from a core of similar size to primary polymer aggregates, suggesting that cooperative interactions at a local level between dissolving calcium phosphate clusters and disassembling polymer segments are responsible for the secondary growth process. A hollow spherical morphology was obtained at pH∼7.0 and 9.0. Such calcium phosphate/polymer monohybrids with complex morphologies are interesting and might be useful as novel drug delivery carriers, ceramics precursors, reinforcing fillers or biomedical implants.