Inorganic/Organic Mesostructures with Complex Architectures: Precipitation of Calcium Phosphate in the Presence of Double-Hydrophilic Block Copolymers

Abstract

Double-hydrophilic block copolymers consisting of a long poly(ethylene oxide) block and a short poly(methacrylic acid) block, modified by partial alkylation with dodecylamine (PEO-b-PMAA-C 12 ) were employed as dispersed templates for the controlled precipitation of calcium phosphate from aqueous solution at different pH values. Two new and nontrivial superstructures of an organized inorganic/organic hybrid material were characterized by ultracentrifugation, small-and wide-angle X-ray analysis, and electron microscopy. At pH 3.5 and 4.0, and pH 4.5, 5.0, and 6.3, two different types of discrete nested structures are obtained which consist of hybrid nanofilaments arranged to give an unusual neuronlike morphology. The fibers originate from a core of similar size to the primary polymer aggregates, suggesting that cooperative interactions at a local level between dissolving calcium phosphate clusters and disassembling polymer units could be responsible for the highly anisotropic nature of the secondary growth process.Aging of the nanofilaments grown in acidic solution results in a second hybrid morphology, consisting of compact aggregates with a diameter of about 130 nm, which show the interlocked layer structure of an ordered inorganic/ organic mesophase with a repeat period of about 3 nm. Such calcium phosphate/ polymer nanohybrids with complex morphologies are interesting from the viewpoint of prebiotic structure formation, and might also be useful as novel ceramics precursors, reinforcing fillers, or biomedical implants.

Calcium phosphate neuron-like structures: a rare case or a common structure?

“…One aspect that should be borne in mind in all the above experiments is that in spite of the remarkable structures that TW can form, bulk crystallization does still take place and the coexistence of the stabilized amorphous neuron-like structure along with the crystallized phase complicates the interpretation of results. The close similarity between the neuron-like structures obtained with this simple surfactant and those discovered by Antonietti et al 20 using DHBCs clearly prove that such structures are not specic of the complex copolymer. To further reinforce this aspect, additional experiments were performed using a different surfactant.…”

“…[16][17][18][19] Particularly in the calcium phosphate system, Antonietti et al reported back in 1998 the stabilization of delicate neuron-like calcium phosphate structures with the help of a DHBC consisting of a long poly(ethylene oxide) block and a short poly(methacrylic acid) block partially alkylated with dodecylamine (PEO-block-PMMA-C12). 20 These structures were again replicated by Tjandra et al in 2005 using a similar design of the DHBC (PEO-block-PMMA) but with a longer PMMA chain. 21 The need for the complex DHBC seemed mandatory for the formation of such structures, and the mechanism of formation was believed to occur via formation of micelle aggregates of the copolymer.…”

“…In the work of Antonietti et al, analytical ultracentrifugation (AUC) studies were performed to prove their existence. 20 In our case, eld emission scanning electron microscopy (FE-SEM) was used to further corroborate their existence. Our hypothesis was that if these structures were artifacts, they would result from the assembly of laments deposited on the grid upon drying, and they should form 2D structures, however, if the neuron-like structures would be real, laments would be surrounding the whole structure forming a 3D assembly.…”

“…20 They observed that when the hydrophobic contribution (C 12 alkanes) was not present in the DHBC, the neuron-like structures could not form. The hydrophobic moieties were suggested to induce aggregation of the polymer.…”

“…The apparently contradictory result from the lack of neuron-like structures in a condition where most of the carboxylic groups of PA were de-protonated (high pH), and therefore bound to calcium, can be explained by a too strong driving force towards HA precipitation at this particular pH. 20 The continuous formation of HA crystals would lead to the gradual depletion of the calcium bound to PA to further contribute to HA precipitation.…”