Title

A mechanistic approach to the synthesis of biomimetic polymer-inorganic hybrids

Date of Completion

January 1996

Keywords

Engineering, Biomedical|Engineering, Materials Science

Degree

Ph.D.

Abstract

Study of mineral deposition from solution is important for understanding not only the mineralization of tissues such as nacre and bone, but also the formation of scale in organs, such as kidney stones, scale as well as in boilers and scale in reactors. Promoting or suppressing mineral deposition requires an understanding of the interactions between the mineral solid phase and the substrate surface.^ Heterogeneous nucleation of calcium carbonate crystals from a super-saturated solution on chitosan-film surface is required, and homogeneous or spontaneous nucleation in solution must be suppressed to synthesize two dimensional chitosan-calcium carbonate hybrids by mimicking nacre mineralization via biomimetic processing. The heterogeneous nucleation of calcium carbonate crystals on a chitosan surface is influenced greatly by the chitosan surface properties. Several acidic materials have been tested as additives, and polyacrylic acid with molecular weight 2,000 (PAA2K) has proved to be an appropriate promoter of heterogeneous nucleation of calcium carbonate crystals on the chitosan film and to suppress the homogeneous nucleation at the same time. Promotion of heterogeneous nucleation is concentration dependent. In the absence of the polyacrylic acid, spontaneous nucleation occurs in the solution. At the high concentration of the PAA2K, crystallization of calcium carbonate is suppressed both in solution and on the chitosan film. Local supersaturation around the chitosan film, a crucial requirement for heterogeneous nucleation on the film, cannot be achieved at high PAA2K concentration due to the high affinity of mobile PAA2K chains to calcium ions in solution. PAA2K penetrated into the chitosan film and protonated nitrogen and carboxylic anions were formed. Nucleation is initiated from these charges. Chitosan-calcium carbonate hybrids with strong interaction at the interface has been achieved by biomimetic synthesis.^ A multi-layer model has been proposed which mimics nacre to produce three dimensional inorganic-polymer hybrids. Poly (L-lactide (PLLA)-co-polydimethylsiloxane (PDMS)) copolymers have been synthesized as polymeric materials, and rod-like nanohydroxyapatite have also been prepared as the ceramic part. Solidification is one characteristic of L-lactide bulk polymerization at a reaction temperature of 120$\sp\circ$C. A bimodal molecular weight distribution occurs when using a low concentration of PDMS due to the existence of two initiation and propagation mechanisms. As a consequence, the mixture of ABA block copolymer and PLLA homopolymer is obtained. A high concentration of PDMS and low content of catalyst, viz.: stannous octoate, favor the formation of ABA block copolymers. ^