Orientation in Biomimetic Polymer-Mineral Composites
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ORIENTATION IN BIOMIMETIC POLYMER-MINERAL COMPOSITES JEREMY BURDON AND PAUL CALVERT Arizona Materials Laboratories, 4715 E. Fort Lowell Rd., Tucson AZ, 85712.
ABSTRACT Mechanisms for the production of oriented precipitates are reviewed in the context of the orientation in mineralized biological tissues. Organic crystals have been oriented in polymer matrices by crystallization in a temperature gradient and by crystallization within a drawn crystalline polymer. Elongated oxide particles can be formed in a polymer by drawing of a material which contains phase-separated droplets of alkoxide, followed by hydrolysis of the alkoxide. Highly elongated rod-like or disc-like particles can be formed. Alternatively drawn films of a polymer blend can be swollen with alkoxide and hydrolysed to produce elongated chains of particles.
INTRODUCTION Mineralized biological tissue microstructures show a number of aspects which would be beneficial if they could be reproduced in equivalent synthetic materials. In bone the reinforcing hydroxyapatite crystals are small platelets (4nm x 50 nm) with a high aspect ratio, aligned parallel to the collagen fibrils. The mineral is present in volume fractions up to 70%. This high aspect ratio gives good reinforcement while the alignment allows the properties to be optimized to the local stress directions. The volume fraction is higher than can be achieved with comparable synthetic discontinuous fiber composites since dense packing of fibers or plates requires some form of imposed order. In shell and tooth enamel, the volume fractions of mineral are above 90% and ceramic-like properties, with a high modulus and low extension to break, are obtained. We have been studying precipitates which are formed in a polymer matrix by reaction between a soluble inorganic compound and a reagent diffused into the polymer. This can be seen as parallel to many biomineralization processes where a tissue matrix is stiffened by deposition of a carbonate or apatite mineral which forms within and under the control of the organic matrix. In previous studies we have explored the types of chemical reactions that can be used to induce precipitation of various metals and oxides in polymer matrices [1], the factors that control the size of the precipitated particles [2], and the extent to which a matrix can catalyze the precipitation reaction [3]. In this paper we will discuss the development of orientation in the precipitate.
ORIENTATION OF CRYSTALLINE PRECIPITATES In discussing orientation of crystalline precipitates in a matrix, we are interested in both the alignment of the particle with respect to any preferred direction in the matrix and the orientation of the crystal lattice. In bone, hydroxyapatite crystals are lath shaped with the long axis parallel to the collagen fibrils. It is believed that the hydroxyapatite is nucleated in gaps within the collagen fibrils where some specific match between the crystal lattice and the polymer periodicity gives rise Mat. Res. Soc. Symp. Proc. Vol. 255. (01992 Materials Research
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