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The effect of type and amount of hydroxyapatite on the setting kinetics of an experimental bone cement based on poly(methyl methacrylate-co-styrene) was studied. The average molecular weights of the polymeric beads synthesized were determined by SEC and the average particle size was determined by Optical Microscopy. Three types of hydroxyapatites were synthesized in the laboratory and then characterized by ICP, FTIR and X-ray diffraction. To obtain more compatible fillers, the hydroxyapatites were treated with 3-trimethoxysilylpropylmethacrylate. Bone cements formulations filled with 0, 10, 30, and 50 weight % of hydroxyapatite powders were prepared and the kinetics of setting was followed by Differential Scanning Calorimetry. The presence of hydroxyapatite decreased the reaction rate and increased the degree of conversion, which could be beneficial for the long time stability of the implant.

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SYNOPSISA butyl acrylate monomer with a small amount of photosensitizer was diffused into a slightly crosslinked polystyrene matrix. After a certain swelling time, the diffusion gradient was either fixed (by rapid photopolymerization in situ) to form a gradient polymer or changed to form an interpenetrating polymer network (IPN), once thermodynamic equilibrium was achieved. Chemical compositions were determined by FTIR. Mechanical and rheological measurements were performed using a universal testing machine and dynamic rheometer, respectively. Impact properties were evaluated with a Falling Dart Impact Tester. Compared to the IPN materials, the gradient polymers exhibit higher moduli, considerable strain, and similar impact resistance. They are also able to retain plastic properties at higher temperatures. Differences in chemical structure of equivalent layers for gradient and IPN polymers yield such differences in properties. 0 1995 John Wiley & Sons, Inc. INTRODUCTIONIn recent years, the production of about 2 billion pounds of polymer blends per year in the United States was reported.' Furthermore, while the annual growth rate of the plastics industry was 2-496, that of the polymer alloys and blends was 9-11%.2 That production and growth rate has been based mainly on research made over the last two decades on multicomponent polymers. Scientific interest and the technical importance of polymer blends is due to several reasons: First, no new polymers need to be synthesized; in addition, based on thermodynamic studies of phase equilibria in multicomponent polymer systems, blending feasibility has become less o b~c u r e .~-~ Besides, because of their heterogeneous nature, polymer blends lead to systems with two or more phases whose properties depend not only on the individual characteristics of the components, but also on the method of p r e p a r a t i~n .~-~ The long-term * To whom correspondence should be addressed.Journal of Applied Polymer Science, Vol. 58, 2207Vol. 58, -2212Vol. 58, (1995 stability of the properties of such systems can noticeably be affected if the components have the tendency to phase-separate. Blend structure stability and low birefringence can be enhanced if phase separation is restricted to the microscopic level: Furthermore, if the phases are interconnected, the morphology leads to a synergistic performan~e.6,~~~-" For that reason, slightly crosslinked interpenetrated polymer networks offer a good way of making stable products of better performance than that of copoly m e r~.~. " Even, further, if a continuous change in the composition structure of interpenetrated macromolecules is achieved throughout a polymer blend, a continuous change in individual properties would be expected.13 The spatial gradient formed in that way opens up the possibility of making polymer systems with tailor-made properties for different apOne method to prepare such structures is by sequential polymerization. A convenient profile throughout the sample can be reached by diffusing a monomer into a polymer mat...

ABSTRACT:To be able to control composition structure in gradient polymers prepared by sequential polymerization, diffusion phenomena has to be considered, particularly for the first 100% weight increment in a glassy polymer matrix. With that purpose, an analytical model to predict diffusion in that region has been developed for amorphous polymers. The inclusion of a relaxation time to estimate surface concentration changes during sorption led to diffusion coefficients one order of magnitude higher than Fickian coefficients. However, adding a volume increment term to account for polymer swelling, diffusion coefficients went up to 48 times the Fickian values. Experimentally, butyl acrylate with a small amount of photosensitizer was diffused into a slightly crosslinked polystyrene slab matrix at different temperatures in the glassy region. After fixing the gradient composition by photopolymerization, chemical structures throughout the slab were determined by FTIR. The proposed model was confronted with experimental sorption showing a close fit at the different temperatures in the region of interest.