SIZE CONTROL OF NANOPARTICLES IN SEMICONDUCTOR-POLYMER COMPOSITES .2. CONTROL VIA SIZES OF SPHERICAL IONIC MICRODOMAINS IN STYRENE-BASED DIBLOCK IONOMERS

The size control of semiconductor nanoparticles via primary aggregate sizes in ionomers has been extended to include the precipitation of cadmium sulfide (CdS) within the spherical ionic cores of PS-b-PACd diblock ionomers of various ionic block lengths. This procedure yields clusters in the range of 2R = 29-50 Angstrom, using ionic block lengths of 4-32 repeat units. The sizes of CdS clusters formed in water-plasticized ionic cores, determined by UV-vis spectroscopy, were found to increase linearly with the size of the original core (correlation coefficient = 0.99) and scale with the ionic block length as N-B(3/5), in agreement with the Halperin model of ''star micelles''. Comparisons between CdS agglomeration numbers and the number of countercations in the original core were used to show that, except in the largest ionic cores, a single particle per core was obtained. The stability of the CdS-containing micelles in a selective solvent can be enhanced considerably by reneutralizing the poly(acrylic acid) layer surrounding the clusters within the core. These stabilized micelles can be dissolved in an organic solvent, precipitated into methanol, and redissolved several times, without significant changes in the UV-vis absorption spectrum. It was also found possible to process the CdS-polymer composites by casting from toluene or compression molding at 130 degrees C, without affecting the nanoparticle sizes. Cluster sizes can be increased by reneutralizing the CdS-containing micelle cores with cadmium acetate, followed by secondary treatment with H2S. By this method, clusters of 37 Angstrom in diameter were enlarged to 56 Angstrom in two steps.