Shell cross-linked knedels: A synthetic study of the factors affecting the dimensions and properties of amphiphilic core-shell nanospheres
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Shell cross-linked knedels (SCK's), nanometer-sized spherical particles that are composed of a core-shell morphology, were prepared by intramicellar polymerization of side chain functionalities along the backbone of the hydrophilic block of amphiphilic polystyrene-b-poly(vinylpyridine) block copolymers organized into micellar assemblies. Control over the size of the SCK's was demonstrated through variation of the chemistry and processing conditions. The solid-state SCK diameters (from atomic force microscopy) ranged from 8 to 30 nm, depending upon the chemical composition and micellization time. The relative amount and nature of hydrophilicity in the amphiphilic block copolymer precursor to the SCK's affects the propensity for aggregation, leading to differences in the aggregation numbers of polymer chains within the micelles and to differences in the dimensions of the SCK's. Therefore, increased lengths of the PS (hydrophobic) block in comparison to the PVP (hydrophilic) block gave larger aggregation numbers and larger SCK's. Alternatively, the incorporation of more quaternized sites along the PVP backbone (increased hydrophilicity) gave smaller SCK's. Quaternization of the PVP using poly(ethylene oxide) (PEO) to give water soluble side chain grafts also resulted in decreased SCK diameters and more narrow size distributions, in comparison to the SCK with no PEO. Allowing longer times for micelle formation prior to cross-linking gave larger SCK's and broader size distributions, due to coalescence of the polymer micelles.