Hydrogel-coated glassy nanospheres: A novel method for the synthesis of shell cross-linked knedels
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A novel method was developed to prepare shell-cross-linked knedel-like structures (SCK's), which are double-layered polymer nanospheres, in a convenient three-step procedure: (1) polystyrene-b-poly(acrylic acid) ((DP(n)PS = 130, (DP(n))(PAA) = 120) copolymer was prepared by sequential anionic polymerization of styrene and tert-butyl acrylate, followed by hydrolysis; (2) the amphiphilic block copolymer was allowed to self-assemble into spherical multimolecular micelles in a water and THF solvent mixture; (3) the poly(acrylic acid), comprising the shells of micelles, was cross-linked by amidation using several di- and multiamino linkers (2,2'-(ethylenedioxy)bis(ethylamine), hexakis(ethylene glycol)diamine, hexamethylenediamine, and triethylenetetramine). The extent of cross-linking was controlled by the relative amount of cross-link reagent used. The SCK's were spherical, core-shell structures composed of polystyrene cores and cross-linked hydrogel-like shells. The sizes, shapes, and structural differences between the polymer micelles and the SCK's were studied by atomic force microscopy (AFM), transmission electron microscopy (TEM), and dynamic light scattering (DLS) in aqueous solution. The non-cross-linked polymer micelles deformed substantially upon adsorption onto mica and became ellipsoidal upon drying on a carbon surface, whereas the SCK's remained as stable, spherical structures under all conditions. The polymer micelles had a number average height of 17 nm from AFM and 26 nm diameter from TEM, while the SCK's prepared from 2,2'-(ethylenedioxy)bis(ethylamine) cross-linkers had a number average height of 24 nm from AFM and a number average diameter of 28 nm from TEM. The SCK shell thickness swelled 2-3-fold in water, as determined from comparison of the number average diameter obtained by DLS (37 nm) with that by TEM. Spectroscopic methods and thermal analysis were utilized to further characterize the SCK's.
