Single molecules and associates of heteroarm star copolymer visualized by atomic force microscopy

Single molecule conformations and associate morphologies for polystyrene/poly(2-vinylpyridine) (PS7-P2VP(7)) heteroarm star copolymer, formed in controlled environment and deposited onto mica or Si wafers, have been studied by atomic force microscopy (AFM) with molecular resolution. At concentrations below 0.01 mg/mL PS7-P2VP(7) exists in the molecularly dissolved state in both selective (acidic water, toluene) and common good (chloroform, tetrahydrofuran) solvents. In acid conditions PS7-P2VP(7) forms either unimolecular or multimolecular micelles depending on concentration and pH. The core of the micelles is constituted of the collapsed PS arms surrounded by protonated P2VP shell. PS7-P2VP(7) undergoes inverse intramolecular segregation upon addition of toluene. In this case, P2VP arms form the dense core of the micelles embedded in the swollen PS shell. The transition between those two inverse types of micelles is strongly modified by interaction with the mica substrate. The micelles deposited onto mica from acidic water are trapped via P2VP extended arms. Upon treatment of the trapped micelles with toluene the PS core is swollen and PS arms gradually adapt an extended conformation whereas P2VP trapped arms retain their extended conformation due to the strong interaction with the mica substrate. The obtained "squash" structures exhibit a unique conformation that does not exist in any solvent and could not be obtained upon a simple adsorption procedure. Surface-supported metallization of the PS7-P2VP(7) on Si wafers improves AFM resolution due to the selective deposition of the Pd clusters along the P2VP chains. Thus, the number of P2VP arms of the unimers as well as the aggregation number of the multimolecular micelles and morphological details of the structures were directly analyzed.