Effect of Block Copolymer Architecture on Lead(II) Bromide (PbBr2) Dissociation and Complexation via Surface...

In this study, we systematically investigate how the molecular weight and block volume fraction of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymers (diBCPs) influence PbBr2 dissociation and complexation through surface adsorption in o-xylene under stirring. o-Xylene is a selective solvent, good for PS but poor for both PEO and PbBr2. Five PSmk-b-PEOnk diBCPs (PS10k-b-PEO11.5k, PS33k-b-PEO13.5k, PS33k-b-PEO35k, PS38k-b-PEO102k, and PS140k-b-PEO141k) with different block ratios and molecular weights were examined, where the subscripts denote the molecular weights of the constituent blocks in kg mol-1. The polymer concentration was fixed at 1 mg/mL. Under this condition, PS-b-PEO is interpreted to exist predominantly as dispersed chains with collapsed PEO segments and swollen PS segments, as well as chain-associated clusters, rather than forming well-defined micelles. As a result, surface adsorption is dominated by dispersed chains: PEO segments act as adsorbing blocks through coordination between ether groups and Pb2+ cations, while PS segments behave as nonadsorbing blocks swollen by o-xylene. Upon addition of PbBr2, PS10k-b-PEO11.5k and PS33k-b-PEO13.5k effectively induce PbBr2 dissociation and complexation to form [PbxBry]2x-y complexes, whereas PS33k-b-PEO35k, PS38k-b-PEO102k, and PS140k-b-PEO141k exhibit a much lower extent of these processes, resulting in negligible complex formation. Two key factors are proposed to account for this behavior. The first is the chain conformation of the adsorbed copolymers. In the proposed adsorption model, PEO blocks adopt train- and loop-like conformations, while PS blocks form tail-like conformations. Short PEO chains favor conformations with a high density of trains and a low density of loops, whereas long PEO chains exhibit the opposite trend. Because PbBr2 dissociation and complexation occur at the interface between adsorbed PEO segments and PbBr2 surfaces, a higher train density promotes the formation of [PbxBry]2x-y complexes. In contrast, long PEO chains favor adsorption conformations dominated by loops with fewer trains, further reducing effective interfacial contact and rendering PbBr2 dissociation and complexation sluggish. The second proposed factor is the ability of PS blocks to screen unfavorable PEO-solvent interactions. When PS chains are sufficiently longer than PEO chains, they effectively shield the adsorbed PEO segments from o-xylene, thereby stabilizing adsorption and promoting dispersion. In contrast, when PS chains are much shorter than PEO chains, they fail to provide adequate screening, leading to a high interfacial energy penalty that suppresses dispersion and hinders surface adsorption.