Effect of Charged Block Length Mismatch on Double Diblock Polyelectrolyte Complex Micelle Cores

Polyelectrolyte complex micelles are hydrophilic nanoparticlesthat self-assemble in aqueous environments due to associative microphaseseparation between oppositely charged blocky polyelectrolytes. Inthis work, we employ a suite of physical characterization tools toexamine the effect of charged block length mismatch on the equilibriumstructure of double diblock polyelectrolyte complex micelles (D-PCMs)by mixing a diverse library of peptide and synthetic charged-neutralblock polyelectrolytes with a wide range of charged block lengths(25-200 units) and chemistries. Early work on D-PCMs suggestedthat this class of micelles can only be formed from blocky polyelectrolyteswith identical charged block lengths, a phenomenon referred to aschain length recognition. Here, we use salt annealing to create PCMsat equilibrium, which shows that chain length recognition, a longstandinghurdle to repeatable self-assembly from mismatched polyelectrolytes,can be overcome. Interestingly, D-PCM structure-property relationshipsdisplay a range of values that vary systematically with the chargedblock lengths and chemical identity of constituent polyelectrolytepairings and cannot be described by generalizable scaling laws. Wediscuss the interdependent growth behavior of the radius, ionic pairaggregation number, and density in the micelle core for three chemicallydistinct diblock pairings and suggest a potential physical mechanismthat leads to this unique behavior. By comparing the results of theseD-PCMs to the scaling laws recently developed for single diblock polyelectrolytecomplex micelles (S-PCMs: diblock + homopolymer), we observe thatD-PCM design schemes reduce the size and aggregation number and restricttheir growth to a function of charged block length relative to S-PCMs.Understanding these favorable attributes enables more predictive useof a wider array of charged molecular building blocks to anticipateand control macroscopic properties of micelles spanning countlessstorage and delivery applications.