Effect of Corona Block Length on the Structure and Chain Exchange Kinetics of Block Copolymer Micelles

Abstract

The effect of corona block length on micelle structure and molecular chain exchange kinetics has been investigated for a series of dilute poly­(styrene)-b-poly­(ethylene-alt-propylene) (SEP) diblock copolymer micelles with constant PS core block length (⟨N core⟩ ≈ 255) but different PEP corona block lengths (⟨N corona⟩ = 256–2080), using a combination of dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and time-resolved small-angle neutron scattering (TR-SANS). Smaller core radii and aggregation numbers, but significantly thicker corona layers (proportional to N corona 0.7), were observed with increasing corona block length. Furthermore, 2 orders of magnitude more rapid chain exchange was observed in SEP micelles with the longest corona block compared to the shortest. This effect is attributed to the entropic gain arising from the relief of corona chain stretching upon chain expulsion. We further extend a previous theoretical model by explicitly including a corona term associated with the entropy change in the chain exchange process, which successfully explains the influence of the corona blocks on chain exchange. Our results are in excellent agreement with simulation results of Li and Dormidontova but are apparently contradictory with Halperin and Alexander’s theory for hairy micelles and with experimental observations in two other systems that the exchange kinetics slow down with increasing corona block length. These discrepancies reveal unanticipated complexity regarding the role of the corona block in chain exchange.

Interplay of Solvation and Size Effects Induced by the Counterions in Ionic Block Copolymers on the Basis of Hofmeister Series

“…The self‐assembly of amphiphilic block copolymers (BCPs) in selective solvents is increasingly attracting the attention of researchers in the past decades due to their potential applications in biological drug delivery, microreactor chemistry, and in novel nanostructural templating . In selective solvents, neutral BCPs can form various morphologies such as spheres, cylinders, vesicles, and lamellae, which can be regulated by many factors, such as block length, solvent composition, pH, and inorganic salt . Self‐assembly behaviors of these BCPs are well documented …”

Tale of Two Tails: Molecular Exchange Kinetics of Telechelic Polymer Micelles

“…Moreover, the effect of the corona block length on the exchange kinetics is disputed in the literature. While some works report an accelerated exchange of molecules with longer corona blocks because of a reduced G mic [137,143], others report slower exchange due to coronal crowding or slower diffusion because of the molecular size [106,115,118]. By mixing length-mismatched corona chains at different ratios and following their individual exchange in TR-SANS experiments, one might distinguish between the effect of diffusion and crowding.…”

“…In contrast, both Li and Dormidontova and Wang et al observed accelerated molecular exchange in micelles with longer corona blocks. [137,143] They proposed that the longer chains experience a greater entropic penalty in the micellized state, lowering the free energy gain of micellization and thus facilitating expulsion. A unifying explanation for these contradictive findings has yet to be found.…”

Role of hydration and micellar shielding in tuning the structure of single crystalline iron oxide nanoparticles for designer applications

“…The spherical structure consists of loosely associated dynamic unimers, that are regularly exchanging between the micelle and the solvent. [78][79][80] Above the critical micelle concentration, the unimers coalesce into a micelle, triggered in a non-polar solvent by entrapped water molecules. [81] The size and shape of the micelles in solution is driven by the molecular weight of the polymer chains, their interaction with the surrounding solvent, and the amount of entrapped water/solvent.…”