Molecular Exchange in Ordered Diblock Copolymer Micelles

Molecular exchange kinetics of diblock copolymers forming spherical micelles packed on a body-centered cubic (bcc) lattice were investigated using time-resolved small-angle neutron scattering (TR-SANS). Ordered arrays of polystyrene spheres were prepared by mixing 15 vol % poly(styrene-b-ethylene-alt-propylene) (PS-PEP) in squalane, a highly selective solvent for the PEP block. Two pairs of diblock copolymers were examined, characterized by two different PS core block molecular weights (< M-PS > = 26500 and 42800). Each pair contained two nearly identical diblock copolymers, one with a deuterated PS block (dPS-PEP) and the other with a protonated PS block (hPS-PEP). Protonated and deuterated squalane were blended to achieve a contrast-matched condition with uniformly mixed dPS and hPS (50/50 by volume) core chains. Beginning with a statistically random array of pure dPS and hPS cores distributed on the bcc lattice, molecular exchange was monitored at multiple temperatures by TR-SANS. Exchange of deuterated and protonated chains results in a decay in SANS intensity, which yields a kinetic function revealing a broad spectrum of relaxation times. These results are qualitatively consistent with our previously reported findings for molecular exchange between PS-PEP micelles in a dilute (1 vol %) squalane solution. However, the measured characteristic time constant for the concentrated, ordered system is more than an order of magnitude slower than in the dilute, disordered dispersion.