Effect of Core Crystallization and Conformational Entropy on the Molecular Exchange Kinetics of Polymeric Micelles

Here we systematically study the equilibrium molecular exchange kinetics of a series of amphiphilic n-alkyl-poly(ethylene oxide) (C-n-PEO) micelles containing partly crystallized cores. Using differential scanning calorirnetry (DSC), we determined the melting transition and extracted the enthalpy of fusion, Delta H-fus of the n-alkyl chains inside the micellar core. Molecular exchange kinetics was measured below the melting point using a time-resolved small-angle neutron scattering technique (TR-SANS) based on mixing deuterated and proteated but otherwise identical micelles. Comparing both kinetic and thermodynamic data, we find that crystallinity within the micellar cores leads to significant enthalpic and the entropic contributions to the activation barrier for molecular exchange. While the former leads to an enhanced stability, the positive entropic gain favors the process. Interestingly, the entropic term contains an excess term beyond what is expected from the measured entropy of fusion. Based on calculations using the Rotational Isomeric State (RIS) model, we suggest that the excess entropy is due to the gain in conformational entropy upon releasing the chain from the confined state in the core. The study thus provides deep insight into the fundamental processes of micellar kinetics and which might be relevant also to other semicrystalline soft matter and biological systems including lipid membranes.