We present the equilibrium chain exchange kinetics in spherical micelles formed by n-alkane-PEOx (CnH2n+1-PEOx with n = 18, 21, 24, 27, 30 and x = 5, 10, 20, 40 kg/mol) block copolymers in aqueous solution. Due to the very well-defined system we could systematically study the effect of polymer composition on the molecular exchange kinetics. To access the kinetics we applied a sophisticated contrast variation technique by using time-resolved small-angle neutron scattering (TR-SANS) [1,2,3].
In our study, it was found that the exchange process dramatically depends on the alkane chain length, while the molecular exchange for C18-PEO5 is too fast, C30-PEO5, is too slow to be measured. We will present kinetic results of Cn-PEO5 with n= 21, 24, 27 and 28. As expected from theories [4], we find a first-order kinetic process characterized by a single-exponential relaxation function. From temperature dependent measurements we deduced the activation energies for such thermal activated kinetic processes. Furthermore, we show that the relaxation is independent of micelle concentration in the dilute limit, supporting that single-unimer exchange is the dominant kinetic mechanism. Very interestingly, the exchange slows down with increasing PEO chain length.
In our presentation a detailed description of the experiment will be given including a thorough discussion of the main results.
References:
[1] T. Zinn, L. Willner, R. Lund, V. Pipich and D. Richter, Soft Matter 2012, 8, 623-626
[2] R. Lund, L. Willner and R. Richter Adv. Polym Sci. 2013, 259, 51-158
[3] S.-H. Choi, T. P. Lodge and F. S. Bates, Phys. Rev. Lett. 2010, 104, 047802
[4] A. Halperin and S. Alexander, Macromolecules 1989, 22, 5072-5074
