Equilibrium exchange kinetics in n-alkyl–PEO polymeric micelles: single exponential relaxation and chain length dependence

Abstract

In this communication we present first results on the chain exchange kinetics of n-alkyl-PEO polymeric micelles by time-resolved small angle neutron scattering. We found that the rate strongly depends on the alkyl-chain length and that the relaxation function almost perfectly follows the single exponential decay predicted by theory. The key achievement of this study is the experimental verification that core block polydispersity accounts for the almost logarithmic time decay in block copolymer micelles as recently suggested by Choi et al. The results thus directly show that unimer exchange is the main mechanism for molecular exchange in block copolymer micelles.An important issue in the understanding of the self-assembling behavior of diblock copolymers in selective solvents is the kinetics of chain exchange between individual micellar entities in thermodynamic equilibrium.1 The exchange can be monitored by timeresolved small angle neutron scattering (TR-SANS) applying a sophisticated contrast variation technique. Significant perturbations as often required for other methods here are limited to simple hydrogen/deuterium isotope labelling.2-9 In an earlier work by using this technique we have studied the chain exchange kinetics of poly (ethylene-alt-propylene)-poly(ethylene oxide) (PEP-PEO) micelles in water/N,N-dimethylformamide mixtures.4,5,10 There we found that the measured relaxation curves follow a logarithmic time dependence in contradiction to the single exponential decay predicted by the theory of Halperin and Alexander.11 A single exponential decay was also observed by dissipative particle dynamics simulation 12 but, in addition to single unimer exchange, contributions from small aggregate fragmentation/merging and unequal size fusion/fission as additional kinetic mechanisms were found. In order to explain the logarithmic decay, Lund et al.4,5 discussed different mechanisms including possible effects of polydispersity but none of them finally could sufficiently explain the observed behavior. More recently Choi et al.8 succeeded in describing the logarithmic relaxation by taking properly into account the polydispersity of the core forming block. Following these ideas also the data of Lund et al.7 could be successfully re-evaluated thereafter. As put forward by Choi et al. the enormous effect of polydispersity on the kinetics can be rationalized by a double exponential dependence of the exchange rate on the core chain length: R(t) ¼ exp(Àkt) with k $ exp(ÀE a /k B T) where the activation energy E a is a function of the degree of polymerization N of the core block. Thus polydispersity effects become crucial even though M w /M n (where M w and M n the weight and number average molecular weight) is generally very small in micelle forming model polymers. However, direct experimental evidence is still missing due to the lack of suitable model compounds which should ideally consist of a monodisperse core polymer. As polymerization is a statistical process inherently leading to materials with a chain length dist...

Spatiotemporal control and superselectivity in supramolecular polymers using multivalency

“…In contrast, the exchange rate is significantly faster at higher temperatures. These results resemble observations on self-assembled polymeric micelles, in which monomer expulsion and insertion is the rate-determining step of system dynamics (20,21). Moreover, monomer polymerization and depolymerization are reported to be the main exchange mechanism of supramolecular polymers in organic solvents (22).…”

Effects of Intermicellar Interactions on the Dissociation of Block Copolymer Micelles: SANS and NMR Studies

“…Similar to low‐molecular‐weight surfactants, diblock copolymers in a selective solvent exhibit a critical micellar concentration (CMC) and temperature (CMT) . In comparison to surfactant micelles, the dynamics of block copolymer micelles at equilibrium show more complex behaviors, and are affected by additional factors, such as polydispersity and accessible conformations, due to the nature of polymer chains . The micellization of diblock copolymers studied by computer simulation suggested that both the stepwise insertion–expulsion of single polymer chains and fusion/fission of whole micelles contributed to the redistribution of the micellar size to the equilibrium one .…”

3-Helix Micelles Stabilized by Polymer Springs

“…27,28 Unimer desorption is the rate-limiting step and its activation energy barrier depends on the length of the hydrophobic block and the intermolecular interactions and steric hindrance between each component of both the head and the tail block. 29–33 Increasing the hydrophobic chain length beyond certain values may lead to kinetically trapped heterogeneous aggregates. Chemically crosslinking the headgroups effectively reduces the unimer desorption kinetics and stabilizes micelle.…”