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...

aHere we present an extensive small-angle neutron scattering (SANS) structural characterization of micelles formed by poly(ethylene oxide)-mono-n-alkyl ethers (C n -PEOx) in dilute aqueous solution. Chemically, C n -PEOx can be considered as a hybrid between a low-molecular weight surfactant and an amphiphilic block copolymer. The present system, prepared through anionic polymerization techniques, is better defined than other commercially available polymers and allows a very precise and systematic testing of the theoretical predictions from thermodynamical models. The equilibrium micellar properties were elaborated by systematically varying the n-alkyl chain length (n) at constant PEO molecular weight or increasing the soluble block size (x), respectively. The structure was reminiscent of typical spherical starlike micelles i.e. a constant core density profile, $r 0 , and a diffuse corona density profile, $r À4/3. Through a careful quantitative analysis of the scattering data, it is found that the aggregation number, N agg initially rapidly decreases with increasing PEO length until it becomes independent at higher PEO molecular weight as expected for star-like micelles. On the other hand, the dependency on the n-alkyl length is significantly stronger than that expected from the theories for star-like block copolymer micelles, N agg $ n 2 similar to what is expected for surfactant micelles. Hence the observed aggregation behavior suggests that the C n -PEOx micelles exhibit a behavior that can be considered as a hybrid between low-molecular weight surfactant micelles and diblock copolymer micelles.

The rate of molecular exchange in diblock copolymer micelles is strongly dependent on the chain length of the core-forming insoluble block. Less is known about the influence of the soluble block forming the micellar corona. In this study we present a time-resolved small angle neutron scattering (TR-SANS) study exploring systematically the effect of corona chain length on the dynamics of chain exchange. As a model system we have taken amphiphilic AB diblock copolymers of the type C27H55-poly­(ethylene oxide)x (C27–PEOx) with varying x between 4 and 36 kg/mol in aqueous solution in which well-defined spherical micelles with partially crystalline cores are formed. The TR-SANS results show that the chain exchange slows down considerably upon increasing PEO molecular weight, while the characteristic “attempt time” constant, τ0, was found to increase with a power law dependence τ0 ∼ M PEO 9/5. The results are in excellent agreement with the Halperin and Alexander model and can be attributed to a reduced diffusion rate through the micellar corona. Our results clearly demonstrate that the rate for molecular exchange is not directly coupled to the solubility of the amphiphile and the critical micellar concentration, as has previously been indicated.

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 calorimetry (DSC), we determined the melting transition and extracted the enthalpy of fusion, ΔH 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.