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...

The impact of the ovo-proteins ovalbumin and lysozyme—present in the first stage of egg shell formation—on the homogeneous formation of the liquid-amorphous calcium carbonate (LACC) precursor, was studied by a combination of complementing methods: in situ WAXS, SANS, XANES, TEM, and immunogold labeling. Lysozyme (pI = 9.3) destabilizes the LACC emulsion whereas the glycoprotein ovalbumin (pI = 4.7) extends the lifespan of the emulsified state remarkably. In the light of the presented data: (a) Ovalbumin is shown to behave commensurable to the ‘polymer-induced liquid precursor’ (PILP) process proposed by Gower et al. Ovalbumin can be assumed to take a key role during eggshell formation where it serves as an effective stabilization agent for transient precursors and prevents undirected mineralization of the eggshell. (b) It is further shown that the emulsified LACC carries a negative surface charge and is electrostatically stabilized. (c) We propose that the liquid amorphous calcium carbonate is affected by polymers by depletion stabilization and de-emulsification rather than ‘induced’ by acidic proteins and polymers during a polymer-induced liquid-precursor process. The original PILP coating effect, first reported by Gower et al., appears to be a result of a de-emulsification process of a stabilized LACC phase. The behavior of the liquid amorphous carbonate phase and the polymer-induced liquid-precursor phase itself can be well described by colloid chemical terms: electrostatic and depletion stabilization and de-emulsification by depletion destabilization.

Mineralization of calcium carbonate in aqueous solutions starting from its initiation was studied by time-resolved small-angle neutron scattering (SANS). SANS revealed that homogeneous crystallization of CaCO 3 involves an initial formation of thin plate-shaped nuclei which subsequently reassemble to 3-dimensional particles, first of fractal and finally of compact structure. The presence of the egg-white protein ovalbumin leads to a different progression of mineralization through several stages; the first step represents amorphous CaCO 3, whereas the other phases are crystalline. The formation and dissolution of the amorphous phase is accompanied by Ca (2+)-mediated unfolding and cross-linking of about 50 protein monomers showing the characteristic scattering of linear chains with a large statistical segment length. The protein complexes act as nucleation centers for the amorphous phase because of their enrichment by Ca (2+) ions. SANS revealed the sequential formation of CaCO 3 starting from the amorphous phase and the subsequent formation of the crystalline polymorphs vaterite and aragonite. This formation from less dense to more dense polymorphs follows the Ostwald-Volmer rule.

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The phase separation mechanism in semidilute aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions is investigated with small-angle neutron scattering (SANS). The nature of the phase transition is probed in static SANS measurements and with time-dependent SANS measurements after a temperature jump. The observed critical exponents of the phase transition describing the temperature dependence of the Ornstein-Zernike amplitude and correlation length are smaller than values from mean-field theory. Time-dependent SANS measurements show that the specific surface decreases with increasing time after a temperature jump above the phase transition. Thus, the formation of additional hydrogen bonds in the collapsed state is a kinetic effect: A certain fraction of water remains as bound water in the system. Moreover, H-D exchange reactions observed in PNIPAM have to be taken into account.