A time-resolved SANS method was developed for the determination of unimer exchange kinetics of polymeric micelles under equilibrium conditions. Measurements were performed on the system PEP-PEO/DMF exploiting the large difference in scattering length density of protonated and deuterated species. In contrast to theoretical predictions the observed kinetics consists of two well-separated processes. From temperature-dependent measurements the corresponding activation energies EA could be determined. A comparison on the basis of a scaling prediction of Halperin and Alexander indicates that the fast process can be associated with a single unimer insertion/expulsion mechanism. The slow process, on the other hand, could not be explained on the basis of available microscopic models.

The synthesis of amphiphilic AB block copolymers, having 1,4-polydiene or corresponding hydrogenated aliphatic polyolefin blocks and poly(ethylene oxide) (PEO) blocks, is described. The materials are obtained in a two-step process. In the first step isoprene is polymerized anionically in a hydrocarbon solvent with sec-butyllithium or tert-butyllithium as initiator. The end-capping of the living polymer with ethylene oxide (EO) yields polyisoprene, which is functionalized with a hydroxyl end group (PI-OH). This product can be hydrogenated to the aliphatic polyolefin poly(ethylene-co-propylene) (PEP-OH). In the second step cumylpotassium is used to deprotonate the OH groups, leading to the macroinitiators PI-OK and PEP-OK. PI-PEO and PEP-PEO block copolymers are obtained by initiating the polymerization of EO with the macroinitiators in THF. The characterization reveals narrow molecular weight distributions, absence of homopolymer in the block copolymers, and complete conversion of monomers in both polymerization steps. The materials form water soluble aggregates even at a PEO content of 41% and a total molecular weight of Mn ) 35 400. In n-decane all block copolymers synthesized were soluble, the one with the smallest hydrophobic part having a PI content of 24% and a total molecular weight of Mn ) 17 700.

The aggregation behaviour of six amphiphilic poly(ethylene-propylene)-blockpolyethylene oxide (PEP-PEO) copolymers in water was investigated with the aim to study systematically the influence of a growing soluble block on the structural properties of the micelle. The polymers were prepared such that they differ in the PEO content over a large range of molecular weights, while the PEP size was kept constant. The micelles were characterized by small-angle neutron scattering (SANS) involving contrast variation experiments. Data analysis was performed by a spherical core/shell model with either constant or radial decreasing density profile in the corona and constant density in the core for both cases. The main new result is that we have observed a coronal shape crossover from a micelle with homogeneous density distribution for polymers of low PEO content to a starlike micelle with a hyperbolic density profile for polymers of high PEO content. With respect to the aggregation number this corresponds to a crossover from the mean-field to the scaling behavior of a starlike micelle.Introduction. -It is well known that above a critical polymer concentration (cmc) diblock copolymers in a selective solvent self-assemble into micellar structures. Usually, these structures have a spherical shape in which the insoluble block forms a closely packed inner core and the soluble block an outer corona swollen by the solvent. Essential characteristics of the micelle are the equilibrium aggregation number, P , the core radius, R C , and the overall radius, R M . For a given block copolymer/solvent system the structural parameters can be influenced in particular by changing block copolymer molecular weight and composition.A number of theoretical models were developed to predict the micellar properties and their dependence on block copolymer characteristics [1][2][3][4][5][6][7]. Generally, two different approaches may be distinguished. The first uses mean-field approximations to estimate the enthalpic and entropic contributions to the free energy of micellization. This concept generally is polymer specific and allows a quantitative calculation of micellar parameters but is strictly speaking limited to micelles having thin and dense coronas with roughly uniform concentration profiles, n(r) const. The second approach is based on scaling considerations, which estimate the different contributions to the free energy. In particular two scaling regimes of micellar structures are discussed: i) starlike micelles, where the degree of polymerization, N B , of the soluble block surpasses greatly that of the unsoluble block, N A , and ii) an intermediate regime,

Micelles of a poly(ethylene-co-propylene)-block-Poly(ethylene oxide) (PEP−PEO) copolymer in water were investigated by small angle neutron scattering (SANS), dynamic light scattering (DLS), and viscometry. The block copolymer was built of a partially deuterated PEP block and a hydrogeneous PEO block with an overall molecular weight of 11.1 × 103. The SANS measurements were performed at four contrasts in order to get detailed structural information about the micellar aggregates. The different scattering curves could be described very well by a spherical core−shell model with constant density in both parts. The sharp edges of the polymer density distribution were smeared by multiplication with a Gaussian in Q-space. The model yielded a core radius of R C = 176 Å and an overall micellar radius of R M = 294 Å. The smearing ranges were 11 and 32 Å, respectively. The fit further yielded an aggregation number of P = 2430. DLS measurements reveal a narrow monomodal distribution of relaxation frequencies, indicating the existence of only one aggregated species. The obtained hydrodynamic radius, R H = 339 Å, as well as the radius determined by viscosity measurements, R V = 324 Å, are consistent with the result of the model fitting. The unusually large aggregation number could be explained by the large interfacial tension between water and PEP in terms of simple thermodynamic models.

Amphiphilic block copolymers of vinyl ethers containing 2-hydroxyethyl vinyl ether (HOVE) and partially deuterated n-butyl vinyl ether (NBVE) were synthesized by living cationic polymerization. Four block copolymers with the same hydrophilic length but different hydrophobic lengths were prepared. The internal structures of the micelles formed by these copolymers in aqueous solution were investigated by the contrast variation method of small-angle neutron scattering (SANS) measurement. The molar volume of HOVE was estimated to be quite small from the dependence of the forward scattering intensity on the contrast, which comes from the hydration effect in the micellar shell. The SANS data were well described by the theoretical form factor of a core−shell model. The micellar shape was strongly dependent on the hydrophobic chain length of the block copolymer. The polymer with the shortest hydrophobic chain was suggested to form spherical micelles, whereas the scattering curves of the longer hydrophobic chain polymers showed the q -1 dependence, reflecting the formation of rodlike micelles. These scattering curves could be described well by a sphere−rod coexistence model. The volume fraction of the rodlike micelle was found to increase with increasing hydrophobic chain length.