A systematic study of the equilibrium chain exchange kinetics of a tunable model system for starlike polymeric micelles is presented. The micelles are formed by well-defined highly asymmetrical poly-(ethylene-propylene)-poly(ethylene oxide) (PEP-PEO) diblock copolymers. Mixtures of N,N-dimethylformamide (DMF) and water are used as selective solvents for PEO. With respect to PEP this solvent mixture allows the interfacial tension, γ, to be tuned over a wide range. The equilibrium chain exchange between these micelles has been investigated using a novel time-resolved small-angle neutron scattering (TR-SANS) technique. The results show that the exchange kinetics is effectively frozen for large interfacial tensions but can be readily tuned to accessible time scales (minutes to hours) by lowering γ. Independent of temperature and concentration, the corresponding relaxation functions show an extremely broad and heterogeneous logarithmical decay over several decades in time. We explicitly show that such broad relaxation cannot be explained by polydispersity or a classical distribution of activation energies. Instead, the logarithmic time dependence points toward a complex relaxation picture where the chains are slowed down due to mutual topological and geometrical interactions. We propose that the behavior stems from constrained core dynamics and correlations between the expulsion probability of a chain and its conformation.

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We have studied the aggregation behavior of polyethylene−poly(ethylenepropylene) (PE−PEP) diblock copolymers dissolved in decane. For this purpose PE−PEP diblock copolymers of various molecular weights, compositions, and degrees of deuteration were synthesized via an anionic route. The structure and morphology of the aggregates was studied by small angle neutron scattering varying both the contrast as well as the polymer labeling. We found a hierarchy of structures:  The PE component crystallizes in lamellar sheets (thickness 40−80 Å) surrounded on both sides by a PEP brush which exhibits a close to parabolic density profile. Different aggregates form macroaggregates of needlelike shape with the PE lamellar planes in the long direction. This macroaggregation is well described by a paracrystalline structure factor. The structural parameters depending on composition and molecular weights can be well understood in terms of a free energy of formation based on a scaling model. A quantitative evaluation of the different contributions to the free energy reveals an important role of defect structures resulting from the ethylene side branches in the polyethylene component. Finally, we show in a semiquantitative approach that the van der Waals energy between the brushes is large enough to facilitate macroaggregation.

In this paper, we have addressed the question of the dynamic miscibility in a blend characterized by very different glass-transition temperatures, Tg, for the components: poly(ethylene oxide) and poly(methyl methacrylate) (PEO/PMMA). The combination of quasielastic neutron scattering with isotopic labeling and fully atomistic molecular dynamics simulations has allowed us to selectively investigate the dynamics of the two components in the picosecond-10 nanoseconds scale at temperatures close and above the Tg of the blend. The main focus was on the PEO component, i.e., that of the lowest Tg, but first we have characterized the dynamics of the other component in the blend and of the pure PEO homopolymer as reference. In the region investigated, the dynamics of PMMA in the blend is strongly affected by the alpha-methyl rotation; an additional process detected in the experimental window 65 K above the blend-Tg can be identified as the merged alphabeta process of this component that shows strong deviations from Gaussian behavior. On the other hand, pure PEO displays entropy driven dynamics up to very large momentum transfers. Such kind of motion seems to freeze when the PEO chains are in the blend. There, we have directly observed a very heterogeneous and moreover confined dynamics for the PEO component. The presence of the hardly moving PMMA matrix leads to the creation of little pockets of mobility where PEO can move. The characteristic size of such confined islands of mobility might be estimated to be of approximately 1 nm. These findings are corroborated by the simulation study, which has been an essential support and guide in our data analysis procedure.

We have investigated the dynamic structure factor for single-chain relaxation in a polyethylene melt by means of molecular dynamics simulations and neutron spin echo spectroscopy. After accounting for a 20% difference in the chain self-diffusion coefficient between simulation and experiment we find a perfect quantitative agreement of the intermediate dynamic structure factor over the whole range of momentum transfer studied. Based on this quantitative agreement one can test the experimental results for deviations from standard Rouse behavior reported so far for only computer simulations of polymer melt dynamics. [S0031-9007(98)05363-0] PACS numbers: 61.25.HqThe dynamics of polymer chains in a dense melt could be supposed to pose a theoretical problem requiring a very complex and mathematically involved description. We have to describe a liquid of intertwined threads where each of them has on average excluded volume interactions with p N other threads, where N is the degree of polymerization of the chains. According to all experimental evidence so far, e.g., Refs. [1-3], however, it seems that all these complex topological interactions can be completely neglected as long as the degree of polymerization of the chains is below some critical value, the so-called entanglement molecular weight N e . For chains longer than N e the entanglements have to be taken into account [4-7] but for shorter chains the simple Rouse theory [8] is supposed to describe the chain dynamics. Computer simulations of abstract [9,10] as well as atomistic [11] polymer models, on the other hand, show systematic deviations from the Rouse behavior, which can be traced to the interactions between the chains in the melt.We will show in this paper the first detailed quantitative comparison between a molecular dynamics (MD) simulation of the melt dynamics of an atomistic polymer model and a neutron spin echo (NSE) determination of the single-chain dynamics in the same polymer melt. By establishing the quantitative agreement between simulation and experiment for the internal dynamics of the chains we can then draw conclusions about the validity or shortcom-ings of the Rouse model from the combined information of simulation and experiment.Simulations and experiments were performed on a dense polyethylene melt of n-C 100 H 202 at 509 K. Experimentally we had already obtained information on the dynamic behavior of longer chain polyethylene (PE) samples at the same temperature from neutron scattering studies [1,2], and we had validated a united atom (UA) model (CH 2 groups treated as one superatom) [12] as well as an explicit atom (EA) model [13] by simulations of shorter chain alkanes. The C 100 chains are slightly shorter than the entanglement length of PE at this temperature (N e 136 [2]) and long enough to show Gaussian chain statistics in their conformations [14], thereby making them the ideal test system for a description by the Rouse model. After equilibration for 3 ns we performed a NVT (constant number of particles, volume, and temperature) mole...