One of the main goals of polymer science has been to relate the structure of macromolecular chains to their macroscopic properties. In particular, it has been hoped that one could relate the sizes of polymer coils to the degree to which they entangle with one another and thus to their viscoelasticity in the melt. In recent years, the availability of model polymers with nearly monodisperse molecular weight distributions and precisely controlled chemical structures has allowed for improved data both on rheology and on the dimensions of the chains. This has now allowed us to determine the correlations between such properties as chain dimensions, density, and plateau modulus and to show that some quite simple relations exist between them. The main body of these data is on polymers that can be considered to be models for polyolefins. These have been made by the hydrogenation of polydienes synthesized by anionic polymerization techniques. In this way the molecular weight distribution can be made to be nearly monodisperse (Afw/M" < 1.1) and the chemical structure is well controlled. For example, models of a wide range of ethylene-butene copolymers have been made by the saturation of polybutadienes with a range of vinyl content. Such polymers can be made at many molecular weights as well. The viscoelastic properties of these polymers have been measured very precisely, and their chain dimensions have been determined by small-angle neutron scattering. To a high degree of correlation, we find that the mean-square unperturbed end-to-end distance,

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...

We have examined doxorubicin's (DOX) physical state in solution and inside EPC/cholesterol liposomes that were loaded via a transmembrane pH gradient. Using cryogenic electron microscopy (cryo-EM) we noted that DOX loaded to 200-300 mM internal concentrations in citrate containing liposomes formed linear, curved, and circular bundles of fibers with no significant interaction/perturbation of the vesicle membrane. The individual DOX fibers are putatively comprised of stacked DOX molecules. From end-on views of bundles of fibers it appeared that they are aligned longitudinally in a hexagonal array with a separation between fibers of approx. 3-3.5 nm. Two distinct small angle X-ray diffraction patterns (oblique and simple hexagonal) were observed for DOX-citrate fiber aggregates that had been concentrated from solution at either pH 4 or 5. The doxorubicin fibers were also present in citrate liposomes loaded with only one-tenth the amount of doxorubicin used above (approx. 20 mM internal DOX concentration) indicating that the threshold concentration at which these structures form is relatively low. In fact, from cryo-EM and circular dichroism spectra, we estimate that the DOX-citrate fiber bundles can account for the vast majority (>99%) of DOX loaded via a pH gradient into citrate buffered liposomes. DOX loaded into liposomes containing lactobionic acid (LBA), a monoanionic buffer to control the internal pH, remained disaggregated at internal DOX concentrations of approx. 20 mM but formed uncondensed fibers (no bundles) when the internal DOX concentration was approx. 200 mM. This finding suggests that in the citrate containing liposomes the citrate multianion electrostatically bridged adjacent fibers to form the observed bundles. 13C-NMR measurements of [1,5-13C]citrate inside liposomes suggested that citrate 'bound' to the DOX complex and 'free' citrate rapidly exchange indicating that the citrate-DOX interaction is quite dynamic. DOX release into buffer was relatively slow (<4% at 1 h) from liposomes containing DOX fibers (in citrate loaded to a low or high DOX concentration or in LBA liposomes loaded to a high internal DOX concentration). LBA containing liposomes loaded with disaggregated DOX, where the internal DOX concentration was only approx. 20 mM, experienced an osmotic stress induced vesicle rupture with as much as 18% DOX leakage in less than 10 min. The possible implications for this in vivo are discussed.

A neutron scattering study of the structure of amorphous glucoseWe present the results of a combined experimental ͑neutron scattering͒ and theoretical ͑computer simulation͒ effort to investigate structural properties of polycarbonate melts and glasses in the wave vector regime of Qр2.2 Å Ϫ1 . The experimental part consists of advanced spin polarized scattering experiments, allowing us to extract the coherent scattering for protonated and deuterated samples. The simulations employ recently developed novel mapping procedures, which allow us to efficiently equilibrate complex polymer melts, thereby reproducing the experiment in much closer detail than earlier attempts.

Small-angle neutron scattering was used to measure the influence of temperature on the radius of gyration in the melt of atactic polypropylene and poly(1-butene). The chain dimension temperature coefficient, d In (RZ)ddT, was found to be respectively -0.1 X and 0.4 X le3 degl. These values, with one exception, disagree with the rotational isomeric state predictions as well as with thoee values obtained via intrinsic viscosity measurements either at 8 conditions or in perceived athermal solvents. Conversely, excellent agreement was obtained with the values available from both thermoelastic and rheological measurements on atactic poly(1-butene).