Hydrogen–hydrogen intermolecular structure of polyethylene in the melt

Multiscale Modeling of Poly(ethylene oxide)-Poly(propylene oxide)-Poly(ethylene oxide) Triblock Copolymer Micelles in Aqueous Solution

We present a multiscale modeling approach for simulation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer micelles in aqueous solution. We rely on systematic elimination of computationally expensive degrees of freedom yet retain implicitly their influence on the remaining degrees freedom in a coarser-grained model. Quantum chemistry (QC) calculations, atomistic explicit solvent (AES) molecular dynamics (MD) simulations, and coarse-grained implicit solvent (CGIS) simulations have been applied to investigate physical properties of these important self-assembling triblock copolymers. High-level QC calculations have been used to parametrize classical atomistic force fields that implicitly take into account and reproduce the important energetic and structural features due to correlations of electronic degrees of freedom. AES MD simulations utilizing the QC-based force fields have been used to provide structural and conformational properties of polymers in aqueous solution which were subsequently used for parametrization of the CGIS model using the Inverted Boltzmann method. The CGIS simulations were then employed to investigate structural properties of two PEO-PPO-PEO micelles (EO13-PO30-EO13 and EO99-PO65-EO99 also known as Pluronic L64 and F127, respectively) in aqueous solution.

Characterization of the "simple-liquid" state in a polymeric system: coherent and incoherent scattering functions

By means of time-of-flight neutron scattering, we have characterized the dynamic structure factor and the hydrogen motions of polyethylene above its melting point. As signatures of simple-liquid dynamics, we observe that (i) in the explored dynamic window, the intermediate scattering functions display a single-step decay, without reminiscence of cage effects, and (ii) the structural relaxation as observed at the intermolecular structure factor peak shows a weak temperature dependence, indicating a low degree of interchain cooperativity. However, stretched functional forms are observed for all length scales and temperatures investigated. An apparent direct crossover from the microscopic regime to Rouse-like dynamics suggests an essential role of connectivity in the observed stretching. Finally, while at momentum-transfer values above the structure factor peak the relation between coherent and incoherent characteristic times is reasonably described by the de Gennes narrowing, at larger length scales it is not reproduced by any existing approach.