Membrane remodelling plays an important role in cellular tasks such as endocytosis, vesiculation and protein sorting, and in the biogenesis of organelles such as the endoplasmic reticulum or the Golgi apparatus. It is well established that the remodelling process is aided by specialized proteins that can sense as well as create membrane curvature, and trigger tubulation when added to synthetic liposomes. Because the energy needed for such large-scale changes in membrane geometry significantly exceeds the binding energy between individual proteins and between protein and membrane, cooperative action is essential. It has recently been suggested that curvature-mediated attractive interactions could aid cooperation and complement the effects of specific binding events on membrane remodelling. But it is difficult to experimentally isolate curvature-mediated interactions from direct attractions between proteins. Moreover, approximate theories predict repulsion between isotropically curving proteins. Here we use coarse-grained membrane simulations to show that curvature-inducing model proteins adsorbed on lipid bilayer membranes can experience attractive interactions that arise purely as a result of membrane curvature. We find that once a minimal local bending is realized, the effect robustly drives protein cluster formation and subsequent transformation into vesicles with radii that correlate with the local curvature imprint. Owing to its universal nature, curvature-mediated attraction can operate even between proteins lacking any specific interactions, such as newly synthesized and still immature membrane proteins in the endoplasmic reticulum.

We present a hierarchical approach that combines atomistic and mesoscopic simulations that can generally be applied to vinyl polymers. As a test case, the approach is applied to atactic polystyrene (PS). First, a specific model for atactic PS is chosen. The bonded parameters in the coarse-grained force field, based on data obtained from atomistic simulations of isolated PS dimers, are chosen in a way which allows to differentiate between meso and racemic dyads. This approach in principle allows to study isotactic and syndiotactic melts as well. Nonbonded interactions between coarse-grained beads were chosen as purely repulsive. The proposed mesoscopic model reproduces both the local structure and the chain dimensions properly. An explicit time mapping is performed, based on the atomistic and CG mean-square displacements of short chains, demonstrating an effective speed up of about 3 orders of magnitude compared to brute force atomistic simulations. Finally the equilibrated coarse-grained chains are back mapped onto the atomistic systems. This opens new routes for obtaining well equilibrated high molecular weight polymeric systems and also providing very long dynamic trajectories at the atomistic level for these polymers.

A quantitative understanding and prediction of the dynamics of entangled polymer melts is a long-standing problem. In this work we present results about the dynamical and rheological properties of atactic polystyrene melts, obtained from a hierarchical approach that combines atomistic and coarse-grained dynamic simulations of unentangled and entangled systems. By comparing short chain atomistic and coarse-grained simulations, the time mapping constant is determined. Self-diffusion coefficients, after correcting for the chain end free volume effect, show a transition from Rouse to reptation-like behavior. In addition, the entanglement molecular weight is calculated through a primitive path analysis. All properties are compared to experimental data.

Results are presented from 300 ns long atomistic molecular dynamics (MD) simulations of polyethylene (PE) melts, ranging in molecular length from C78 to C250. Above C156, the self-diffusion coefficient D is seen to exhibit a clear change in its power-law dependence on the molecular weight (M), significantly deviating from a Rouse (where D ∼ M -1) toward a reptation-like (where D ∼ M -2.4) behavior. The mean-square displacement (msd) of chain segments and the dynamic structure factor is also calculated and the crossover from the Rouse to entangled behavior is again observed above C156. A novel strategy is also developed for projecting atomistic chain configurations to their primitive paths and thereby mapping simulation trajectories onto the reptation model. Results for the friction factor ζ, the zero-shear rate viscosity η0 and the self-diffusion coefficient D are found to be internally consistent and in agreement with experimental rheological data.

An atomistic modeling approach is presented for simulating the interface between a polymer melt and a crystalline solid substrate. As a test case, a thin film of polyethylene (PE) melt confined between a semiinfinite graphite phase on the one side and vacuum on the other is considered. The simulation is carried out in the NPT statistical ensemble with an efficient Monte Carlo (MC) algorithm based on state-of-the-art variable connectivity moves. The atomistic simulations are conducted by describing the PE chains with a united atom model, which considers each methylene (CH2) and methyl (CH3) group along the chain backbone as single interaction sites. To calculate the potential energy of interaction between polymer atoms and the semiinfinite graphite substrate, the method designed by Steele was implemented, capable of incorporating the exact crystallographic structure of graphite. The new approach has allowed us to analyze structural and conformational properties on the length scale of just a few angstroms from both surfaces. Detailed results are presented for the local mass density, structure, and conformation of PE at the two interfaces, obtained from simulations with model, strictly monodisperse PE samples of molecular length up to C400. Additional structural features of the adsorbed layer, such as the distribution of skeletal carbon atoms in train, loop, and tail conformations and their statistics, are also analyzed in detail and compared with the predictions of the lattice-based Scheutjens−Fleer self-consistent mean-field theory in the limit of zero solvent concentration (melt case). Our atomistic simulation data demonstrate a stronger dependence of these descriptors of adsorbed layer structure on chain length than what is calculated by the mesoscopic Scheutjens−Fleer lattice model. In a second step, thoroughly equilibrated configurations of the confined model PE melt films are subjected to detailed molecular dynamics (MD) simulations in the NPT ensemble to analyze their dynamic behavior. The MD simulations are carried out with the rRESPA multiple-time-step algorithm and have allowed us to monitor segmental and chain center-of-mass mean-square displacements over time scales on the order of a few hundreds of nanoseconds. Results from the MD simulations are presented in the companion paper.

We present a detailed study of a new, optimized coarse‐grained (CG) model of polystyrene (PS) and compare it with a recently published one (Harmandaris et al., Macromolecules 2006, 39, 6708). By implementing a different mapping scheme, the new model, augmented with softer nonbonded interactions, better reproduces the local chain conformations and melt packing observed in atomistic simulations of atactic PS. Both models properly predict the bonded distributions and are capable of simulating different tacticities without needing sidegroups. Both CG models fit dynamic data from long atomistic simulations after determining the scale factor for the simulation time. Together with a rigorous back‐mapping procedure from the mesoscopic to atomistic description, this opens up a very feasible way for generating very long atomistic trajectories.magnified image

We present a molecular coarse-graining approach applied to polystyrene which obtains both the bonded and nonbonded interactions of the coarse-grained model from the sampling of isolated atomistic chains and pairs of oligomers. Atomistic melt properties are not used in the parametrization. We show that the coarse-grained polystyrene model not only predicts melt properties, including the melt packing and the density between 400 and 520 K, in satisfactory agreement with the atomistic model, but also reproduces the local chain conformations of atactic as well as stereoregular polystyrene. The model takes into account and reproduces correlations between neighboring bonded degrees of freedom and therefore reproduces the conformations of detailed atomistic chains in the melt on all length scales.

Well-relaxed atomistic configurations of polydisperse, linear polyethylene (PE) melts, obtained with the end-bridging Monte Carlo algorithm, have been subjected to detailed molecular dynamics simulations in both the canonical (NVE) and microcanonical (NVT) ensembles. Three different systems have been investigated, characterized by mean molecular lengths C24, C78, and C156, and by the same polydispersity index I of about 1.09. Results are presented for the static and (mainly) dynamic properties of these melts at P = 1 atm and T = 450 K. The diffusion coefficient D, determined for various chain lengths, N, is in very good agreement with experimentally measured values. The friction coefficient ζ D is extracted from D by invoking the Rouse model; it is seen to increase from a relatively small value characteristic of short alkanes to a chain-length-independent plateau, reached in a region of N = 60−80. The friction coefficient ζτ is also obtained by analyzing the decay of the time autocorrelation function for the normal modes X p at various chain lengths; the values thus extracted are consistent with those obtained from D for N above 40. Although the decay of the autocorrelation function of the end-to-end vector is very well described by the Rouse model, individual Rouse modes show some deviation from theoretical predictions. Even for chains sufficiently long to be in the asymptotic ζ regime, only the first two normal modes fully conform to Rouse theory in terms of their squared amplitudes and correlation times. Zero-shear viscosities computed from ζ D values by means of the Rouse model are in excellent agreement with available experimental data for N = 90.