Macromolecular Theory and Simulations最新文献

In the present study, the dielectric and electrical properties of the carbon nanotube/polydimethylsiloxane (CNT/PDMS) composite are theoretically analyzed for various doping concentrations. For both single-walled and multiwalled CNTs (SCNTs and MCNTs), the work is done between 75 and 750 THz. The behavior of the dielectric constant, loss factor, and conductivity are analyzed as functions of frequency. It is observed that there is no appreciable change in the real part of the dielectric constant at high frequencies in single-walled CNT. The loss tangent is high at lower frequencies, and the loss peak is observed at a particular frequency. The Cole–Cole plot is used to interpret the static- and high-frequency dielectric constants and relaxation time of the composite. With increasing concentrations of SCNT and MCNT, the conductivity at the obtained peak maximum shifts to a lower frequency.

Front Cover: Free energy of polymer adsorption is sampled in a series of windows – each constrains the system with a harmonic bias potential. Efficiency of this umbrella sampling technique can be significantly improved by allowing the sampling windows to vary along the reaction coordinate. This is reported by Naveen Kumar Vasudevan, Dongyang Li, and Li Xi in article number 2300057.

A new concept called isoconversional state diagram, which can be used to predict the kinetics of single-step condensed phase reactions, is introduced. A state represents a certain extent of conversion degree α in a reaction. The construction of the isoconversional state diagram is based on the isoconversional state equation, which is a piecewise linear equation about 1/T and lnβ, where T is the temperature and β is the heating rate. The slope of the linear equation is controlled by the activation energy E_α and its intercept contains the inherent information of the kinetic triplet, i.e., the pre-exponential factor A_α, the activation energy E_α and the reaction model f(α). Consequently, the geometric methods for nonisothermal and isothermal kinetic predictions are derived. The latter reflects the physical meaning of the relationship between reactions under isothermal and nonisothermal conditions, i.e., the time to advance from α_i to α_i+1 at isothermal temperature T_iso is equal to the time to heat from T_iso to $�T_{�{\alpha }_i�+�1�}$ under heating rate $�{\beta }_{{\alpha }_i}$, where T_iso, $�T_{�{\alpha }_i�+�1�}$ and $�{\beta }_{{\alpha }_i}$must be determined from the isoconversional state diagram.

While many studies are performed on the effect of ligands on the adhesion and endocytosis of NPs, the effects of ligand length and surface density on the NPs' interaction with lipid membranes are poorly investigated. Here, a computational investigation is presented, based on molecular dynamics of a coarse-grained implicit-solvent model, of the interaction between ligand-decorated spherical NPs and lipid membranes. Specifically,the case is considered where the ligands interact attractively with lipid membranes only through their ends. In particular, the effects of ligand grafting density, ligand length, and strength of ligand-lipid interaction is investigated on the degree of wrapping of the NP by the membrane and on the morphology of the membrane close to the NP. Whereas the degree of wrapping is found to increase with increasing the grafting density for a given interaction strength and ligand length, it decreases with ligand length for a given grafting density and interaction strength. For moderate values of the adhesion strength and long ligands, it is found that the end ligands form long linear clusters, which lead to an anisotropic conformation of the membrane around the NP. For short ligands, the wrapping of the membrane around the NP is almost complete, with the wrapped NP showing a regular faceted structure for high adhesion strength.

Free energy calculation in molecular simulation is an computationally expensive process. Umbrella sampling (US) is a go-to method for obtaining the potential of mean force (PMF) along a reaction coordinate. Its computational cost increases drastically as the molecular system gets more complex. For many polymeric and biomolecular systems, adequately sampling all configurational degrees of freedom is computationally prohibitive. Using the adsorption of a short-chain methylcellulose on a cellulose crystalline surface as the test case, this study shows that the sampling time required for reliable results is much higher than typical choices made in the literature. The accuracy of the PMF profile is strongly affected by sampling inadequacy in a few regions along the reaction coordinate. Non-uniform windows and sampling parameters are proposed to enhance the sampling in difficult regions. Sampling windows that vary with the local PMF steepness are allocated with a new algorithm. Parameters in this algorithm are optimized for the best sampling efficiency. It is demonstrated that significantly less computer time will be required to achieve the same sampling accuracy if computational resources are optimally distributed along the reaction coordinate.

A new kind of perturbation ring element (PRE) is first proposed to introduce a repetitive topology of splitting and recombination across the intermeshing zones of a co-rotating non-twin screw elements (NTSE) with a speed ratio of 2. A numerical simulation is performed using finite element method (FEM) along with the mesh superposition technique (MST). Post-treatment codes are successfully developed where fourth-order Runge–Kutta scheme is used to achieve particle tracking. For the tracer particle groups released initially from the upper and bottom intermeshing regions, mixing is characterized in terms of the evolution of tracer particles, mixing variance index, and residence time distribution (RTD). The numerical results revealed for a given output, the larger the screw speed, the larger the dividing ratio, and the better distributive mixing is. PRE achieved the best distributive mixing owing to the changing of flow topology. In TSE there are Komogorov-Arnold-Moser (KAM) tubes in which the tracer particles are confined to prevent better mixing from occurring.