Macromolecular Theory and Simulations最新文献

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.

Robust mathematical treatment of the Ozawa/Flynn/Wall isoconversion method is conducted to determine the value and uncertainty of the activation energy and pre-exponential factor for the degradation of polypropylene in thermogravimetric analysis experiments at constant heating rates. The present work employs mathematical models and uncertainty propagation techniques based on the Guide to the Expression of Uncertainty in Measurement to estimate the Arrhenius activation energy and pre-exponential factor due to the uncertainty of the integration constant b, both in a linear and a third-degree reciprocal polynomial model with respect to x. The error arising from Doyle's linear approximation in the improper integral of temperature in the Arrhenius equation is examined, and an alternative method is proposed to correct this error, reducing it to 0.032% in the working interval of −200 ≤ x ≤ −15, where x = −E/RT. Given the increased sensitivity of modern thermogravimetric analysis equipment, these improvements are considered essential for obtaining reliable results that align with experimental precision limits compared to previous works. Thus, this allows for the development of an enhanced quality assurance framework by providing more robust uncertainty estimation and a better understanding of the method. Moreover, this approach can be applied to other similar polymer system.

Front Cover: Visualizing higher probability sites for end segments forming a matrix phase formed by B-component in a BCC lattice structure from an AB diblock copolymer. The red surfaces are the A/B interfaces, and the higher probability isosurface of end B-segments is displayed in green. The end segments are localized on the frame of the Kelvin's tetrakaidecahedron, truncated octahedron. This is reported by Jiro Suzuki and Yushu Matsushita in article number 2300016.

The structure of a thin film of symmetric ABA triblock copolymer melts in an external in-plane DC or AC electric field is studied theoretically. The situation is considered when the triblock copolymer forms a hexagonal morphology of standing cylinders in bulk in the absence of an external field. Self-consistent field theory calculations are carried out to determine the most thermodynamically favorable thin film structure among the cylindrical phases perpendicular and parallel to the substrate and the lamellar phase perpendicular to the substrate. The results are presented as phase diagrams with the film thickness and electric field energy on the axes and as distributions of the local composition, which serve as an order parameter in the system. It is confirmed that electric fields only weakly affect the spinodal curves of block copolymers but they can reorient or markedly modify microphase-separated morphologies in those systems. Such restructuring is consistent with a considerable modulation of the free surface of a copolymer film and it can lead to the coexistence of different phases that appear in the film areas with different local thicknesses.

The suitability of dissipative particle dynamics simulations is investigated to predict the dynamics of polymer chains in dilute polymer solutions. The authors find that the predictions depend on the value of the repulsive parameter for bead-bead pairwise interactions used in the DPD simulations (a_ij). For all systems, the chain sizes and the relaxation time spectrum are analyzed. For a_ij = 0, theta solvent behaviour is obtained, whereas the dynamics at equilibrium agrees well with the predictions of the Zimm model. For higher values of aij, the static properties of the chain show good solvent behaviour. However, the scaling laws for the chain dynamics at equilibrium show wide variations, with consistent results obtained only at an intermediate value of a_ij = 25. At higher values of the repulsive parameter (a_ij ⩾ 25), the simulations are also able to predict the abrupt cut-off in the relaxation spectrum, which has been observed earlier in experiments of dilute solutions. To verify further, the chain dynamics in shear flow using DPD simulations is studied. Specifically, the variation of the chain is analysed stretch and end-over-end tumbling with shear rates. Overall, the trends obtained from DPD simulations agree well with those observed in earlier BD simulations.

Monte Carlo simulations that examine the partitioning of dilute solutions of ring or linear chains into a slit pore using two chain models, the random-walk (RW) model and the self-avoiding walk (SAW) model are presented. The partitioning coefficients K for the ring and the linear chains at the surface interaction both under the critical adsorption point (CAP) and above the CAP are compared. In both chain models, K for the ring remains larger than K for the linear chains. The ring chain crosses over the point K = 1 at a weaker surface interaction than the linear chain. When extrapolated to infinite chain length, the cross-over point for the ring and linear chain becomes the same (within statistical errors) for the RW model but remains different for the SAW model. The density profiles of the ring within the pore reveal the development of humps near the wall as the surface interaction crosses over the CAP. The excluded volume interaction in the SAW model additionally impacts the partitioning of chains in a solution consisting of a binary mixture of ring and linear chains and makes the K values in a binary mixture differ from the monodispersed solutions.