Macromolecular Theory and Simulations最新文献

Polymers are known to exhibit hysteresis in their thermal and volumetric properties between cooling and heating at the glass transition. A thorough investigation of this hysteresis using atomistic simulation is not proposed until now. In this work, therefore, the glass transition is studied through heating and cooling protocols at constant rate for two polymers, polystyrene (PS) and polyethylene oxide (PEO), with different molecular weights. To achieve this objective, the analysis is carried out by plotting against temperature, specific volume, coefficient of thermal expansion, total energy, and constant volume heat capacity. The calculated properties for PS and PEO are found to be in good agreement with experimental data, confirming the accuracy of the TraPPE force field for these polymers. The glass transition temperature (T_g) range remains the same regardless of the properties. Moreover, the difference in properties between heating and cooling processes systematically leads to a peak at the same temperature, associated with T_g. Finally, starting from a low temperature, the polymer chains remain mainly in a potential well as the temperature rises, while during cooling the exploration of the configuration space continues up to the temperature where no torsional changes are observed.

Front Cover: The effects of 4010NA content on the damping and aging properties of hydrogenated nitrile butadiene rubber (HNBR)/nitrile butadiene rubber (NBR) matrix are studied via molecular simulation and experiments. With an increase in the 4010NA content, there is a decrease in the free volume fraction (FFV), attributed to the hydrogen bond network between 4010NA and the HNBR/NBR matrix. This is reported by Meng Song, Qi Qin, and co-workers in article number 2200072.

Simulations are performed for long hard-sphere polymer chains using a recently developed binary-tree based Monte Carlo method. Systems in two to five dimensions with free and periodic boundary conditions and up to 10⁷ repeat units are considered. The analysis is focused on scaling properties of the end-to-end distance and the entropy and their dependence on the sphere diameter. To this end new methods for measuring entropy and its derivatives are introduced. By determining the Flory exponent ν and the weakly universal amplitude ratio of end-to-end distance to radius of gyration we find that the system generally reproduces the behavior of self-avoiding lattice walks in strong support of universality.

Polymer designs, especially monomer designs, can be performed with machine learning and artificial intelligence using a polymer dataset, however, it is meaningless if the designed monomer structures cannot be synthesized and the polymer compound cannot be polymerized. In this study, a retrosynthesis prediction model based on sequence-to-sequence (Seq2Seq) with attention is developed, which is originally used in language transformation, to predict reactants from monomer structures corresponding to polymers. In addition, Seq2Seq with an attention-based synthetic reaction prediction model that predicts monomer structures from reactants is also developed to propose monomer structures with free bonds for polymer design. Through case studies using an actual polymer dataset, it is confirmed that appropriate polymer designs can be achieved by using the proposed method, including the generation of valid monomer structures, the selection of the monomer structures with promising polymer properties, and the prediction of reactants for the monomer structures.

Linear alkanes (n-alkanes) are chemically the most simple linear hydrophobic molecules in nature. Studying the incorporation of n-alkanes into lipid membranes is therefore a good starting point toward understanding the behavior of hydrophobic molecules in lipid membranes and to assess how accurately molecular dynamics models describe such systems. Here, the miscibility and structure of different n-alkanes—n-decane (C10), n-eicosane (C20), and n-triacontane (C30)—in dipalmitoylphosphatidylcholine membranes are investigated using two of the most used force fields for lipid membrane molecular dynamics simulations (CHARMM36 and Slipids). The n-alkanes are miscible in the membrane up to a critical volume fraction, ϕ_c, that depends on the force field interaction parameters used. ϕ_c is dependent on alkane chain length only for the model with more disordered chains (Slipids). Below ϕ_c, a comparison with ²H nuclear magnetic resonance (NMR) spectra indicates that a more realistic structure of the longer alkane molecules (C20 and C30) is obtained using the Slipids force field. On the other hand, for the shorter alkane (C10), Slipids simulations underestimate molecular order and CHARMM36 simulations enable a precise prediction of its experimental spectrum. The predicted ²H NMR spectra are highly sensitive to 1–4 electrostatic interactions, and suggest that a reduction of the partial charges of the longer alkanes and acyl chains in CHARMM36 results in a better performance. The results presented indicate that lipid membranes with incorporated alkanes are highly valuable systems for the validation of force fields designed to perform lipid membrane simulations.

The rational development of sustainable polymeric materials demands tunable properties using mixtures of polymers with chemical variations. At the same time, the sheer number of potential variations and combinations makes experimentally or numerically studying every new mixture impractical. A direct predictive tool quantifying how material properties change when molecular features change provides a less time- and resource-consuming route to optimization. Numerically solving Flory–Huggins theory provides such a tool for mono-disperse mixtures with a limited number of components, but for multi-component systems the large number of equations makes numerical computations challenging. Approximate solutions to Flory–Huggins theory relating miscibility and solubility to molecular features are presented. The set of approximate relations show a wider range of accuracy compared to existing approximations. The combination of the analytical, lower-order, and more accurate higher-order approximations together contribute to a broader applicability and extensibility of Flory–Huggins theory.

Proteins and peptides exhibit an immense variety of structures, which are generally classified according to simple structural motifs (mainly α helices and β sheets). Considerable efforts have been invested in understanding the relationship between chemical structure (primary structure) of peptides and their spatial motifs (secondary structure). However, little is known about the possibility to interfere intentionally in these structural driving forces, for example, by inserting (short) artificial polymer chains in the peptide backbone. Structure formation on such hybrid synthetic/biochemical polymers is still an emerging field of research. Here, molecular dynamics simulations are used to illustrate the influence of inserted polyethylene segments on the secondary structure of several peptide homopolymers. A loss of structure of ≈50% when the peptide chain length drops to ten amino acids and a practically complete absence for even shorter peptide segments.

Focusing on the formation of hierarchical structure under cylindrical confinement, the self-assembly of A(BC)₂B multiblock copolymer of chain length N in a nanopore with size R is studied using the self-consistent field theory. The hierarchical concentric ring (HC_k), hierarchical perforated cylinder (HP_k), hierarchical helix (HH_k), and even hierarchical disk (HD_k) is obtained with different number of mid-thin layers k via a proposed design principle. The results show that large pore size and χ_AB favor the hierarchical structure with more k, while χ_BC prefers hierarchical structure with less k, consistent with the results of hierarchical structure in bulk. By investigating the effect of the volume fraction of the tail A block (f_A), the phase transition sequence, HC_k → HP_k → HH_k → HD_k is explored, which shares the same transition of multiblock copolymer in bulk with L_k → G_k → C_k → S_k. Finally, the phase diagram with respect to the f_A and R is explored, where the stability regime of these hierarchical structures is well understood. The results provide a compelling panacea for the fabrication of hierarchical 3D nanostructures under confinement.