Macromolecular Theory and Simulations最新文献

Often the errors in the measurement of copolymerizations are not accurately determined or included in the calculation of reactivity ratios. Some knowledge of the errors in the initial monomer ratio, conversion, and copolymer composition is however essential to obtain reliable (unbiased) reactivity ratios with a realistic uncertainty. It is shown that the errors serve a trifold purpose; they can serve as weighing factors in the fit, they can be compared with the fit residues to decide whether the chosen model is adequate for the data and they can be used to construct a realistic joint confidence interval for the reactivity ratios. The best approach is to have an estimate of the individual errors in the copolymer composition, either from a thorough error propagation exercise or from replicate measurements. With these errors, the χ²-joint confidence intervals can then be constructed which gives a realistic estimate of the errors in the reactivity ratios. Utilizing the Errors in Variables Method (EVM) is correct and useful, but only if the individual errors in all the variables in each experiment are more or less known.

Light-responsive shape-changing polymers are photonastic materials: they can convert light into mechanical energy through macroscopic transformations. Indeed, photochromic molecules embedded in these polymer films present light-induced structural modifications that can trigger a significant macroscopic deformation. In this theoretical study based on molecular dynamics simulations, analysis tools ranging from atomic to supramolecular scales are developed to investigate this photonastic phenomenon. To this purpose, a model system built upon an azobenzene photochrome embedded in different environments (tetrahydrofuran, cis-1,4-polybutadiene and hydroxyl-terminated polybutadiene) is considered. First, the impact of the environment on the photochrome properties is discussed through the analysis of the structural properties, ultra-violet visible (UV–vis) absorption spectra and dynamical properties of the photoswitch. Then, the impact of the presence of the photochrome on the polymer is studied. At the atomic scale, the radial distribution functions show some differences between the cis and trans isomers due to geometrical effects. At the molecular scale, the analysis of the size and shape of the polymer chains reveals that the photochrome has no impact on the chain properties. Finally, at the macroscopic scale, the cohesive energy density shows that the polymer is stabilized by the presence of photochrome molecules.

Metal-organic frameworks (MOFs) have emerged as versatile materials with exceptional properties, including high porosities, large surface areas, and remarkable stabilities, making them attractive for various applications. MOF-5 stands out for its thermal stability and surface area, making it promising for diverse applications, including drug delivery and gas adsorption. This study explores the potential of amino acid MOF (AA-MOF) composites, integrating phenylalanine, tryptophan, and tyrosine, for selective CO₂ and CH₄ adsorption using grand canonical Monte Carlo (GCMC) simulations. The impact of amino acid composition and spatial arrangement within MOF-5 on selective CO₂ and CH₄ adsorption performance have been investigated. The results indicate that tryptophan-MOF-5 exhibits the highest CO₂ uptake due to the interaction between CO₂ and tryptophan, while phenylalanine-MOF-5 demonstrated the lowest affinity for gas adsorption. Radial distribution function (RDF) analysis reveals distinct gas distribution patterns within the composites, with tryptophan playing a dominant role in gas adsorption. Additionally, analysis of total energy, enthalpy of adsorption, and Henry's coefficient provide insights into the thermodynamic aspects of gas adsorption onto AA-MOF composites. This study enhances the understanding of the fundamental mechanisms underlying CO₂ and CH₄ selective adsorption in amino acid MOF composites, facilitating the development of efficient gas separation technologies.

The critical frequency and the relaxation time are analyzed through deformation and displacement during electrostriction which is induced by the electrical field at different frequencies. First, when the frequency is 50 Hz and the field strength is 2.5 kV mm⁻¹, the electrostrictive displacement of polyethylene is 6.72 × 10⁻⁴ mm. After the data fitting, it is found that the displacement increases linearly with the square of field strength and that the proportional coefficient of 50 Hz is 1.08 × 10⁻⁴. Second, due to the influence of relaxation polarization and power loss, with the increase of frequency, the displacement and the proportional coefficient first increases then decreases, and when the frequency is 10 kHz, the displacement of 2.20 × 10⁻⁶ mm and the proportional coefficient of 3.51 × 10⁻⁷ have minimum values, which are 99.67% and 99.68% lower than that of 50 Hz, respectively. There is the critical frequency. Finally, based on the characteristic of anomalous dispersion, the relaxation time of polyethylene is 9.19 × 10⁻⁶s, which is in the time range of thermionic relaxation polarization and consistent with the actual situation. This analysis confirms the quantitative relationship between electrostrictive characteristics, field strength, and polarization. In addition, the relationship between frequency and strain is discussed, and the critical frequency in polymer and the relaxation time are confirmed.

The entanglement length plays a key role in deciding many important properties of thermoplastics. A number of computational techniques exist for the determination of entanglement length. In Ahmad et al.,^[1] a method is proposed that treats a macromolecular chain as a 1D open curve and identifies entanglements by computing the linking number between two such interacting curves. If the curves wind around each other, a topological entanglement is detected. However, the entanglement length that is measured in experiments is assumed to be between rheological entanglements, which are clusters of such topological entanglements that collectively anchor the interacting chains strongly. In this article, the method of clustering topological entanglements into rheological ones is further elaborated and the robustness of the method is assessed. It is shown that this method estimates an entanglement length that depends on the forcefield chosen and is reasonably constant for chain lengths longer than the entanglement length. For shorter chain lengths, the method returns an infinite value of entanglement length indicating that the sample is unentangled. Moreover, in spite of using a geometry-based algorithm for clustering topological entanglements, the estimated entanglement length retains known empirical connections with physical attributes associated with the ensemble.

The design of block-copolymer-based functional materials, including mesoporous membranes and nanoparticles, requires a comprehensive understanding of the hierarchical assembly of block copolymers in selective solvents into micelles and subsequent ordered phases. It is hypothesized that micellar ordering and characteristic assembly can be described using a set of phase parameters that account for entropic and enthalpic interactions. Dissipative particle dynamics (DPD) simulations are used to systematically investigate the self-assembly of semidiluted block copolymers, resembling isoporous membrane preparation conditions. The effect of Flory–Huggins interaction parameters, block lengths, and concentration on the morphology and polydispersity of the micelles is evaluated. The interaction parameters are mapped into Flory–Huggins theory by considering the block's conformation. These results reveal the effect of polymer concentration and solvent affinity on the morphological transition of the aggregates, in agreement with existing experimental evidence. It is identified that monodisperse-spherical micelles in solution are fundamental to stabilize ordered states. Weak solvent segregation of the largest block, curvature of the core-corona interface, and stretching of the corona-forming one are found to be key to stabilize monodisperse assemblies. These conditions can be predicted using spherical-micelles packing considerations and a global phase parameter from the Flory–Huggins theory. This study provides valuable insights into the self-assembly of diblock copolymers and offers a potential way to optimize the preparation of mesoporous ordered structures and micelle ordering in semidiluted systems.

Step-growth polymerized systems of type “A3 + A1” are considered. The monomers bear, respectively, 3 or 1 reactive A group. During the reaction, an A group on one monomeric unit might react with an A group on another such unit, thus chemically coupling the two units involved. Complexly structured polymeric molecules are formed. The A3's act as branching points; the A1's as end cappers. At the end of the reaction, the population of molecules present in the reactor vessel varies in size and branching structure. A method is presented to calculate the bivariate (molecular size) × (square radius of gyration) number distribution. It is shown that within the class of molecules of the same size, their square radius of gyration follows a shifted gamma distribution. Two new molecular parameters are introduced: the D index and the G index. The method uses bivariate generating functions.

A time–temperature-transformation diagram is created for the curing reaction of a diglycidylether bisphenol A (DGEBA)-based epoxy resin. It results from a kinetic analysis performed by means of dynamical differential scanning calorimetry (DSC) measurements; a gelation curve determined with isothermal and dynamical rheological tests; and a vitrification curve obtained from temperature-modulated dynamic DSC measurements. The resulting diagram is validated by comparison of isothermal measurements with the corresponding calculated curves.