Macromolecular Theory and Simulations最新文献

Front Cover: A continuous transition function that approximates the macroscopic-phenomenological behavior of amorphous polymers at glass transition is obtained from an exactly-solvable Riccati equation. This ordinary differential equation is derived within thermodynamics with internal state variables. More details can be found in article 2400052 by Claudio Corbisieri.

Studying the macroscopic-phenomenological behavior of amorphous polymers at glass transition is often subject to limitations because the ordinary differential equations that describe the material behavior require numerical solution. To avoid these limitations, ad-hoc-formulated models of the glass transition have been proposed. However, their scope of application is expected to be limited due to insufficient theoretical foundation. This work establishes a theoretical framework for models that use the logistic function to approximate the macroscopic-phenomenological behavior of amorphous polymers at glass transition. For this purpose, an exactly-solvable Riccati equation is derived within thermodynamics with internal state variables. A closed-form expression in terms of mathematical functions for the temperature derivative of a single internal state variable is the result. This closed-form expression contains the logistic function thus featuring a continuous transition region centered around a pressure and cooling-rate dependent transition temperature. Based on comparison of existing models with the exact solution derived from the Riccati equation, generalized models that approximate the thermal expansion coefficient and heat capacity at glass transition are proposed. This work thus demonstrates the validity of the logistic function in glass transition models of amorphous polymers and provides suggestions as to how existing models can be extended in their applicability.

Front Cover: The rheological parameters are important factors influencing mixing of polymer blends. The tracer particle method is used to represent the mixing effect of the melt, the flow of virtual material/thermoplastic polyurethanes with high and low viscosities in dynamic mixers are simulated. The patterns of zero shear viscosity ratio, relaxation time ratio and non-Newtonian index ratio influencing the mixing are discovered. More details can be found in article 2400031 by Jiankang Wang and co-workers.

Polymer flooding, using hydrolyzed polyacrylamide (HPAM), is crucial in enhanced oil recovery technology. The effect of the HPAM and NaCl concentration on the stability of the simulated emulsions was assessed through multiple light scattering experiments. The results demonstrated that HPAM significantly enhanced the stability of both oil-in-water (O/W) and water-in-oil (W/O) emulsions. The HPAM concentration escalated from 200 mg L⁻¹ to 1000 mg L⁻¹, increasing from 1.24% to 1.31% at 60 minute in the average backscattering of W/O emulsions. The average transmittance of O/W emulsions exhibited a significant decline from 2.54% to 0.12%. The NaCl concentration had a small effect on the stability of the emulsions. Molecular dynamics simulations revealed that HPAM adsorbed at the oil water interface by the point-like nature, with stronger interaction between its amide group and the oil molecule than its carboxyl group. The hydrogen bond number and the hydrogen bond lifetime of HPAM-H₂O and HPAM-HPAM increase with increasing the number of HPAM molecules at the oil-water interface, slowing diffusion coefficient of water molecules and increasing the interface thickness. Increasing salinity can weaken the HPAM-water interaction, reducing the emulsification stability. This work provides insights into the emulsification characteristics and mechanisms of HPAM.

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.