Macromolecular Theory and Simulations最新文献

Statistical learning is employed to present a principled framework for the establishment of quantitative structure-property relationships (QSPR). Property predictions of industrial polymers formed by multiple reagents and at varying molecular weights are focused. A theoretical description of QSPR as well as a rigorous mathematical method is developed for the assimilation of experimental data. Results show that these methods can perform exceptionally well at establishing QSPR for glass transition temperature and intrinsic viscosity of polyesters.

According to recent reviews and experiments, some key dynamic properties of cyclic polymers from neutron spin echo spectroscopy, molecular dynamics simulations, and rheological measurements are at variance with the predictions from theories based on motions restricted by fixed obstacles. These dynamic properties including non-Gaussianity, heterogeneity, and subdiffusive center of mass mean square displacements turn out to be hallmarks of cooperative dynamics found in entangled linear polymers, and in other many-units interacting systems that are not polymers. The current situation suggests new theory emphasizing that cooperative many-chain dynamics is needed to explain the properties. The Coupling Model is such a theory. Its predictions are applied to the dynamic properties of cyclic polymers here to show consistency with experiments and simulations.

The effect of spatial inhomogeneity on the dimensions of network polymers is investigated by using model networks containing highly crosslinked domains. It is found that the dimensions of network architecture consisting of densely crosslinked domains connected by long chains are larger than those of loosely crosslinked domains connected by short chains, given the cycle rank is the same. The cases with the domains connected by the domains are also investigated. In all cases, the dimensions are larger than the corresponding randomly crosslinked homogeneous networks. This is because the loosely crosslinked regions dominate the dimensions of network polymers. The master curve relationship found for the statistical networks is applicable also for the present types of spatial inhomogeneous network polymers when the cycle rank is increased to make the network well-developed in a homologous series of networks.

Spatial gradient materials occupy an important research position in the field of functional materials with their unique porous structure. Gradient changes in pore size and density distribution have received extensive attention in the fields of biomimetic and smart materials. The gradient transition law is mathematically related to the driving force of isomerization reaction and component phase separation. In this study, a dissipative particle dynamics simulation is used to introduce photoisomerization reactions into the system. Lambert's law is used to construct a reaction model for the variation of light intensity with irradiation depth, and a gradient structure with a spatial transition law is obtained. The effects of the extinction coefficient ε, the initial reaction probability Pr₀, and the interactions α(A,B) between the isomerized molecules as well as the viscosity on the formation of the gradient structure are investigated in detail. Furthermore, the mathematical proportionality between the size of the phase region and interfacial energy of the two phases is elucidated. This study provides preliminary computational insights into the factors affecting the photoinduced phase separation process of polymeric gradient materials. It may help to develop effective strategies to improve the phase separation and properties of polymer gradient materials in subsequent studies.

The conformation of macromolecules attached to a surface is influenced by both their excluded volume and steric forces. Here, self-avoiding random walk simulations are used to evaluate the occurrence of various conformations as a function of the number of monomeric units to estimate the effect of conformational entropy of a tethered chain. Then, a more realistic scenario is assessed, which can more accurately reproduce the shape of a tethered macromolecule. The simulations presented here confirm that it is more likely for a polymer to undergo a collapse conformation rather than a stretched one, as a collapse conformation can be realized in more different ways. Also, they confirm the “mushroom” shape of polymers close to a surface. From this simple approach, the conformation entropy of a model 100-unit polymer close to a surface is estimated to contribute with over 129 $��k_B�T�$ toward its collapse. This conformation entropy is higher than that of typical hydrogen bonds and even barriers that keep proteins folded. As such, entropic collapse of macromolecules plays an important role in realizing the mushroom shape of attached polymers and can be the driving force in protein folding, while the polypeptide chain emerges from the ribosome.

The macroscopic properties of polymer nanocomposites (PNC) rely largely on the interphase between the polymer chains and the filler particles. One significant difficulty to solve this issue is to quantitatively model the structure-property correlations due to the interfacial region in these complex materials. While dielectric spectroscopy (DS) measurements are routinely used to characterize the effective permittivity of filled polymers, fitting standard effective medium models and mixing equations to these data remains notoriously difficult to interpret. This is due to the absence of explicit reference to internal length scales characterizing the interfaces in the PNC. As an illustrative example, a two-level homogenization framework is proposed which enables the extraction of useful information on the impact of a thin interphase confined on a nanometer length scale based on broadband DS data. This model leads to new ways of tuning the interphase so as to optimize the material's response to electric field, a situation relevant for electromagnetic shielding. This approach provides guidance on how to observe directly and experimentally the actual properties of the interface between the phases (as opposed to model-based inference). Aside from its secure physical foundation in the theory of effective medium, a significant advantage of this approach is that a genetic algorithm (GA) technique applied to this physics-based model enables the uniqueness of the fit parameters to be considered, as the GA method is robust in terms of finding globally optimum solutions, therefore placing confidence in non-universal values of the percolation exponents. Recent work in physics-informed machine learning indicates that the effective dielectric properties of PNC with many degrees of freedom due to their complex morphology can be described by considering only a few degrees of freedom describing the interface features between the phases in these composites.

Front Cover: A repetitive topology of splitting and recombination is introduced into the flow channel of a dual-speed non-twin screws with the speed ratio of 2. A new kind of perturbation ring elements is proposed to achieve the above purpose. The mixing is numerically characterized in terms of evolution of tracer particles, mixing variance index and residence time distribution. This is reported by Baiping Xu, Ruifeng Liang, Shuping Xiao, Yanhong Feng, and Huiwen Yu in article number 2300048.

Stockmayer's formula for the weight average of cross-linked primary chains is extended to the z-average degree of polymerization $��D�P_z�$. This average is a function of the weight- and z-average degree of polymerization λ_w and λ_z of the primary chain distribution and the branching density α: $��D�P_z�=�\frac{�{\lambda }_z�{�(�1�+�\alpha �)�}^3�-�{\lambda }_w^2�{\alpha }^2��(�3�+�\alpha �)��}{�{�(�1�-�\alpha ��(�{\lambda }_w�-�1�)��)�}^2��(�}$

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.