Population balance modeling of assembly formation; accounting for the H-NS protein oligomerization and DNA binding mechanisms

The results obtained in this study demonstrate H-NS oligomerization in solution is not sufficient in forming large structures of H-NS-DNA assemblies indicating oligomerization along bacterial DNA is crucial. We present a comprehensive and novel multi-scale modeling scheme to study protein-protein and protein-DNA interactions, with the focus on population balance modeling of assemblies formed by the H-NS protein and bacterial DNA. The model helps understanding how the concentration of H-NS influences the formation of H-NS-DNA assemblies. The presented model covers the two underlying molecular mechanisms involved in assembly formation, H-NS oligomerization in solution and H-NS-DNA binding. Within protein oligomerization interactions, protein molecules are entitled to dimerize, propagate, recombine and deform (break). In addition, DNA binding and unbinding interactions are included to account for formation of filaments along DNA. All mechanisms have been modeled with their associated forward (formation) and backward (deformation) interactions. The results obtained agree well with former experimental studies.</p

Simulations mapping the influence of oxygen, extruder residence time, and mechanical shear on low-density polyethylene structure during recycling

We present a computational study of the development of structural properties for recycled low-density polyethylene (LDPE). First, we apply the population balance method together with the method of moments to study the mutual influences of scission and crosslinking reactions. Then, we study the combined effects of extruder residence time and oxygen concentration as the initiator of thermo-mechanical degradation in the recycling process of LDPE in a twin-screw extruder. Further, we extend the models to study the influence of mechanical scission. We explore various case study scenarios to incorporate the effect of extruder residence time and mechanical shear. It is concluded that the competition between random scission and crosslinking, as well as the presence of oxygen, play important roles in the degradation of LDPE during recycling. Random scission is a key factor in terms of altering molecular weight distribution (MWD) during a recycling extrusion process. The various explored scenarios with mechanical scission demonstrate the sensitivity of molecular structure to the combination of competing mechanisms during recycling

Modeling Crystallization Kinetics and Resulting Properties of Polyamide 6

This paper provides a model of the crystallization kinetics of polyamide 6 (PA6), including primary and secondary crystallization, lamellar thickness distribution, and the evolution of the mobile and rigid fractions of the amorphous phase. The kinetics includes the two-phase structure, the monoclinic α-phase and the pseudo-hexagonal γ-meso phase, of which the fractions depend on the thermal history during solidification. The model is parameterized with experimental results from the literature. The thickness of the rigid amorphous layer was the only parameter to be estimated. The obtained results indicate that the fraction of the amorphous rigid fraction depends not only on the thermal history but also on the crystalline phase and if the rigid amorphous layer was formed in combination with the α- or γ-meso phases. These results provide the bases for predicting and controlling mechanical properties, which can strongly depend on processing conditions as, for example, experienced during injection molding

Computational study of the structural properties of recycled low-density polyethylene

We present a comprehensive model to computationally study molecular structure development in the recycling extrusion process of low-density polyethylene (LDPE) and provide initial results versus reported experiments as guidance. Molecular weight distribution (MWD) and branching distribution (BD) are modeled by applying a combination of population balance modeling, the method of moments and branching pseudo distributions. Special cares have been taken to incorporate the effects of scission and crosslinking reactions. Further, effective strategies have been provided to mitigate the computational complexity of solving balance equations. The models have been applied to the case of increasing residence time in a recycling extruder. The simulated MWD of the recycled LDPE exhibits a shift towards lower chain lengths for residence times up to 400s, while for higher residence times the high MW tail forms. The evolution of the simulated BD during recycling reveals important modifications versus the original virgin LDPE with implications for derived properties

Branching determination from radius of gyration contraction factor in radical polymerization

This paper proposes a set of models to calculate contraction factor, which to maximum extent accounts for the kinetics of radical polymerization with transfer to polymer and recombination termination. The models are alternatives to the Zimm and Stockmayer's (1949) analytical expression of contraction factor for molecules with terminal branching. The results, being representative for polymers like as low-density Polyethylene (IdPE), show significantly stronger contraction than predicted by the model of Zimm and Stockmayer. In the case of termination by disproportionation only, molecular sizes turn out to be smaller by a factor of almost two. In presence of recombination termination molecules are less compact. It is shown that the interpretation of contraction factors as measured by the Size Exclusion Multi-Angle Light Scattering to find the branchedness of IdPE, with the new model would lead to a considerably lower estimate of branching than by using Zimm and Stockmayer's model. (C) 2015 Elsevier Ltd. All rights reserved