4 research outputs found

    Influence of Cohesive Energy and Chain Stiffness on Polymer Glass Formation

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    The generalized entropy theory is applied to assess the joint influence of the microscopic cohesive energy and chain stiffness on glass formation in polymer melts using a minimal model containing a single bending energy and a single (monomer averaged) nearest neighbor van der Waals energy. The analysis focuses on the combined impact of the microscopic cohesive energy and chain stiffness on the magnitudes of the isobaric fragility parameter <i>m</i><sub><i>P</i></sub> and the glass transition temperature <i>T</i><sub>g</sub>. The computations imply that polymers with rigid structures and weak nearest neighbor interactions are the most fragile, while <i>T</i><sub>g</sub> becomes larger when the chains are stiffer and/or nearest neighbor interactions are stronger. Two simple fitting formulas summarize the computations describing the dependence of <i>m</i><sub><i>P</i></sub> and <i>T</i><sub>g</sub> on the microscopic cohesive and bending energies. The consideration of the combined influence of the microscopic cohesive and bending energies leads to the identification of some important design concepts, such as iso-fragility and iso-<i>T</i><sub>g</sub> lines, where, for instance, iso-fragility lines are contours with constant <i>m</i><sub><i>P</i></sub> but variable <i>T</i><sub>g</sub>. Several thermodynamic properties are found to remain invariant along the iso-fragility lines, while no special characteristics are detected along the iso-<i>T</i><sub>g</sub> lines. Our analysis supports the widely held view that fragility provides more fundamental insight for the description of glass formation than <i>T</i><sub>g</sub>

    Molecular Dynamics Investigation of the Relaxation Mechanism of Entangled Polymers after a Large Step Deformation

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    The chain retraction hypothesis of the tube model for nonlinear polymer rheology has been challenged by the recent small-angle neutron scattering (SANS) experiment (Wang, Z.; Lam, C. N.; Chen, W.-R.; Wang, W.; Liu, J.; Liu, Y.; Porcar, L.; Stanley, C. B.; Zhao, Z.; Hong, K.; Wang, Y., Fingerprinting Molecular Relaxation in Deformed Polymers. <i>Phys. Rev. X</i> <b>2017</b>, <i>7</i>, 031003). In this work, we further examine the microscopic relaxation mechanism of entangled polymer melts after a large step uniaxial extension by using large-scale molecular dynamics simulation. We show that the unique structural features associated with the chain retraction mechanism of the tube model are absent in our simulations, in agreement with the previous experimental results. In contrast to SANS experiments, molecular dynamics simulations allow us to accurately and unambiguously determine the evolution of the radius of gyration tensor of a long polymer chain after a large step deformation. Contrary to the prediction of the tube model, our simulations reveal that the radius of gyration in the perpendicular direction to stretching increases monotonically toward its equilibrium value throughout the stress relaxation. These results provide a critical step in improving our understanding of nonlinear rheology of entangled polymers

    Effects of Chain Rigidity on the Adsorption of a Polyelectrolyte Chain on Mixed Lipid Monolayer: A Monte Carlo Study

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    We apply Monte Carlo simulation to explore the adsorption of a positively charged polyelectrolyte on a lipid monolayer membrane, composed of electronically neutral, monovalent anionic and mulvitalent anionic phospholipids. We systematically assess the influence of various factors, including the intrinsic rigidity of the polyelectrolyte chain, the bead charge density of the polyelectrolyte, and the ionic strength of the saline solution, on the interfacial structural properties of the polyelectrolyte/monolayer complex. The enhancement of the polyelectrolyte chain intrinsic rigidity reduces the polyelectrolyte conformational entropy loss and the energy gains in electrostatic interaction, but elevates the segregated anionic lipid demixing entropy loss. This energy-entropy competition results in a nonmonotonic dependence of the polyelectrolyte/monolayer association strength on the degree of chain rigidity. The semiflexible polyelectrolyte, i.e., the one with an intermediate degree of chain rigidity, is shown to associate onto the ternary membane below a higher critical ionic concentration. In this ionic concentration regime, the semiflexible polyelectrolyte binds onto the monolayer more firmly than the pancake-like flexible one and exhibits a stretched conformation. When the chain is very rigid, the polyelectrolyte with bead charge density <i>Z</i><sub>b</sub> = +1 exhibits a larger tail and tends to dissociate from the membrane, whereas the one with <i>Z</i><sub>b </sub>= +2 can still bind onto the membrane in a bridge-like conformation. Our results imply that chain intrinsic rigidity serves as an efficient molecular factor for tailoring the adsorption/desorption transition and interfacial structure of the polyelectrolyte/monolayer complex

    Effects of Concentration and Ionization Degree of Anchoring Cationic Polymers on the Lateral Heterogeneity of Anionic Lipid Monolayers

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    We employed coarse-grained Monte Carlo simulations to investigate a system composed of cationic polymers and a phosphatidyl-choline membrane monolayer, doped with univalent anionic phosphatidylserine (PS) and tetravalent anionic phosphatidylinositol 4,5-bisphosphate (PIP<sub>2</sub>) lipid molecules. For this system, we consider the conditions under which multiple cationic polymers can anchor onto the monolayer and explore how the concentration and ionization degree of the polymers affect the lateral rearrangement and fluidity of the negatively charged lipids. Our work shows that the anchoring cationic polymers predominantly bind the tetravalent anionic PIP<sub>2</sub> lipids and drag the PIP<sub>2</sub> clusters to migrate on the monolayer. The polymer/PIP<sub>2</sub> binding is found to be drastically enhanced by increasing the polymer ionization fraction, which causes the PIP<sub>2</sub> lipids to form into larger clusters and reduces the mobility of the polymer/PIP<sub>2</sub> complexes. As expected, stronger competition effects between anchoring polymers occur at higher polymer concentrations, for which each anchoring polymer partially dissociates from the monolayer and hence sequesters a smaller PIP<sub>2</sub> cluster. The desorbed segments of the anchored polymers exhibit a faster mobility on the membrane, whereas the PIP<sub>2</sub> clusters are closely restrained by the limited adhering cationic segments of anchoring polymers. We further demonstrate that the PIP<sub>2</sub> molecules display a hierarchical mobility in the PIP<sub>2</sub> clusters, which is regulated by the synergistic effect between the cationic segments of the polymers. The PS lipids sequester in the vicinity of the polymer/PIP<sub>2</sub> complexes if the tetravalent PIP<sub>2</sub> lipids cannot sufficiently neutralize the cationic polymers. Finally, we illustrate that the increase in the ionic concentration of the solution weakens the lateral clustering and the mobility heterogeneity of the charged lipids. Our work thus provides a better understanding of the fundamental biophysical mechanism of the concentration gradients and the hierarchical mobility of the anionic lipids in the membrane caused by the cationic polymer anchoring on length and time scales that are generally inaccessible by atomistic models. It also offers insight into the development and design of novel biological applications on the basis of the modulation of signaling lipids
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