4 research outputs found
Influence of Cohesive Energy and Chain Stiffness on Polymer Glass Formation
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
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
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
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