We use molecular dynamics simulations to investigate the control of electroosmotic flow by grafting polymers onto two parallel channel walls. The effects of the grafting density and the electric field strength on electroosmotic flow velocity, counterion distribution and conformational characteristics of grafted chains have been studied in detail for athermal, good, and poor solvent cases. The simulation results indicate that in the range of grafting densities investigated, increasing the grafting density induces a different change tendency of electroosmotic flow velocity for three different solvent qualities. These tendencies are demonstrated to be related to counterion distribution, polymer coverage, and interactions between monomers and solvent particles. It is found that counterions tend to move toward the interface between polymer layer and solvent as the grafting density increases. Especially in the poor solvent case, most of the counterions gather near the interface at high grafting densities. A similar behavior is also observed when enhancing the electric field strength at a fixed grafting density.

Despite many merits, biobased and biodegradable poly(lactic acid) (PLA) suffers from inherently high flammability, significantly impeding its practical applications in the areas of packaging, fibers, electric and electronics, etc. Phosphoramidesbased flame retardants have demonstrated exceptional flame retardancy in PLA only at very low loading levels. Unfortunately, exiting synthetic processes of phosphoramides often involve the use of a huge amount of organic solvents, presenting a potential threat to the ecosystem and human beings, in addition to high production costs. Herein, we report a one-pot, solvent-and catalyst-free, and green approach for the synthesis of a polyphosphoramide (DM-H) via two-step polycondensation between dimethyl methylphosphonate (DMMP) and 1,6-hexanediamine (HDA). As-synthesized DM-H exhibits exceptional flame-retardant efficiency in the PLA while preserving the mechanical properties of the PLA. The addition of only 2 wt % DM-H enables the PLA to achieve a desirable V-0 rating during vertical burning tests and a high limited oxygen index (LOI) value of 29.7%, because the DM-H can act both in the gas and condensed phases. Moreover, the tensile strength is of 59.7 MPa comparable to 62.1 MPa of the bulk PLA. This work offers an innovative and green approach for the synthesis of polyphosphoramides to create high-performance sustainable PLA materials with balanced flame retardancy and mechanical performances.

No abstract

Molecular dynamics simulations were done to study the electroosmotic flow (EOF) transport in a nanochannel grafted with polyelectrolytes under the control of an electric field normal to the channel wall. This study first addresses some problems on the interplay between complex EOF and non-equilibrium conformational behavior of polyelectrolyte brushes at a molecular level. We demonstrated that changing the normal electric field has a significant impact on the conformational transition of polyelectrolytes and ion distributions, further leading to some new flow phenomena. The coupling mechanisms of polyelectrolyte chain dynamics and electrohydrodynamics were discussed. A remarkable result obtained is that fluid flux depends nonmonotonically on the normal electric field. Our work provides fundamental understanding of the EOF modulation using polyelectrolyte brushes and guidance for the design of smart nanofluidic channels.

Using coarse-grained molecular dynamics simulations, we study the behavior of a DNA-nanosphere complex in the absence and presence of an external stretching force exerted on two ends of DNA chain. In this work, we use an accurate coarse-grained model for double-stranded DNA chain recently developed by Savelyev and Papoian [Biophys. J. 96, 4044 (2009)]. Charged particles are uniformly distributed on the surface of the sphere. Without a stretching force, an ordered or disordered complex is formed depending on the surface charge density and the salt concentration. It is found that DNA wraps randomly around the sphere only at an intermediate salt concentration and high surface charge density. Additionally, the DNA folding around the sphere induces a reduced distance between DNA monomers close to the spherical surface. When an external force is applied, the force-extension relation reveals a discontinuous transition of DNA stretching during the unwrapping process. Moreover, the discrete change becomes more obvious for a higher salt concentration.

We report a molecular dynamics study on non-equilibrium dynamics of polyelectrolyte brushes under external electric fields. In this work, the effects of chain stiffness and salt concentration on static and dynamic responses of the brushes are addressed in detail. Our simulations indicate that varying these parameters induce rich electro-responsive behavior of the brushes. The increase of salt concentration results in the enhancement of an opposite electric field formed by non-equilibrium distribution of cations and anions, which resists stretching or shrinkage of grafted chains. At strong positive electric fields, the flexible brushes are more sensitive to the change of salt concentration. When reversing the electric field, the stiff brushes undergo a conformational transition from collapse to complete stretching. At high salt concentrations, dynamic responsive magnitude of the brush thickness to added electric field is strongly reduced. It was found that the fall time for the stiff brush becomes much shorter than that for the flexible brush. Additionally, increasing ion concentration leads to an excess extension or shrinkage of flexible brushes. For strongly stiff brushes, such phenomenon occurs in the presence or absence of salt.

We have performed dissipative particle dynamics (DPDs) simulations of electroosmotic flow (EOF) through a polymer-grafted nanopore. In this model, charged particles including salt ions and counterions are not included explicitly, and EOF is created using an effective boundary condition. The screening effect of polymer layer on EOF is investigated in detail under different solvent qualities and boundary electroosmotic velocities. Results show that the solvent quality has a significant effect on the conformational properties of polymer chains and the flow characteristics of the solvent. The polymer layer undergoes a collapsed transition when decreasing the solvent quality from good to poor. Under different solvent qualities, enhancing the EOF leads to a different variation tendency of the layer thickness. The solvent-induced permeability change is inconsistent with the steady velocity away from the surface. The minimum value of the solvent permeability occurs at an intermediate solvent quality. However, the layer thickness drops gradually to a smallest value (corresponding to the largest effective pore radius) in the poor solvent condition. It is also found that the polymer inclination and stretching length exhibit a complex behavior under the combined effect of solvent quality and electroosmosis-induced shear.

No abstract