Some aspects of a two-pore translocation problem

We report Brownian dynamics (BD) simulation results for a coarse-grained (CG) model semi-flexible polymer threading through two nanopores. Particularly we study a “tug-of-war” situation where equal and opposite forces are applied on each pore to avoid folds for the polymer segment in between the pores. We calculate mean first passage times (MFPT) through the left and the right pores and show how the MFPT decays as a function of the off-set voltage between the pores. We present results for several bias voltages and chain stiffness. Our BD simulation results validate recent experimental results and offer avenues to further explore various aspects of multi-pore translocation problem using BD simulation strategies which we believe will provide insights to design new experiments.

An unexpected N-dependence in the viscosity reduction in all-polymer nanocomposite

Adding small nanoparticles (NPs) into polymer melt can lead to a non-Einstein-like decrease in viscosity. However, the underlying mechanism remains a long-standing unsolved puzzle. Here, for an all-polymer nanocomposite formed by linear polystyrene (PS) chains and PS single-chain nanoparticles (SCNPs), we perform large-scale molecular dynamics simulations and experimental rheology measurements. We show that with a fixed (small) loading of the SCNP, viscosity reduction (VR) effect can be largely amplified with an increase in matrix chain length $$N$$N, and that the system with longer polymer chains will have a larger VR. We demonstrate that such $$N$$N-dependent VR can be attributed to the friction reduction experienced by polymer segment blobs which have similar size and interact directly with these SCNPs. A theoretical model is proposed based on the tube model. We demonstrate that it can well describe the friction reduction experienced by melt polymers and the VR effect in these composite systems.

Solubility of Polymers in Supercritical Carbon Dioxide

A great deal of information is known about the solvent character of CO2 with a wide range of polymers and copolymers based on well-characterized and systematic solubility studies that are available in the literature (Kirby and McHugh, 1999). Nevertheless, the prediction of polymer solubility in CO2, or any solvent for that matter, presents a formidable challenge since contemporary equations of state are still not facile enough to describe the unique characteristics of a long-chain polymer in solution. The difficulty resides in accounting for the intra- and intersegmental interactions of the many segments of the polymer connected to a single backbone relative to the small number of segments in a solvent molecule. An additional challenge exists to describe the density dependence of the intermolecular potential functions used in the calculations since SCF–polymer solutions (SCF, supercritical fluid) can be highly compressible mixtures. In this brief review, the solvent character of CO2 is described using the principles of molecular thermodynamics and also using a select number of phase behavior studies to reveal the impact of polymer architecture on solubility. To form a stable polymer–SCF solvent solution at a given temperature and pressure, the Gibbs energy, shown in eq. 7.1, must be negative and at a minimum. . . . ΔGmix = ΔHmix − T ΔSmix (7:1) . . . where ΔHmix and ΔSmix are the change of enthalpy and entropy, respectively, on mixing (Prausnitz et al., 1986). Enthalpic interactions depend predominantly on solution density and on polymer segment–segment, solvent–solvent, and polymer segment–solvent interaction energies. The value of ΔSmix depends on both the combinatorial entropy of mixing and the noncombinatorial contribution associated with the volume change on mixing, a so-called equation-of-state effect (Patterson, 1982). The combinatorial entropy always promotes the mixing of a polymer with a solvent. However, the noncombinatorial contribution can have a negative impact on mixing as a result of monomer–monomer interactions that arise due to the connectivity of the segments in the backbone of the polymer chain.