Kinetics of high density functional polymer nanocomposite formation by tuning enthalpic and entropic barriers

Competing Effects of Temperature and Polymer Concentration on Evolution of Re-entrant Interactions in the Nanoparticle-Block Copolymer System

An interesting evolution of the re-entrant interaction has been observed in an anionic silica nanoparticle (NP)-block copolymer (P85) dispersion due to mutually competing effects of temperature and polymer concentration. It has been demonstrated that a rise in the temperature leads to an evolution of attraction in the system, which interestingly diminishes on increasing the polymer concentration. Consequently, the system exhibits a re-entrant transition from repulsive to attractive and back to repulsive at a given temperature but with respect to the increasing polymer concentration, within a selected region of concentration and temperature. The intriguing observations have been elucidated based on the temperature/concentration-dependent modifications in the interactions governing the system, as probed by contrast-variation small-angle neutron scattering. The initial transition from the repulsive to attractive system is attributed to the temperature-driven enhancement in the hydrophobicity of the amphiphilic triblock copolymer (P85) adsorbed on nanoparticles. The strength and range of this attraction are found to be more than van der Waals attraction while relatively less than electrostatic interaction. At higher polymer concentrations, the saturation of polymer adsorption on nanoparticles introduces additional steric repulsion along with electrostatic interaction between their conjugates, effectively reducing the strength of the attraction. However, with a significant increase in temperature (>75 °C), the attraction again dominates the system, which eventually leads to the particle aggregation at all the measured polymer concentrations (>0.1 wt %). Our study provides useful inputs to develop smart NP-polymer composites having capabilities to respond to external stimuli such as temperature/concentration variation.

Enhanced efficiency of water desalination in nanostructured thin-film membranes with polymer grafted nanoparticles

Polyamide composite (PA-TFC) membranes are the state-of-the-art ubiquitous platforms to desalinate water at scale. We have developed a novel, transformative platform where the performance of such membranes is significantly and controllably improved by depositing thin films of polymethylacrylate [PMA] grafted silica nanoparticles (PGNPs) through the venerable Langmuir-Blodgett method. Our key practically important finding is that these constructs can have unprecedented selectivity values (i.e., ∼250-3000 bar^-1, >99.0% salt rejection) at reduced feed water pressure (i.e., reduced cost) while maintaining acceptable water permeance A (= 2-5 L m^-2 h^-1 Bar^-1) with as little as 5-7 PGNP layers. We also observe that the transport of solvent and solute are governed by different mechanisms, unlike gas transport, leading to independent control of A and selectivity. Since these membranes can be formulated using simple and low cost self-assembly methods, our work opens a new direction towards development of affordable, scalable water desalination methods.

Dispersion and orientation patterns in nanorod-infused polymer melts

Introducing nanorods into a polymeric matrix can enhance the physical and mechanical properties of the resulting material. In this paper, we focus on understanding the dispersion and orientation patterns of nanorods in an unentangled polymer melt, particularly as a function of nanorod concentration, using molecular dynamics simulations. The system is comprised of flexible polymer chains and multi-thread nanorods that are equilibrated in the NPT ensemble. All interactions are purely repulsive except for those between polymers and rods. Results with attractive vs repulsive polymer-rod interactions are compared and contrasted. The concentration of rods has a direct impact on the phase behavior of the system. At lower concentrations, rods phase separate into nematic clusters, whereas at higher concentrations more isotropic and less structured rod configurations are observed. A detailed examination of the conformation of the polymer chains near the rod surface shows extension of the chains along the director of the rods (especially within clusters). The dispersion and orientation of the nanorods are a result of the competition between depletion entropic forces responsible for the formation of rod clusters, the enthalpic effects that improve mixing of rods and polymer, and entropic losses of polymers interpenetrating rod clusters.