Molecular dynamics study of wetting behavior of grafted thermo-responsive PNIPAAm brushes

Abstract

In this work, the effect of temperature on the contact angle of a water droplet on grafted thermo-responsive poly-(N-isopropylacrylamide) (PNIPAAm) polymer brushes is studied using all-atom molecular dynamics simulations in the temperature range of 270-330 K. A shift from 55° to 65° in contact angle values is observed as the temperature increases from 300 K to 310 K, which is in line with the experimental reports. The behavior of a water droplet on PNIPAAm brushes is analyzed using hydrogen bond analysis, water diffusion, radial distribution functions, the potential of mean force, excess entropy and the second virial coefficient (B2). The thermo-responsive behavior of PNIPAAm brushes, quantified using the excess entropy and B2 of PNIPAAm-water and water-water interactions, is mainly governed by polymer-water interactions. In particular, the excess entropy and B2 of PNIPAAm resulting from the PNIPAAm-water interactions are found to increase with increasing temperature. The dehydration of PNIPAAm brushes and the increase in the contact angle of water were confirmed to be entropy driven processes.

Molecular Structure Inhibiting Synergism in Charged Surfactant Mixtures: An Atomistic Molecular Dynamics Simulation Study

“…In a synergistic system, – T Δ mix S tr shows an apparent decrease upon mixing, while in a nonsynergistic system the two-body excess entropy of a mixed system turns out to be lower than that of the corresponding pure systems ( T Δ mix S tr greater than 0). Consequently, the two-body excess entropy of the hydrocarbon tails (which has also been used to estimate the tendency of a system to aggregate) discriminates between synergistic and nonsynergistic behavior in the systems examined in this work.…”

Temperature-Responsive Polymer Brush Coatings for Advanced Biomedical Applications

“…The LCTS-based transition of TRPBCs appears as a drastic change in wettability [ 1 , 47 , 48 , 92 , 93 , 94 ]. To determine the temperature of the transition between the states of the extended hydrophobic chain and the collapsed hydrophilic globule, the contact angles of water at different temperatures are often studied [ 1 , 47 , 48 , 92 , 93 , 94 ].…”

Molecular-scale description of interfacial mass transfer in phase-separated aqueous secondary organic aerosol

“…We propose an equation which can serve as a qualitative descriptor of the entropy related to the orientation of the molecules based on equations for translational (Bhandary et al, 2016) and translational-orientation entropy terms (Piaggi and Parrinello, 2018). With a simple exchange of the radial distribution function in Bhandary et al (2016) with the angular distribution function (g(θ )) of the angle (θ ) between the dipole vector of the water molecules and the surfacenormal axis, we obtain an expression for orientational entropy: ) This expression is evaluated for angular distribution functions calculated in the first two molecular layers and the bulk of the aqueous phase, to highlight the effect of increased molecular order on the free-energy profiles. The separation of interfacial water molecules and those constituting the sec- ond layer is performed by two consecutive repetitions of the ITIM algorithm, using the output bulk phase of the first one as input for the second one.…”