Molecular dynamics study of wetting behavior of grafted thermo- responsive PNIPAAm brushes
D Bhandary and Z Benkova and MNDS Cordeiro and JK Singh, SOFT MATTER, 12, 3093-3102 (2016).
In this work, the effect of temperature on the contact angle of a water droplet on grafted thermoresponsive 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 551 to 65 degrees 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 (B-2). The thermo-responsive behavior of PNIPAAm brushes, quantified using the excess entropy and B-2 of PNIPAAm-water and water-water interactions, is mainly governed by polymer-water interactions. In particular, the excess entropy and B-2 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.
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