A molecular dynamics study of poly(N-isopropylacrylamide) endgrafted on a model cylindrical pore surface

Structure and dynamic behavior of the thermo-responsive polymer poly(N-isopropylacrylamide) (PNIPAM) endgrafted onto the inner surface of a simple cylindrical pore model that resembles a carbon nanotube (CNT) with a diameter of 8.4 nm is studied as a function of temperature, of surface-polymer interaction strength, and of pore water content. A free PNIPAM chain in water shows thermo-responsive behavior with a lower critical solution temperature (LCST) of about 305 K. We have investigated two different strengths of PNIPAM-pore interactions. In the strong interaction case, which corresponds to force-field parameters taken without change from the AMBER force field, the endgrafted PNIPAM chain collapses onto the surface at all temperatures studied and hence does not adopt a brush structure. In the weak interaction case the PNIPAM-pore interaction strengths were scaled by a factor of 10, and the temperature-responsive behavior of the PNIPAM chain re-emerges. End-to-end distances, radii of gyration, density profiles, number of hydrogen bonds, and radial distribution functions demonstrate the temperature-dependent structural changes of endgrafted PNIPAM in the pore. Analysis of the translational motion of water molecules in the pore shows that the ratio of the water self diffusion coefficient in a pore with a free pore surface relative to the self diffusion coefficient in a pore containing an end-grafted PNIPAM molecule is less strongly reduced above the LCST than below the LCST, where the chain is in a more extended state.