A coil-to-globule transition capable coarse-grained model for poly(N-isopropylacrylamide)

We present a model for mesoscopic molecular dynamics simulations of poly(N-isopropyl-acrylamide) (pNIPAM). The model uses a coarse-grained scheme based on the explicit-solvent Martini force field. The mapping of the polymer accounts for three beads per monomer. Similarly to the Martini water bead, the amide moieties of the polymer include an electric dipole. The model is tested by building polymer chains of different sizes and proved to accurately capture the thermal response of pNIPAM without including any temperature-dependent parameters. The critical temperature of the model is observed at (302.1 ± 1.1) K for a 35-mer and it keeps invariant when increasing the chain length. We deployed a series of replica-exchange molecular dynamics simulations that evidence the oligomer reaches thermodynamic equilibrium irrespective of the starting configuration. Finally, the model is applied to a membrane structure of pNIPAM where a good agreement with previous atomistic simulations is observed.