Multiscale modeling of poly(2-isopropyl-2-oxazoline) chains in aqueous solution

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Official URL: http://dx.doi.org/10.1016/j.eurpolymj.2016.10.013

Poly(2-isopropyl-2-oxazoline) (PIPOX) is a thermo-responsive polymer exhibiting lower critical solution behaviour in water close to the human body temperature. PIPOX chains form crystalline nanoribbons in aqueous solutions above the critical temperature (Tc). Chain conformations in water prior to the crystallization is still much debated. In this study, a multiscale computational approach is used to investigate the conformations of PIPOX chains well-below and well-above Tc. Molecular dynamics (MD) simulations are performed to obtain single chain behaviour while dissipative particle dynamics (DPD) simulations are utilized to estimate the mesoscale morphologies of the polymer-solvent systems. Atomistic information is imposed on mesoscale beads by reverse-mapping, followed by MD simulations for relaxation. The most probable conformation obtained from the reverse-mapping is a helix with a pitch length of 15 monomers. The helical PIPOX chains are used to construct a triclinic unit cell that well-reproduce the experimental X-ray diffraction pattern for crystalline PIPOX nanoribbons formed in water above Tc. A pair of interacting helical PIPOX chains maintain their relative conformations at Tc, stabilized by hydrophobic interactions of the isopropyl side groups. The results are important in identifying a precursor helical conformation for PIPOX prior to crystallization in water above Tc. The new structure paves the way in using the interactions and phase transitions of thermo-responsive polymers for design purposes, in light of their potential in biomedical applications.