Solution behaviour of poly(N-isopropylacrylamide) stereoisomers in water: a molecular dynamics simulation study
The water affinity of poly(N-isopropylacrylamide), PNIPAM, is tuned by tacticity, since the hydrophobicity rises with the increase of the degree of isotacticity. On the basis of this experimental evidence, atomistic molecular dynamics simulations of pairs of PNIPAM stereoisomers in 1.6% w/w polymer aqueous solution, a condition intermediate between the dilute and semidilute regimes, were carried out to comparatively investigate the solution behaviour and hydration of atactic and isotactic-rich PNIPAMs, both below and above the lower critical solution temperature, LCST. 30-mers with contents of meso dyads, m, of 45% and 59%, built assuming a Bernoullian dyad distribution, are used as models since their stereochemical composition corresponds to that of experimentally characterized PNIPAM stereoisomers. The simulation results at 283 K, below the LCST, show a slight influence of tacticity on the chain size, but a higher propensity for inter-chain association of the meso-dyad-rich system, in agreement with the experimental results. Junctions between chains are formed because of hydrophobic interactions and are stabilized by a layer of hydrogen bonded water molecules, whose mobility is reduced as compared to that observed for the same meso-dyad-rich stereoisomer at infinite dilution. At 323 K, above the LCST, simulations detect both the coil–globule transition and the aggregation of chains. Under these conditions, the influence of tacticity on the characteristics of PNIPAM aggregate is negligible.