Study of the Polymer Length Dependence of the Single Chain Transition Temperature in Syndiotactic Poly(N-isopropylacrylamide) Oligomers in Water

Aqueous solutions of poly(N-isopropylacrylamide) (PNIPAM) exhibit a temperature responsive change in conformation. When the temperature is increased, the polymer transitions from an extended coil conformation to a collapsed structure. We performed molecular dynamics simulations of aqueous solutions of single chain, syndiotactic PNIPAM oligomers over a wide range of temperatures and varying degrees of polymerization to elucidate the effect of oligomer length on the single chain transition temperature, T-1. We have reproduced recent measurements of the transition temperature increasing with decreasing oligomer chain length. Examination of the chain structure reveals that conformations above T-1 bend to bring hydrophobic segments together to shield them from the water. The constraints of the dihedral dynamics require elevated temperatures for shorter chains to bend sharply enough in order to undergo the transition. This result is confirmed by calculations of the solvent accessible surface area, which shows an increase in shielding of the hydrophobic groups with increasing oligomer length above T-1.