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.

We present atomistic simulations of a single PNIPAM-alkyl copolymer surfactant in aqueous solution at temperatures below and above the LCST of PNIPAM. We compare properties of the surfactant with pure PNIPAM oligomers of similar lengths, such as the radius of gyration and solvent accessible surface area, to determine the differences in their structures and transition behavior. We also explore changes in polymer-polymer and polymer-water interactions, including hydrogen bond formation. The expected behavior is observed in the pure PNIPAM oligomers, where the backbone folds onto itself above the LCST in order to shield the hydrophobic groups from water. The surfactant, on the other hand, does not show much conformational change as a function of temperature, but instead folds to bring the hydrophobic alkyl tail and PNIPAM headgroup together at all temperatures. The atomic detail available from these simulations offers important insight into understanding how the transition behavior is changed in PNIPAM-based systems.

The long-time correlation functions of an infinitesimally thin needle moving through stationary point scatterers-a so-called Lorentz model-exhibits surprisingly long-time tails. These long-time tails are now seen to persist in a two-dimensional model even when the needle has a finite thickness. If the needles are too thick, then the needles are effectively trapped at all nontrivial densities of the scatterers. At needle widths approximately equal to or smaller than sigma = epsilon/20 where ε is the average spacing between scatterers, the needle diffuses and exhibits the crossover transition from the expected Enskog behavior to the enhanced translation diffusion seen earlier by Höfling, Frey and Franosch [ Phys. Rev. Lett. 2008 , 101 , 120605 ]. At this needle width, an increase in its center-of-mass diffusion with respect to increasing density is seen after a crossover density of n* approximately 5 is reached. (The reduced density n* is defined as n* = nL(2) where n is the number density of particles and L is the needle length.) The crossover transition for needles with finite thickness is spread over a range of densities exhibiting intermediate behavior. The asymptotic divergence of the center of mass diffusion is suppressed compared to that of infinitely thin needles. Finally, a new diminished diffusion regime, apparently due to the increased importance of head-on collisions, now appears at high scattering densities.

The existence of three regimes in the dynamics of a thin needle-like particle diffusing through a two-dimensional random array of scatterers as the needle length is varied relative to the scatterer density was previously seen in a series of simulations. The first regime occurs at low density when the needle's diffusion follows the expected Enskog behavior. An intermediate regime gives rise to enhanced diffusion after a critical density of scatterers is reached, a manifestation of the suppression of librational motion as the needle is confined to effectively thinner but longer tubes. The third regime occurs at high scatterer density with the particle dynamics characteristic of a glass. In this article, we investigate whether the tubes seen in the second regime persist in a three-dimensional array of scatterers. The fact that the enhanced regime is not observed in a three-dimensional random array of scatterers suggests that the effective tubes formed by the moving needle are fragile structures highly dependent on dimensionality.

Monte Carlo simulations have been used to construct free energy surfaces of 1,2-dichloroethane dissolved in methanol confined in hydrophobic spherical cavities of varying size (10-15 A) and solution density (0.6-0.79 g/cm3). The free energy surfaces are functions of two variables: the (center-of-mass) distance from the cavity wall of 1,2-dichloroethane and the Cl-C-C-Cl dihedral angle. Umbrella sampling and the weighted histogram analysis method were used to obtain accurate results for the free energy in these two degrees of freedom. Our results indicate that the conformational equilibrium and the barrier to internal rotation of the 1,2-dichloroethane depend on the position in the cavity. The results are discussed in the context of the solvent density, orientational distributions, and packing effects.