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Our previous work demonstrated that liquid water can freeze continuously into either pentagonal or helical solid-like ice nanotubes in a single-walled carbon nanotube (SWCNT) with a tube diameter of 1.2 nm, depending on the strengths of an external electric (E) field applied along the tube axis. In this study, the structure and the structure transition behavior of water confined in a wider SWCNT (diameter = 1.31 nm) under the influence of E-fields are investigated by molecular dynamics simulations using the TIP4P model for water at atmospheric pressure. We find that confined water can freeze into three different polygonal (including hexagonal, heptagonal, and mixed hexagonal-heptagonal) ice nanotubes through a first-order phase transition at lower E (<0.75 V/nm), while form a helical ice nanotube encapsulating a helical water nanoline through a continuous phase transition at higher E (1.0 < E < 2.0 V/nm), different from the phase transition behavior of water in a SWCNT with a diameter = 1.2 nm. The populations of the three different polygonal ice nanotubes are modulated by both temperature and electric field. In addition, an E-induced discontinuous solidsolid phase transition between polygonal and helical ice nanotubes is observed at low temperature (T < 230 K) with a significant electric hysteresis loop of 1.0 V/nm. Finally, we present a rich phase diagram of confined water as a function of temperature and electric field, in which the boundaries of the first-order phase transition at lower E and the continuous phase transition at higher E are connected by a connecting line which corresponds to the hysteresis region. [ABSTRACT FROM AUTHOR]

The effect of polydispersity on surface segregation of a lower molecular weight polymer component in a higher molecular weight linear polymer melt host is investigated theoretically. We show that the integrated surface excess zM of a polymer component of molecular weight M satisfies a simple relation zM=2Ue(M/Mw-1)φM, where Mw is the weight averaged molecular weight, φM is the polymer volume fraction, and Ue is the attraction of polymer chain ends to the surface. Ue is principally of entropic origin, but also reflects any energetic preference of chain ends to the surface. We further show that the surface tension γM of a polydisperse melt of high molar mass components depends on the number average degree of polymerization Mn as, γM=γ∞+2UeρbRT/Mn. The parameter γ∞ is the asymptotic surface tension of an infinitely long polymer of the same chemistry, ρb is the bulk density of the polymer, R is the universal gas constant, and T is the temperature. The predicted γM compare favorably with surface tension values obtained from self-consistent field theory simulations that include equation of state effects, which account for changes in polymer density with molecular weight. We also compare the predicted surface tension with available experimental data. [ABSTRACT FROM AUTHOR]