No abstract

A variety of measurements using a differential scanning calorimeter were used to probe how single-walled carbon nanotubes (SWCNTs) affect polymer dynamics associated with the glass transition. Tubes were dispersed in N,N-dimethylformamide containing dissolved polystyrene, and precipitation was quickly forced by the addition to large amounts of water. The percolation threshold was found to be less than 0.5 wt %, indicating good dispersion of the tubes. The glass transition temperature (T g ) increased at low nanotube fractions to a constant value about 6-7°C higher than the T g of pure polystyrene, and did not change further as the nanotube amount changed from 1 to 30 wt %. The heat capacity change at the glass transition decreased with increasing nanotube concentration, except at very high SWCNT contents (>10 wt %), where the heat capacity change began to increase. The decrease of heat capacity at low nanotube contents indicates that a fraction of the polymer is made immobile via the addition of SWCNTs; while the large increase at high contents suggests that nanotubes are participating in the molecular motion that is the glass transition. The relaxation rate as determined by the change in limiting fictive temperature with annealing time showed the same qualitative behavior as the glass transition, a decrease in polymer mobility at very low nanotube fractions followed by a constant value. Surprisingly, one measure of the activation energy increased at low nanotube contents (<0.5 wt %) and dropped at high nanotube contents to an energy that looks to be slightly higher than that for pure polystyrene. In other words, in the region where the formation of a continuous network occurs the activation energy is highest.

The efficient operation of polymer-and perovskite-based photovoltaic devices depends on selective charge extraction layers that are placed between the active layer and electrodes. Herein, we demonstrate that integration of a tetra-n-butyl ammonium iodide-doped zwitterionic fulleropyrrolidine derivative, C 60 -SB, as a cathode modification interlayer significantly improves the photovoltaic device performance. Compared to the intrinsic (undoped) zwitterionic material, which is an efficient interlayer itself, the doped interlayers further improve average power conversion efficiencies from 8.37% to 9.68% in polymer-based devices and from 12.53% to 15.31% in perovskite-based devices. Ultraviolet photoelectron spectroscopy revealed that doping increases the interfacial dipole at the C 60 -SB/Ag interface, i.e., reduces the effective work function of the resultant composite cathode. This effect originates from the population of negative polaron states in C 60 -SB by extrinsic charges that prevent directional charge transfer from Ag to the integer charge-transfer states in C 60 -SB, pinning the Fermi level at higher energy. The reduced resistivity of the doped interlayer, as measured by impedance spectroscopy, enables efficient device operation with a broad range of interlayer thicknesses, thus simplifying the solution-based device fabrication process.

a b s t r a c tThe thermophysical properties of mixtures of galactitol and mannitol were examined via differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD) analysis. The aforementioned sugars were found to form a eutectic mixture at a 30:70 molar ratio of galactitol and manntitol, and displayed a melting point of 153°C while maintaining a high latent heat of fusion (DH fus = 292 J g À1 ). The XRD data revealed that the eutectic mixture contained the a, b, and d forms of mannitol with the d form being the major component. By varying the temperature ramp rates utilized in the DSC measurements from 0.5°C min À1 to 20°C min À1 , the heat of crystallization as well as the crystallization temperature increased (c.f., DH crys : 64 J g À1 ? 197 J g À1 ; T c : 68°C ? 105°C). In addition, the temperature and the enthalpy of crystallization were also improved by up to 34% through the addition of small quantities (up to 0.5 wt%) of nucleating agents, such as graphite powder or silver iodide. After 100 heating/cooling cycles under an atmosphere of nitrogen, the heat of fusion of the eutectic mixture decreased by only 4% with no change in the melting point, and the mixture appeared to be chemically stable according to a Fourier transform infrared (FT-IR) spectroscopic analysis. Collectively, these data indicate that the eutectic mixture exhibits excellent cyclic stability under ambient atmospheres and offers potential for use in thermal energy storage applications.

Positron annihilation lifetime spectroscopy (PALS) was employed to study the free volume properties of polystyrene (PS) containing three different types of carbon nanoparticles: polystyrenegrafted single wall carbon nanotubes (SWCNTs-g-PS), single wall carbon nanotubes (SWCNTs), and carbon nanofibers (CNFs). The glass transition temperature measured via PALS was significantly lower than that from differential scanning calorimetry (DSC), although qualitatively the two methods agreed in that the T g measured increased as nanotubes were added to the material. There were some specific differences between the two measurements which may have been related to the fact that DSC does not measure T g of a portion of the material which is immobilized on the surface of the particle, while PALS measures all polymer, whether immobilized or not. PALS was also used to measure the thermal expansion coefficient and distributions of the free volume of the polystyrene.