Effect of substrate interactions on the melting behavior of thin polyethylene films

Polymer films have been known to change their physical properties when film thickness is decreased below a certain value. The cause of this phenomenon is still unclear but it has been suggested that interactions and/or chain free-volume changes at the surface of the films are largely responsible for this behavior. In this paper, the effect of substrate interactions on the behavior of polymer thin films is evaluated quantitatively. The infrared spectra of nanothin polyethylene (PE) films were recorded as a function of temperature and amount of substrate covering the surface of the film. The evolution of specific bands in the CH₂ rocking region of the spectra was used to determine the melting temperature (T _m ) of the material. Results show different variations in T _m depending on the nature of the substrate, indicating that interactions dominate free-volume considerations in PE thin films. By varying the amount of surface coverage, a quantitative estimate of the heat of interaction was determined, which confirmed the importance of surface interactions.     

The melting line for ethylene on graphite

Using the minimum in the nuclear magnetic spin-lattice relaxation time versus temperature as an indicator of melting we have mapped out the solid-fluid phase boundary for ethylene adsorbed on graphite. At low coverages the ethylene forms a self-bound monolayer solid with a melting temperature of about 68 K. The molecules in the solid retain orientational mobility down to 55 K, the lowest temperatures explored.     

Isotropic graphite melting under pulsed heating

Experiments on pulsed heating (few microseconds) of graphite with measurements of the liquid carbon resistivity are described. It is confirmed that heating in water at atmospheric pressure do not allow production and study of liquid carbon; in the best case, the liquid state region beginning is achieved. Heating in sapphire tubes results in pulsed pressure (to ten of kbar) as expanding graphite bears against the tube wall. This increasing (during few microseconds) pressure makes it possible to study the carbon liquid state in a limited volume. Isochoric heating resulted in the possibilities ofmeasuring the liquid carbon resistivity at high specific energies (to ～32 kJ/g) and high pressures. Such measurements are extremely expensive at stationary studies.     

Wetting behavior of magnesite and dolomite surfaces

Magnesite and dolomite are salt-type minerals that show similar chemical composition and flotation behavior due to same crystal structure, and sparingly soluble nature. The surface properties of minerals play a major role in determining their separation from each other in processes such as flotation. During flotation process, selectivity problem arises between magnesite and associated gangue minerals such as dolomite. There is a close relationship between floatability of minerals and their contact angles. Therefore, surface hydrophobicity of magnesite and dolomite minerals was investigated by contact angle measurements in the absence and presence of flotation reagents.Magnesite and dolomite show hydrophilic properties and they have got a small contact angle (magnesite ∼10.4° and dolomite ∼6.6°) in distilled water in the absence of any surfactant. The contact angle values at the magnesite and dolomite surfaces remained at 9.7°-10.9° in the presence of petroleum sulphonates (R825 and R840) while sodium oleate affected hydrophobicity of magnesite, and the contact angle value increased up to 79°. The contact angle value of 39° at dolomite surface was obtained in the solution of sodium oleate, respectively.     

The multilayer melting transition in methane adsorbed on graphite

High resolution heat capacity measurements of multilayer methane adsorbed on graphite are presented and analyzed. The evidence indicates the presence of two wetting transitions: a first-order dewetting transition at T_w = 90.48 K, and a continuous wetting transition at the triple point, T_t = 90.66 K. This behavior is to be expected in connection with the melting transition in any system where both solid and liquid wet the surface. Heat capacity measurements can provide a valuable diagnostic tool for the wetting behavior of films too thick to be investigated by other means. In the thin film limit, we find that the latent heat of melting vanishes at about 4 layers.     

Surface reactivity of oxygen on cleaved and sputtered graphite

The characterization of reactions which occur during the oxidation of highly oriented pyrolytic graphite (HOPG) was performed using high-resolution electron energy loss spectroscopy (HREELS) temperature programmed desorption (TPD), and scanning tunneling microscopy (STM). The results of this investigation provide spectroscopic evidence for the presence of semi-quinone functionalities on sputtered and oxidized HOPG STM images are presented to quantify the increase in defect sites after oxygen ion sputtering, and to correlate defect site density with reactivity.