Papers by Keyword: Glass Transition

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Authors: Ming Horng Su, Hung Chang Chen
Abstract: This paper studies the phase transitions of Cu and Ni alloys as they cool from melting temperature to room temperature under high-pressure conditions. The interatomic forces acting between the atoms are modeled by the tight-binding potential. Control over the environmental pressure and the cooling temperature is maintained by a canonical ensemble (N, P, T) system. The numerical results confirm that the metal phase transition is influenced significantly by the pressure conditions, even in the case of pure Cu and Ni metals. Three specific transition pathways are identified for the Cu and Ni alloys as they cool from melting temperature to room temperature, namely a transition at the melting temperature to a crystalline structure, a transition at the glass transition temperature to a glass (amorphous) structure, and finally solidification at the melting temperature followed by a subsequent transition at the glass transition temperature. The results reveal that glass transition generally occurs at lower pressures in alloys with higher Cu compositions, while glass transition following prior solidification tends to takes place at higher pressures in alloys with higher Ni compositions.
Authors: Ran Huang
Abstract: Since the first paper by Keddie et al. published on 1994 [21], the glass transition of polymer systems on surface/thin film has been an active research field and attracted many groups interests. Numerous works have been done, in both experimental and computation approaches, to investigate this subject. In this paper we reviewed the milestone findings in the last twenty years. Generally with only minor disagreements in the mechanism all the mainstream works are consistent in the conclusions that: 1) Geometric confinement in thin film or on surface reduces the glass transition temperature Tg comparing to the bulk behavior; 2) For supported film the substrate-film interaction is critical and its effect may surpass the geometry effects and rise increase on Tg; 3) Chain mobility and molecular weight are critical but the detailed phenomena vary with systems. Notwithstanding the achievement has been made, due to the controversy of glass transition itself and technology limitation on characterization on glass transitions on thin film, the research in this field is still a long-marching effort and breakthrough findings are expected for the development in materials science and engineering and feedback knowledge to understand the glass transition on the theoretical base.
Authors: Kai Chen Kuo, Pee Yew Lee, Jai Yush Yen
Abstract: In the current study, the amorphization behavior of mechanically alloyed Ni57Zr20Ti22Ge1 powder was examined in details. The conventional X-ray diffraction results confirm that the fully amorphous powders formed after 5 hours of milling. The thermal stability of the Ni57Zr20Ti22Ge1 amorphous powders was investigated by differential scanning calorimeter (DSC). As the results demonstrated, the glass transition temperature (Tg) and the crystallization temperature (Tx) are 761 K and 839 K, respectively. The supercooled liquid region ΔT is 78 K. The appearance of wide supercooled liquid region may be mainly due to the Ge additions which cause the increasing differences in atomic size of mechanically alloyed Ni57Zr20Ti22Ge1 powders.
Authors: Glaura Goulart Silva, Patterson Patrício de Souza, Ana Júlia Silveira Mizher, Marcos A. Pimenta
Abstract: Changes in glass transition and ionic association of PPG/LiClO4 samples with increasing salt concentration have been studied using temperature modulated differential scanning calorimetry (TMDSC)and micro Raman spectroscopy, respectively. PPG, of average molar mass of 3000, was used as host for LiClO4 over the range O:Li = 35 – 4. The analysis of the Raman band, obtained at room temperature, associated with the ν1 symmetric stretching mode of ClO4 - anions, shows the presence of ionic association (ion pair) for electrolytes with salt concentration higher than O:Li = 8. The glass transition changes in the electrolytes were investigated using the differential of heat capacity with respect to temperature [dCp/dT], obtained from TMDSC. The analysis of the dCp/dT signal, by fitting with Gaussian curves, showed that there is a glass transition splitting in samples with concentrations higher than O:Li = 16, which indicates the presence of nanoheterogeneities in these amorphous electrolytes. Raman results pemit to affirm that this nanoheterogeneities were associated mainly with solvent separated ion pairs regions of different concentrations, and not with aggregate of ionic species.
Authors: Qamar Nawaz, Y. Rharbi
Abstract: The effects of nanoconfinement on the dynamic and the glass transition (Tg) of polymers remains the focus of a lot of research since over a decade. Particularly, the glass transition temperature (Tg) and the dynamic of polystyrene (PS) were found to be altered by nanoconfinement in thin films and on the bulk free-surface. However, the dynamic of polymer nanoconfined in nanoparticles has not been investigated, even though the close-packed nanoparticle geometry is commonly used in many applications such as waterborne coatings. We investigate the dynamic of polystyrene in nanoparticles by monitoring the closure of voids (interstices) between PS nanoparticles in the close-packed structure. Void-closure during the passage from the close-packed particles to bulk PS is monitored using small angle neutron scattering at the bulk Tg (100 °C). The relaxation time (τ) and the apparent viscosity (η) of nanoconfined polystyrene estimated from the void-closure decay is found to decrease only by ~2 times for particle diameters between 93 nm and 42 nm. These results infer that dynamic of nanoconfined PS in nanoparticles at the bulk Tg is not different from that of bulk polystyrene.
Authors: Eugen R. Neagu, José N. Marat-Mendes, Radu Neagu
Abstract: The apparent activation energy W, the temperature Ta at which the molecular movement is assumed frozen (the relaxation time is infinitely large) and the preexponential factor t0 were determined for Nylon 11 using the data obtained by fractional polarization thermally stimulated discharge current technique. The aim was to investigate the temperature dependence of the relaxation time in the range of the glass transition temperature. An experimental thermogram can be analyzed equally well using the Vogel-Fulcher-Tammann-Hess relationship or the Arrhenius equation. Experimental results and simulations, for nearby peaks, demonstrate that the relaxation time at peak temperature changes from about 100 to 500 s revealing a very narrow distribution in relaxation times. On contrary, W and especially t0 varies in large limits and the values for t0 are in many situations physically meaningless. It was found that the temperature at which the molecular movement can be assumed frozen for electric effects is about 150 K below the glass transition temperature in Nylon 11. A small change in W (in the limit ± kTm which represents the minimum uncertainty interval in W) induces a change in t0 of about two decades, indicating (i) that any analysis assuming t0 constant should be avoided and (ii) the selectivity limit of the method.
Authors: U. Geyer, S. Schneider, Y. Qiu, M.P. Macht, T.A. Tombrello, William L. Johnson
Authors: Yi Long, Rong Chang Ye, Yong Qing Chang, Farong Wan
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