Melts structure

Since amorphous alloys can be regarded as metallic solids with a frozen-in melt structure, the liquid structure freezes at different temperatures... 

Such a model of the melt structure does not contradict conductivity data [324], if plotted against the composition of the KF - TaF5 system. Fig. 63 presents isotherms of molar conductivity, in which molar conductivity of the ideal system was calculated using Markov s Equation [315], and extrapolation... 

Melt structure High shear at a temperature not far above the melting point may cause a melt to take on too much molecular order. In turn, distortion could result. 

Weakly segregated systems, Todt > Tc > Tg with soft confinement. In this case, crystallization often occurs with little morphological constraint, enabling a breakout from the ordered melt MD structure and the crystallization overwrites any previous melt structure, usually forming lamellar structures and, in many cases, spherulites depending on the composition [10-18],... 

We show typical examples for the melt structure factor and for the single-chain structure factor in Figure 7. The upper panel is for a chemically realistic simulation of PB,111 where the scattering was calculated with the... 

Figure 7 Comparison of melt structure factor and single-chain structure factor for PB (upper panel, calculated as scattering from the united atoms only) and a bead-spring melt (lower panel, in Lennard-Jones units).

We can see from Figure 7 that for momentum transfers larger than about 3 A-1 in PB, i.e., starting around the second maximum, one observes only intramolecular correlations in the melt structure factor112-114 when one considers only scattering from the united atom centers. The melt structure factor can always be decomposed into a chain contribution (Sck(q)) and a contribution that captures the correlations between distinct melt chains (S,nj(c])). 

The chemically realistic simulations we are discussing have been performed using a united atom representation of PB, which leads to the question How does one actually measure a CH vector reorientation for such a model The answer to this question is to use the trick we discussed in the analysis of the pressure dependence of the melt structure factor of PB. Hydrogen atoms are placed on the backbone carbons at their mechanical equilibrium positions for each structure that has been sampled along the MD trajectory. The CH vector dynamics we are showing in Figure 16 is solely from the backbone reorientations of the chain. 

In the discussion on the dynamics in the bead-spring model, we have observed that the position of the amorphous halo marks the relevant local length scale in the melt structure, and it is also central to the MCT treatment of the dynamics. The structural relaxation time in the super-cooled melt is best defined as the time it takes density correlations of this wave number (i.e., the coherent intermediate scattering function) to decay. In simulations one typically uses the time it takes S(q, t) to decay to a value of 0.3 (or 0.1 for larger (/-values). The temperature dependence of this relaxation time scale, which is shown in Figure 20, provides us with a first assessment of the glass transition... 

Properties of a Simulated Supercooled Polymer Melt Structure Factors, Monomer Distributions Relative to the Center of Mass, and Triple Correlation Functions. 

To better understand diffusion in silicate melts, we first briefly review silicate melt structure. In natural silicate melts from basalt (about 50% Si02) to rhyolite... 

The dependence of diffusivity in silicate melts on composition is related to how melt structure (including degree of polymerization and ionic porosity) depends on composition. One the one hand, as Si02 concentration increases, the melt becomes more polymerized and the viscosity increases. Hence, diffusivity of most structural components, such as Si02 and AI2O3, decreases from basalt to rhyolite. On the other hand, as Si02 content increases, the ionic porosity increases. The increasing He diffusivity from basalt to rhyolite to silica, opposite to the viscosity... 

The first detailed ionic model of melt structure is that of Endell and Hellbriigge (J4)- According to these workers, the addition of a metal oxide to silica causes the progressive breakdown of the three dimensional network. The reaction may be formally represented by ... 

W AXS-pattern of a polymer melt if these Boltzmann-weighted distance fluctuations are not accounted for. They characterize a relevant feature of polymer melt structures. We treat this in terms of the Rotational Isomeric State Approximation (RISA) discussed in the next section. 

Fig. 5.15 (a) SAXS intensity profiles along the lamellar normal for a PE-FEE diblock (M = 81 kgmor. /pE - 0.35) (Hamley el al. 1996b). (b) Schematic of the epitaxial growth of a hexagonal-packed cylinder melt structure from a lamellar solid structure. The direction of incidence of X-rays is along the direction of the arrow. 

Fig. 5.17 SAXS patterns for PEQwPBO.w showing (a) the ordered melt structure (T = 90 °C) (b) a metastable structure at T = 42 °C (c) the equilibrium once-folded structure grown at T - 50°C by a self-seeding process (Ryan et al. 1997). Numbers indicate the order of reflection from a lamellar structure and the arrow indicates the position of the peak in the ordered melt. The calculated repeat lengths for possible molecular conformations are indicated.

Park CB, Padareva V, Lee PC, Naguib HE (2005) Extmded open-celled LDPE-based foams using non-homogeneous melt structure. J Polym Eng 25 239-260... 

Crystal field spectral measurements of transition metal ions doped in a variety of silicate glass compositions (e.g., Fox et al., 1982 Nelson et al., 1983 Nelson and White, 1986 Calas and Petiau, 1983 Keppler, 1992) have produced estimates of the crystal field splitting and stabilization energy parameters for several of the transition metal ions, examples of which are summarized in table 8.1. Comparisons with CFSE data for each transition metal ion in octahedral sites in periclase, MgO (divalent cations) and corundum, A1203 (trivalent cations) and hydrated complexes show that CFSE differences between crystal and glass (e.g., basaltic melt) structures,... 

The conditions where the last domain vanishes is found as follows. A condition for equilibrium between a two-phase structure and a homogeneous-melt structure is that the chemical potential of a repeat unit in the domain and in the monomolecular melt state pA are the same ... 

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