Super-cooled melt

Before we examine in more detail the dynamics of a super-cooled melt of coarse-grained chains and of PB chains, respectively, let us first compare the structure of these two glass-forming systems. Structure is obtained experimentally from either the neutron or the X-ray structure factors. The melt (or liquid) structure factor is given as110... 

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... 

The general theory of phase transition by crystallisation was developed by Gibbs, and later extended by Becker and Doring (1935), Avrami (1939/1941), Turnbull and Fisher (1949) and Hoffman et al. (1958/1966). The theory is based on the assumption that in super-cooled melts there occur fluctuations leading to the formation of a new phase. The phase transformation begins with the appearance of a number of very small particles of the new phase (nucleation). 

The problems in getting a "seed" to form in the Bridgeman crucible was, perhaps, the basis for a new and different type of crystal growth based upon a variation of the Kjnropolous method. It was noted that if one could get the melt to "super-cool" (that is- not crystallize when the melt temperature was lowered just slightly below the crystallization temperature), one could then induce rapid crystallization by "touching" the surface of the super-cooled melt with a seed of the same material. In... 

Incipient Crystal Freezing in a Super-Cooled Melt... 

Rapid crystallisation from super-cooled melts and supersaturated solutions often results in a "fluffy" tree-like crystal formation called dendrites. The main crystal... 

F. T. Wallenberger, N. E. Weston and S. D. Brown, Calcia-alumina glass fibers drawing from super-cooled melts versus Inviscid melt spinning. Mat. Letters, 11 [89], 229-235 (1991). 

AY [)mr were measured by a Rheovibron (Toyo Seiki) at a heating rate of 2°C/min and a frequency of lOH over the temperature range from 150 C to 200° C. These phenomenological results suggest that the activation energy for the molecular transport term in polymer crystallization is associated with molecular diffusion in the super-cooled melt above T. This indicates that the activation process in molecular transport in polymer crystalUzation could be similar to that in the confined molecular motion in the rubbery state above T... 

Degree that melt will "super-cool"... 

Let us now turn to a discussion of the relation of the temperature dependence of the polymer melt s configurational entropy with its glass transition and address the famous paradox of the Kauzmann temperature of glass-forming systems.90 It had been found experimentally that the excess entropy of super-cooled liquids, compared with the crystalline state, seemed... 

Finding that the scattering functions at low temperature are amenable to an MCT description, we are faced with a dilemma. On the one hand, the high-temperature mean-square displacement curves lead us to conclude that dihedral barriers constitute a second mechanism for time scale separation in super-cooled polymer melts besides packing effects. On the other hand, the... 

Of primary environmental interest are the melting point, boiling point (the temperature at which the vapor pressure equals atmospheric pressure), and related vapor pressure at environmental temperatures. Chapters 1,2, and 3 discuss these properties. Also of interest is the super-cooled liquid vapor pressure, i.e., the vapor pressure which a solid substance would have if it were liquid at environmental temperatures. This vapor pressure, which is shown dashed in the figure, can be obtained by extrapolating the liquid s vapor pressure below the melting point. It cannot be measured directly. For example, naphthalene melts at 80°C, well above environmental temperatures. Its measured solid vapor pressure depends on the stability of the crystal structure of the pure substance, symmetrical molecules... 

Glass is a super-cooled liquid. Unlike crystalline solids which have sharp melting points, glass softens when heated, flows, and thus can be worked. Bending, molding, and blowing are standard operations in glassworking. 

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