Definition of a Glass

Effect of temperature on the enthalpy of a glass forming melt 

Draw a volume/enthalpy diagram showing the behavior of a melt which cools to form (a) glass and (b) crystals. Label each section of this diagram. Show the curves for rapidly- and slowly-cooled melts. 

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The strict definition of a phase is any homogeneous and physically distinct region that is separated from another such region by a distinct boundary . For example a glass of water with some ice in it contains one component (the water) exhibiting three phases liquid, solid, and gaseous (the water vapour). The most relevant phases in the oil industry are liquids (water and oil), gases (or vapours), and to a lesser extent, solids. 

Since the definition of the glass phase is fuzzy as Reid et al. [1.17] note, and the methods of measurement can be chosen, the results for Tg- will have a range of variations. For freeze drying one can summarize ... 

No thermodynamic signature at Tg is evident in specific heat data at equilibrium, but a peak is observed under nonequilibrium conditions and is often taken as the definition of the glass transition. Unfortunately, this nonequilibrium peak cannot be addressed within the LCT of glass formation. We strictly avoid a discussion of the specific heat, given the complications of interpreting these data for polymer materials and the omission of the important vibrational component in the LCT treatment. 

These glasses differ macroscopically in volume and, therefore, in structure. This mental experiment intends to point out, that correctly speaking, there does not exist a glass of definite structure. A glass of the composition x at the temperature T can be produced in different ways which result in glasses of different structure. The characteristic behavior of the plasticizer in its molecular interaction with the chain molecules determines structure and macroscopic behavior of the glass. 

Experiment 52. — Dissolve a small piece of sodium hydroxide in an evaporating dish half full of water. Slowly add dilute hydrochloric acid, until a drop taken from the dish by means of a glass rod reddens blue litmus paper. Then evaporate to dryness by heating over a piece of wire gauze. Since the residue mechanically holds traces of the excess of hydrochloric acid added, it is necessary to remove this acid before applying any test. Heat the dish until all the yellow color disappears, then moisten the residue carefully with a few drops of warm water and heat again to remove the last traces of acid. This precaution is essential to the success of the experiment. Test a portion of the residue with litmus paper to find whether it has acid, basic, or neutral properties. Taste a little. Test (a) a solution of the residue for a chloride, and (b) a portion of the solid residue for sodium. Draw a definite conclusion from the total evidence. 

Glass plays an important role in pharmaceutics as well as everyday life. It functions as a barrier against the environment, protects us from harmful rays and preserves our foods and medicines. It is useful but can be a problem for some products. Unfortunately, a clear definition of the glass required for each of these purposes is still lacking. To date, no compendium has included a definition even though they are replete with warnings to "protect from light."... 

When the formulated solution predominantly contains polyols, sugars, or polymers, the interstitial phase does not usually crystallize out upon cooling but increases progressively in viscosity as a glass-like system. In the case where the interstitial phase has effectively the structure of a glass, the frozen system becomes fully rigid once the glass transition temperature (Fg) is reached. In contrast, some amorphous systems may show no such definite transition, but they eventually become very stiff at low temperature, as shown by electrical resistance studies. 

Fig. 3. Arrhenius plot of the viscosity of several supercooled liquids. The horizontal dotted line, where the viscosity reaches 10 P, is commonly used as a definition of the glass transition. (Reprinted with permission from C. A. Angell. Formation of glasses from liquids and polymers. Science (1995) 267 1924. Copyright (1995), American Association for the advancement of Science.)...

Since glasses are inherently in a non-equilibrium state around Tg, thermodynamic formulation relevant for this region has to include one important additional feature and that is a time scale. Even in laboratory time scales profound changes in properties can occur. It is the existence of this time scale, which led to the definition of a "fictive temperature" by Tool (Tool and Eichlin, 1931 Tool, 1946). If we call this reference or fictive temperature as 7/ (which is more accurately written as T/J) since it is time-dependent), then the Gibb s free energy of a glass can be described in terms of three parameters, T, P and Tf. Also the heat balance equation in... 

Al the basis of the polyclustcr model, there lies the assumption of the presence of one or several types of atom LO in a solid. At this point, the polycluster model is close to the so-called stereochemical models [6.8]. Besides, it includes the definition of a cluster as a set of locally ordered atoms and the definition of the boundaries as the closing of this set. Finally, the assumption that clusters conjoin along common boundaries completes the definition of a polyclustcr structure. The polycluster model includes a rather wide set of structures. This model was suggested in [6.26, 27] and developed and applied while describing various properties of metallic glasses [6.28 33]. 

A complete specification of the structure of a glass is impossible, but for a crystal it is only necessary to give the details of one unit cell. Substances are said to have the same structure if the arrangement of atoms within a unit cell is essentially similar, although the interatomic distances and the dimensions of the cell are different. Structure types are named after a particular example, frequently naturally occurring minerals thus we talk of the rocksalt structure of NaCl or the rutile structure of Ti02. Specifying a definite mineral rather than the compound formula is important, as... 

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