Liquid, definition glass transition,

The lower limit of the elastic range, the glass transition temperature, can be easily determined by refractometric, volumetric, or other well known methods. The upper limit suffers from an exact definition the transition from the fixed liquid to the liquid state occurs without transformation. But as the viscosity decreases exponentially with the temperature it is very convenient to define a 1 flow-temperature by penetrometer measurements. If the rate of temperature rise is kept constant, this temperature is reproducible within 1° or 2°C. The penetrometer indicates a temperature where macroscopically one would call the substance liquid. ... 

The definitions of the elastic parameters are given in Table 13.1 (Eqs. (13.4)-(13.7) see also commentary to the definitions of E and S). The three elastic moduli have the dimension force per unit area, so in the S.I.-system N/m2 or Pa. For practical reasons the numerical values below the glass transition temperature are usually given in GPa. The Poisson ratio is dimensionless theoretically it varies from -1 to Vi, and in practice from 0 to Vi (incompressible rigid solids and liquids). 

When a liquid supercools (i.e., does not crystallize when its temperature drops below the thermodynamic melting point), the liquidlike structure is frozen due to the high viscosity of the system. The supercooled liquid is in a so-called viscoelastic state. If the crystallization can be further avoided as the ten ierature continues to drop, a glass transition will happen at a certain temperature, where the frozen liquid turns into a brittle, rigid state known as a glassy state. A well-accepted definition for glass transition is that the relaxation time t of the system is 2 X10 s or the viscosity / isio Pas (an arbitrary standard, of course). 

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.)...

From the earlier definition of the glass transition, it might be expected that below Tg, crystallisation is inhibited, and this is generally the case, at least for practical time scales. There are, however, reports describing crystallisation of polymers in real time at temperatures well below the glass transition.It must of course be remembered that, on a molecular basis, glasses are undercooled liquids, and they differ from ordinary liquids mainly in the rates of their transport properties. These differences can be usefully exploited in the formulation of pharmaceutical preparations, as will presently be discussed. 

Such a definition is equivalent to what is customary in glass physics, where the transition from an equilibrium liquid to a non-equilibrium supercooled liquid (a glass) is characterized by a glass transition temperature, Tg, which is typically defined as the temperature at which the a-relaxation time scale approaches a certain laboratory time scale, typically 100 s. 

A preliminary, operational definition of the solid state is given within the box of Fig. 1.6. It will be expanded upon and linked to the material properties throughout the book. For materials, the transitions between solid and liquid are basic and determine their utility. Similarly, the evaporation characteristics need to be known to choose a molecule for a given application. The new classification scheme for molecules of Fig. 1.6 is, thus, much more useful than the earlier, arbitrary distinction that rehed upon the abihty or inability of living organisms to synthesize a particular substance. The bottom brackets give a rather unique explanation of the glass transition which is detailed in Sect. 2.5. 

However, by making use of this definition, one has to conclude that many systems which look like a gel are in fact not covered by this definition aqueous poly(vinyl alcohol)/borate systems, which are known to show liquid-like behaviour at low frequencies [7], solutions of phase separated atactic polystyrene at temperatures above the glass transition temperature of the swollen polystyrene crosslinks, solutions of ABA block copolymers above the glass transition temperature of the swollen A-blocks, and even gelatin, which also shows creep behaviour, as shown by Ross-Murphy et al. [8,9] and by Kramer et al. (private communication), and a relaxation mechanism at extremely low frequencies, as is shown in Fig. 7 of the Section on gelatin, and possibly ako poly(vinyl chloride) in plasticizers [10-13]. The advantage of the approach of Kramer et al. [3] is that these systems certainly are covered by their practical definition. 

The glass transition aspect of this definition allows the exclusion of substances that are so viscous as to be glassy at the temperature of interest, so it simply adds a level of clarification to the word liquid. Consequently, some level of ionic conductivity is the only property that we can always expect to be generic in ionic liquids. 

Fig. 3A-3 Definition of the glass transition temperature Tg by a diagram of length versus temperature during a measurement, as the intersection of the tangents to the elastic region B and to the liquid region to the right of C. In the Uansformation range C, the glass softens...

---