Glassy state

In the usual schedule solid - liquid the solid is crystalline and passes into the liquid state at the melting point, Tm. This transition is, in nearly all cases, accompanied by an increase in volume (one of the exceptions is water ), and with an increase in the heat content (enthalpy), the heat of melting. 

Some substances are, however, not able to crystallize, for instance normal glass, as a result of a too irregular molecular structure. When such a substance is cooled down from the liquid state, and follows the line AB, then from B to D it still remains a fluid, which solidifies at D without showing a jump in volume. The line then continues as DE, with about the same slope as CF the matter is, however, not in a crystalline condition, but in an unordered, amorphous, glassy state, and has, therefore, a greater volume. 

The transition at D is called the glass transition, occurring at the glass transition temperature, Tg. It follows that Tg is always lower than the melting point, Tm. It is very important to distinguish very carefully between Tg and Tm  

Polymers are sometimes wholly amorphous and non-crystallizable they then follow the line ABDE. However, when such a polymer is heated up to above its Tg it is not immediately transferred into a liquid state, but first into a rubbery state, which, upon further heating, gradually passes into a fluid. Tg is, therefore, called the glass-rubber 

Sometimes crystallization is possible in the solid state polymers are, however, never totally crystalline, but are still partly amorphous. The amorphous part may be above or below Tg, i.e. in the rubbery or in the glassy condition. 

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Let us first consider two identical polymers, one deuterated and the other not, in a melt or a glassy state. The two polymers (degree of polymerization d) differ from each other only by scadermg lengths and b. If the total number of molecules is N, x is the volume fraction of the deuterated species x = N- / N, with Aq -t = A). According to equation (B1.9.116), we obtain... 

At very short times the modulus is on the order of 10" ° N m comparable to ordinary window glass at room temperature. In fact, the mechanical behavior displayed in this region is called the glassy state, regardless of the chemical composition of the specimen. Inorganic and polymeric glasses... 

At very short times the compliance is low and essentially constant. This is the glassy state where chain motion requires longer times to be observed. 

At longer times an increase in compliance marks the relaxation of the glassy state to the rubbery state. Again, an increase of temperature through Tg would produce the same effect. 

The average polymer Enjoys a glassy state, but cools, forgets To slump, and clouds in closely patterned minuets. 

A variety of experimental techniques have been employed to research the material of this chapter, many of which we shall not even mention. For example, pressure as well as temperature has been used as an experimental variable to study volume effects. Dielectric constants, indices of refraction, and nuclear magnetic resonsance (NMR) spectra are used, as well as mechanical relaxations, to monitor the onset of the glassy state. X-ray, electron, and neutron diffraction are used to elucidate structure along with electron microscopy. It would take us too far afield to trace all these different techniques and the results obtained from each, so we restrict ourselves to discussing only a few types of experimental data. Our failure to mention all sources of data does not imply that these other techniques have not been employed to good advantage in the study of the topics contained herein. 

New photochromic dyes with electrocycHc reactions have been proposed on the basis of 1,5-electtocycHzation of heterogenous pentadienyl—anions (124). StiH newer are investigations into the photocycHzation of 2,4,6-tri-isoptopylbenzophenones for vinyl polymers ia the glassy state (133). 

Fig. 19. Generalized modulus—temperature curves for polymeric materials showing the high modulus glassy state, glass-transition regions for cured and uncured polymers, plateau regions for cross-linked polymers, and the dropoff in modulus for a linear polymer.

The Metravib Micromecanalyser is an inverted torsional pendulum, but unlike the torsional pendulums described eadier, it can be operated as a forced-vibration instmment. It is fully computerized and automatically determines G, and tan 5 as a function of temperature at low frequencies (10 1 Hz). Stress relaxation and creep measurements are also possible. The temperature range is —170 to 400°C. The Micromecanalyser probably has been used more for the characterization of glasses and metals than for polymers, but has proved useful for determining glassy-state relaxations and microstmctures of polymer blends (285) and latex films (286). 

Transition region or state in which an amorphous polymer changed from (or to) a viscous or rubbery condition to (or from) a hard and relatively brittle one. Transition occurs over a narrow temperature region similar to solidification of a glassy state. This transformation causes hardness, brittleness, thermal expansibility, specific heat and other properties to change dramatically. 

With plastics there is a certain temperature, called the glass transition temperature, Tg, below which the material behaves like glass i.e. it is hard and rigid. As can be seen from Table 1.8 the value for Tg for a particular plastic is not necessarily a low temperature. This immediately helps to explain some of the differences which we observe in plastics. For example, at room temperature polystyrene and acrylic are below their respective Tg values and hence we observe these materials in their glassy state. Note, however, that in contrast, at room temperature, polyethylene is above its glass transition temperature and so we observe a very flexible matoial. When cooled below its Tg it then becomes a hard, brittle solid. Plastics can have several transitions. 

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