Molecular glasses glass transition phenomenon

In this chapter, the glass transition phenomenon is treated in moderate detail, including phenomenological aspects, molecular theories, and the effect of molecular structure. In addition, relaxation occurring in the glassy state below the glass transition temperature (Tg) is discussed. 

DS has been demonstrated to be a useful tool for the analysis of the glass transition phenomenon in both natural and synthetic polymers, especially under the influence of water, and its application on composites molecular dynamics analysis was also demonstrated [5-7]. This chapter addresses the glass transition phenomenon from an experimental standpoint by exploring a dielectric method used for the characterization of the glass transition phenomenon in natural and synthetic polymers. 

In polymers, the glass transition phenomenon has been related to the dielectric a-relaxation processes through the Vogel-Fulcher-Tammann (VET) equation [9], and it can be characterized by means of their molecular dynamics analysis. 

The glass transition phenomenon is characteristic of the amorphous state and occurs regardless of molecular weight. However, Tg is greatly reduced when the molecular weight M is decreased ... 

D. S. Kaplan, Structure-Property Relationships in Copolymers to Composites Molecular Interpretation of the Glass Transition Phenomenon, J. Appl. Polym. Sci. 20,2615 (1976). Reviews morpholgy of IPNs. 

The glass transition phenomenon has been the object of many molecular theories, that of Ferry [29], further developed by Bueche [30], on the free volume concept being widely accepted in the polymer field. The volume occupied by a chain of amorphous polymer consists partly of free space, i.e., the volume excluded by the movements of segments about their equilibrium position. As shown in Fig. 1.8, the temperature coefficient for the polymer volume at constant pressure dV dT)p is higher for the viscoelastic state (curve b) than for the glassy state (curve a), and it changes abruptly at Tg this is the mathematical definition of the glass transition temperature. It can also be defined as the... 

Remarks The glass transition phenomenon is not restricted to polymers. Due to a molecular structure that lacks symmetry, certain low molar mass molecules cannot crystallize quickly and can thus be cooled below their melting point, leading to amorphous systems that undergo the said phenomenon. 

The cohesive properties thus play an important role in the glass transition phenomenon. Indeed, molecular interactions decrease the mobility of the chains which, according to the kinetic theory, must contribute to an increase of Tg and can be beneficial for certain applications. The comparison of the glass transition temperature of polypropylene-[CH2-CH(CH3)]n-, whose Tg is -10°C, with that of poly(vinyl alcohol)-[CH2-CHOH]n-, whose Tg is +70°C, gives an idea of the importance of interactions. 

A crucial question for understanding a glass transition phenomenon relates to a cause of freezing molecular motion. There are three main groups of theoretical models dealing with the glass transitions kinetic theories, free-volume theories, and thermodynamic theories. 

The iatroduction of a plasticizer, which is a molecule of lower molecular weight than the resia, has the abiUty to impart a greater free volume per volume of material because there is an iucrease iu the proportion of end groups and the plasticizer has a glass-transition temperature, T, lower than that of the resia itself A detailed mathematical treatment (2) of this phenomenon can be carried out to explain the success of some plasticizers and the failure of others. Clearly, the use of a given plasticizer iu a certain appHcation is a compromise between the above ideas and physical properties such as volatiUty, compatibihty, high and low temperature performance, viscosity, etc. This choice is appHcation dependent, ie, there is no ideal plasticizer for every appHcation. 

Two different views have been adopted in the molecular interpretation of the glass transition. One view considers conditions when relaxation processes occur so slowly that the glass transition can be treated as time independent phenomenon. 

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