химический каталог




Компьютерное материаловедение полимеров

Автор А.А.Аскадский, В.И.Кондращенко

is obtained in case when a small constant force acts on a sample while the temperature grows continuously with time. The temperature area of the test on a typical thennomechanical curve for amorphous polymeric bodies is subdivided into three intervals (Fig. 18): in the first interval the body is in the glassy state, in the second interval it is in the rubbery state, and in tlie third interval it is in the viscous flow state. Tlie corresponding temperatures of transition from the glassy state to the rubbery state and from the rubbery state to the viscous flow state have received the names of "tlie glass transition temperature Tg" and "the temperature of transition into the viscous state T/' respectively. These temperatures on the thermomechani-cal curve are determined eiUier from tlie deformation e0 that is taken beforehand under the scheme presented in Fig 22, or from the points of intersection of the tangents to two appropriate branches of the curve (Fig.23). In Table 14 a range of glass transition temperature for a series of polymers is shown as well as the influence of chemical structure of polymers upon Tg. It should be pointed out that from the thennomechanical curve it is also possible to determine a mechanical segment, i.e tlie value of molecular mass Mc beginning with which the rubbery state of polymer occurs (in Fig. 20 the molecular mass of the mechanical segment is equal to M,).

1

?s

J

Summary

The value of the mechanical segment can be determined according to the Kargin-Sloiiyrnskii's formula (58) using three mermornechanical curves of a polymer with the known molecular mass А/or by the calculation according to the formula (62) on the basis of the polymer chemical structure

We should focus our particular attention on die correct interpretation of the tliermomechanical curve obtained In particular, as a proof of development of a large deformation of the polymer in the area of high temperatures owing to its flow rather than degradation, it is necessary to simultaneously apply the thermogravim-etry method of research. To confirm the presence of the area of the rubbery state of a polymer a reversible character of its deformation should be proved, whereas during analysis of the thermomeclvanical curve some important information can be obtained from preliminary X-ray analysis of a polymer. Along with the thennomechanical method of determination of the glass transition temperature and the temperature of transition into the viscous state of the polymer, dilatometric methods, and dynamic mechanical analysis, as well as the method of measurement of the diffusion coefficient of ttte polymer are used

There are two basic points of view drat do not contradict each other as to the mechanism of the glass transition process of polymers, namely - the relaxation mechanism and formation of a physical network of intermolecular bonds. According to the first conception, the relaxation mechanism of glass transition is based on the assumption of a possibility of transition of the polymer kinetic unit from one energy level which is typical for the glassy state of a polymer to another energy level that is peculiar to the rubbery state of a polymer. The time of such transition (the time of relaxation) depends on the temperature and is determined by die value of the energy barrier which it has to overcome in order that such a transition can take place. Tlie more flexible is the macromolecule the smaller is the value of the energy barrier of the system constructed of such macromolecules. According to the second concept, the leading role in the glass transition process is given to intermolecular bonds in polymers: at cooling a polymeric body Uvat is in its rubbery state the quantity of emerging intermolecular bonds grows, and with a certain temperature which is the glass transition temperature the number of such cross-linkage bonds becomes sufficient to fix separate macromolecules in relation to each other and to prevent their translation mobility (Fig.41). At tins temperature the mobility of macromolecules is lost to a sufficient degree and hence the polymeric body becomes solid, i.e. it transits into the glassy state. As will be shown below, the display of intermolecular interaction is necessary to be taken into account when tlie calculated glass transition temperature of polymers is determined on the basis of Uieir chemical structure.

The glass transition temperature of linear polymers is detenmiied from Equation (84) or, with regard to influence of links connection type in a chain of a polvSummary 501

mer ("head-to-head" or "head-to-tail"), from Equation (87). For copolymers Equation (84) takes tlie fonn (95) or (97) if the glass transition temperatures of ho-mopolymers are known. A good agreement between calculated and experimental values Tg is observed (Table 19). Tlie described approach is known as "atomic" and has an essential advantage over the wide-spread method of calculation based on contribution

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