химический каталог




Компьютерное материаловедение полимеров

Автор А.А.Аскадский, В.И.Кондращенко

sur-S face tension and heat capacity of polymers depending on the number of repeating units in a polymer chain, beginning with и = 1. The analysis of the calculations made (their results being cited in Table 49 and Figs. 101-104) shows that the approximation to asymptotic values of polymer properties under study does depend on the type of end group and on the average is reached at the number of repeating units in a polymer chain и = 10-20. At the same time the action of chemical structure of end groups upon molecular characteristics and properties of polymers can prove to be significant and quite different. Thus, the presence of bulky end groups first of all affects Van-der-Waals volume of an "averaged" unit, molar refraction and heat capacity of a polymer. At the same time the presence of some groups (hydroxyl-type) small in volume, but possessing a specific intermolecular interaction critically tells upon molar cohesion energy, surface tension and solubility parameter, but is insignificant for Van-der-Waals volume and molar refraction. Therefore, it has been shown how important the chemical structure of end groups is in study of regular dependence of polymer properties upon their molecular mass.

Chapter XVI Thermo-physical Properties of Polymers

Thermo-physical properties of polymers are their heat capacity, temperature conductivity and heat conductivity.

In a relatively wide temperature range heat capacity grows linearly with an increase in temperature. During transition from a glassy state to a rubbery state heat capacity jumps to a larger value. After such a physical transition a weak increase of heat capacity begins again together with growing temperature.

Heat capacity of polymers depends upon their chemical structure (Table 50). Thus, when hydrogen atoms are substituted for polar groups, their heat capacity increases. A substantial growUi of heat capacity is observed in transition from aliphatic to aromatic polymers. Formulas for calculation of polymer heat capacity on the basis of their chemical structure have been obtained from the assumption that contribution of each atom of a polymer repeating unit into heat capacity is proportional to its Van-der-Waals volume. Molar heat capacity under the constant pressure C;, for a polymer in a glassy state С is calculated according to Equation (429), and for a polymer in a mbbery state Cp according to Equation (430) The accuracy of molar heat capacity calculation Cp and CJ, can be seen from Table 50.

514

Summary

Summary

515

Temperature conductivity a is determined from relation (431) and serves as a characteristic of temperature propagation ina substance under the actionof thermal flow at unstationary thermal conditions. For solid (glassy and crystalline) polymers temperature conductivity weakly decreases with the growing temperature. However, when a polymer transits from its glassy state into a rubbery state, a sharp fall in temperature conductivity is observed. Temperature conductivity depends upon the degree of crystallicity, molecular mass, pressure and, as it follows from Table 52, chemical structure of polymers.

Heat conductivity X characterizes the capacity of polymer bodies to transfer heat from more heated elements of the body to less heated. Heat conductivity of polymers depends on their physical and phase states as well as on chemical structure of polymers (Table 53) Within the same physical state the action of the chemical structure of a polymer upon its heat conductivity versus temperature may be different. For one series of polymers the slope of this curve is positive, whereas for another series it is negative. A weak maximum of heat conductivity is observed in the area of physical transition from the glassy state to the rubbery one.

CHAPTER XVII

MOLECULAR DESIGN AND COMPUTER SYNTHESIS OF POLYMERS WITH PREDETERMINED PROPERTIES

In die preceding chapters of this monograph it has been demonstrated that the basic physical properties of polymers can be calculated issuing from chemical structure of repeating unit in a linear polymer or a repeating fragment of a polymer network. Nowadays such calculations are made on computers. In that way it is possible to solve principal and inverse problems, which are the basic ones, and some auxiliary problems.

The principal problem is to calculate properties of a polymer from the data of chemical structure of a polymer repeating unit or a fragment of a polymer network.

The most complicated inverse problem is to predict chemical structure of a repeating unit or network fragment of a polymer that would meet the required set of properties.

Solution of such problems by a simple exhaustion of a large number of polymers and their properties contained in a database obtained by increment methods, other calculation method or experimentally does not have any predictive capacity for structures which are absent in the computer memory. For efficient solution of the principal and inverse problem

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