химический каталог




Компьютерное материаловедение полимеров

Автор А.А.Аскадский, В.И.Кондращенко

8). The porous structure of a polymer is determined by its macro- and microporosity which, in its turn, depends both on the method of polymer synthesis and methods of determining the characteristics of porosity: total volume of pores, their size distribution as well as specific surface of pores. Equations (23), (27), and (28) predetermine the relationship between molecular packing coefficient, empty volume and porosity of polymer.

498

Summary

The connection between molecular packing and volume variation of the system m lite process of polymerization referred to as contraction lias been analyzed. Depicted in Ihe book is the influence of a monomer and an associated polymer on the amount of contraction of chemical structure as well as on the factors caused by reduction of Van-der-Waals volumes of atoms owing to their "compressing" and the factor caused by a greater packing density of polymeric chains as compared to packing of monomer molecules, the faclor of greater packing density of polymeric chains making the largest contribution to contraction of the system.

The characteristics of microporous structure of polymers are determined by the methods of equilibrium adsorption, capillary condensation, mercury porosimetry and, recently, by the positron annihilation method. The last mentioned method possesses a number of important advantages. First, no disruption is caused to the mi-croslructure of polymer as a result of its swelling (during sorption) or deformation of pores underpressure (during mercury pressing into pores) and, second, it allows lo determine characteristics of the micropores commensurate to molecule sizes and beyond the reach of other methods of measurements In particular, it lias been found by the positron annihilation method that in the process of stress relaxation, rearrangement of the microporous structure of polymer takes place. The rearrangement is expressed in terms of recombination of tlie finest pores and their merging with formation of long-living microcavities of a greater size.

Chapter III Thermal Coefficient of Volumetric Expansion

Thermal expansion of bodies is a consequence of anltarmonicity of thermal oscillations of their particles. The thermal expansion of polymers has a number of peculiarities connected wiUt various physical transitions occurring in a polymer as temperature is increased. The thermal expansion of polymers is characterized by the thermal coefficient of volumetric expansion determined from a dilalometric curve (Fig. 13) for the areas below (coefficient Од) and above (coefficient uv.) the glass transition temperature Tg. Coefficient u^, as a rule, is not a co^mut value. Ii depends on temperature (Fig. 14) and can be calculated more precisely from Equation (39). Since thermal expansion of bodies is a consequence of anliaimonicily of thermal fluctuations of Iheirparticles, it is natural to assume that tlie thermal coefficient of polymer volumetric expansion consists of die contributions made by various oscillatory movements of such particles. In particular, as it is calculated, the role of weak dispersion interaction and strong intenuolccular interactions stipulating the dipole-dipolc interaction (ester and nitrile groups, halogens, substituting hydrogen atoms etc.) and die initiation of hydrogen bonds (amide, urelhane. hy-droxyl, acid groups, etc.) is taken into account.

499

Summary

In the calculation scheme [28, 43] to account for weak dispersion interactions during determination of the thermal coefficient of volumetric expansion, it was assumed that die contribution of each atom is proportional to a fraction of its Van-der-Waals in die total Van-der-Waals volume of repeating unit and, in the first approximation, tlie contribution of any dipole-dipolc interaction is determined by the same parameter (ij irrespective of chemical structure of the group. Contribution of hydrogen bonds is also estimated by one parameter p j, characterizing the energy of hydrogen bonds. With due regard for these parameters, whose values are given in Table 13, the thermal coefficient of volumetric expansion for the polymers in the glassy state is determined from Equation (44), and for copolymers it is determined from Equation (50) or (51). A presentation of thermal coefficient of volumetric expansion of a copolymer in terms of the known similar coefficient of appropriate homopolymers is given by Equation (52) or, in a more compact form, by Equation (53). At the same time thermal coefficient of volumetric expansion forpolymers in the rubbery state is, as a rale, constant and equal to = 6.3 10"4 K."1.

Chapter IV Glass Transition Temperature of Polymers

Glass transition temperature of polymers Ts is most often determined from a thennomechanical curve which is built in accordance with the results of tests on polymer samples under a constant or varying loading determining deformation of samples at each fixed temperature and selected time of force action. A similar curve

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