химический каталог




Компьютерное материаловедение полимеров

Автор А.А.Аскадский, В.И.Кондращенко

326) and (327) was confirmed by the experiments devoted to measurement of kinetics during sorption of free films and coatings on the basis of cured epoxy resin ED-20 (Fig.88). The best fit between calculated and experimental values is reached by using memory function T2(z). This shows that the limiting stage of the process serving as a base for sorption is the process of relaxer diffusion (self-diffusion). The use of memory function obtained permits to approximate sorption kinetic curves with a good fit for almost all of their period which is not possible when applying the Fick equation witii a constant diffusion coefficient for this aim. In this case sorption process can be approximated widt a sufficient accuracy only in a small initial time period.

Chapter XII Solubility of Polymers

Tlie problem of polymer solubility prediction has remained urgent for many years Oneway of preliminary evaluation of a polymer solubility is a juxtaposition of the values of Hildebrand solubility parameter 5as defined by relation (329) fora

I

510

Summary

Summary

511

polymer bp and for a solvent 6S. If the values of bp and 5, are in a substantial divergence one may firmly predict that this polymer is insoluble in the given organic liquid. Experimental determination of 8 value is made only for low molecular liquids which are evaporate without decomposition For polymers which cannot be evaporated without decomposition the value of &p is determined by indirect methods or by calculation, which is preferable because of difficulties in realization of experiments Calculation formula (331) for determination of solubility parameter 5 for liquids and polymers has been attained allowing for the type of molecular packing, cohesion energy components AE",as typical for each atom and type of intermolecular interaction of a polymer repeating unit (Table 43). A similar relation for copolymers is written as Equation (331") or, if the values of solubility parameters for homopolymers are known or calculated, as Equation (331""). Table 44 shows calculated and experimental values of solubility parameters for polymers of diverse chemical structures. As we see, the value of S increases when strong polar groups are present such as —CI, -COO-, -CN. The solubility parameter is especially enhanced by hydrogen bonds emerging when-OH and -HNCO- groups are present as well as aromatic rings.

When solubility is predicted for a particular polymer a sufficient number of poly mere remain for which o^, = 8, but die polymer would solve only in nearly a half of them.

To create a solubility criterion having a sufficiently high predictive capability the supermolecular structure of the polymer should be accounted for. Such a solubility criterion taking account of the globular supermolecular structure has been obtained for amorphous polymers, both unswelling (criterion (343)) and swelling (criterion (345)). Both parts of the solubility criterion are calculated taking into consideration chemical structures of the polymer and the solvent.

It follows from those equations that a polymer is, as a rule, soluble if the polymer and the solvent have similar values of cohesion energy density, molarvolumes and coefficients of surface tension. The results of an experimental test for solubility criterion (345) made on approximately 50 organic liquids and near 300 poly mer-solvent systems are shown in Fig. 94. As we see, the share of polymer-solvent systems not meeting criterion (345) (black dots are lowerlhan the dotted line, whereas white poinls are overil) is very small, notexceeding 15 % of the total number of the systems studied.

Appearance of "foreign points" may be caused by non-swelling of a polymer in a given solvent (in tins case criterion (343) should be used), or поп-accounting for the degree of polymerization of the polymer, or presence of another type of supermolecular structure. For these cases respective solubility criteria have been obtained which show thai the larger a polymer molecular mass is (criterion (359)) the less soluble is an amorphous polymer with globular structure. The presence of a fibrillary supermolecular structure (criterion (371)) decreases the solubility of a polymer having an identical chemical structure but different in the presence of a globular supermolecular structure.

Chapter XIII Surface Properties of Organic Liquids and Polymers

The value of surface tension у of liquids and polymers determines many of their individual properties, among them mutual compatibility and mutual solubility. Among the existing methods of surface tension calculation the one most often used is the additive calculation scheme from Expression (372) based on summation of parachors P, characterizing the contribution of individual atoms into surface tension. Equation (372) permits to calculate the value of у to a sufficient accuracy but due to physical uncertainty of the P, values it does not allow to analyze the influence of weak Van-der-Waals

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