химический каталог




Компьютерное материаловедение полимеров

Автор А.А.Аскадский, В.И.Кондращенко

s from separate groups of atoms [214]. Actually, if in polymer there is a group whose contribution to the glass transition temperature is unknown, the calculation of Г^Ьу the method of group contributions becomes impossible. In the approach that is the subject of our description, on the contrary, the contribution of any group is simply summed up from contributions of atoms in such a group and from contributions of specific interactions, if these groups are polar. Consideration of influence of various types of branchings of the macromolecule chains (Fig.44) upon the glass transition temperature of polymer shows that, under frequent and regular arrangement of branches with an increase in their length, the properties of polymers vary insignificantly and approach the properties of homopolymers having chemical structure identical to that of the branchings themselves. If the branchings are less frequent and they are distributed along the backbone of a polymer in a random way, their chemical nature being different from that of the backbone, then the influence upon the properties of polymer is more significant. However, the greatest influence upon the glass transition temperature of a poly mer is rendered by short branchings, a great number of which brings about qualitative changes into the chemical structure of a polymer. During calculation of the glass transition temperature of polymer networks it is necessary to take into account the number of repeating units between cross-linked points m; their number being reduced in high-cross-linked networks, the glass transition temperature begins to grow sharply (Fig.54). For such polymers the glass transition temperature is determined by the number of functional connections of a cross-linked point, its structure, distribution of crosslinks in a network, and by presence of defects of a network in the form of dangled, branched chains or isolated loops Equation (109) has been obtained that takes into account on a par witii chemical structure of a polymer the influence from both linear fragments and cross-linked points upon the glass transition temperature of a polymer Using Equation (109) for various types of polymer cross-linked points and different chemical structure of linear fragments between them some examples of calculation of glass transition temperature are given; the value of the temperature approaching the experimental values of Tg. Taking polystyrene and divinyl-benzene networks as examples, the influence of cross-links distribution witiiin the network upon Tg has been shown: at the identical degree of cross-linking a glass transition temperature increases during transition from the most irregular distribution of cross-links to the random one and then to their regular distribution (Fig.57). On the example of polyrhmethylsiloxane networks having an identical structure of

502 Summary

linear fragments, but different structure of cross-linked points it has been shown that the action of structure of the said points upon Tg becomes essential when the number of polydimethylsiloxane units is reduced to n = 1-4 (Fig.60). By using a hypothetical polyethylene-based network it is found out that, for instance, such a type of network defect as a dangled chain decreases the glass transition temperature of polymer networks (Figs,62 and 63).

Chapter V

Temperature of Transition into the Viscous Flow State for Amorphous Polymers

The value of amorphous polymer temperature of transition into the viscous flow state Tj determines the scope of rubbery state temperature range which should be known to process polymers. Using Expression (166) determining the ratio between zero (Newtonian) shear viscosity and the molecular mass Formula (168) was obtained to determine a polymer Tj from its chemical structure. Temperature of transition into the viscous flow state Tj being dependent upon a poly mer molecular mass Л/, at M < Mc (Mc being molecular mass of a mechanical segment) equation Tg = Tf will be valid and glass transition temperature Tg will depend upon the polymer molecular mass as well (it should be noted that the actual glass transition temperature of the polymer will not be reached). On the other hand, the molecular mass value determining the value of Tj is limited by the temperature of onset of intense thermal degradation Td (see Chapter VII), thus, a polymer cannot be transferred into a viscous flow state at any temperature. Limiting values of M allowing to completely transfer a polymer into a viscous flow state are determined from Expression (168) and given in Table 20. It must be noted tliat the rubbery stale temperature ranges obtained by this calculation are 15-20 %higher than the actually observed values of those ranges. It is explained by the fact thai due lo polydisper-sity of polymer chains some of them come to a viscous flow state earlier than others, which causes the longer chains overstress and breakage The broken chains can be found from the results of analysis

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