Chemically specific molecular-structure models of amorphous polymers

2 Chemically specific molecular-structure models of amorphous polymers 

The development of chemically specific polymer molecular-structure models employing primarily the static-energy-minimization (SEM) technique was pioneered by Suter and co-workers (for a-polypropylene (PP) by Theodorou and Suter 

An important feature of these models, which is particularly relevant for the understanding of their mechanical response, is the very high levels of their atomic site stress tensors associated with the disordered glassy state measured by a technique pioneered by Egami and Vitek (1983) in amorphous metals and discussed in Section 1.4.1. For example, Theodorou and Suter (1986) have calculated these atomic site stresses in polypropylene for the three different atomic environments of C, H, and CH3 by the method of Egami and Vitek. Their results for the average pressure field (p) and deviatoric shear field (r) (= d/VJ) for the three environments are given in Table 2.1 for the structure model of Theodorou and Suter (1985). 

Considering that for PP the fractions, c, of the atomic environments are given by, for C, Cl = cc = 0.333, for H, C2 = ch = 0.5, and for CH3, C3 = cr = 0.167, where R represents CH3 as a pseudo-atom, the excess enthalpy AH, or elastic strain energy, stored in the atomic site pressure fields AH and the atomic site deviatoric fields can be readily estimated as 

from the values of the average atomic site pressure and deviatoric shear fields given in Table 2.1 one obtains = 0.198 x 10 J/m and 

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