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The origin of the elastic behaviour of the bioelastomer elastin has been a matter of some controversy in recent years, as has been the degree of stable secondary structure present in the native state. Available data on the primary sequence of the protein chains constituting the elastin network has indicated a significant incidence of three repeat peptides of length four, five and six, with all containing a Pro-Gly pair within them. The present investigation involved the calculation of the temperature coefficient of the mean-square end-to-end distance 〈r2〉0 of random-coil model chains composed exclusively of one of the three repeat peptides. This configuration-dependent property is directly related to the experimentally obtained energetic component of the elastic force. The values of dln 〈r 2 〉 0 d T for each model chain were evaluated through the use of semi-empirical energy calculations on terminally-blocked residues present in the aforementioned repeat peptides, and subsequent use of established matrix techniques for calculating 〈r2〉0 for polypeptides within a scheme involving independently rotating virtual bonds. Three levels of hydrogen bonding inclusion were investigated to help establish the importance of this contribution. The small positive value of the energetic component of the elastic force for elastin in the literature was found to be adequately reproduced in these calculations on the random-coil model chains.

Elastomeric networks prepared from atactic polypropylene were crosslinked with γ-irradiation. The stress-optical behavior of the unswollen networks was studied over a variety of elongation ratios and temperatures. At constant length, the stress f/A increases markedly at the glass transition temperature Tg, but the value of Tg does not change discernibly with increasing elongation. The unswollen networks exhibit negative birefringence for values of the elongation ratio λ below 1,8; however, for values of λ > 1,8, the birefringence becomes positive. This behavior was attributed to the presence of crystallites in the unswollen networks. Polypropylene networks, swollen with 1,3,5-triethylbenzene and tetradecane, were also studied with regard to their stress-temperature and birefringence-temperature relationships. Whereas both the birefringence and stress of elongated networks swollen with the former solvent show a linear dependence on temperature, the birefringence of elongated networks swollen with tetradecane exhibits an anomalous increase at low temperatures which was attributed to poorly developed crystallinity. Values of the birefringence and stress corresponding to swollen networks were used to calculate the values of the optical configuration parameter Δa and its temperature coefficient. Theoretical calculations carried out with the rotational isomeric state model were used to evaluate Δa as a function of the stereoregularity of the chains. It was found that Δa decreases as the proportion of meso diads Pm increases, remaining almost constant for values of Pm > 0,7. The theoretical results of Δa and d In Δa/dT obtained for Pm > 0,4 reproduce very satisfactorily the experimental results obtained on the networks swollen with 1,3,5-triethylbenzene.