Melting, strain induced

The copolymerization of butadiene in trans configuration with suitable comonomers represents a second route for obtaining a wide range of strain induced crystallizable elastomers, with melting point tailorable in a wide range of temperatures. These copolymers can be used, in particular, in blends with other crystallizable rubbers (e.g. synthetic cis-l,4-polyisoprene) in order to improve their "green strength". 

Differential Thermal Analysis (DTA). One of the characteristics of a rubber useful in tire rubber compounds is that it is amorphous at room temperature but readily undergoes strain induced crystallization. For this reason, copolymers were prepared in order to appropriately adjust the crystalline melt temperature. 

Strain-induced crystallization would presumably further improve the ultimate properties of a bimodal network. It would therefore obviously be of considerable importance to study the effect of chain length distribution on the ultimate properties of bimodal networks prepared from chains having melting points well above the very low value characteristic of PDMS. Studies of this type are being carried out on bimodal networks of polyethylene oxide) (55), poly(caprolactone) (55), and polyisobutylene (56). 

F. Boue, J. Bastide, M. Buzier, A. Lapp, J. Herz and T.A. Vilgis, Strain induced large fluctuations during stress relaxation in polymer melts observed by small-angle neutron scattering. Lozenges , butterflies , and related theory, Coll. Polym. Sci., 269, 195 (1991). 

The question may arise whether the same changes in material properties can also be obtained by stretching the filaments after the polymerization has taken place. In our experience the anisotropy was never so great in such cases, whereas the isotropic state was almost completely recovered when the samples were heated above the melting temperature. At this temperature the post-polymerization drawn filaments retained the original dimensions they possessed before stretching. Apparently some strain-induced crystallization yielded a metastable anisotropy which was lost under the combined action of entropy and strained crosslinks when the crystalline areas were melted. 

High flexibility in the equilibrium sense means high conformational randomness in the amorphous state, and thus high entropy of fusion and low melting point. This entropy can be reduced by stretching, in what is called strain-induced crystallization. Such strain-induced crystallization has been studied extensively, both experimentally and theoretically. " The crystallites thus generated can be very important since they... 

PDMS networks are unsuitable for characterizing the effects of bimodality on strain-induced crystallization, because of their very low crystallization temperatures. Poly(ethylene oxide), however, has a relatively high melting point ( 65 C) and thus readily undergoes strain-induced crystallization. Decreasing temperature increases the extent to which the ultimate strength of the... 

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