Polymer and Materials Science
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
The crystallization of cross-linked cis-1,4-polybutadiene, stretched at room temperature, is investigated by wide and small angle X-ray diffraction techniques, at temperatures ranging from -40°C to the melting point, Tm, and for extension ratios, λ, from 1 to 7,7. The experimental conditions adopted should approximate thermodynamic equilibrium. All the experimental findings, namely, crystallinity, preferred orientation of the crystalline and amorphous fractions, unit-cell dimensions and others, are consistent with the formation of two types of crystals: one depending mainly on stretching, the other on temperature. The former is characterized by extended chain crystals with Tm higher than the isotropic melting point (Tm0 = -3°C); the latter by a lamellar (bulk-like) morphology which is stable only below Tm0. A slight increase of Tm occurs for λ 〈 3, when the axes of the crystallized chains are tilted with respect to the strain direction; only when untilted crystals are formed (λ 〉 3) a remarkable increase of Tm is observed as a function of λ. As a qualitative interpretation, the two-fold crystallization mechanism is based on non-uniform extensibility of the segments included between adjacent junctions of the network.
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