Correlation between Orientation of Crystallites and Stress‐Induced Phase Transformations in Copolymers of Isotactic Poly(Butene) with Ethylene

The spontaneous transformation in isotactic poly(butene) (iPB) of kinetically favored form II into the thermodynamically stable form I at room temperature leads to dimensional instability due to changes of density and strength and has prevented industrial development of iPB. This transformation is accelerated by tensile deformation. This study investigates the correlation between the form II-form I transition occurring during tensile deformation and orientation of relative crystallites in 1-butene/ethylene (C4C2) isotactic copolymers. During stretching, form II transforms into form I in all samples. Both the critical strain at which the form II-to-form I transition begins (ε_c) and the strain at which 50% of the initial form II is transformed into form I (ε_0.5) increase with increasing ethylene (C2) content. For samples with C2 content ≤ 7.6 mol%, form II crystals adopt an off-axis orientation at ε_0.5. In contrast, for higher C2 content, form II crystallites remain isotropic at ε_0.5. Form I crystals adopt an off-axis orientation at ε_0.5 only in the two samples with lowest C2 content (1.7 and 4.3 mol%). Crystals of form II and form I begin to orient in the standard fiber orientation at progressively earlier stages of the form II-to-form I transition as the ethylene content increases.