Stretching‐induced changes in the crystal structure of isotactic polybutene‐1: Temperature and stretching speed dependence

To better understand the two synchronous processes of lamella orientation and crystal transition of iPB-1 during stretching, the stretching-induced changes in the crystal structure of iPB-1 were investigated at different temperatures (room temperature, 65, and 85 degrees C) and stretching speeds (5, 10, and 20 mu m/s). It was discovered that the acceleration of crystal transition occurred almost simultaneously with strain-hardening, and that lamella orientation and crystal transition made a different response to the stretching speeds and temperatures. This is attributed to two modes of energy dissipation: (1) the motion and orientation of the lamellae; and (2) the lattice displacement within the individual lamellae, resulting in the crystal transition from form II to I. At high temperatures, the reduced resistance to motion makes the lamellae more inclined to move in the amorphous phase in the stretching direction, which caused a high orientation of the lamellae but a retardation of the crystal transition. When iPB-1 was stretched at a fast speed, the tie molecules failed to immediately transfer sufficient stress to the individual lamellae to induce the helix displacement of the lattice; thus, the orientation of the lamellae was a dominant trait. As a result, both a rapid crystal transition and a high yield of crystal form I were achieved when iPB-1 was stretched at a slow speed at room temperature.