Mechanism of Fast Phase Transition from Form II to Form I of Polybutene-1 during Solvent Vapor Annealing

The mechanism of acceleration of the phase transition of isotactic polybutene-1 (iPB-1) from form II to form I during solvent vapor annealing was explored by in situ wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. During n-hexane vapor annealing, the (200) and (213) diffraction peaks of form II in WAXD shift toward lower angles first and then backward to larger angles. The phenomenon suggests that substantial internal stress is generated within the sample during solvent vapor annealing. The internal stress was quantitatively characterized using an elastic model, showing an initial increase to 0.6 MPa over time, followed by a decrease and stabilization at a plateau of 0.2 MPa. Unexpectedly, the internal stress introduced by the solvent vapor exerted compression along the c-axis and inhibition in the a,b-axis, hindering the phase transition. Indeed, the penetration of solvent molecules into the amorphous region causes expansion and induces disentanglement, thereby weakening the stabilizing effect of form II and creating favorable conditions for the rapid transition to form I.