Crystallization of Polybutene-1 into Forms I′, II, and III Under High Pressure CO2: In situ Synchrotron X-Ray Diffraction, WAXD Mapping, and DSC Investigations

The crystallization behavior of polybutene-1 (PB-1) into its I ', II, and III polymorphs, under supercritical carbon dioxide (CO2), has been investigated by means of in situ synchrotron X-ray diffraction, wide angle X-ray diffraction mapping, and differential scanning calorimetry techniques. At low CO2 pressures, crystals of stable forms I ' and III as well as metastable form II are all observed, but form II crystals transform into form I crystals quickly after their formation. With increasing CO2 pressure, the direct crystallization of both form II and form III is suppressed. The suppressed growth of form II at higher CO2 pressures is due to its highest nucleation barrier, owing to its large critical nucleus size. Consequently, there is competition between nucleation of forms I ' and III, which possess smaller nuclei sizes under supercritical fluids (SCFs), as is ascribed to the solvent effect of SCFs. At a CO2 pressure of 10 MPa, form III is observed, being randomly dispersed in the system. It is proposed that random chain coils are transformed into a 4/1 helix conformation, thereby promoting the growth of form III. With further increasing the SCF pressure, more random coils develop into helices, while short helices extend into longer ones. Here, form III with a "shorter" and "wider" chain conformation is no longer the most favorable conformation for the well-developed long helices. As a result, the growth of form I ' overtakes the growth of form III. This lays the framework for control of crystal polymorphism in PB-1 using a simple, environmentally friendly supercritical fluid approach.