Stability of form II of syndiotactic polypropylene confirmed by cold and melt crystallization in supercritical carbon dioxide

The form II of syndiotactic polypropylene (sPP) has been found more thermodynamically stable than form I when melt crystallized at pressures above 150 MPa, while the reverse occurs below 150 MPa. In the present study, through the cold and melt crystallization in supercritical CO₂ the stability of various polymorphic forms of sPP, especially form II, was confirmed by using Fourier-transform infrared spectroscopy and wide-angle X-ray diffraction. Compared with the formation of pure form I at high temperatures under ambient condition, a mixture of forms I and II was formed by both the cold and melt crystallization in supercritical CO₂. This atmosphere changed the relative stability of forms I and II, and made the form II more thermodynamically stable than form I. The increased solubility parameters of the surroundings, at which the form II was formed, also confirmed the stability of form II over form I in supercritical CO₂. The incubation pressure was the key factor affecting the formation and amount of form II. Supercritical CO₂ provides a combining severe condition to obtain the form II crystal, although its pressure was much lower than the elevated pressures (>150 MPa) reported before.