Direct melt-crystallization of isotactic poly-1-butene with form I′ using high-pressure CO2

In this work, we found a new method to obtain isotactic poly-1-butene (iPB-1) with form I′ through direct melt-crystallization using high-pressure CO2. The non-isothermal melt-crystallization behaviors of iPB-1 under atmospheric N2 and 0.5-10 MPa CO2 at cooling rates ranging from 0.25 to 5 °C/min were carefully studied using high-pressure differential scanning calorimeter (DSC) and analyzed using the modified Avrami method. Wide-angle X-ray diffraction (WAXD) measurements showed that the crystal structure of non-isothermally melt-crystallized iPB-1 changed from form II under atmospheric N2 and 0.5-8 MPa CO2 to form I′ under 10 MPa CO2. In-situ high-pressure Fourier transform infrared (FTIR) was also used to investigate the non-isothermal melt-crystallization at CO2 pressure up to 18 MPa at the cooling rate of 1 °C/min. Likewise, it was found that form II crystallized under atmospheric N2 and 0.5-8 MPa CO2, and form I′ melt-crystallized directly at CO2 pressures higher than 10 MPa, which was confirmed by the followed DSC and WAXD characterizations on the iPB-1 films after FTIR measurements. The crystal morphology of the melt-crystallized iPB-1 films, characterized by using polarized optical microscopy (POM), showed that the Maltese cross pattern of iPB-1 spherulite became more diffuse with increasing CO2 pressure, and the spherulite size decreased abruptly at the CO2 pressure of 10 MPa.

Graphical abstractIn-situ high-pressure FTIR was applied to detect the crystal modification change during the non-isothermal melt-crystallization process of isotactic poly-1-butene (iPB-1) in atmospheric N2 and high-pressure CO2 at the cooling rate of 1 °C/min. Under atmospheric N2 and low-pressure CO2, e.g., 6 MPa CO2, the characteristic band at 904 cm−1 generated at the start of crystallization, and its intensity increased with decreasing temperature, indicating that the form II crystallized directly. At 8 MPa, 904 cm−1 band appeared at 67 °C, and its intensity increased continuously with decreasing temperature until to 55 °C, at which the form I (or I′) characteristic band at 924 cm−1 appeared, indicating that the form II crystallized from the melt and then quickly transformed into the form I with the assistance of CO2. Under 10 MPa CO2, only was the IR characteristic band of form I (or I′) at 924 cm−1 detected during the cooling, indicating the form I (or I′) crystallized directly from the melt.Download full-size image