Synergistic Effect of Pressurization Rate and β-Form Nucleating Agent on the Multi-Phase Crystallization of iPP

Using a homemade pressure device, we explored the synergistic effect of pressurization rate and beta-form nucleating agent (beta-NA) on the crystallization of an isotactic polypropylene (iPP) melt. The obtained samples were characterized by combining small angle X-ray scattering and synchrotron wide angle X-ray diffraction. It was found that the synergistic application of pressurization and beta-NA enables the preparation of a unique multi-phase crystallization of iPP, including beta-, gamma- and/or mesomorphic phases. Pressurization rate plays a crucial role on the formation of different crystal phases. As the pressurization rate increases in a narrow range between 0.6-1.9 MPa/s, a significant competitive formation between beta- and gamma-iPP was detected, and their relative crystallinity are likely to be determined by the growth of the crystal. When the pressurization rate increases further, both beta- and gamma-iPP contents gradually decrease, and the mesophase begins to emerge once it exceeds 15.0 MPa/s, then mesomorphic, beta- and gamma- iPP coexist with each other. Moreover, with different beta-NA contents, the best pressurization rate for beta-iPP growth is the same as 1.9 MPa/s, while more beta-NA just promotes the content of beta-iPP under the rates lower than 1.9 MPa/s. In addition to inducing the formation of beta-iPP, it shows that beta-NA can also significantly promote the formation of gamma-iPP in a wide pressurization rate range between 3.8 to 75 MPa/s. These results were elucidated by combining classical nucleation theory and the growth theory of different crystalline phases, and a theoretical model of the pressurization-induced crystallization is established, providing insight into understanding the multi-phase structure development of iPP.

Automated summary

Using a homemade pressure device, the synergistic effect of pressurization rate and beta-form nucleating agent on the crystallization of isotactic polypropylene melt was explored. Different crystal phases of iPP were found to be influenced by the pressurization rate, with a competitive growth between beta- and gamma-iPP observed at specific rates. The presence of mesophase was also determined by the pressurization rate, with a theoretical model of pressurization-induced crystallization established to explain the process.