CO₂-induced Manipulation Of Isotactic Poly-1-butente Crystal Modification

CO2 has been extensively used in polymer processing owing to its attractive properties. Dissolution of CO2 in polymer will affect the polymer properties in both melt and solid states. Moreover, the activity of CO2 can be easily tuned by changing temperature and pressure, which provides us a technique to manipulate crystalline modification transition and crystallization of polymers. Isotactic poly-1-butene (iPB-1) is a polymorphous semi crystal polyolefin with many outstanding properties. However, the relatively complex phase transition restricts its commercialization. Manipulation of the crystal modification of iPB-1 by using CO2 provides a potential solution to overcome the drawback. In this work, the crystal modification transformation and crystallization of iPB-1 under CO2 is systematically studied for the first time. The details are shown as follows:CO2-Inducted crystal phase transition from form?to I in iPB-1. CO2 substantially promoted the phase transition rate of form?to?, and form?transformed completely to the form?during the melting process under 5 MPa. The total crystallinity of iPB-1 samples before and after CO2 treatment did not seem to change dramatically, which indicated the phase transition was only occurred in the crystal region. Meanwhile, in-situ high-pressure Fourier transform infrared spectroscopy (FTIR) was used to investigate the phase transition of form?to?at various CO2 temperatures and pressures. It was shown that the phase transition rate was increased with increased pressure and decreased with increased temperature. The phase transition of form?to form?completed in several minutes under low temperature and high pressure CO2, which was the most efficient ever reported method to accelerate the phase transition.CO2-Induced polymorphous phase transition of iPB-1 with form III. The application of CO2 promoted more form?to transform into form I', and also made the phase transformation of form?to I'occur at a lower temperature. The phase transition of form III to ' and the perfection of form I'crystal lowered the free energy barrier for melting of form III and I', thereby making forms?and I'melt directly. The phase transition kinetics of CO2-induced phase transition of form?to I'was studied by in-situ high-pressure FTIR. It was shown that the phase transition of form?to I'might be comprised two stages:the initial instantaneous nucleation and random nucleation. Most of form?transformed into form I'in the first stage. Meanwhile, the formation of form?was changed from the solid-solid phase transition under ambient N2 to the melt-recrystallization under CO2. The crystalline morphology of the C2-treated iPB-1 with form III was also investigated using the polarized optical microscopy (POM). To obtain the strong orientation, the formation processes of form II displayed the following order:melt crystallization at ambient condition> melt recrystallization under CO2>phase transition upon annealing at ambient condition.Effect of CO2 on the recrystallization of form?from iPB-1 with form?during heating. The formation of form?during heating of form?comprised two processes:the solid-solid phase transition and the melt-recrystallization. The solid-solid phase transition of form?was inhibited gradually with increasing CO2 pressure, making relatively more form?generate through the melt-recrystallization. The nonisothermal recrystallization kinetics of form?was then analyzed by the modified Avrami method. Significant changes in the Avrami parameters at CO2 pressure of 3 MPa indicated a change in the recrystallization mechanism. In-situ high-pressure FTIR was also applied to detected the phase transition of form?under compressed CO2. The results showed that the form?recrystallization mechanism did change at 3 MPa, at which form?was recrystallized from the completely melt state. The directly melt of form?into the melt state at 4 MPa was ascribed to the plasticization effect of CO2.Effect of CO2 on the nonisothermal crystallization behaviors of iPB-1 melt. The nonisothermal crystallization peak temperature of iPB-1 decreased linearly with increasing CO2 pressure. The crystallized crystal structure of iPB-1 changed from form?under 0.5-8 MPa CO2 to form?' under CO2 at above 10 MPa. A new approach to obtain form?' was found during the nonisothermal crystallization of iPB-1 under high pressure CO2. Furthermore, the modified Avrami method was used to analyze nonisothermal crystallization kinetics of iPB-1 from the melt. The results showed that the gradually reduced n in the nonisothermal crystallization of iPB-1 with the increased CO2 pressure, indicating the possible change in the crystallization mechanism with increasing CO2 pressure. In situ high pressure FTIR measurement of the nonisothermal crystallization evidenced the form?'was directly generated from iPB-1 melt under 10-18 MPa CO2. The gradually changed crystallization process and mechanism was also directly detected by using POM.Effect of CO2 foaming process on the crystal modification of iPB-1. The high pressure differential scanning calorimeter (DSC) was used to determine the temperature window for foaming iPB-1 with forms?or?'. The foaming process of iPB-1 using CO2 inhibited the crystallization of form?' from the melt and made the iPB-1 melt directly transform into form?. It indicated that the deformation of the iPB-1 matrix changed the crystallization behavior, and the porous stable form?was directly generated after the foaming.