Crystallization and gelation of isotactic poly(4-methyl-1-pentene) in n-pentane and in n-pentane + CO2 at high pressures

High pressure miscibility and phase separation of isotactic poly (4-methyl-1-pentene) in n-pentane and n-pentane + carbon dioxide mixtures have been studied in the pressure range from 10 to 50 MPa. For solutions containing 1.0, 3.0, 4.6 and 6.5 wt.% polymer in n-pentane no liquid–liquid phase separation was observed over a temperature range from 298 to 423 K. The fluid–solid phase boundaries (crystallization and melting temperatures) for these solutions were determined at a range of selected pressures. The experiments in n-pentane + CO2 were carried out with a 4.6 wt.% polymer solution containing 5, 10, 20, 40 and 50 wt.% CO2. In the systems containing 5–20 wt.% CO2, no liquid–liquid phase separation boundary could be identified. In solutions containing 40 and 50% carbon dioxide, instead of crystallization, a sol–gel type phase transformation was observed. NMR characterization showed that the original polymer was of crystal modification Form I with a DSC melting transition at 506 K. When the polymer is recrystallized from n-pentane at ambient pressures, it transforms to Form III displaying a DSC endotherm at 347 K, and a multiple melting endotherm clustered around 506 K. When recrystallized at higher pressures, it transforms to Form II crystal modification, in which the 347 K endotherm disappears, but an endotherm at 406 K appears. Crystallization in solutions containing up to 20 wt.% carbon dioxide led to Form II crystal modification. The polymer samples collected after sol–gel transformation showed crystalline features of Form I. The SEM results revealed that the polymer transforms from a lacy appearance after crystallization in n-pentane to more porous structures in after sol–gel transformation in n-pentane + carbon dioxide solutions.