Utilization of supercritical CO2 for controlling the crystal phase transition and cell morphology of isotactic polybutene-1 foams

Isotactic polybutene-1 (iPB-1) is a semi-crystalline polymer with multiple crystal forms. In this work, we found a novel method to obtain and adjust the expansion ratio and bimodal cellular structures of iPB-1 foams with annealing and foaming using supercritical CO2. The form I′ can be obtained via the supercritical CO2 annealing, and the ratio of form I and I′ can be adjusted by controlling simply the annealing time. The effects of crystal form ratio on the thermal behavior, rheological behavior and cell morphology of foams were analyzed. Energy storage modulus (G′) and complex viscosity (|η*|) decrease with the increased content of form I. Crystal form I′ has a lower foaming temperature window and bubble growth resistance than form I, resulting in larger cell diameter and expansion ratio. The bimodal cell structure can be observed in the samples with 1:1 form I′/form I, where the average large and small cell size are 122 and 40 µm at 109 °C and 10 MPa CO2, respectively. To regulate the cell distribution, various ratios of mixed crystal phases were applied to investigate their effect on the foaming behavior and bimodal cells structure at different temperatures. Even with a similar expansion ratio, significantly different cell distributions of bimodal cell structure can be easily prepared by simply adjusting the content of form I′, which can broaden the application range of iPB-1 foams.