Open-pore morphology of i-PP copolymer crystallized from a gel state in supercritical propane

Open-pore morphology was produced in low-density polypropylene (i-PP) by first crosslinking the i-PP and then crystallizing it from a highly swollen state in supercritical propane. The extent of crosslinking, expressed by an increasing gel fraction and an increasing compression modulus, was found to have a strong effect on pore size but less so on crystal structure. The resulting pore structure, studied using scanning electron microscopy, showed a decrease in pore size with an increase in gel fraction. Typical pore sizes are between 1 and 10 μm. The polymer morphology in the wall, analyzed using wide-angle X-ray diffraction, indicated the presence of α-PP crystals. The experimental results showed that mostly loose chains (sol fraction) contribute to the crystallinity of the polymer. For our specific i-PP, the crosslinked network devoid of sol fraction (after extraction) collapses during cooling and does not produce any open pore structure.