Porous structure of crystalline polymers by exclusion effect of carbon dioxide

We investigated the morphology of high-density polyethylene (HDPE) and poly(vinylidene fluoride) (PVDF) crystallized under carbon dioxide (CO2) by light scattering measurements and microscopic observations. The crystallization of HDPE was delayed and the ordering of the spherulite was smaller by dissolving CO2 rather than air at ambient pressure. A fine-layered porous structure having a size of 500 nm was obtained in HDPE, while a large rod-like porous structure radiating in the spherulite was obtained in PVDF. Such a characteristic porous structure is attributed to the exclusion of CO2 from the crystal growth front to the intercrystalline amorphous region and the growth of bubbles by the supersaturation of CO2 in the constrained amorphous region. The exclusion effect is covered by the Keith–Padden theory through consideration of the self-diffusion in polymer–CO2 systems; the exclusion and the growth of bubbles occur as lamellar stacks in HDPE whereas they occur as bundles of lamellar stacks in PVDF.