CO2 pressure effects on melting, crystallization, and morphology of poly(vinylidene fluoride)

Supercritical CO2 fluids (SCF CO2) assisting melting of poly(vinylidene fluoride) (PVDF) and the SCF CO2 pressure affecting surface and bulk morphology, melting and crystallization of PVDF were investigated by means of SEM, AFM, FTIR, WAXD, DSC and SAXS. Three SCF CO2 conditions at 84, 283, and 476 atm all at 140 °C for 30 min were studied. Morphological changes, induced by melting of PVDF under SCF CO2 and recrystallization during depressurization of CO2, were found. The level of the CO2-assisted melting of PVDF was found to increase with increasing pressure. SEM and AFM images showed that the 84 atm of CO2 assisted melting on the surface of PVDF film while both 283 and 476 atm of CO2 gave rise to melting of the whole film. FTIR spectra and WAXD patterns found that the hot-pressed PVDF film exhibited predominant α-crystalline form, which is one of the reported four crystalline forms including α, β, γ, and δ forms, and did not transform to other crystalline form(s) upon the SCF CO2 treatments although they lowered the bulk crystallinities of PVDF. SEM images showed that the SCF CO2 treatments at 283 and 476 atm resulted in foam formations in PVDF, with smaller foam cells resulting from the lower pressure treatment. SAXS data found that the thickness of crystalline layer in the lamellar stacks increased while that of amorphous layers insignificantly changed after SCF CO2 treatments at 283 and 476 atm, as compared with untreated PVDF. SAXS and DSC data suggested the presence of a bimodal distribution of crystal size of PVDF after SCF CO2 treatments.