Impact of rapid expansion of supercritical solution process conditions on the crystallinity of poly(vinylidene fluoride) nanoparticles

• The RESS process was successfully applied to produce PVDF particles smaller than 50 nm.
• This study demonstrates that RESS induces a crystal phase transformation from α to the desired β phase.
• Simple static treatment with supercritical CO2 does not induce crystal phase transformation of PVDF samples.
• Static scCO2 treatment and subsequent expansion yields to submicron particles with higher molar mass, degree of crystallinity and lower dispersity.

The rapid expansion of supercritical solution (RESS) process was used to produce poly(vinylidene fluoride) (PVDF) nanoparticles. PVDF was chosen since five different crystal phases (α, β, γ, δ, ε) have been reported in literature. The experiments were performed with a low molar mass polymer (Mn = 1500 g mol−1) that consists exclusively of α phase. The main objectives of this study were to examine the influence of process parameters, i.e. inner nozzle diameter, pre-expansion temperature and pressure on polymer crystallinity and crystal phase properties. Before and after micronization, the polymer was characterized by scanning electron microscopy, size-exclusion chromatography, differential scanning calorimetry and X-ray diffraction.The findings of our investigations clearly show that RESS enables the formation of submicron particles. Furthermore, under certain process conditions, i.e. higher expansion and cooling rates and thus small nozzle diameters, RESS induces a crystal phase transformation from α to the desired β phase. In opposite thereto, simple static treatment with scCO2 does not induce crystal phase transformation. Furthermore, average molar mass, dispersity and crystallinity of the RESS product was reduced due to the preferred extraction of low molar mass fractions.

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