Characterisation of micro polymer inclusion beads by thermogravimetric analysis

Abstract

In this study micro polymer inclusion beads (μPIBs), containing di-(2-ethylhexyl)phosphoric acid (D2EHPA) as the extractant and poly(vinyl chloride) (PVC) or poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) as the base polymer, were characterised by thermogravimetric analysis (TGA) for the first time and their extraction performance for La3+ was assessed. μPIBs are a novel polymer-based extracting material fabricated via a recently developed phase inversion microfluidic method. However, since the μPIBs are formed under aqueous conditions and D2EHPA can be slightly water soluble at pH values higher than its pKa of 2.8, the final composition could differ from the theoretical one because of D2EHPA leaching during fabrication. Hence, a TGA method was developed to determine the exact composition of μPIBs, using polymer inclusion membranes (PIMs) of known composition as reference materials. For μPIBs and PIMs containing PVDF-HFP as the base polymer, a linear temperature ramp of 10 °C min−1 was applied and the mass loss of D2EHPA and PVDF-HFP was observed at temperatures up to 270 and 400 °C, respectively. PVDF-HFP-based μPIBs fabricated to theoretically incorporate 40, 50 and 60 wt% D2EHPA were found to contain 40.2, 48.9, and 59.6% of D2EHPA, respectively. In the case of PVC-based μPIBs and PIMs, an isothermal step at 220 °C held for 35 min was required to separate the degradation features of D2EHPA and PVC. PVC-based μPIBs with intended D2EHPA concentrations of 40, 50 and 60 wt% were determined to contain 41.4, 49.6, and 60.8% D2EHPA, respectively. Thus, the TGA degradation profiles indicated that PVC- or PVDF-HFP-based μPIBs underwent negligible D2EHPA leaching during fabrication. TGA studies involving the PVC-based μPIBs revealed that Na+, extracted by D2EHPA during the fabrication process, slowed the thermal degradation of D2EHPA at 220 °C and this affected the accuracy of the composition assessment. Moreover, the presence of Na+ in both PVC- and PVDF-HFP-based μPIBs was found to have a negative impact on the extraction of La3+ which could be overcome by washing the μPIBs with 1 M sulfuric acid after fabrication to remove the Na+.