Impacts of ions on the plasticization of cellulose triacetate by fluorinated ionic liquids: Thermal properties, microscopy, Raman spectra, and sorption of pure enantiomers

Abstract

Cellulose triacetate (CTA) is a bio-based chiral polymer with the rare property of forming homogenous solid blends with several compounds. The controlled modification of the conformational freedom of the polymer chains by plasticizers could enable the tuning of its selectivity towards optical isomers. We survey the blends of CTA with four individual ionic liquids (ILs), with a focus on the action of the individual counter ions. Blends of CTA with four ILs being, the combinations of two cations (BMIM and BMPy) and two anions (Tf2N and TfO), were studied using thermal analysis methods (TGA, DMA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and sorption tests. Blends containing the ILs with Tf2N, which undergoes cis-trans stabilization, were less effective plasticizers of CTA than the TfO-based ILs when sensed as decreases in the temperatures of glass transition (Tg) and degradation (Td). The exchange of the cations caused discernible but far smaller changes of Tg and Td. All studied ILs caused the disappearance of the Raman band of CTA at 702 cm−1, which we assign to the skeletal vibrations of the pyranose ring using the ab-initio calculation, and pronounced wavenumber shifts of the bands of the methoxy group (at 1163 cm−1) and pyranose ring (at 1075 and 1085 cm−1) in CTA. This indicates the increased intra-chain mobility and physical interactions between CTA and IL. TEM analysis confirmed the existence of interactions between the ILs and CTA chain, and the amplification of plasmon resonance. Sorption uptakes of some enantiomers (carvone, 1-phenylethanol and 1-amino-2-propanol) confirmed the chiral behavior of the CTA-based polymers. For instance, the sorption uptake of (S)-(−)-1-phenylethanol in pure CTA was by 30 % higher than that of (R)-(+)-1-phenylethanol. Moreover, the sorption uptakes of (R)-(+)- and (S)-(−)- carvone enantiomers increased by 47 % and 260 %, respectively, due to blending CTA with 20 wt % of [BMPy][TfO].