Influence of hydrogen bonding on the crystallization behavior of poly(ethylene oxide)/ionic liquids mixtures

Two types of imidazolium-based ionic liquids (ILs) [BMIM][BF4] and [BMIM][NTf2] and their mixtures were blended with poly(ethylene oxide) (PEO) matrix to tune the strength of intermolecular hydrogen bonding in the system. With the addition of ILs, particularly increasing [BMIM][NTf2] ratio, significantly reduced overall crystallization kinetics in the order of pure PEO > P80B20N0 > P80B16N4 > P80B6N14 > P80B0N20 (P80BxNy represents the mixture containing 80 wt% PEO, x wt% [BMIM][BF4] and y wt% [BMIM][NTf2]) were observed, which is mainly dominated by the enhanced hydrogen bonding. The early stage crystallization is accompanied by the breaking of hydrogen bonds as evidenced by in situ FT-IR, and much slower wavenumber shift of C-O-C groups on PEO backbone in P80B0N20 compared to that of P80B20N0 further demonstrates that the stronger hydrogen bonding between PEO and [BMIM] [NTf2] could significantly inhibit the nucleation process. The fold surface free energy estimated from Lauritzen-Hoffman model is about 1.4 times in P80B0N20 than that in P80B20N0 indicating the stronger hydrogen bonding could increase the energy penalty associating to the nucleation and crystal growth. From SAXS, it is seen that hydrogen bonding only functions to the amorphous layer L-a rather than the lamellar thickness, that is, stronger hydrogen bonding results in larger L-a and more imperfect lamellar structure.