Structural studies on transition metal ion complexes of polyethylene oxide-natural rubber block copolymers

Two shot solution polymerised NR/PEO block copolymer (BC) was used as an absorbent in this study. This polymer has got polyethylene oxide (PEO) immobilised on hydrophobic natural rubber and it was used for complexation studies with the selected 3d transition metal ions. The prepared complexes were subjected to various analytical techniques such as energy dispersive X-ray spectroscopy (EDX), Fourier Transform infrared spectroscopy(FTIR), Raman spectroscopy, X-ray diffraction (XRD) studies, extended X-ray absorption fine structure (EXAFS) analysis and X-ray absorption near edge spectroscopy (XANES). EDX analysis confirms presence of the respective metal ion in each complex. FTIR spectroscopy reveals the 72 helical conformation of polyethylene oxide segments in BC which is retained with some deformation upon complexation. The BC-metal ion interaction is confirmed by broadening of the C-O-C triplet peak. XRD analysis revealed that PEO lattice undergoes expansion during complexation inorder to accommodate the respective metal ion. From the EXAFS results it was observed that each metal ion shows only one peak that corresponds to the oxygen shell indicating the presence of only one type of metal ion bonding. The EXAFS gives hexa coordinated pattern for Co(II), Ni(II) and Zn(II) complexes while a tetra coordination for the Cu(II) complex. Metal-oxygen distance in a given complex is constant and unique which varies with the metal ion. XANES shows a distorted octahedral symmetry (Oh) with sp3d2 hybridisation for the hexa coordinated complexes and a square planar symmetry with dsp2 hybridisation for the tetra coordinated complex. Feasibility of 1 s → 3d transition confirms +2 oxidation state of the metal ions. The combined result of EXAFS and FTIR shows the best fit structure of the complexes in which metal ions are encapsulated within the PEO helical tunnel.