The effect of counter cation species on the formation of various crystal forms and their phase transition behavior of poly(vinyl alcohol)-iodine complex

Abstract

Poly(vinyl alcohol) [PVA, -(CH2CH(OH))n-] forms the crystalline complexes I and II with iodine ions (I3−) when PVA is immersed in the KI/I2 aqueous solutions of 0.1–0.5 and 1–3 mol/l concentration, respectively. On the basis of the X-ray diffraction and the ultraviolet-visible, IR and Raman spectral data, we have found that the immersion of PVA films into the iodine aqueous solutions containing the different type of counter ions (K+, Na+, Li+, H+, or Cs+) creates the various types of the crystal modification. When PVA sample was immersed in the solution of relatively low concentration, all the types of counter ion gave the complex I structure. When the concentration of the solution was increased to 1–3 mol/l, the sample immersed for several hours showed the formation of the complex II. In the case of H+ ion, the longer immersion was found to give a new crystal form or complex III with the different packing mode of PVA and I3− ions as determined by the X-ray structure analysis. In the cases of Na+ and Li+ ions, the complex II was disordered by the longer immersion in such a highly-concentrated solution. As clarified by the infrared spectral data, the hydrogen bonds of O–H ⋯ I type are formed in the iodine complex, the strength of which was found also to be different depending on the type of counter ion species. In this way, the present paper has revealed the important role of the counter cations on the formation and stability of the PVA-iodine complexes for the first time.