Iodine-complex formation behavior of vinyl alcohol-vinyl acetate copolymers as studied by X-ray diffraction and vibrational spectroscopic data analyses

The iodine complex formation behavior of a series of vinyl alcohol-vinyl acetate copolymers (P(VA-co-VAc), -[CH2CH(OH)+CH2CH(OCOCH3)]n-) has been studied systematically on the basis of X-ray diffraction and IR/Raman spectroscopic data. The VA segments in the copolymers were found to start to crystallize for the first time when the sequential segmental length is beyond the critical sequential length of 3–4 VA monomeric units. Below this critical length, the OH units on VA segments exist as isolated groups or as being connected with O=C bonds of VAc segments. The latter type of hydrogen bonds, –C=O⋯HO-, was found to be formed in the copolymers of a wide range of VA content of 30–90 mol%. The immersion of these VA-VAc copolymers into the iodine solution gave the crystalline iodine complexes. In the VA range of 0–40%, the PVAc-I5- complex (β form) is formed mainly. Once when the VA content is beyond the critical sequential length, the PVA-I3- complex of form II starts to be created and develops into the larger and regular crystalline complex in addition to the PVAc-iodine complex. [Display omitted] •Structural evolution behavior has been clarified for a series of VA-VAc copolymers and their iodine-complexes.•Critical sequential length of VA units was evaluated, which is necessary for crystallization of P(VA-co-VAc) copolymers.•Characteristic iodine complex structure created in the VA and VAc monomer sequences have been clarified by X-ray analysis.