Spin-Label Investigation of the Dynamic Behavior of Stearic Acid Additives in Poly (vinyl chloride)

Three kinds of spin label compounds having a DOXYL-radical substituent at different positions of the hydrocarbon chain of stearic acid were added to the poly (vinyl chloride) (PVC) including dioctyl phthalate (DOP) as plasticizer, and the order parameter (S) of each radical in PVC resin was determined by the ESR measurement. The values of S which is a parameter of dynamic state of spin label compounds in PVC decreased by increasing the quantity of DOP added to PVC. Similarly, the values of S decreased by increasing the temperature. The decrease of S may be explained by the decrease of rigidity of PVC resin, due to the addition of DOP and the increase of the temperature. The half-decrease temperature of the order parameter (TS1/2) for each radical decreased by increasing the quantity of DOP added to PVC and by decreasing the density (d) and the glass transition temperature (Tg) of PVC resin. The stable structures for each interaction model (PVC/stearic acid additive model, PVC/DOP model) were estimated by molecular orbital (MO) calculation.