Plasticizer-level study of poly(vinyl chloride) ion-selective membranes.

Highly plasticized poly(vinyl chloride) (PVC) membranes (200 per hundred resin [phr]) form the basis of one class of ion-selective electrodes (ISEs). In previous work on the mechanical properties of membranes, the optimal ratio of plasticizer level employed to minimal level required for complete plasticization (phr(exp)/phrmin) was found to be 2.0. The current study was designed to determine whether this ratio is necessary or sufficient for proper ISE function. Dynamic mechanical analysis (DMA) was used to examine the effects of five plasticizers on the dynamic mechanical properties of membranes at three frequencies (110, 11.0, and 1.1 Hz) as a function of temperature (-100 degrees C to +100 degrees C); dioctyl sebacate (DOS), epoxidized propylene glycol dioleate (PGDO), ortho-nitrophenyl octyl ether (o-NPOE), epoxidized soybean oil (ESO), and epoxidized linseed oil (ELO). The glass transition temperature of PVC, which was found to be +77.1 degrees C at 11.0 Hz, was depressed by the addition of 200 phr of each plasticizer from a high of -1.4 degrees C (PGDO at 110 Hz) to a low of -70.2 degrees C (DOS at 1.1 Hz). DMA and electromotive-force (EMF) measurements on membranes plasticized with o-NPOE through a range of phr(exp)/phrmin from 0.5 to 9.3 indicated that a "transition window" occurs between phr(exp)/phrmin of 2.0 and 3.3 in which the membranes change from minimally plasticized polymer films to predictable ion-selective membranes, coinciding with the optimal mechanical properties observed previously. Based on dynamic mechanical properties and EMF response data, the optimal phr(exp)/phrmin++ ratios for membranes as a function of plasticizer were proposed: 0.8 for ESO and ELO, 1.3 for PGDO, 1.7 for DOS, and 3.0 for o-NPOE.