**5. Plasticizer and di-electric constant**

The plasticizer showed an effect on the dielectric constant of a memebrane. **Kumar and Sekhon,** *European Polymer Journal*, **2002**, studied such effect by addition of plasticizer to the polyethylene oxide (PEO)–ammonium fluoride (NH4F) polymer electrolytes. They found to result in an increase in conductivity value and the magnitude of increase has been found to depend upon the dielectric constant of the plasticizer. The addition of dimethylacetamide as a plasticizer with dielectric constant (*ϵ* =37.8) higher than that of PEO (*ϵ* =5) results in an increase of conductivity by more than three orders of magnitude whereas the addition of diethylcarbonate as a plasticizer with dielectric constant (*ϵ* =2.82) lower than that of PEO does not enhance the conductivity of PEO–NH4F polymer electrolytes. The increase in conductivity has further been found to depend upon the concentration of plasticizer, the concentration of salt in the polymer electrolyte as well as on the dielectric constant value of the plasticizer used. The conductivity modification with the addition of plasticizer has been explained on the basis of dissociation of ion aggregates formed in PEO–NH4F polymer electrolytes at higher salt concentrations.

Similar study was applied earlier by **William Robert**, *Ploymer* **1998**. They studied the influence of plasticizer on the dielectric characteristics of highly plasticized PVC.Three citrate-related compounds [Citroflex A-4 (CFA4), Citroflex A-6 (CFA6), and Citroflex B-6 (CFB6)] and six sebacate-related compounds [dimethyl sebacate (DMS), diethyl sebacate (DES), dibutyl sebacate (DBS), dioctyl sebacate (DOS), dioctyl azelate (DOZ), and dioctyl adipate (DOA)] were used to evaluate the effects of configurational changes in plasticizer on the dielectric properties of ion-selective poly(vinyl chloride) membranes. Tridodecylamine (TDDA) and potassium tetrakis-4-chlorophenyl borate (KTpCIPB) were used as neutral charge carriers and negative sites, respectively. Using parallel plate sensors, the dielectric properties [ionic conductivity (σ) and tan δ] of the plasticized PVC membranes were determined at temperatures from − 100 to + 100°C and seven log frequencies (− 1, 0, 1, 2, 3, 4, and 5 Hz). Generally, increasing the amount of plasticizer in the membrane improved the σ and lowered the temperature of the tan δ peak. A positive linear correlation existed between the log σ and the log phr ratio for a given temperature and frequency, when no data was included for membranes below the melting temperature of the plasticizer. When plotted *versus* temperature, the slopes of all these lines passed through a maximum between 0 and 60°C. The intercepts of all these lines increased monotonically with increasing temperature. These intercepts were highly dependent on the frequency at low temperatures, becoming less frequency dependent as the temperature increased. Having established that configurational changes of the plasticizers had no effect above the melting point of each plasticizer, global nomograms were only required for the citrate- and sebacate-related plasticizers, respectively. Using the appropriate nomogram for a selected plasticizer, the σ could be predicted at a given phr ratio, temperature, and frequency.

#### **6. Plasticizers and the physical properties of membrane**

Puncture tests quantified five mechanical properties for at least eight levels of seven plasticizers were applied (**Gibbons et al**, *Polymer*, **1997)**. Using Citroflex B-6 at a phr ratio of 0.31, the strength and secant stiffness peaked at 9.63 N and 1250 N m−1, respectively. At a phr ratio of 0.6 the toughness peaked at 48 N mm. These three properties decreased at higher phr ratios for all plasticizers. Tangent stiffnesses were generally 1.7 times secant stiffnesses. They concluded that for all plasticizers, ductility increased to a constant value of 15 mm at a phr ratio of two. The molecular structures of the plasticizers influenced the mechanical properties. For a given phr ratio, plasticizers having lower hydrodynamic volumes increased the strengths, stiffnesses, and toughnesses of the membranes. Compared to prior dielectric testing, the strength, toughness, and stiffness increased as the ionic resistivity increased. In electrodes and biosensors phr ratios should be reduced to a minimum of one.
