Effect of carboxyl end groups content on the thermal and electrical properties of poly(propylene terephthalate)

Poly(propylene terephthalate) (PPT) samples with different carboxyl terminal groups content were synthesized in bulk and characterized in terms of chemical structure and molecular weight. The thermal behavior was examined by thermogravimetric analysis and differential scanning calorimetry. All the polymers under investigation show a good thermal stability, however decreasing with increasing carboxyl terminal groups content. No significant change of the glass transition temperature as well as melting temperature values was found in the samples under investigation; on the contrary, the crystallization rate of PPT was found to be affected by carboxyl terminal groups content, regularly decreasing as the amount of –COOH terminal groups is increased. This trend was interpreted on the basis of the interactions among the terminal groups of the polymeric chains, which determine a decrease in the chain mobility. Direct current (dc) electrical behavior was also investigated. The dc charging/discharging currents and electrical conductivity are studied as a function of temperature, time of applied voltage and amount of –COOH terminal groups. The conductivity values were found to increase as the content of –COOH end groups was increased, due to an increment of amount of ionic charge carriers.