Morphology and thermal properties of low-density polyethylene/graphite composite films as potential pH sensors prepared via heat treatment and natural drying

To develop sensing materials, this study used cheap and readily available graphite, with decalin as solvent to disperse the graphite in low-density polyethylene (LDPE). Two methods of solvent removal (heat treatment at 100 °C and natural drying) were applied. Afterward, two kinds of composite LDPE/graphite films with different proportions of graphite were obtained. Different samples of the two films were compared in terms of the differences in appearance, thermal properties, and resistances. SEM showed that the appearance of heat-treated composite films presented a complete and smooth structure and good bendability relative to untreated films. Thermal gravimetric analysis and differential thermogravimetry showed that composite films with heat treatment had a higher thermal stability. Differential scanning calorimetry results showed that the melting points of the two composite films decreased with increasing graphite content. Finally, these composite films were used to measure the induced voltage values corresponding to changes in pH of different samples. The results showed that stable voltage values could not be obtained in the case of composite materials with very low conductivity, and that an appropriate amount of graphite mixed with LDPE could have better acid–base sensing properties.