Constant Voltage Conductivity Measurements of a Critical Temperature Transition in Low Density Polyethylene

Temperature-dependent constant voltage conductivity measurements of the highly disordered insulating polymeric material low density polyethylene (LDPE) were made to investigate a transition of electrical transport mechanisms from variable range hopping to multiple trapping at a critical temperature. Such a transition is evidenced as a change of slope in a double logarithmic plot of conductivity versus temperature at the critical temperature, $T_{c}$. Below $T_{c}$ variable range hopping, with a T$^{\mathrm{-1/4}}$ dependence, is the dominant mechanism; above $T_{c}$ multiple trapping mechanisms, with linear T$^{\mathrm{-1}}$ dependence dominate. To investigate this transition, the sample temperature, $T$, was varied from \textasciitilde 230 K to 300 K, based on prior experimental evidence which estimated $T_{c}$ to be \textasciitilde 268 K, along with theoretical models which predict $T_{c}_{\mathrm{\thinspace }}$\textasciitilde 255 K. A constant voltage conductivity system was used, with current measured in parallel plate geometry with a steady voltage applied across 25 \textmu m thin film LDPE samples using Ohm's law. Experiments were conducted \textit{in vaccu}, with a lower bound in measurable conductivities of \textasciitilde 1$\cdot $10$^{\mathrm{-21}}$ ($\Omega $-cm)$^{\mathrm{-1}}$ due to fA current resolution. Transitions seen in other electron transport measurements and related structural phase transitions at comparable temperatures are discussed.