# Recent Advances in Flexible Resistive Random Access Memory
## 3 Material System of Flexible Resistive Memory
### Flexible Substrate Materials
#### 3.3.1 Flexible Polymer Substrate  
Polymer materials are currently the most commonly and widely used flexible substrates, including polyimide (PI) [23, 24, 37, 42, 49, 56, 66, 82, 91, 114], polyethylene naphthalene (PEN) [15, 30, 36, 102, 111, 114, 115], polyethylene terephthalate (PET) [8, 10, 18, 19, 22, 25, 27, 40, 45, 51, 52, 53, 54, 59, 68, 70, 71, 73, 74, 75, 76, 77, 78, 79, 86, 87, 90, 93, 95, 101, 104, 105, 109, 110, 112, 114], polycarbonate (PC) [116], polyethersulfone (PES) [58, 117] and so on. The comparison of their mechanical properties is shown in Table 3:  
These polymer materials have good mechanical flexibility, which can withstand repeated bending and twisting, and they also have stable physical and chemical properties. PEN (Figure 12a) and PET (Figure 12b) are currently the most popular polymer substrates in the field of flexible electronic devices, which possess great flexibility and chemical stability. However, their relatively low temperature of glass transition and melting temperatures limit them from producing and operating. In addition, they have high water vapor transmittance (1.1 g/m\({}^{2}\) per day at 45 \({}^{\circ}\)C for 100 \(\upmu\)m thickness [131]), resulting in the role of protecting the functional layer being small, which may lead to the unstable performance of the functional layer. Therefore, if the device requires long-term stability, it is necessary to carry out appropriate packaging, which involves the packaging process and cost. PI (Figure 12c) has a relatively high temperature of glass transition and melting temperature, but its low transmittance and high cost make it difficult to me  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Features** & **PET [118–120]** & **PEN [121–124]** & **PI [120, 125]** & **PC [126–128]** & **PES [129, 130]** \\ \hline tensile strength (flexibility, MPa) & 742 & 708 & 144 & 60 & 44 \\ Young’s modulus (elasticity, GPa) & 2.8 & 9.0 & 2.5 & 2.5 & 0.64 \\ temperature of glass transition (\({}^{\circ}\)C) & 84 & 129 & 320 & 150 & 220 \\ melting temperature (\({}^{\circ}\)C) & 255–265 & 260 & 590 & 220–230 & 330 \\ thermal conductivity (W/m\(\cdot\)K) & 0.3 & 0.164 & 0.8 & 0.2 & 0.17 \\ coefficient of thermal expansion (ppm/\({}^{\circ}\)C) & 60 & 18.45 & 15 & 50 & 65 \\ transmittance & 90.4\% & 88.18\% & 60\% & 90\% & 89\% \\ corrosion resistance & good & good & good & poor & good \\ dimensional stability & good & good & fair & fair & good \\ \hline \hline \end{tabular}
\end{table}
Table 3: Mechanical properties of flexible RRAM polymer substrates.  
ments of low cost and high transparency in industrialization. PC has a high transmittance but also has flame retardant and oxidation resistance, which can play a very protective role on the functional layer. The main defect of PC is low hydrolysis resistance, easy to be corroded by organic chemical solvents or atmosphere, resulting in performance decline or material degeneration. PES has high transparency, but it has problems of poor flexibility and low operating temperature. Moreover, PES has a high coefficient of thermal expansion, which may cause material strain due to mismatching with the thermal properties of electrode materials during cooling or heating up. In a word, the unique advantage of polymer materials is excellent flexibility, but the disadvantage is low operating temperature.