Synthesis of Linear and Star-Shaped Poly[4-(diphenylamino)benzyl methacrylate]s by Group Transfer Polymerization and Their Electrical Memory Device Applications

Hole-transporting triphenylamine (TPA)-based polymers, the linear poly[4-(diphenylamino)benzyl methacrylate] (PTPMA) and the three-armed star-shaped poly[4-(diphenylamino)benzyl methacrylate] (N(PTPMA)3), have been synthesized by group transfer polymerization (GTP). For the synthesis of N(PTPMA)3, the core-first approach coupled with the GTP was adopted using a newly designed silyl ketene acetal initiator with three initiating points. The polymerization results showed that the polymer molecular weights were precisely controlled by the monomer/initiator ratio. The new hole-transporting TPA-based polymers could be blended with electron-accepting [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) for electrical memory device applications. The experimental results showed that pristine PTPMA and N(PTPMA)3 exhibited dynamic-random-access-memory (DRAM) volatile electrical behavior, but their PCBM composite-based devices changed to the write-once-read-many times (WORM) nonvolatile memory characteristic or conductor behavior. The ON or OFF state for the WORM memory devices could remain over 104 s without any degradation. The optical absorption and photoluminescence results indicated that the charge transfer complexation between the TPA and PCBM led to the varied PCBM aggregated size and memory characteristics. The trapping-detraping and field-induced charge transfer effects were used to explain the different memory characteristics. The steric hindrance of the star-shaped N(PTPMA)3 compared to PTPMA probably reduced the electron transport efficiency from Al to the LUMO of PCBM in the polymer/PCBM composite-based devices, such as turn-on voltage and conduction mechanism. The present study reveals the importance of polymer architecture or donor/acceptor composition on tuning memory device characteristics.