Synthesis and Characterization of Nanostructured Functional Brush-Polymers for Electrical Memory

Abstract

Polymer-based resistance random access memory (RRAM) has been increasingly proposed as the promising candidate for next-generation nonvolatile memory devices because of their low-cost fabrication, solution processability, miniaturized dimensions and simple device structure. With extensive investigation of polymer memory, many operating mechanisms have been proposed such as filamentary conduction, charge transfer, charge trapping and conformational change mechanism. However, none of these mechanisms can comprehensively account for all of the observed properties of polymer memory. Therefore, discussion about the operating mechanism still continues and fundamental understanding of switching mechanism is still under exploration. One strategy for defining switching mechanism is to study the structure-property relationship of memory materials. To elucidate the structure-property relationship, we use the brush-type polymers because they reveal various self-assembly nanostructures by varying the chemical structures of components. For synthesising the self-assembly brush polymer, three different chemical structures and synthetic methods are applied. First approach is to incorporate the mesogen moieties into polymer side chain. Second approach is to use the microphase-separated nanostructure of brush block copolymers. Third approach is to use the synthetic polypeptides. In the first part, we explain about the mesogen-containing brush polymer. A new oxadiazole-containing brush polymer, poly(5-phenyl-1,3,4-oxadiazol-2-yl-[1,10-biphenyl]carboxyloxyn-nonyl acrylate) (PPOXBPA), was synthesized as a mesogen-containing brush polymer and its structure and properties (including electrical memory characteristics) were investigated. PPOXBPA was synthesized by nitroxide-mediated radical polymerization (NMP) because the NMP method is metal-free and can be applied to a wide variety of functional groups and lead to interesting polymer architectures with narrow polydispersity. The polymer was easily fabricated by means of conventional solution coating (spin-, roll-, or dip-coating) and subsequent drying, producing high-quality thin films. Interestingly, analysis of grazing incidence X-ray scattering data disclosed that the brush polymer molecules in the thin films showed self-assembly abilities due to the well-defined oxadiazole mesogen in the bristle and formed a molecular multibilayer structure. Furthermore, the ordering and orientation of such multibilayer structures were improved by a thermal annealing process. These changes were found to influence the electrical memory characteristics of the brush polymer film

specifically, the as-cast films revealed excellent volatile memory behavior, whereas the thermally annealed films exhibited excellent nonvolatile memory characteristics. In addition, the switching mechanism and reliability of the observed memory behavior were investigated in terms of the chemical and morphological structures. In the second part, we demonstrate the synthesis of well-defined functional brush diblock copolyethers. Brush-type block copolymers have many advantages because they combine the properties of various functional groups of the side-chain with nanostructures, thereby providing not only novel functionalities but also the possibility of nanostructure formation. When the main chain is a polyether, the flexibility of the polymer backbone helps to produce the self-assembled nanostructure of a terminal functional unit. Therefore, we synthesized well-controlled functionalized linear aliphatic diblock copolyethers from allyl glycidyl ether (AGE) and ethoxyethyl glycidyl ether (EEGE) via room-temperature metal-free anionic ROP with the aid of a catalyst system, 3-phenyl-1-propanol (PPA

an initiator) and 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)phosphoranylidenamino]-2 Λ5 ,4 Λ5-catenadi(phosphazene) ( t -Bu-P 4

a promoter). Anionic polymerizations of the AGE monomer were successfully demonstrated without side reactions over a range of monomer feed ratios in the catalyst system, producing the homopolymer products with a unimodal narrow molecular weight distribution. In contrast, chain transfer reactions during anionic polymerization of the EEGE monomer could be suppressed significantly but could not be eliminated completely. However, the side reactions (which were observed in the polymerization of EEGE) were completely eliminated in the sequential anionic polymerization of AGE and EEGE, providing well-defined linear diblock copolyether products with a unimodal molecular weight distribution. The obtained linear diblock copolyethers were confirmed to undergo selectively deprotection reaction in each block and further post chemical modification for selective functionalization. Moreover, the diblock copolymers nicely demonstrated various nanostructures via favorable phase-separation and molecular ordering, depending on the compositions and selective post-functionalizations. In third part, we demonstrated a synthesis of brush polypeptide and block copolypeptide and an application of brush polypeptides, bearing carbazole side group, to an active material of polymer memory devices and the nano-structure induced memory performance change by controling the alkyl chain lengths of carbazole side groups. By using the ring-opening polymerization, the brush homo and block copolypeptide was synthesized. The brush homo-polypeptide contained carbazole dericatives which have different lengths of alkyl side chains. Carbazole groups of poly(γ-(9H-Carbazole-9-ethyl)-L-Glutamate)20 (Glu-2-CBZ) molecules are strongly bound to polypeptide backbone to make more ordered layer structure, while those of poly(γ-(9H-Carbazole-9-undecyl)-L-Glutamate)20 (Glu-11-CBZ) molecules are more relaxed to have more freedom to move and less ordered layer structure.