Tomoya Higashihara

Polyimide memory: a pithy guideline for future applications

Polymeric materials for use in memory devices have attracted significant scientific interest due to their several advantages, such as low cost, solution processability, high mechanical strength, and possible development of three-dimensional stacking devices. Taking into account the heat resistance for the device fabrication process, polyimides (PIs) are one of the most attractive polymers for memory applications due to their high thermal stability and mechanical strength. In recent years, a large number of studies have revealed that almost all kinds of memory properties from volatile to non-volatile memory can be produced by optimizing the chemical structure of the PIs. Several mechanisms have been discussed in order to explain the switching properties. Among them, two major theories, field induced charge transfer (CT) and filament formation, are thought to explain the phenomena in the PI system. In this article, recent studies of functional polyimides (PIs) for memory applications are reviewed, mostly focusing on the mechanism underlying the switching phenomena. In addition, some progress in the fabrication process for developing high density storage will also be reviewed.

Hyperbranched Polymers with Controlled Degree of Branching from 0 to 100

A linear polymer, hyperbranched polymers with various degrees of branching, and 100% hyperbranched polymers were successfully synthesized by self-polycondensation of 2,2,2-trifluoro-1-[4-(4-phenoxyphenoxy)phenyl]ethanone by using different amounts of trifluoromethanesulfonic acid from the same AB(2) monomer.

Synthesis of hyperbranched polymers with controlled structure

Hyperbranched polymers (HBPs) are highly branched macromolecules with 3-dimensional globular structures, which have unique properties, such as low viscosity, high solubility, and a large number of terminal functional groups compared to linear analogues. They are easily prepared by the polymerization of an ABx monomer in which A and B represent two different functional groups. This facile one-pot synthesis is advantageous over the synthesis of a dendrimer which requires multi-step reactions. For the HBPs, the degree of branching (DB) is one of the most important intrinsic parameters as well as the molecular weight distribution, which has a significant influence on the physical and chemical properties of the polymeric materials. The DB is theoretically ∼50% for a polymer derived from an AB2 monomer; this value is assumed based on the equal reactivity of the two B functional groups of the AB2 monomer. Recently, several groups have successfully achieved a 100% DB, which is a characteristic property of dendrimers, by using several strategies. Moreover, HBPs with a controlled DB from 0 to 100% have been reported by changing the rate constants of the first reaction of B and the second one toward an A functional group. In addition, the molecular weight and molecular weight distribution of the HBPs can also be controlled. In this review, we focus on the recent progress in the synthesis of HBPs in order to control their structures, i.e., 100% DB, molecular weight and molecular weight distribution (≤1.1).

Flexible polymer memory devices derived from triphenylamine–pyrene containing donor–acceptor polyimides

Organic polymer based electrical memory devices have attracted significant scientific interest for flexible electronics. However, molecular design principles of polymers with specified memory characteristics on flexible substrates remain challenges. Herein we developed new triphenylamine–pyrene containing donor–acceptor polyimides (PIs) on flexible poly(ethylene naphthalate) (PEN)/Al/PIs/Al cross-point devices, which showed the memory characteristics changing from volatile to nonvolatile via the relative copolymer ratio. The PIs were prepared from the diamines 4,4′-diamino-4′′-methyltriphenylamine (AMTPA) or N,N-bis(4-aminophenyl)aminopyrene (APAP) and the dianhydride 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), with relative AMTPA/APAP molar compositions of 100/0, 95/5, 90/10 and 0/100. As the APAP content increased, the memory device characteristics changed from volatile to nonvolatile behavior of flash and write once read many (WORM), since the pyrene moiety could stabilize the radical cation of the APAP moieties. The threshold voltage, ON/OFF ratio, and retention ability were within −3 V, more than 104, and 104 s, respectively. Additionally, the endurance and bending cyclic measurements confirmed that the flexible PI memory devices exhibited excellent reliability and mechanical stability. A possible switching mechanism based on the charge transfer interaction was proposed through molecular simulation and fitted with physical conduction models in OFF and ON states. The manipulation of the memory volatility through tailoring the molecular design and plastic electronic devices demonstrated promising applications of donor–acceptor PIs in integrated memory devices.

Combining living anionic polymerization with branching reactions in an iterative fashion to design branched polymers.

This paper reviews the precise synthesis of many-armed and multi-compositional star-branched polymers, exact graft (co)polymers, and structurally well-defined dendrimer-like star-branched polymers, which are synthetically difficult, by a commonly-featured iterative methodology combining living anionic polymerization with branched reactions to design branched polymers. The methodology basically involves only two synthetic steps; (a) preparation of a polymeric building block corresponding to each branched polymer and (b) connection of the resulting building unit to another unit. The synthetic steps were repeated in a stepwise fashion several times to successively synthesize a series of well-defined target branched polymers.

Polymer electrolyte membranes based on poly(m-phenylene)s with sulfonic acid via long alkyl side chains

Polymeric electrolyte membranes (PEM) based on the cross-linked poly(m-phenylene)s with sulfonic acid via long alkyl side chains (c-SPMPs) were successfully obtained by the reaction of the phenyl groups in the sulfonated poly(m-phenylene)s (SPMPs) and 4,4′-methylene-bis[2,6-bis(hydroxyethyl)phenol] (MBHP) as the cross-linker in the presence of methanesulfonic acid. The c-SPMPs produced transparent membranes with a relatively high mechanical property in the dry state, regardless of their high ion exchange capacity (IEC) values. The rigid main chains and the cross-linking structure suppressed the unacceptable water uptake and dimensional change of the c-SPMPs membranes even in the hydrated state. The c-SPMP8-2 membrane with IEC = 2.93 mequiv g−1 showed a higher proton conductivity than Nafion 117 over a wide range of relative humidities (30–95%) at 80 °C. The mean distance between the domain of the hydrophobic main chains and that of the hydrophilic sulfonic acid groups was determined by the wide-angle X-ray diffraction (WAXD) measurements, supporting the formation of proton channels based on phase separation. The c-SPMP membranes might be promising alternatives of the perfluorinated PEM materials based on both advantageous features of a high proton conductive performance and straightforward synthetic method.

Synthesis of all-conjugated donor–acceptor block copolymers and their application in all-polymer solar cells

Electron donor–acceptor triblock and diblock copolymers have attracted considerable interest due to their potential applications in all-polymer solar cells as single-active components or surfactants. However, the number of donor–acceptor block copolymers has lagged behind that of the rod–coil and rod–rod block copolymers due to their synthetic difficulties. This minireview highlights the recent advances in the synthetic strategies for the all-conjugated donor–acceptor block copolymers and their application in all-polymer solar cells.

Synthesis of Branched Polymers by Means of Living Anionic Polymerization, 8. Synthesis of Well-Defined Star-Branched Polymers by an Iterative Approach Based on Living Anionic Polymerization Using 1,1-Diphenylethylene Derivatives

The synthesis of well-defined four- and six-arm star branched polymers in which arms differ either in molecular weight or composition has been achieved via a new iterative approach based on living anionic polymerization using 1,1-diphenylethylene (DPE) derivatives. Each stage in the iteration involves two reactions: a living functionalization reaction of living anionic polymer with DPE derivatives and an in-situ reaction of the resulting linked product having two anions with 1-4-(4-bromobutyl)phenyl]-1-phenylethylene to introduce two DPE moieties into the polymer. In each living functionalization reaction, a 1.2-fold excess or more of living anionic polymer relative to DPE moiety was employed to complete the reaction. Asymmetric A2A′2 and A2A′2A′′2 star-branched polystyrenes as well as A2B2 and A2B2C2 heteroarm star-branched polymers were synthesized by repeating the iteration synthetic sequence two and three times, respectively. Since the polymers obtained by each reaction stage were contaminated with their precursor polymers, they were isolated by SEC fractionation. Their high degrees of compositional, molecular weight and architectural homogeneity were confirmed by the analytical results of SEC, SLS, VPO, 1H NMR and viscosity measurements.

Electrically bistable memory devices based on all-conjugated block copolythiophenes and their PCBM composite films

We explore the memory device characteristics of the all-conjugated diblock copolythiophenes, poly(3-hexylthiophene)-block-poly(3-phenoxymethylthiophene) (P3HT-b-P3PT), and their blends with PCBM. The field-effect transistors prepared from P3HT-b-P3PT showed a significant hysteresis between the forward and backward gate-bias scans in the transfer curve, indicating the occurrence of charge trapping. The charge trapping may have occurred within the amorphous P3PT domains dispersed in the block copolythiophene by preventing charge transport. P3HT525252-bbb-P3PT393939 and P3HT102102-bb-P3PT3737 exhibited dynamic random access memory (DRAM) behavior in the sandwich configuration of ITO/P3HT-b-P3PT/Al, whereas P3HT only showed semiconductor characteristics, suggesting the significant effect of the amorphous P3PT segments on the electrical switching behavior. By blending a small amount (5–10 wt%) of PCBM into P3HT-b-P3PT of different block ratios (P3HT525252-bbb-P3PT393939, P3HT102102-bb-P3PT3737, and P3HT8989-bb-P3PT2323), the memory devices showed a write-once-read-many times (WORM) behavior with the switching voltages of −2.6 to −3.3 V and high ON/OFF ratios (105 to 107). The mechanism associated with the memory characteristics was the charge transfer from the P3HT-b-P3PT donor to the PCBM acceptor, which stabilized the charge separated state and retained the high conductance state for a long time during the ON stage. These experimental results provide a new strategy of designing all-conjugated block copolymers for advanced memory device applications.

Precision synthesis of tailor-made polythiophene-based materials and their application to organic solar cells

AbstractPolymer-based solar cells (PSCs) have been promising candidates as renewable energy resources, having multiple advantages of flexible, low-cost and large-area processing for their mass production. Among them, much attention has been paid to fundamental bulk-heterojunction (BHJ) devices using a blend of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the active layer. However, there are still significant limitations not only in the low power conversion efficiency (PCE), but also in the device stability. The morphological control of BHJ PSCs is one of the most important issues to improve the device performances. On the other hand, P3HT itself has received much attention in many fields, because it is the best class of balanced high-performance materials as a p-type semiconductor in terms of solubility, chemical stability, charge mobility, and commercial availability. The discovery of the quasi-living Grignard metathesis polymerization (or called catalysttransfer polycondensation) system has made it possible to synthesize a wide variety of chain-end-functional P3HT derivatives, their block copolymers and star-branched polymers. Since the competitive research areas including PSC applications have strongly demanded the accelerated developments of new materials and well-defined morphologies related to polythiophenes, the fundamental studies of P3HT have still been targeted by many research groups. In this review, the controlled synthesis of P3HT, the synthesis of P3HT-based block copolymers, their applications to PSCs, as well as the scope and potential of new thiophene-based materials are described.