Han-Sheng Sun

Self-assembled structures in rod-coil block copolymers with hydrogen-bonded amphiphiles

We report the synthesis and the self-assembled morphologies of a series of new rod-coil diblock copolymers, poly[2,7-(9,9-dihexylfluorene)]-b-poly(4-vinylpyridine) (PF-b-P4VP). The rod-coil diblock copolymers were synthesized via Suzuki coupling reaction and living anionic polymerization. Probing by transmission electron microscopy, small-angle X-ray scattering and atomic force microscopy, we found that the microphase separation varies from lamellar to cylindrical and then to spherical, depending on the length of P4VP blocks. The P4VP coil blocks were further hydrogen-bonded with 3-pentadecylphenol (PDP) to form rod-comb block copolymers. Similar to conventional coil-coil block copolymers-based supramolecules, the incorporation of PDP transforms the morphologies from lamellar to cylindrical or from cylindrical to spherical due to the increase of volume fraction of P4VP(PDP) comb blocks. The strategy described here can be used to tune the self-assembled structures of rod-coil block copolymers.

Using a Single Electrospun Polymer Nanofiber to Enhance Carrier Mobility in Organic Field-Effect Transistors toward Nonvolatile Memory

In this work, a single electrospun polymer nanofiber was employed as an additional dielectric in organic field-effect transistors where the active channel was a layer of pentacene. A high field-effect mobility (>1.50 cm(2)/(V·s)) and a high ON/OFF current ratio (>10(6)) could be achieved by the use of such a nanofiber. Probing by electron microscopy, atomic force microscopy, and scattering techniques, we found that the geometry of the fiber is key to induce a pentacene morphology with large and oriented grains that facilitates the charge transport in pentacene layer along the fiber. The feasibility of nonvolatile memory based on this new type of transistor has been explored and the devices showed a fairly high memory window and reliable memory characteristics. In addition to pure polymers, the effects of composite nanofibers with dispersed [6,6]-phenyl-C61-butyric acid methyl ester were also investigated, and the electrical properties and memory characteristics of the transistors were found to be further improved. This study highlights the importance of dielectric geometry to pentacene morphology that is decisive for the performances of organic field-effect transistors.

Oligosaccharide Carbohydrate Dielectrics toward High-Performance Non-volatile Transistor Memory Devices.

Oligosaccharides are one of the most promising biomaterials because they are abundant, renewable, diversified, and biosourced. The use of oligo- or polysaccharides for high-performance non-volatile organic field-effect-transistor memory is demonstrated herein. The charge-storage mechanism is attributed to charged hydroxyl groups that induce stronger hydrogen bonding, thus leading to the stabilization of trapped charges. This study reveals a promising future for green memory devices.

Synthesis, morphology, and electrical memory application of oligosaccharide-based block copolymers with π-conjugated pyrene moieties and their supramolecules

We report the synthesis, morphology, and field effect transistor memory application of maltoheptaose-based block copolymers, maltoheptaose-block-poly(1-pyrenylmethyl methacrylate) (MH-b-PPyMA), and their supramolecules with (4-pyridyl)-acceptor-(4-pyridyl), MH(4Py-Acceptor-4Py)x-b-PPyMA. MH-b-PPyMA was prepared by the combination of the t-Bu-P4-catalyzed group transfer polymerization and the Cu(I)-catalyzed azide–alkyne cycloaddition reaction. After the thermal annealing process, the MH-b-PPyMA bulk sample underwent microphase separation to form the sub-10 nm periodic self-assembled nanostructure. The self-assembled morphologies transform from the hexagonal cylinder packing to the body-centered cubic sphere arrangement and the disordered spherical nanodomain with the increase of the PPyMA segment length. On the contrary, only the spherical nanodomain was observed in the thermo-annealed thin film samples of both MH-b-PPyMA and MH(4Py-Acceptor-4Py)x-b-PPyMA. The electrical characteristics of the p-type pentacene-based OFET memory device were studied using the thermo-annealed polymer thin film as the electret layer. The MH(4Py-Acceptor-4Py)x-b-PPyMA-based organic field effect transistor (OFET) devices had the high hole mobility of 0.20–1.08 cm2 V−1 s−1 and the ON/OFF current (ION/IOFF) ratio of 107–108, in which the acceptor of the benzo[c][1,2,5]thiadiazole (BT) based device possessed the higher hole mobility than that of the isoindigo-based one due to the more ordered packing pentacene crystals. The memory window (ΔVTH) of the supramolecule-based device was increased with enhancing the 4Py-Acceptor-4Py blending composition, and that of the MH(4Py-BT-4Py)1.5-b-PPyMA10-based device had the largest ΔVTH of ca. 9 V, a long-term retention time greater than 104 s and the high ION/IOFF memory ratio of 106–107 (reading at Vg = 0 V) for more than 100 programming/erasing cycles. Our results demonstrate that the bio-related block copolymers and their supramolecular thin film could be used as electret layers for high-performance nonvolatile flash green memory devices.

Synthesis of multifunctional poly(1-pyrenemethyl methacrylate)-b-poly(N-isopropylacrylamide)-b-poly(N-methylolacrylamide)s and their electrospun nanofibers for metal ion sensory applications

We report the synthesis and characterization of multifunctional triblock copolymers, poly(1-pyrenemethylmethacrylate)-block-poly(N-isopropylacrylamide)-block-poly(N-methylolacrylamide) (PPy-b-PNIPAAm-b-PNMA), and their electrospun (ES) nanofibers for temperature or metal-ion sensing. The triblock copolymers are composed of fluorescent and metal-ion-sensitive PPy, thermoresponsive PNIPAAm, and chemically crosslinkable PNMA segments. Non-crosslinked ES nanofibers are initially prepared using the aforementioned PPy-b-PNIPAAm-b-PNMA triblock copolymers followed by thermal crosslinking. It is found that ES nanofibers prepared from PPy-b-PNIPAAm-b-PNMA can self-assemble to form nano-scale spherical aggregates with PNMA located at the core, PNIPAAm at the center layer, and PPy at the shell. This self-assembly characteristic therefore induces a strong excimer emission between the pyrenyl moieties. The crosslinking process between PNMA blocks is then implemented to stabilize the ES nanofibers since the non-crosslinked ones are metastable objects. The resulting crosslinked nanofibers exhibit a predominant wettability and dimension stability under aqueous states, and can perform a detectable photoluminescence transition at different temperatures or toward an Fe3+ ion. In contrast to their counterpart drop-cast film, ES nanofibers with a high surface/volume ratio have obviously higher sensing ability toward thermal stimuli and metal ions, and are expected to be applied as multifunctional sensory devices.

Rod-coil type miktoarm star copolymers consisting of polyfluorene and polylactide : precise synthesis and structure-morphology relationship

A series of rod–coil miktoarm star copolymers consisting of poly[2,7-(9,9-dihexylfluorene)] (PF) and polylactide (PLA) arms as the rod and coil segments, respectively, were synthesized by combining the chain-growth Suzuki–Miyaura coupling polymerization and living ring-opening polymerization (ROP). First, PFs having a hydroxyl group at the α-chain end (PF–BnOH) were prepared by the polymerization of 2-(7-bromo-9,9-dihexyl-9H-fluorene-2-yl)4,4,5,5-tetramethyl-1,2,3-dioxaborolane using the initiating system of 4-(tetrahydropyran-2′-yloxymethyl)-iodobenzene/Pd2(dba)3/t-Bu3P. After a couple of chain end modifications, PFs having one, two, and three hydroxyl groups at the α-chain end (PF–OH, PF–(OH)2, and PF–(OH)3, respectively) were obtained with dispersity (Đ) values of less than 1.29. The obtained PF–OH, PF–(OH)2, and PF–(OH)3 were used as the initiators for the organic base-catalyzed ROP of rac-lactide to produce the AB type linear diblock (PF-b-PLA) and AB2 and AB3 type miktoarm star copolymers (PF-b-(PLA)2 and PF-b-(PLA)3, respectively). The self-assembled nanostructures in the thermally-annealed PF-b-(PLA)xs (x = 1, 2, and 3) were evaluated in the bulk by synchrotron small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The PF-b-PLA, PF-b-(PLA)2, and PF-b-(PLA)3 with the PF weight fractions of 0.52–0.56 were found to form poorly ordered PLA cylindrical structures in the PF matrix due to a strong rod–rod interaction of the PF segment. On the other hand, these BCPs with the PF weight fractions of 0.22–0.23 exhibited the hexagonally close-packed cylinder (Hex) morphologies, in which the cylindrical PF phase was embedded in the PLA matrix. More interestingly, the domain-spacing (d) values for the Hex morphologies decreased with the increasing PLA arm number despite each BCP having a comparable molecular weight and PF weight fraction: d = 26.4 nm for PF-b-PLA, d = 21.7 nm for PF-b-(PLA)2, and d = 18.6 nm for PF-b-(PLA)3.

Field‐Effect Transistors: Oligosaccharide Carbohydrate Dielectrics toward High‐Performance Non‐volatile Transistor Memory Devices (Adv. Mater. 40/2015)

Utilizing sugar materials as dielectrics toward high-performance non-volatile organic field-effect-transistor memory is demonstrated by R. Borsali, W.-C. Chen, and co-workers on page 6257. This not only reveals a promising future for green memory devices but also builds the bridge between biomaterials, such as biostarch from the cellulose of straw, and organic electronics.