Zesheng An

Copolymers of perylene diimide with dithienothiophene and dithienopyrrole as electron-transport materials for all-polymer solar cells and field-effect transistors

Electron-accepting solution-processable conjugated polymers consisting of perylene diimide moieties alternating with dithienothiophene, oligo(dithienothiophene), or N-dodecyl dithienopyrrole units have been synthesized. All these polymers possess excellent thermal stability with decomposition temperatures over 400 °C. The glass-transition temperatures vary from 155 to 263 °C. These polymers show broad absorption extending from 250 to 900 nm with electrochemical and optical bandgaps as low as 1.4 eV; the maximum absorbance increases and the bandgap decreases with increasing the conjugation length of oligo(dithienothiophene), while the bandgap can also be decreased by the replacement of dithienothiophene by dithienopyrrole. The electrochemical onset reduction potentials range from −0.8 to −1.0 V vs. ferrocenium/ferrocene, suggesting that the electron affinities are essentially unaffected by the specific choice of donor moiety, while the onset oxidation potentials (+0.6 to +1.0 V) are a little more sensitive to the choice of donor. The mono dithienothiophene and the dithienopyrrole polymers were found to exhibit electron mobilities as high as 1.3 × 10−2 and 1.2 × 10−3 cm2V−1s−1, respectively, in top-contact organic field-effect transistors. Power conversion efficiencies in the range 0.77–1.1% were obtained under simulated AM 1.5, 100 mW/cm2 irradiation for all-polymer solar cells using the dithienothiophene-based polymers as acceptors in a 1 : 1 ratio with a polythiophene derivative as a donor. The device performance varies with the conjugation length of oligo(dithienothiophene) in the polymer acceptors, and for the best-performing material it can be further optimized to give a power conversion efficiency of 1.5% by increasing the donor/acceptor weight ratio to 3 : 1.

Development of thermosensitive copolymers of poly(2-methoxyethyl acrylate-co-poly(ethylene glycol) methyl ether acrylate) and their nanogels synthesized by RAFT dispersion polymerization in water

Thermosensitive polymeric materials based on copolymers of oligo(ethylene glycol) methacrylates are attracting significant attention in various materials sectors. The preparation of their thermosensitive microgels/nanogels via the aqueous dispersion polymerization process is, however, limited by low monomer loading and thus low solid content of the final colloids. Moreover, the preparation of nanogels by reversible addition-fragmentation chain transfer (RAFT) mediated dispersion polymerization has been further hampered by the poor RAFT control of the polymerization process. In this article, we report the development of thermosensitive copolymers based on poly(2-methoxyethyl acrylate-co-poly(ethylene glycol) methyl ether acrylate) (P(MEA-co-PEGA)) and their use for nanogel synthesis by RAFT dispersion polymerization in water. The thermosensitive copolymers exhibited sharp thermal transitions upon increasing thee temperature above their lower critical solution temperature. The use of MEA as the majority comonomer and poly(N,N′-dimethylacrylamide) as the RAFT agent and stabilizer for the synthesis of nanogels allowed monomer loadings of up to 20%, which significantly improved the solid content of the dispersion polymerization system. Moreover, the dispersion copolymerization of MEA with PEGA was under excellent RAFT control up to complete monomer conversion. The synthesized nanogels showed an unprecedented linear relationship between nanogel size and temperature, suggesting expanded applications of such responsive polymeric materials.

Room-temperature discotic liquid-crystalline coronene diimides exhibiting high charge-carrier mobility in air

Six N,N′,5,11-tetrasubstituted coronene-2,3,8,9-tetracarboxydiimides have been synthesised incorporating 3,4,5-tri(n-dodecyloxy)phenyl or 2-(n-decyl)-n-tetradecyl groups in various positions. Differential scanning calorimetry, polarised optical microscopy, and X-ray diffraction indicate that all form columnar discotic mesophases from around room temperature to around 200 °C. Charge-carrier mobility values, which energetic considerations suggest are electron mobility values, have been determined in non-aligned samples cooled from the isotropic melt using the space-charge-limited current technique. The highest mobility, 6.7 cm2V−1 s−1, was found in N,N′-bis(n-2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecylfluorooctyl)-5,11-bis(3-[{3,4,5-tri(n-dodecyloxy)phenyl}carbonyloxy]-n-propyl)coronene-2,3,8,9-tetracarboxydiimide, which X-ray diffraction suggests is the most highly ordered of the materials examined.

Efficient and versatile synthesis of star polymers in water and their use as emulsifiers

Core cross-linked star polymers of low polydispersity were efficiently prepared in high yield by RAFT-mediated emulsion and dispersion polymerizations in water at high solid content. These star polymers were demonstrated to be effective emulsifiers, and the emulsion was successfully used as template to fabricate polymer particles.

Hydrazine as a Nucleophile and Antioxidant for Fast Aminolysis of RAFT Polymers in Air

The bifunctional role of hydrazine as a potent nucleophile and antioxidant was investigated for the rapid aminolysis of RAFT polymers within minutes in air with effective suppression of the formation of disulfides. Using both dithioesters and trithiocarbonates as model compounds, we showed that hydrazine exhibited a significantly improved aminolysis rate when compared with a commonly used primary alkyl amine. On the basis of the exellent results with CTAs, we further studied the aminolysis of RAFT polymers prepared with either dithioesters or trithiocarbonates. In accord with the aminolysis results on CTAs, hydrazine aminolyzed RAFT polymers in an impressively short time and, more importantly, it significantly suppressed the formation of disulfides as comfirmed with GPC.

One-Step Microwave Preparation of Well-Defined and Functionalized Polymeric Nanoparticles

Well-defined colloidal polymeric nanoparticles are important in advanced biomedical and optical technologies. We report a facile microwave methodology to prepare narrowly dispersed cross-linked polymeric nanoparticles at high solids content through a surfactant-free emulsion polymerization process. The nanoparticle size was controlled by using cross-linkers with enhanced reactivity through a one-step microwaving process, significantly simplifying the nanoparticle synthetic process. The successful size control was realized by confining the cross-linking to intraparticle cross-linking rather than interparticle cross-linking. We also discovered that the superheating/dielectric heating effect associated with microwave irradiation could be utilized to effectively reduce the nanoparticle size.

High electron mobility in nickel bis(dithiolene) complexes

The charge-carrier mobilities for three Ni bis(dithiolene) complexes have been determined using the steady-state space-charge limited current technique. A high mobility of 2.8 cm2 V–1 s–1 was observed for one compound, which exhibits a π-stacked columnar structure, in an annealed unsymmetrical melt-processed device. Energy-level considerations and field-effect transistor measurements suggest that this value represents an electron mobility. However, saturation mobilities measured for this compound in spin-coated field-effect transistors were found to be over two orders of magnitude lower than the space-charge limited current values. X-Ray diffraction shows a difference in morphology between thick melt-processed and thin spin-coated films and, therefore, a significant change in intermolecular packing between the device types may explain the discrepancy in mobilities obtained using the two techniques.

Heterofunctional polymers and core-shell nanoparticles via cascade aminolysis/Michael addition and alkyne-azide click reaction of RAFT polymers.

A convenient methodology involving cascade aminolysis/Michael addition and alkyne-azide click reaction was developed for polymers and polymeric core-shell nanoparticles, synthesized via RAFT-mediated homogeneous and heterogeneous polymerisation processes, respectively, to provide well-defined heterofunctional polymeric materials.

Emerging Synthetic Strategies for Core Cross‐Linked Star (CCS) Polymers and Applications as Interfacial Stabilizers: Bridging Linear Polymers and Nanoparticles

Core cross-linked star (CCS) polymers become increasingly important in polymer science and are evaluated in many value-added applications. However, limitations exist to varied degrees for different synthetic methods. It is clear that improvement in synthetic efficiency is fundamental in driving this field moving even further. Here, the most recent advances are highlighted in synthetic strategies, including cross-linking with cross-linkers of low solubility, polymerization-induced self-assembly in aqueous-based heterogeneous media, and cross-linking via dynamic covalent bonds. The understanding of CCS polymers is also further refined to advocate their role as an intermediate between linear polymers and polymeric nanoparticles, and their use as interfacial stabilizers is rationalized within this context.

pH-responsive high internal phase emulsions stabilized by core cross-linked star (CCS) polymers

Well-defined core cross-linked star (CCS) polymers of poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) were prepared via cross-linking copolymerization in an aqueous-based dispersion polymerization system, mediated by reversible addition–fragmentation chain transfer (RAFT) polymerization. The synthesized CCS polymers were characterized by nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC), dynamic light scattering (DLS), and conductivity and zeta potential measurements. The use of PDMAEMA CCS polymers as effective emulsifiers for oil-in-water emulsions was investigated. Interfacial tension measurements showed that the CCS polymer reduced the interfacial tension between water and oil in a pH-dependent manner. Gelled high internal phase emulsions (HIPEs) were formed at high oil fractions (80–89 vol%) and over a wide range of pH values (2–12). The HIPEs were characterized by conductivity, confocal laser scanning microscopy (CLSM) and rheology. The emulsion properties in terms of oil droplet size, long-term stability and rheology were pH-dependent. Complete demulsification of HIPEs was easily realized 2 min after the addition of base. The CCS-stabilized HIPEs were used as templates to prepare porous hydrophilic polymers by polymerizing the monomers in the continuous aqueous phase. The study presented herein reveals that responsive CCS polymers can be used as effective stabilizers for the formation of responsive HIPEs, which have a wide range of applications.