Highly Tough and Reliable Poly(Amic Acid) with Ballistic Impact-Resistance through Modulation of Hydrogen Bonding Interactions.

Poly(amic acid) (PAA) materials as the precursor of polyimide generally show remarkably poor mechanical properties, thus limiting their application as the engineering plastics. In this study, it is demonstrated that the mechanical properties of PAA materials can be improved significantly for tens of folds with breaking strength >50 MPa, Young's modulus >400 MPa, and elongation at break >300% by incorporation of 20% (mol%) poly(propylene glycol) (PPO) soft segments. The optimization for suitable hard-soft composition with 20% PPO and the existence of various hydrogen bonds with different binding energies can dissipate energies efficiently, which simultaneously improve the material strength and toughness. In addition, PAA82 films exhibit excellent tolerance toward cyclic stretch, and have the capability to resist various harsh conditions including solar radiation testing (1 sun), heat (85 °C), alkalinity (pH 10), and acidity (pH 4) over one month. Noted that PAA82 films can be laminated with Kapton films, which show excellent resistance to ultrahigh (200 °C) and ultralow temperature (-196 °C). The laminated film also exhibits bulletproof property with a thickness of 6 mm. The strategy via modulation of hard-soft compositions and hydrogen bonds in PAA materials shows great potentials to improve the mechanical properties of polymeric materials.

Catalyst-Free One-Step Preparation of Self-Crosslinked pH-Responsive Vesicles.

The fabrication of block copolymer (BCP) vesicles with controlled membrane permeability and promising stability remains a considerable challenge. Herein, a new type of pH-responsive and self-crosslinked vesicle based on a hydrolytically hindered urea bond is reported. This kind of vesicle is formed by the self-assembly of a pH-responsive and hydrolytically self-crosslinkable copolymer poly(ethylene glycol)-block-poly[2-(3-(tert-butyl)-3-ethylureido)ethyl methacrylate-co-2-(diethylamino)ethyl methacrylate] (PEG-b-P(TBEU-co-DEA)). The BCP can be easily synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization of 2-(3-(tert-butyl)-3-ethylureido)ethyl methacrylate (TBEU) and 2-(diethylamino)ethyl methacrylate (DEA) using PEG-based macro-chain transfer agent. The copolymer could self-assemble into stable vesicles by the hydrophobic interaction and in situ cross-linking between amines and isocyanates after the hydrolysis of the hindered urea bonds without any catalyst. Dynamic light scattering (DLS) studies show that the vesicles exhibit enhanced stability against the dilution of organic solvent, and the size can be adjusted through the change of pH values. Moreover, the alkaline phosphatase-loaded vesicles can act as nano-reactor and enable free diffusion of small molecules into the vesicles, followed by the significantly improved fluorescence intensity of phosphate-caged fluorescein. This self-crosslinking and pH-sensitive vesicles may serve as a smart platform in controlled drug delivery and molecular reactor.

Synthesis and characterization of new donor-acceptor type copolymers based on fluorene derivatives for photovoltaic solar cells.

In this paper, we demonstrated the successful synthesis of newly designed copolymers, C1 and C2, with donor-acceptor type structure. Both C1 and C2 copolymers contained 9,9-dioctylfluorene-2,7-bis(trimethyleneboronate) as one constructional unit to improve the solubility in common organic solvents. The other constructional unit was 2,3-bis(5-bromothiophen-2-yl)acrylonitrile (DTDBAL) for C1, while 4,7-dibromobenzo[c][1,2,5]thiadiazole unit, 5,5'-dibromo-2,2'-bithiophene unit and N1, N1-bis(4-bromophenyl)-N4,N4-bis(4-(2-phenylpropan-2-yl)phenyl)benzene-1,4-diamine are for C2. We fabricated photovoltaic devices based on the C1 and the C2 copolymers with Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer, PC70BM layer, TiOx layer, and aluminum (Al) electrode. The bulk heterojuntion photovoltaic devices using these copolymers as electron donor and PC70BM as the acceptor exhibited good device performances when measured at 100 mW cm-2. The power conversion efficiency (PCE) of the C1 device reached 0.45% with Voc, Jsc and FF of 0.51, 2.50 and 35%, respectively. The PCE of the C2 device reached 0.34% with Voc, Jsc, and FF of 0.56, 2.01 and 30%, respectively.

Colorless-to-colorful switching electrochromic polyimides with very high contrast ratio.

Colorless-to-colorful switching electrochromic polymers with very high contrast ratio are unattainable and attractive for the applications of smart wearable electronics. Here we report a facile strategy in developing colorless-to-colorful switching electrochromic polyimides by incorporating with alicyclic nonlinear, twisted structures and adjusted conjugated electrochromophores, which minimize the charge transfer complex formation. It is noted that, by controlling the conjugation length of electrochromophore, the colorless-to-black switching electrochromic polymer film (PI-1a) exhibites an ultrahigh integrated contrast ratio up to 91.4% from 380 to 780 nm, especially up to 96.8% at 798 nm. In addition, PI-1a film with asymmetric structure also demonstrates fast electrochemical and electrochromic behaviors (a switching and bleaching time of 1.3 s and 1.1 s, respectively) due to the loose chain stacking, which provides more pathways for the penetration of counterion. Moreover, the colorless-to-black EC device based on PI-1a reveals an overall integrated contrast ratio up to 80%.

Theoretical Study of Electrochemical and Electrochromic Properties of Novel Viologen Derivatives: Effects of Donors and π-Conjugation.

We propose a linkage approach by merging ambipolar electrochromic (EC) materials in both π-acceptor-π (π-A-π) and donor-acceptor-donor (D-A-D) configurations and investigated their electrochemical and spectroelectrochemical properties using density functional theory calculations. Here, we considered anthracene, toluene, and pyrene as π-conjugated molecules, triphenylamine (TPA) as a donor, and viologen as an acceptor moiety for π-A-π and D-A-D configurations. We have also explored the substitutional effects in the donor moiety on the overall electrochromism during both oxidation and reduction processes. Here, we mainly focused on the relationship between the structure, substitution of functional groups, electronic and spectral properties, as well as redox potential of the designed monomers. Our calculations indicate that the designed monomers have attractive absorption properties and show clear color switching upon the redox process. We find that the substitution of stronger electron-donating and π-spacer groups create new absorption peaks in the oxidation states. These designed viologen derivatives will be potential candidates, which can be used in both oxidation and reduction processes for upcoming EC devices.

Tunable and Processable Shape-Memory Materials Based on Solvent-Free, Catalyst-Free Polycondensation between Formaldehyde and Diamine at Room Temperature.

Compared with traditional thermosets, malleable thermosets have more applications in aerospace, biotechnology, and construction. Here we report a one-step, solvent-free, catalyst-free polycondensation method between diamine and formaldehyde to prepare a series of malleable hemiaminal dynamic covalent networks (HDCNs). The materials have excellent malleability and reprocessability by hot pressing. The Young's modulus and breaking strength of HDCNs obtained by the polycondensation of formaldehyde and 4,4-diaminodiphenylmethane (MDA) are as high as 1.6 GPa and 60 MPa, respectively, which can be facilely adjusted through the introduction of polyetheramine-400 (PEDA). Moreover, the HDCNs feature the shape memory ability with a recovery ratio above 93.5% and can be recycled by the addition of different monomers. This promising HDCN, prepared from economical raw materials, may have vast applications in industries.

Photophysical and Electroluminescence Characteristics of Polyfluorene Derivatives with Triphenylamine.

In this work, polymers of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-triphenylamine] with side chains containing: pyrene (C1), diphenyl (C2), naphthalene (C3), and isopropyl (C6) structures were synthesized via a Suzuki coupling reaction. The structures were verified using NMR and cyclic voltammetry measurements provide the HOMO and LUMO of the polymers. The polymer with pyrene (C1) and naphthalene (C3) produced photoluminescence in the green while the polymer with the side chain containing diphenyl (C2) and isopropyl (C6) produce dual emission peaks of blue-green photoluminescence (PL). In order to examine the electroluminescence properties of the polymers, the solutions were spin-coated onto patterned ITO anode, dried, and subsequently coated with an Al cathode layer to form pristine single layer polymer LEDs. The results are compared to a standard PFO sample. The electroluminescence spectra resemble the PL spectra for C1 and C3. The devices of C2, C3, and C6 exhibit voltage-dependent EL. An additional red emission peak was detected for C2 and C6, resulting in spectra with peaks at 435 nm, 490 nm, and 625 nm. The effects of the side chains on the spectral characteristics of the polymer are discussed.

Digital Memory Characteristics of Aromatic Polyimides Based on Pyridine and Its Derivatives.

Soluble aromatic polyimides and polyvinyls were prepared by incorporating pyridine moiety and its derivatives in the backbone and the side groups, respectively: 6F-Py-i polymers based on the polyimide backbone (6F-Py-1 to 6F-Py-7) and PVPy-i polymers based on the polyvinyl backbone (PVPy-1 to PVPy-4). All polymers were found to be amorphous. The 6F-Py-i polymers were thermally stable up to 511-545 °C; the PVPy-i polymers were stable up to 362-376 °C. Their glass transitions, thin film densities, molecular orbitals, and band gaps were determined. The electrical devices fabricated with the polymers in an electrode/polymer/electrode structure revealed p-type unipolar write-once-read-many times (namely, permanent) or dynamic random access memory or dielectric behavior, depending on the substituents of the pyridine unit and the film thicknesses. In particular, such digital memory characteristics were found to originate from the pyridine moieties possessing a high charge affinity in the polymers. However, the pyridine moieties were found to still need at least two or more aromatic substituents to get enough power to stabilize charges via utilizing the resonance effects provided by the substituents. Overall, this study demonstrated that the pyridine unit conjugated with two or more aromatic substituents is a very useful component to design and synthesize digital memory materials based on thermally stable polyimides and other high performance polymers. The 6F-Py-i polymers have potential for the low-cost mass production of high-performance programmable unipolar permanent memory devices with very low power consumption.

Sorption behavior of polyaramides in relation to isolation of nucleic acids and proteins.

The effect of chemical composition and morphology of the surface layers of new polyaramide-containing sorbents on the mechanism of selective sorption of nucleic acids and proteins was investigated as compared to the previously studied sorbents modified with fluoropolymers and polyaniline (high-throughput materials providing one-step isolation of DNA from biological mixtures). A series of silica-based sorbents modified with polyaramides having consistently varying structure and containing the set of "key" structural elements (aromatic units and nitrogen atoms in the backbone, fluorinated groups), and various donor and acceptor moieties was prepared. The chemical composition of the polymer coatings was evaluated by X-ray photoelectron spectroscopy. The surface morphology was studied by scanning probe microscopy. The sorption properties were investigated by passing the mixtures containing DNA, RNA and proteins of different nature through the cartridges containing the obtained sorbents. All the investigated materials weakly retain double-stranded DNA but effectively retain RNA and proteins. The sorption capacity of the sorbents depends on the protein nature. The observed sorption behavior was shown to be determined by the chemical structure and not by the morphology of the polymer coating. It was proposed that similarity of the sorption properties of the series of chemically different polymers could be determined by similar total input of different sorption mechanisms.

A bifunctional copolymer additive to utilize photoenergy transfer and to improve hole mobility for organic ternary bulk-heterojunction solar cell.

To realize the high efficiency organic photovoltaics (OPVs), two critical requirements have to be fulfilled: (1) increasing the photon energy absorption range of the active layer, and (2) improving charge separation and transport in the active layer. This study reports the utilization of THC8, a novel fluorescence-based polymer containing propeller-shaped di-triarylamine and fluorene moieties in the active layer consisting of poly-3-hexylthiophene and [6,6]-phenyl-C61-butyric acid methyl ester to form a ternary bulk heterojunction. The results showed that the high absorbance and strong fluorescence of THC8 at 420 and 510 nm, respectively, broadened the spectral absorption of the OPV, possibly through Förster resonance energy transfer. In addition, the morphology of the device active layer was improved with the addition of a suitable amount of THC8. Consequently, the charge transport property of the active layer was improved. The best power conversion efficiency (PCE) of the device with THC8 was 3.88%, a 25% increase compared to the PCE of a pristine OPV.

Tristable electrical conductivity switching in a polyfluorene-diphenylpyridine copolymer with pendant carbazole groups.

Tristable electrical conductivity switching and non-volatile memory effects are demonstrated in a conjugated copolymer of poly(2,6-diphenyl-4-((9-ethyl)-9H-carbazole)-pyridinyl-alt-2,7-(9,9-didodecyl)-9H-fluorenyl) (PPCzPF). The indium-tin oxide (ITO)/PPCzPF/Al device can be switched from the low-conductivity (off) state to the first high-conductivity (on-1) state at 1.8 V, with an on/off current ratio of approximately 100. The device can be further switched from the on-1 state to the next higher conductivity (on-2) state at 2.4 V, with an on-2/on-1 current ratio of approximately 20. All the three conductivity states are accessible, stable and non-erasable. The tri-level conductance switching can be explained in terms of field-induced conformational ordering of the polymer chains and enhanced charge-transfer interaction at the PPCzPF/ITO interface.

Synthesis and dynamic random access memory behavior of a functional polyimide.

The device under testing was a plastic dynamic random access memory based on a donor-functionalized polyimide (TP6F-PI), which exhibited the ability to write, read, erase, and refresh the electrical states. The device had an ON/OFF current ratio up to 105, promising minimal misreading error. Both the on and off states were stable under a constant voltage stress of 1 V and survived up to 108 read cycles at 1 V.