Kevlar-like Aramid Polymers from Mixed PET Waste.

This work describes the synthetic approaches, spectroscopic and thermal characterization of aramid polymers prepared from waste polyethylene terephthalate (PET) via sustainable and scalable processes. Direct depolymerization of PET with aliphatic diamines under melt conditions resulted in decomposition without substantial formation of any aramid polymer. The Higashi-Ogata methodology or direct polycondensation of terephthalic acid (TPA) derived from PET waste and p-phenylenediamine, resulted in oligomerization and formation of aramids with a low degree of polymerization. The highest molecular weight polymers were obtained via the acid chloride of TPA, the traditional method. A proprietary solvent enabled the dissolution of most polymers and subsequent size exclusion chromatography analysis in the same solvent. We emphasize that although the soluble polymer compounds are prepared via the traditional route, they are novel. The apparent molecular weights of the soluble polymers ranged between 10-35 kDa (M n ) and 28-81 kDa (M w ). All analogues were prepared with commercially available diamines and diamine combinations. The obtained solid powders were dissolved in D2SO4 and analyzed spectroscopically to qualitatively evaluate the degrees of polymerization, while the solids were characterized via thermogravimetric analysis and differential scanning calorimetry. Many reaction conditions were employed to improve the solution polycondensation reaction, and it was found that addition of pyridine (2 eq) to the NMP reaction medium was crucial in preventing the precipitation of the polymer. Contrary to conventional wisdom, CaCl2 did not play a crucial role in the molecular weight increase of the polymer when oxydianiline was used. Our data indicated that the temperature and absence of CaCl2 provided a boost in molecular weight. Both room temperature and 0 °C reactions generated similar polymers as suggested by nuclear magnetic resonance; however, the cold conditions enhanced gel formation, an important attribute in the future processing of these materials to obtain fibers. All analogues had a high degradation temperature at 5 and 10% weight loss (5% and T10%), above 400 °C, along with high percent char values. A glass transition (T g) was not detected in any of the analogues prepared.

Solvent-Induced Selectivity of Isoprene From Bio-Derived Prenol.

In this work we demonstrate the selective catalytic conversion of prenol, which is an allylic alcohol that can be prepared from renewable resources to isoprene. The catalyst is an inexpensive molybdenum complex (Molyvan L) designed and used as an additive for lubricants. Isoprene is generated under relatively mild reaction parameters at 130-150°C, for 2 h, under vapor pressure conditions that do not exceed 50 psi. Two cases were studied: one in which Molyvan L was dissolved in a base oil at 1% concentration (weight/weight) and then mixed with a solvent and prenol and the other in which neat Molyvan L was introduced in the reaction and the base oil was replaced with the solvent and prenol. We investigated the selectivity of the reaction using the following solvents in both cases: dodecane, dodecanol, isododecane, octane, blendstock for oxygenate blending (BOB3), a fuel surrogate, a polyalphaolefin (PAO4), and methoxy polyethylene glycol (methoxy PEG350). Although conversion of prenol was above 94% in all experiments, isoprene was formed with various degrees of efficiency alongside a prenol isomeric alcohol, diprenyl ether and mixed ether via intramolecular and intermolecular dehydration reactions. Dodecane appeared to have the highest level of selectivity initially in base oil so we studied the effect of various dodecane-like solvents on isoprene yield and product profile. Surprisingly, octane (similar to dodecane) and isododecane (branched alkane) generated insignificant amounts of byproducts, essentially providing the highly desired isoprene with a very high selectivity. Branching of the solvent does not appear to have an effect on selectivity. Another advantage of this catalyst is the low loadings required to effect the transformation; that is, 0.25% (weight/volume) in the cases using neat Molyvan L and 0.5% (weight/volume) in the cases using Molyvan L dissolved in the base oil. Provided that prenol can be produced in large scale from bioresources, this work would enable the sustainable production of isoprene, in good yield, and with very high selectivity.

Carbon Fiber Reinforced Epoxy Vitrimer: Robust Mechanical Performance and Facile Hydrothermal Decomposition in Pure Water.

Conventional carbon fiber reinforced thermosetting polymers (CFRPs) are neither recyclable nor repairable due to their crosslinked network. The rapid growing CFRP market raises a serious concern of the waste management. In this work, a viable method to develop a readily recyclable CFRP based on epoxy vitrimer is introduced. First, a self-catalytic epoxy prepolymer with built-in hydroxy and tertiary amine groups is designed, which upon reaction with an anhydride formed a catalyst-free epoxy vitrimer. The epoxy prepolymer is synthesized from a diamine and an excess of bisphenol A epoxy resin. The hydroxyls and tertiary amines of the epoxy prepolymer efficiently catalyze both curing and the dynamic transesterification of the crosslinked polymer without the need of a catalyst. Then, the epoxy vitrimer is used as the matrix resin to prepare CFRP. The resulting CFRP exhibited a tensile strength as high as 356 MPa. More interestingly, the matrix of the CFRP is efficiently degraded in pure water at above 160 °C. This is because the built-in tertiary amines catalyze the hydrolysis of the ester bonds of the crosslinked network. The simple method developed in this work provides a framework for the development of recyclable CFRP.

Probing the molecular design of hyper-branched aryl polyesters towards lubricant applications.

We report novel polymeric materials that may be used as viscosity index improvers (VII) for lubricant applications. Our efforts included probing the comb-burst hyper-branched aryl polyester architecture for beneficial viscosity and friction behavior when utilized as an additive in a group I oil. The monomer was designed as to undergo polymerization via polycondensation within the architectural construct (AB2), typical of hyperbranched polymers. The monomer design was comprised of aliphatic arms (12 or 16 methylenes) to provide the necessary lipophilicity to achieve solubility in a non-polar medium. Once polymerized, via catalyst and heat, the surface alcohols were functionalized with fatty acids (lauric and palmitic). Controlling the aliphatic nature of the internal arms and peripheral end-groups provided four unique flexible polymer designs. Changing the reaction time and concentration provided opportunities to investigate the influence of molecular weight and branching density on oil-solubility, viscosity, and friction. Oil-solubility was found to decrease with fewer internal carbons, but the number of internal carbons appears to have little influence on the bulk solution viscosity. At concentrations of 2 wt % in a group I base oil, these polymer additives demonstrated an improved viscosity index and reduced friction coefficient, validating the basic approach.

Anion-Tunable Properties and Electrochemical Performance of Functionalized Ferrocene Compounds.

We report a series of ionically modified ferrocene compounds for hybrid lithium-organic non-aqueous redox flow batteries, based on the ferrocene/ferrocenium redox couple as the active catholyte material. Tetraalkylammonium ionic moieties were incorporated into the ferrocene structure, in order to enhance the solubility of the otherwise relatively insoluble ferrocene. The effect of various counter anions of the tetraalkylammonium ionized species appended to the ferrocene, such as bis(trifluoromethanesulfonyl)imide, hexafluorophosphate, perchlorate, tetrafluoroborate, and dicyanamide on the solubility of the ferrocene was investigated. The solution chemistry of the ferrocene species was studied, in order to understand the mechanism of solubility enhancement. Finally, the electrochemical performance of these ionized ferrocene species was evaluated and shown to have excellent cell efficiency and superior cycling stability.

Radical Compatibility with Nonaqueous Electrolytes and Its Impact on an All-Organic Redox Flow Battery.

Nonaqueous redox flow batteries hold the promise of achieving higher energy density because of the broader voltage window than aqueous systems, but their current performance is limited by low redox material concentration, cell efficiency, cycling stability, and current density. We report a new nonaqueous all-organic flow battery based on high concentrations of redox materials, which shows significant, comprehensive improvement in flow battery performance. A mechanistic electron spin resonance study reveals that the choice of supporting electrolytes greatly affects the chemical stability of the charged radical species especially the negative side radical anion, which dominates the cycling stability of these flow cells. This finding not only increases our fundamental understanding of performance degradation in flow batteries using radical-based redox species, but also offers insights toward rational electrolyte optimization for improving the cycling stability of these flow batteries.

TEMPO-based catholyte for high-energy density nonaqueous redox flow batteries.

A TEMPO-based non-aqueous electrolyte with the TEMPO concentration as high as 2.0 m is demonstrated as a high-energy-density catholyte for redox flow battery applications. With a hybrid anode, Li|TEMPO flow cells using this electrolyte deliver an energy efficiency of ca. 70% and an impressively high energy density of 126 W h L(-1) .

Multi-electron redox reaction of an organic radical cathode induced by a mesopore carbon network with nitroxide polymers.

An organic radical based composite cathode comprised of poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA)-Ketjenblack was developed by a simple solvent-less electrode fabrication method. The composite cathode demonstrated a two-electron redox reaction of PTMA that is from an aminoxy anion (n-type) via a radical to an oxoammonium cation (p-type) with the corresponding redox potential at 2.8-3.1 V and 3.5-3.7 V vs. Li/Li(+) when evaluated in lithium half cells. Moreover, the PTMA-Ketjenblack composite electrode exhibits fast electrode reaction kinetics and an enhanced solid electrolyte interface by cyclic voltammetry and electrochemical impedance spectroscopy measurements. These improved electrochemical properties contribute to increased capacity (300 mA h g(-1)), a high rate (50% capacity retention after 100 C rate excursions) and a long cycle life in the cell performance evaluations. Morphological and compositional characterization indicates a unique mesopore network of Ketjenblack with the PTMA matrix, which highly facilitates the interaction between the conductive media and the radical species, resulting in the performance enhancement of the PTMA-Ketjenblack composite cathode.

Injectable and thermogelling hydrogels of PCL-g-PEG: mechanisms, rheological and enzymatic degradation properties.

We report new injectable and thermosensitive hydrogels from polycaprolactone-graft-polyethylene glycol (PCL-g-PEG). The PCL-g-PEG polymer aqueous solution was injectable and formed a physical hydrogel at human body temperature. The rheological properties, sol-gel transition mechanisms, and in vitro degradation properties of PCL-g-PEG hydrogels were investigated. Rheological results demonstrate that hydrogels with tunable storage moduli (G') that span four orders of magnitude, from 0.2 to 5500 Pa, can be obtained by varying polymer concentrations. Hydrophobic dye solubilization, dynamic light scattering, and X-ray diffraction results suggest that micelle aggregation and partial crystallization of the polycaprolactone segment lead to the sol-gel transition with increasing temperature. The degradation of PCL-g-PEG hydrogels was slow in the absence of the enzyme lipase, but can be substantially increased by lipase in a concentration-dependent manner. The PCL-g-PEG hydrogel has a low critical gelation concentration, high storage modulus, and easily handled solid morphology, representing great advantages over our previously developed structurally analogous PLGA-g-PEG. The results presented showcase the potential biomedical application of the versatile PCL-g-PEG hydrogels.

Anthraquinone with tailored structure for a nonaqueous metal-organic redox flow battery.

A nonaqueous, hybrid metal-organic redox flow battery based on tailored anthraquinone structure is demonstrated to have an energy efficiency of ~82% and a specific discharge energy density similar to those of aqueous redox flow batteries, which is due to the significantly improved solubility of anthraquinone in supporting electrolytes.

Injectable and thermosensitive PLGA-g-PEG hydrogels containing hydroxyapatite: preparation, characterization and in vitro release behavior.

Here we report the design and characterization of injectable and thermosensitive hydrogel composites comprised of poly(lactic acid-co-glycolic acid)-g-poly(ethylene glycol)(PLGA-g-PEG) containing hydroxyapatite (HA) for potential application in bone tissue engineering. Inclusion of HA into the hydrogels would provide both enhanced mechanical properties and bioactivity to the composites. The effects of HA on the properties of the hydrogels were investigated in terms of storage modulus, sol-gel transition properties, pH and in vitro dye release behavior. The hydrogel composites were also studied by scanning electron microscopy (SEM), x-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The results revealed that hydrogel composites preserved their sol-gel transition properties in the presence of HA. The storage modulus of the hydrogels was enhanced in a HA-content dependent manner, and the acidic pH environment of the hydrogel was neutralized by HA, both representing great advantages over the hydrogel alone. SEM images showed that HA particles were well dispersed and distributed within the hydrogel matrix. The composites showed a sustained release of a small molecule model dye for up to two weeks with slight increase of release with addition of HA. This work demonstrates the formation of novel thermogelling composites of PLGA-g-PEG and HA that are injectable and promote controlled release.

7,7,8,8-tetracyanoquinodimethane-based molecular dopants for p-type doping of OLEDs: a theoretical investigation.

The array of organic conductivity dopants used for organic light-emitting devices (OLEDs) to reduce the operating voltage and improve power efficiency is extremely limited. Here we report a comparative theoretical study between newly proposed analogues and the standard state-of-the-art conductivity dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). We used density functional theory to determine the bond lengths, bond angles, and electronic properties, such as the energy of the highest occupied molecular orbital (E(HOMO)) and the lowest unoccupied molecular orbital (E(LUMO)) states. The ground state structures of the proposed molecules were optimized at the B3LYP/6-31G* level. The results show that substitution of one or two fluorine groups in the F4-TCNQ core with a substituted phenyl ring or other electron-withdrawing moieties, will not substantially affect the geometry of the molecule or its electronic ability to accept electrons. The most significant finding was that the phenyl substitutions onto the TCNQ core are nearly perpendicular to the TCNQ plane, and thus there is no electronic communication between the two rings. This is extremely important, as such extension of the π conjugated system would negatively affect the E(LUMO) and thus the electron affinity of the molecule.

Synthesis and application of pyridine-based ambipolar hosts: control of charge balance in organic light-emitting devices by chemical structure modification.

We studied the influence of a pyridine moiety versus a phenyl moiety when introduced in the molecular design of an ambipolar host. These pyridine-based host materials for organic light-emitting diodes (OLEDs) were synthesized in three to five steps from commercially available starting materials. The isomeric hosts have similar HOMO/LUMO energies; however, data from OLEDs fabricated using the above host materials demonstrate that small structural modification of the host results in significant changes in its carrier-transporting characteristics.