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We report a study of photoinduced charge separation in poly(1,4-phenylenevinylene) (PPV) derivatives with enhanced exciton dissociation using light-induced electron paramagnetic resonance (EPR). Four polymers were studied; all contained alkoxy and fluorenyl side chain units, and two of them also contained a nitro group on the 7-positon of the fluorenyl moieties. Pulsed EPR detected a light induced polaron for all the polymers with the intensity significantly higher for those with nitro groups. A second center was observed for these materials and spectra could be fitted assuming a radical anion was localized on the nitro groups. The fraction of radical anion and polaron centers was comparable, consistent with intramolecular exciton dissociation for the PPV derivatives containing the nitro groups. © 2008 Elsevier B.V. All rights reserved.

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Chemistry

(SoNS) School of Natural Sciences

Organic Electronics

Photoinduced charge separation in poly(1,4-phenylenevinylene) derivatives studied by electron paramagnetic resonance

We report a study on the feasibility of using an intramolecular dipole to enhance charge separation for organic photovoltaic applications. We have developed a poly(1,4-phenylenevinylene) derivative, poly{2-[(9,9-di-n-propyl-9H-7-nitrofluoren-2-yl)]-5-(2-ethylhexyloxy)-1,4-phenylenevinylene} 12, which has electronically asymmetric chromophores as a result of the para attachment of alkoxy and nitrofluorenyl groups on the phenyl units of the polymer backbone. The luminescence of the polymer with the electronically asymmetric chromophores was found to be smaller by a factor of eight when compared to the equivalent polymer but without the nitro group attached to the fluorenyl unit. Light-induced electron spin resonance experiments showed that the polymer with the electronically asymmetric chromophores had more photo-induced spins, showing that the quenching of the luminescence was due to intramolecular charge separation. Single layer photovoltaic devices containing neat films of 12 showed efficiencies similar to other single layer devices and this was ascribed to poor transport of the separated charges. This was confirmed by a strong improvement in device power efficiency to 0.52% at AM1.5 by blending the polymer with an electron transporting material. © 2007 Elsevier B.V. All rights reserved.

Electronically asymmetric poly(1,4-phenylenevinylene)s for photovoltaic cells

The reaction of poly(allyl azide) (PAA; number-average molecular weight = 1800 g/mol) with N-phenyl maleimide, 4,4′-bis(maleimidodiphenyl) ether, 4,4′-(bismaleimidodiphenyl) sulfone, and 1,6-bismaleimido hexane is described in this article. As indicated by Fourier transform infrared studies, the azido group reacted with the highly electron-deficient olefinic bond of N-phenyl maleimide or bismaleimides in a 3 + 2 cycloaddition reaction under mild reaction conditions. The curing of PAA with bismaleimides (10-30 phr) was monitored with differential scanning calorimetry. In freshly prepared samples of PAA and bismaleimides, a broad exothermic transition was observed in the temperature range of 150-280°C and could be attributed to the loss of N 2 from PAA and the curing of bismaleimides. The heat of the exothermic reaction (ΔH) depended on the structure and composition and was in the range of 800-1413 J/g. Isothermal heating in an air oven at 40°C (2 days) followed by 60°C (5-6 days) resulted in the conversion of a sticky, viscous liquid into an elastic, nonsticky material. No significant difference in the ΔH values of uncured and cured samples was observed, and this indicated the reaction of fewer azido groups with the bismaleimides (dipolarophiles). In the thermogravimetry traces, a mass loss was indicated in the temperature range of 130-280°C due to the loss of nitrogen from the azido group. © 2006 Wiley Periodicals, Inc.

Journal of Applied Polymer Science

Curing and thermal behavior of poly(allyl azide) and bismaleimides

The synthesis, characterization and physical properties of energetic azido binders were discussed. The energetic composites of glycidyl azide polymer (GAP)/nitroplasticizers and polyethylene glycol with HMX in various particle sizes were studied and compared with hydroxy terminated polybutadiene (HTPB) composites. The particle size of the oxidizer played an important role in determining the performance of the propellants. It was found that the flame structure of the GAP propellants was altered by the nature of the oxidizers.

Journal of Macromolecular Science - Polymer Reviews

Azido Polymers - Energetic Binders for Solid Rocket Propellants

The paper describes the synthesis of low molecular mass poly(allyl chloride) (PAC) (Mn = 856-3834 g mol-1) using Lewis acid (AlCl3, FeCl3, TiCl4) and Al powder. Branching in PAC was indicated on the basis of elemental analysis and 1H-NMR spectroscopy. Azidation of PAC could be carried out at 100°C by using NaN3 and DMSO as solvent. Curing of poly(allyl azide) (PAA) by cyclic dipolar addition reaction with EGDMA (ethylene glycol dimethacrylate, 5-45 phr) was investigated by differential scanning calorimetry and structure of cured polymer was confirmed by FTIR. A two-step mass loss was exhibited by uncured and cured PAA in nitrogen atmosphere. A mass loss of 20-28% (155-274°C) and 50-61% (330-550°C) was observed.

Journal of Thermal Analysis and Calorimetry

Thermal behaviour of poly(allyl azide)

A dendritic polyester based on pentaerythritol and 1,2,4 - benzenetricarboxylic acid anhydride followed by modification with glycidyl methacrylate was synthesised by using a divergent approach. The dendrimer (D-2) thus obtained had approximately eight double bonds. Several blends of D-2 with varying proportions of bismethacryloxy derivative of diglycidyl ether of bisphenol-A having styrene as a reactive diluent (VE resin) were prepared. The Brookfield viscosity of VE resin increased with an increase in D-2 content in the blends. Curing behaviour of these resins in the presence of AIBN was investigated by DSC. No significant change in curing exotherm was observed by addition of D-2 to the VE resin. Isothermal curing of resin samples in the presence of AIBN was done at 60 °C for 5 h and then at 150 °C for 90 mins. A significant improvement in char yield at 600 °C in nitrogen atmosphere was observed in samples containing D-2. Mechanical properties of glass fiber reinforced laminates with 31 ± 3% of resin content (w/w) were also determined. Smoke density of laminates increased with decrease in D-2 content in the resin, whereas only a marginal effect on limiting oxygen index was observed.