Joke Vandenbergh

Synthesis of poly(p-phenylene vinylene) materials via the precursor routes

Poly(p-phenylene vinylene)s (PPVs) are an important class of conjugated polymer materials that have in the last few years gained significant interest in the polymer community. (Precursor) Synthesis routes to obtain high-molecular weight PPVs are reviewed with respect to the applicability of the reactions towards specific synthesis goals and materials as well as structural integrity of the obtained polymers, which affect the applicability of these compounds in electronic devices.

Use of a continuous-flow microreactor for thiol–ene functionalization of RAFT-derived poly(butyl acrylate)

This study describes the synthesis of functionalized RAFT-derived poly(n-butyl acrylate) polymers via the use of a continuous-flow microreactor, in which aminolysis as well as thiol–ene reactions are executed in reaction times of just 20 minutes. Poly(n-butyl acrylate) (Mn = 3800 g mol−1, PDI = 1.10) with a trithiocarbonate end group was prepared via a conventional RAFT process. The polymer was then functionalized via aminolysis/thiol–ene reactions in the micro-flow reactor with isobornyl acrylate, propargyl acrylate, poly(ethylene glycol) methyl ether acrylate and pentaerythritol tetraacrylate. To optimize the reaction time and reaction temperature of the micro-flow reactor, freshly collected samples were studied with soft ionization mass spectrometry. With this technique, efficient and very fast aminolysis and subsequent thiol–ene reactions take place on the RAFT-precursor polymer, yielding quantitative end group conversion within 20 min and functionalized polymers of 3700–4000 g mol−1, depending on the type of acrylate coupled. The use of a continuous-flow microreactor opens the pathway towards upsizing lab scale methods into larger processes without suffering from problems associated with reproducibility and tedious optimization issues.

Improved Livingness and Control over Branching in RAFT Polymerization of Acrylates: Could Microflow Synthesis Make the Difference?

The superior capabilities of structured microreactors over batch reactors are demonstrated for reversible addition-fragmentation chain transfer (RAFT) solution polymerization of n-butyl acrylate with the aid of simulations, explicitly accounting for the chain length distribution of all macrospecies types. Since perfect isothermicity can be established in a microreactor, less side products due to backbiting and β-scission are formed compared to the batch operation in which ineffective heat removal leads to an undesirable temperature spike. For a given RAFT chain transfer agent (CTA), additional microstructural control results under microflow conditions by optimizing the reaction temperature, lowering the dilution degree, or decreasing the initial molar ratio of monomer to RAFT CTA.

Efficiency assessment of single unit monomer insertion reactions for monomer sequence control: kinetic simulations and experimental observations

The reaction efficiency of single unit monomer insertion (SUMI) reactions via the reversible addition fragmentation chain transfer (RAFT) method is investigated in detail by the determination of obtained product yields of optimized batch and microflow synthesis procedures in combination with kinetic simulations of the radical insertion process. A method is developed to obtain exact concentration information on different SUMI products from calibration of the corresponding electrospray ionization mass spectra that are recorded on-line during synthesis. Experimental data show that isolated yields decrease for each subsequent SUMI reaction. This effect is investigated via kinetic modelling to understand which parameters have a beneficial or negative influence on the reaction outcome. Although most reaction conditions (such as monomer concentration or radical flux) do not play a considerable role in the obtainable yield of the insertion reaction, the model clearly shows that the propagation rate coefficient must display a strong chain-length dependency in order to explain the experimental observations. When taken into account, the simulations very well fit the experimental data obtained from optimized microreactor flow synthesis and recommendations for SUMI reactions are formulated. Finally, the optimized SUMI conditions obtained from microreactor experiments and kinetic modelling insights have been applied to upscale the SUMI synthesis reactions in a mesoflow reactor. This demonstrates the simple upscalability of continuous flow reactions and opens the pathway towards future synthesis of longer sequence controlled oligomers.

Hysteresis-free electron currents in poly(p-phenylene vinylene) derivatives

The interpretation of electron currents in conjugated polymers is strongly hindered by the occurrence of hysteresis. We investigate the transport of electrons in electron-only devices based on derivatives of poly(p-phenylene vinylene) (PPV) for various hole-blocking bottom electrodes as well as purification of the polymer. The use of a variety of hole blocking bottom contacts, as metallic electrodes and n-type doped polymers, did not give any improvement in the observed hysteresis. By purification of the PPV, hysteresis free electron-only currents can be obtained. The deep traps responsible for hysteresis, with a concentration in the 10 16 cm -3 range, are not responsible for the trap-limited electron transport as observed in purified PPV.

Phase behavior of PCBM blends with different conjugated polymers

In this work the phase behavior of [6,6]-phenyl C(61)-butyric acid methyl ester (PCBM) blends with different poly(phenylene vinylene) (PPV) samples is investigated by means of standard and modulated temperature differential scanning calorimetry (DSC and MTDSC) and rapid heat-cool calorimetry (RHC). The PPV conjugated polymers include poly(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene) (MDMO-PPV), High T(g)-PPV which is a copolymer, and poly((2-methoxy-5-phenethoxy)-1,4-phenylene vinylene) (MPE-PPV). Comparisons of these PPV:PCBM blends with regioregular poly(3-hexyl thiophene) (P3HT):PCBM blends are made to see the different component miscibilities among different blends. The occurrence of liquid-liquid phase separation in the molten state of MDMO-PPV:PCBM and High T(g)-PPV:PCBM blends is indicated by the coexistence of double glass transitions for blends with a PCBM weight fraction of around 80 wt%. This is in contrast to the P3HT:PCBM blends where no phase separation is observed. Due to its high cooling rate (about 2000 K min(-1)), RHC proves to be a useful tool to investigate the phase separation in PPV:PCBM blends through the glass transition of these crystallizable blends. P3HT is found to have much higher thermal stability than the PPV samples.

Synthesis of well-defined PPV containing block polymers with precise endgroup control by a dual-initiator strategy

Poly(2,5-bis(2-ethylhexyloxy)p-phenylene vinylene)-b-poly(tert-butyl acrylate) (BEH-PPV-b-P(t-BuA)) block copolymers with various block compositions have been synthesized by a dual initiator strategy. PPV polymerizations are performed via the anionic polymerization mode of the so-called sulfinyl precursor synthesis route with a dedicated initiator carrying a moiety that is able to reinitiate polymer chains under the conditions of a single electron transfer living radical polymerization (SET-LRP). In order to prove that such a route can be taken, a detailed study on the initiator efficiency in the anionic polymerization based on post-mortem analysis of materials with ESI-MS is presented. Low molecular weight PPVs in the range of 2000 Da are synthesized by means of high initiator to monomer concentrations with the use of two differently functionalized anionic initiators. Almost quantitative incorporation of the initiator in the alpha position of the chains is confirmed. Block copolymers are subsequently obtained from a BEH-PPV building block with an apparent Mn of 5300 g mol−1. Block copolymers with apparent Mn between 6800 and 25u2006000 g mol−1 are synthesized. Hydrolysis of the acrylate ester block yields amphiphilic block copolymers with a poly(acrylic acid) block, which can be self-assembled in methanolic solution to form micelles with a mean diameter of 81 nm. The micelles respond to changes in pH and ionic strength, leading to significant expansion of the micelles in both cases.

Combustion deposition of MoO3 films: from fundamentals to OPV applications

A systematic study of a combustion precursors thermal decomposition pathway is undertaken, in both powders and thin films, to obtain insights in the various parameters influencing the combustion process. The study focuses on MoO3 as a hole transporting layer (HTL) for applications in organic photovoltaics (OPV). Via evolved gas analysis, it was found that fuel volatility occurs prior to the actual combustion reaction of the acetylacetonate based precursor, affecting the optimal oxidizer to fuel ratio. Moreover, close investigation showed that the high rate combustion reaction disappears with increasing surface to volume ratio. Nonetheless, thermal analysis performed on films suggests that with the right heating rate, an oxidative complete decomposition still occurs in films, exemplified by the film composition and specific morphological differences in the resulting layers and through analysis of the evolved components. Finally, the discussed synthesis route allows organic free, crystalline MoO3 films to be obtained affording organic solar cell devices with promising current density–voltage characteristics.

Controlled synthesis of MDMO-PPV and block copolymers made thereof

Poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene-vinylene] (MDMO-PPV) with various average molecular weights was directly synthesized via the sulfinyl precursor route by employing excess amounts of CBr4 as control agent. Bromine functional materials were obtained in the range of a few thousand to close to 80u2006000 g mol−1. Also, block copolymers have become available via reactivation of the chains in ATRP.

Optical detection of deep electron traps in poly(p-phenylene vinylene) light-emitting diodes

The trap-limited electron currents in poly(p-phenylene vinylene) (PPV) derivatives can be modeled using a Gaussian trap distribution that is positioned approximately 0.75u2009eV below the lowest unoccupied molecular orbital (LUMO) of PPV. Photothermal deflection spectroscopy measurements and internal photo-emission spectroscopy measurements confirm the claim of a Gaussian shaped trap distribution centered at 0.75u2009eV below the LUMO of PPV. Additionally, two PPV derivatives that differ in the number of conformational defects incorporated during synthesis exhibit identical electron trapping behavior, showing that the traps do not originate from extrinsic impurities of the synthesis or defects in the polymer chains.