Block copolymer strategies for solar cell technology

A simple overview of the methods used and the expected benefits of block copolymers in organic photovoltaic devices is given in this review. The description of the photovoltaic process makes it clear how the detailed self-assembly properties of block copolymers can be exploited. Organic photovoltaic technology, an inexpensive, clean and renewable energy source, is an extremely promising option for replacing fossil fuels. It is expected to deliver printable devices processed on flexible substrates using high-volume techniques. Such devices, however, currently lack the long-term stability and efficiency to allow organic photovoltaics to surpass current technologies. Block copolymers are envisaged to help overcome these obstacles because of their long term structural stability and their solid-state morphology being of the appropriate dimensions to efficiently perform charge collection and transfer to electrodes.

Electrospinning pH-responsive block copolymer nanofibers

Electrospinng of a fibrous triblock copolymer consisting of poly(methyl methacrylate-block-poly[2-(diethylamino) ethyl methacrylate]-block-poly(methyl methacrylate) (PMMA-b-PDEA-b-PMMA) has been discussed. A mixed co-solvent system of tetrahydrofuran (THF) and dimethylformamide (DMF) was used to electrospin fibrous PMMA-b-PDEA-b-PMMA and its influence on surface morphology and diameter of the electrospun fiber was also investigated in an attempt to control the fiber diameter. The concentration range between 20 and 40 wt % was found suitable for electrospinning of PMMA-b-PDEA-b-PMMA in a THF/DMF system. It was also observed that the average fiber diameter decreased as the content of DMF was increased. A significant decrease in fiber diameter was observed when moving from a THF solution to a THF/DMF system at a ratio of 70:30.

Block co-polymerization by transformation reactions

The aim of this study was to use the transformation of anionic to metathesis polymerization to produce block co-polymers of styrene-b-pentenylene using WC16 /PStLi and WC16/PStLi/ AlEtC12 catalyst systems. Analysis of the products using SEC and 1H and 13C NMR spectroscopy enabled mechanisms for metathesis initiation reactions to be proposed. The initial work involved preparation of the constituent homo-polymers. Solutions of polystyryllithium in cyclohexane were prepared and diluted so that the [PStLi]o<2x10-3M. The dilution produced initial rapid decay of the active species, followed by slower spontaneous decay within a period of days. This was investigated using UV / visible spectrophotometry and the wavelength of maximum absorbance of the PStLi was found to change with the decay from an initial value of 328mn. to λmax of approximately 340nm. after 4-7 days. SEC analysis of solutions of polystyrene, using RI and UV / visible (set at 254nm.) detectors; showed the UV:RI peak area was constant for a range of polystyrene samples of different moleculor weight. Samples of polypentenylene were prepared and analysed using SEC. Unexpectedly the solutions showed an absorbance at 254nm. which had to be considered when this technique was used subsequently to analyse polymer samples to determine their styrene/ pentenylene co-polymer composition. Cyclohexane was found to be a poor solvent for these ring-opening metathesis polymerizations of cyclopentene. Attempts to produce styrene-b-pentenylene block co-polymers, using a range of co-catalyst systems, were generally unsuccessful as the products were shown to be mainly homopolymers. The character of the polymers did suggest that several catalytic species are present in these systems and mechanisms have been suggested for the formation of initiating carbenes. Evidence of some low molecular weight product with co-polymer character has been obtained. Further investigation indicated that this is most likely to be ABA block copolymer, which led to a mechanism being proposed for the termination of the polymerization.

Synthesis of polymers and copolymers of lactide through titanium initators

The research described in this thesis explored the synthesis tlnd characteristltion of biocompatible and biodegradable polymers of lactide through non-toxic titanium alkoxide nitiators. The research objectives focused on the preparation of polylactides in both solvent and solventless media, to produce materials with a wide range of molecular weights. The polylactides were fully characterised using gel permeation chromatography and 1H and 13C NMR spectroscopy. NMR spectroscopy was carried out in the study the reaction mechanisms. Kinetic studies of the ring opening polymerisation of lactide with titanium alkoxide initiators were also conducted using NMR spectroscopy. The objectives of this research were also focused on the enhancement of the flexibility of the polymer chains by synthesising random and block copolymers of lactide and ε-caprolactone using Ti(0-i-Pr)4 as an initiator, This work involved extensive characterisalion of the synthesised copolymers using gel permeation chromatography and 1H and 13C NMR spectroscopic analysis. Kinetic studies of the ring opening polymerisation of ε-caplrolactone and of the copolymerisation of lactide and ε-caprolactone with Ti(O-i-Pr)4 as an initiator were also carried out. The last section of this work involved the synthesis of block and star-shaped copolymers of lactide and poly(ethylene glycol) [PEG]. The preparation of lactide/PEG block copolymers was carried out by ring opening polymerisation of L-Iactide using Ti(O-i-Pr)4 as an initiator and hydroxyl-terminated PEG's with different numbers of hydroxyl groups as co-initiators both in solution and solventless media. These all-in-one polymersations yielded the synthesis of both lactide homopolymer and lactide/PEG block copolymer. In order to selectively synthesise copolymers of lactide and PEG, the experiment was carried out in two steps. The first step consisted of the synthesis of a titanium macro-initiator by exchanging the iso-propoxide ligands by PEG with different numbers of hydroxyl groups. The second step involved the ring opening polymerisation of lactide using the titanium macrocatalyst that was prepared as an initiator. The polymerisations were carried out in a solventless media. The synthesis of lactide/PEG copolymers using polyethylene glycol with amino terminal groups was also discussed. Extensive characterisation of the lactide block copolymers and macroinitiators was carried out using techniques such as, gel permeation chromatography (GPC), NMR spectroscopy and differential scanning calorimeter (DeS).

Reciprocating power generation in a chemically driven synthetic muscle

A scalable synthetic muscle has been constructed that transducts nanoscale molecular shape changes into macroscopic motion. The working material, which deforms affinely in response to a pH stimulus, is a self-assembled block copolymer comprising nanoscopic hydrophobic domains in a weak polyacid matrix. A device has been assembled where the muscle does work on a cantilever and the force generated has been measured. When coupled to a chemical oscillator this provides a free running chemical motor that generates a peak power of 20 mW kg 1 by the serial addition of 10 nm shape changes that scales over 5 orders of magnitude. It is the nanostructured nature of the gel that gives rise to the affine deformation and results in a robust working material for the construction of scalable muscle devices.

Self-assembly-driven electrospinning:the transition from fibers to intact beaded morphologies

Polymer beads have attracted considerable interest for use in catalysis, drug delivery, and photo­nics due to their particular shape and surface morphology. Electrospinning, typically used for producing nanofibers, can also be used to fabricate polymer beads if the solution has a sufficiently low concentration. In this work, a novel approach for producing more uniform, intact beads is presented by electrospinning self-assembled block copolymer (BCP) solutions. This approach allows a relatively high polymer concentration to be used, yet with a low degree of entanglement between polymer chains due to microphase separation of the BCP in a selective solvent system. Herein, to demonstrate the technology, a well-studied polystyrene-poly(ethylene butylene)–polystyrene triblock copolymer is dissolved in a co-solvent system. The effect of solvent composition on the characteristics of the fibers and beads is intensively studied, and the mechanism of this fiber-to-bead is found to be dependent on microphase separation of the BCP.

Fullerene-capped copolymers for bulk heterojunctions: device stability and efficiency improvements

A fullerene end-capped polymer-compatibilizer based on poly(3-hexylthiophene) (P3HT) was synthesized and demonstrated to have a remarkable effect on both the stability and efficiency of devices made from exemplar P3HT and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). P3HT with ethynyl chain-ends and α-azido-ω-bromo-PS were prepared via Grignard metathesis (GRIM) and atom transfer radical polymerisation, respectively. “Click” chemistry resulted in the preparation of poly(3-hexylthiophene)-block-ω-bromo-polystyrene (P3HT-b-PS-Br), and subsequent atom transfer radical addition chemistry with fullerene (C60) yielded the donor–acceptor block copolymer P3HT-b-PS-C60. Both P3HT-b-PS-Br and P3HT-b-PS-C60 were considered as compatibilizers with P3HT/PCBM blends, with the study detailing effects on active-layer morphology, device efficiency and stability. When used at low concentrations, both P3HT-b-PS-Br (1%) and P3HT-b-PS-C60 (0.5%) resulted in considerable 28% and 35% increases in efficiencies with respect to devices made from P3HT/PCBM alone. Furthermore, P3HT-b-PS-C60 (0.5%) resulted in an important improvement in device stability.

A hydrothermally stable transition alumina by condensation-enhanced self-assembly and pyrolysis crystallization:application in the steam reforming of methane

The preparation of a steam-based hydrothermally stable transition alumina is reported. The gel was derived from a synthetic sol-gel route where Al-tri-sec-butoxide is hydrolysed in the presence of a non-ionic surfactant (EO20PO70EO20), HCl as the catalyst and water (H2O/Al = 6); the condensation was enhanced by treating the hydrolysed gel with tetrabutylammonium hydroxide (TBAOH), after which it was dried at 60 °C by solvent evaporation. The so-obtained mesophase was crystallized under argon at 1200 °C (1 h) producing a transition alumina containing δ/α, and possibly θ, alumina phases. Due to its surface acidity, the pyrolysis conditions transform the block copolymer into a cross-linked char structure that embeds the alumina crystallites. Calcination at 650 °C generates a fully porous material by burning the char; a residual carbon of 0.2 wt.% was found, attributed to the formation of surface (oxy)carbides. As a result, this route produces a transition alumina formed by nanoparticles of about 30 nm in size on average, having surface areas in the range of 59-76 m2 g-1 with well-defined mesopores centered at 14 nm. The material withstands steam at 900 °C with a relative surface area rate loss lower than those reported for δ-aluminas, the state-of-the-art MSU-X γ-alumina and other pure γ-aluminas. The hydrothermal stability was confirmed under relevant CH4 steam reforming conditions after adding Ni; a much lower surface area decay and higher CH4 conversion compared to a state-of-the-art MSU-X based Ni catalyst were observed. Two effects are important in explaining the properties of such an alumina: the char protects the particles against sintering, however, the dominant effect is provided by the TBAOH treatment that makes the mesophase more resistant to coarsening and sintering. This journal is © the Partner Organisations 2014.

Thermal detemplation of SBA-15 mesophases. Effect of the activation protocol on the framework contraction

The potential use of the solvothermal extraction (SE) as a preliminary step to calcination for detemplating SBA-15 mesophases is investigated; aiming to reduce the amount of organics to be burnt and thereby the corresponding structural shrinkage. A systematic study was carried out by soxhlet extraction on mesophases hydrothermally aged between 90 and 130 C. The mesophases containing variable amounts of template were then treated by calcination or pyrolysis/calcination. TGA was applied to quantify the template amount after the various treatments. The as obtained materials were characterized by SAXS and Ar ad/desorption for structural and textural information while 1H NMR gave information about the integrity of the as-recycled template. The study shows that solvothermal conditions remove considerably the template, typically from 50 to 10-20 wt.%, mainly extracted from the primary mesopores. Possible reuse of the extracted template is questionable as it is poor in polyethyleneoxide compared to the synthesis block-copolymer, Pluronic P123. For all thermal protocols applied (direct calcination, calcination after solvent-extraction or pyrolysis/calcination after solvent extraction), the thermal shrinkage decreases with the aging temperature; that is consistent with the condensation degree of the silica. For each mesophase, it was found that the thermal shrinkage becomes less pronounced when the material is fully templated; thus the template can serve as structural support or can control the mass transfer of O2 and thereby the oxidation rate of the template burning. © 2013 Elsevier Inc. All rights reserved.

Synthesis and characterisation of vinyl block copolymers

A study has been made of the anionic polymerisation of methyl methacrylate using butyllithium and polystyryl lithium as initiators and the effects of lithium chloride and aluminium alkyls on the molecular weight and molecular weight distributions. Diblock copolymers of styrene-b-methyl methacrylate were synthesised at -78oC in THF in the presence of lithium chloride, and at ambient temperatures in toluene in the presence of aluminium alkyls. Studies in the presence of lithium chloride showed that the polymerisation was difficult to control; there was no conclusive evidence of a living system and the polydispersity indices were between 1.5 and 3. However, using relatively apolar solvents, in the presence of aluminium alkyls, homopolymerisation of methyl methacrylate showed characteristics of a living polymerisation. An investigation of the effects of the structures of the lithium and aluminium alkyls on the efficiency of initiation showed that a t-butyllithium/triisobutylaluminium initiating system exhibited an efficiency of 80%, compared with lower efficiencies (typically 30%) for systems based on butyllithium/triethylaluminium.The polydispersity index was found to decrease from ∼2.2 to ∼1.5 when butyllithium was replaced by t-butyllithium. The efficiency of the initiator was found to be solely dependent on the size of the alkyl group of the aluminium component, whereas the polydispersity index was found to be solely dependent on the size of the alkyl group on the lithium component. The aluminium alkyl is thought to be co-ordinated to the ester carbonyl groups of both the monomer and polymer. There is a critical degree of polymerisation, at which point the rate of polymerisation decreases, which probably relates to a change in structure of the active chain end. Characterisation of poly(styrene )-b-poly(4-vinylpyridine) and poly(styrene)-b-poly(4-vinylpyridine methyl iodide) diblock copolymers using static light scattering techniques, showed the formation of star-shaped 'reverse' micelles when placed in toluene. Temperature effects on micellization behaviour are only exhibited for the unquaternised micelles, which showed characterisically lower aggregation numbers than their quaternised counterparts. A suitable solvent was not obtained for characterisation of the styrene-b-methyl methacrylate diblock copolymers synthesized.

Synthesis, thermal processing, and thin film morphology of poly(3-hexylthiophene)-poly(styrenesulfonate) block copolymers

A series of novel block copolymers, processable from single organic solvents and subsequently rendered amphiphilic by thermolysis, have been synthesized using Grignard metathesis (GRIM) and reversible addition-fragmentation chain transfer (RAFT) polymerizations and azide-alkyne click chemistry. This chemistry is simple and allows the fabrication of well-defined block copolymers with controllable block lengths. The block copolymers, designed for use as interfacial adhesive layers in organic photovoltaics to enhance contact between the photoactive and hole transport layers, comprise printable poly(3-hexylthiophene)-block-poly(neopentyl p-styrenesulfonate), P3HT-b-PNSS. Subsequently, they are converted to P3HT-b-poly(p-styrenesulfonate), P3HT-b-PSS, following deposition and thermal treatment at 150 °C. Grazing incidence small- and wide-angle X-ray scattering (GISAXS/GIWAXS) revealed that thin films of the amphiphilic block copolymers comprise lamellar nanodomains of P3HT crystallites that can be pushed further apart by increasing the PSS block lengths. The approach of using a thermally modifiable block allows deposition of this copolymer from a single organic solvent and subsequent conversion to an amphiphilic layer by nonchemical means, particularly attractive to large scale roll-to-roll industrial printing processes.

Autonomous volume transitions of a polybase triblock copolymer gel in a chemically driven pH-oscillator

A pH-responsive ABA triblock copolymer, comprising poly(methyl methacrylate)-b/ock-poly(2-(diethylamino)ethyl methacrylate)-block-poly(methyl methacrylate) [PMMA-b-PDEA-b-PMMA], has been cast Into thin films with a well-defined microstructure. Small Angle X-ray Scattering (SAXS) and Atomic Force Microscopy (AFM) studies confirm that this copolymer forms a hydrogel consisting of PMMA spheres embedded within a polybase PDEA matrix, with the PMMA domains acting as physical cross-links. The hydrogel has a pH-reversible coil-globule transition at around pH 4.5. This responsive physical property was exploited by immersing a sample of copolymer hydrogel in an aqueous solution containing a cyclic pH-oscillating reaction, whereby the pH was continuously oscillated above and below the transition pH so as to induce autonomous volume transitions. The changes in microscopic and macroscopic length scales correlate closely during (de)swelling cycles, with affine behaviour occurring over five orders of magnitude. Copyright © 2007 WILEY-VCH Verlag GmbH & Co. KGaA,.