Non-conjugated Blocks Research Articles

(Third Place Poster Award) Synthesis and Characterization of Block Copolymer Dispersants for Semiconducting Single-Walled Carbon Nanotubes

To leverage the full potential of the outstanding electronic properties of semiconducting single-walled carbon nanotubes (s-CNTs), it is essential to disperse and sort high purity s-CNTs from a heterogeneous mixture. Due to the inherent insolubility of CNTs, various organic dispersing agents that non-covalently interact with CNTs have been successfully used. These dispersants include small molecule surfactants and aromatic compounds, conjugated polymers, DNA, and polyelectrolytes. Of the macromolecular dispersants, polymers with conjugated backbones are widely studied because they can wrap around or strongly adsorb on CNTs to create very stable dispersions in organic solvents. Most of these conjugated polymers are based on polythiophenes or polyfluorenes, which depending on the structure and composition can sort specific chiralities of s-CNTs with over 99.99% semiconducting purity.In this work, we explore conjugated polymer-based block copolymers for sorting and dispersing s-CNTs. Non-conjugated block copolymer-based dispersants have been reported by us and others with varying degrees of success in aqueous and organic solvents. Here we present our recent studies on the synthesis of ABA triblock copolymer-based CNT dispersants where B is the conjugated block and A is the non-conjugated block. The synthesis of the ABA triblock can be achieved by separately synthesizing the rod segments and the coil segments, each with reactive chain end functionalities, and then coupling the rod segments with the coil segments via reaction of the chain ends using, for example, a click reaction. Alternatively, the rod segment or the coil segment can be synthesized first and used as a macroinitiator for the polymerization of the other segment. Each of these two methods offers some advantages and disadvantages. A series of these ABA triblocks with fixed B block length and varying A block lengths were synthesized and evaluated for their ability to sort and disperse ArcD and HiPCO CNTs. A second series of ABA triblocks with fixed A block lengths but increasing B block lengths were also synthesized. We will present the challenges associated with the synthesis and purification of these block copolymer dispersants. As the middle-conjugated B block wraps around the CNTs, we anticipate the two peripheral A blocks to interact with the organic solvent toluene to further solubilize the CNTs. Our studies show that the presence of the A blocks significantly increases the sorting efficiency while preserving the chirality selectiveness of the B block. As the length of the B block increases, the sorting yields can be significantly improved as well. We further discuss the role of the dispersity of the B block if any on the dispersion of s-CNTs.

Rapid formation and real-time observation of micron-sized conjugated nanofibers with tunable lengths and widths in 20 minutes by living crystallization-driven self-assembly.

Preparing well-defined semiconducting nanostructures from conjugated polymers is of paramount interest for organic optoelectronic devices. Several studies have demonstrated excellent structural and size control from block copolymers (BCPs) containing non-conjugated blocks via crystallization-driven self-assembly (CDSA); however, the precise control of their size and shape remains a challenge due to their poor solubility, causing rapid and uncontrolled aggregation. This study presents a new type of fully conjugated BCP comprising two polyacetylene derivatives termed poly(cyclopentenylene-vinylene) to prepare semiconducting 1D nanofibers. Interestingly, the widths of nanofibers were tuned from 12 to 32 nm based on the contour lengths of their crystalline core blocks. Their lengths could also be controlled from 48 nm to 4.7 μm using the living CDSA. Monitoring of the growth kinetics of the living CDSA revealed the formation of micron-sized 1D nanofibers in less than 20 min. The rapid CDSA enabled us to watch real-time growth using confocal fluorescence microscopy.

Polystyrene-block-Poly(ionic liquid) Copolymers as Work Function Modifiers in Inverted Organic Photovoltaic Cells.

Interfacial layers play a critical role in building up the Ohmic contact between electrodes and functional layers in organic photovoltaic (OPV) solar cells. These layers are based on either inorganic oxides (ZnO and TiO2) or water-soluble organic polymers such as poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)] and polyethylenimine ethoxylated (PEIE). In this work, we have developed a series of novel poly(ionic liquid) nonconjugated block copolymers for improving the performance of inverted OPV cells by using them as work function modifiers of the indium tin oxide (ITO) cathode. Four nonconjugated polyelectrolytes (n-CPEs) based on polystyrene and imidazolium poly(ionic liquid) (PSImCl) were synthesized by reversible addition-fragmentation chain transfer polymerization. The ratio of hydrophobic/hydrophilic block copolymers was varied depending on the ratio of polystyrene to the PSImCl block. The ionic density, which controls the work function of the electrode by forming an interfacial dipole between the electrode and the block copolymers, was easily tuned by simply changing the PSImCl molar ratio. The inverted OPV device with the ITO/PS29-b-PSImCl60 cathode achieved the best power conversion efficiency (PCE) of 7.55% among the synthesized block copolymers, exhibiting an even higher PCE than that of the reference OPV device with PEIE (7.30%). Furthermore, the surface properties of theblockcopolymers films were investigated by contact angle measurements to explore the influence of the controlled hydrophobic/hydrophilic characters on the device performances.

Morphology Control of Selenophene–Thiophene Block Copolymers through Side Chain Engineering

We report the synthesis of three fully π-conjugated diblock copolymers containing selenophene- and thiophene-based repeating units. All of these diblock copolymers undergo phase separation, and by systematically changing the compatibility of the two blocks through side chain modification, we are able to access different thin film morphologies. Introducing a bulky 2-ethylhexyl side chain increases solubility while retaining crystallinity of the selenophene block. While poly(3-hexylselenophene)-b-poly(3-hexylthiophene) and poly(3-(2-ethylhexyl)selenophene-b-poly(3-(2-ethylhexyl)thiophene) form more disordered fibrillar structures, poly(3-hexylthiophene)-b-poly(3-(2-ethylhexyl)selenophene) forms long (1–2 μm) solid state fibrillar structures that are reminiscent of the lamellae that are formed by nonconjugated block copolymers. We further use electron energy loss spectroscopy to visualize thiophene- and selenophene-rich domains at the nanometer scale in each of these examples. By studying new polymer compositions and relating them to solid state structure, we further our understanding of heterocycle induced phase separation and phase separation in general.

Folate-conjugated amphiphilic block copolymers for targeted and efficient delivery of doxorubicin

In this paper, novel biodegradable amphiphilic block copolymers based on folate-conjugated poly(ethylene glycol)-b-copolycarbonates (FA-PEG-b-P(MAC-co-DTC)) and methoxy poly(ethylene glycol)-b-copolycarbonates (mPEG-b-P(MAC-co-DTC)) were successfully synthesized for targeted and efficient delivery of doxorubicin (DOX) to cancer cells. Immobilized porcine pancreas lipase (IPPL) was employed as the catalyst to perform the ring-opening copolymerization in bulk, while the folate-conjugated poly(ethylene glycol) (FA-PEG) or methoxy poly(ethylene glycol) (mPEG) was used as the initiator. The resulting copolymers, characterized by 1H NMR and GPC, could self-assemble to form nano-sized micelles in aqueous solution by dialysis method. P(MAC-co-DTC) acted as the hydrophobic core, thereby aggregating hydrophilic PEG chains as the outer shell with FA as targeting ligand located at the surface of the polymeric micelles. Transmission electron microscopy (TEM) observation showed that the micelles dispersed in spherical shape with nano-size before and after DOX loading. Both the FA-conjugated and non-conjugated block copolymers showed low cellular cytotoxicity. Furthermore, as compared to the non-conjugated copolymers, much more efficient cellular uptake of the FA-conjugated copolymers via FA-receptor-mediated endocytosis could be observed by confocal laser scanning microscopy (CLSM), while MTT assays also demonstrated highly potent cytotoxic activity against HeLa cells.

Blue-Emitting Self-Assembled Polymer Electrolytes for Fast, Sensitive, Label-Free Detection of Cu(II) Ions in Aqueous Media

We have developed a light-emitting material based on nonconjugated block copolymers that contain polystyrene sulfonate (PSS) chains. The confinement of the PSS chains within nanosized domains appeared to be a powerful means of achieving enhanced fluorescence signals. High fluorescence quantum yield, with a maximum value of 37%, was obtained by adjusting the types of self-assembled morphologies of PSS-containing block copolymers; in contrast, the fluorescence quantum yield was merely 5% for the PSS homopolymer lacking organization. The wavelength of fluorescence emission was tunable by rational molecular design. In addition, significant self-quenching behavior was not noticed in diverse forms of this material such as solutions, thin films, and free-standing membranes. Notably, the light-emitting self-assembled block copolymer electrolytes exhibited high sensitivity toward Cu(2+) ions, with a detection limit of parts per billion levels, rapid response time of ≤1 min, and insignificant interference of other competitive metal ions.

Supramolecular assembly of conjugated polymers: From molecular engineering to solid-state properties

Abstract π-Conjugated polymers are central ingredients in the development of organic electronics and it is well-known that the degree of order in thin films of those materials is a major factor governing the performances of the devices (field effect transistors, light-emitting diodes, photovoltaic cells). In this review, we present some approaches for the control of the molecular assembly of conjugated polymers into well-defined nanostructures. We consider self-assembly leading to thin deposits from compounds molecularly dispersed in a solution and we focus on the relationship between the supramolecular interactions between the individual π-conjugated molecules/polymers and the microscopic morphology appearing upon aggregation. We describe approaches to control the self-assembly of conjugated systems in the solid-state via (i) the interactions with the substrate surface; (ii) the design of a well-defined molecular structure; and (iii) the use of block-copolymers comprising conjugated and non-conjugated blocks. To understand how supramolecular interactions influence the microscopic morphology, we developed a joint experimental–theoretical approach, combining scanning probe microscopy characterization of thin deposits and force-field molecular modeling of supramolecular aggregates and adsorbates. With this methodology, a strong correlation is established between the polymer molecular structure, the degree of supramolecular order, and the solid-state photoluminescence properties (for light-emitting applications), or the charge transport properties (for the design of field-effect transistors). Since the construction of small-size devices requires that the spatial growth of the thin deposits be perfectly controlled, we illustrate in the last section of the paper how soft lithographic methods and nanorubbing can be used to orient the formation of the self-assembled conjugated nanostructures in a confined space.

Synthesis and characterization of new blue‐photoluminescent copolymer derived from bisphenol A

AbstractA new copolymer with short alternating conjugated and nonconjugated blocks, derived from bisphenol A (BPAEt2‐PPV) containing separated phenylenevinylene (PV) units has been synthesized by Wittig condensation. The copolymer is fully soluble in common organic solvents and has a number‐average molecular weight of 3200 with a polydispersity index of 1.53. The structure of the polymer was confirmed by 1H NMR, 13C NMR, FTIR and Raman spectroscopic analysis. Thermogravimetric analysis and differential scanning calorimetry indicate that BPAEt2‐PPV is stable up to 400°C in air and displays a glass transition temperature of 107°C. The fluorescence spectrum of the polymer film shows an emission in the blue region (428, 456, and 488 nm). The band‐gap energy, estimated from the edge absorption of the film, is about 2.9 eV. A single‐layer diode device of the configuration ITO/BPAEt2‐PPV/Al has been fabricated and has a relatively low turn‐on voltage of 3.7 V. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci, 2006

Color tuning and efficiency enhancement through interfacial energy transfer and thickness control in light-emitting diodes

We report a possible interfacial energy transfer phenomenon in double-layer light-emitting diodes (LEDs) consisting of a poly(p-phenylene vinylene) (PPV) hole-transport layer and an emitting polymer layer containing either of two blue emitting polymers (I and II). The polymers in film form have almost the same photoluminescence (PL) spectra though they have different band gaps as determined from their absorption and PL spectra. This is due to intermolecular∕intramolecular energy transfer in polymer II films since the latter structure contains two differing oligomeric chromophores regularly interspersed by long nonconjugated blocks. In double-layer LEDs, PPV functions not only as a hole-transport layer which improves hole injection but also as a chromophore participating in the tuning of the LED emission color through interfacial energy transfer. The significant spectral overlap between the absorption spectrum of PPV and the emission spectra of I or II supports this energy transfer. The presence of PPV can modify the LED color from blue to blue-green or green and also contributes to a high external quantum efficiency. Poly(3,4-ethylenedioxythiophene)∕poly(styrene sulfonate) was also used as a hole-transport layer and its LEDs were compared to PPV-containing devices. Thicker films of the conjugated polymer chromophores produced red shifts in both PL and electroluminescence (EL) spectra which was attributed to molecular packing and self-absorption effects. At higher applied voltage, these LEDs showed blue shifts, and the emission corresponding to a higher energy transition was enhanced at the expense of emissions from the lower energy transitions. Chromophore heating is also held responsible for some of the blueshifts in EL. The thicker LEDs showed lower efficiency because of self-absorption.

Synthesis and properties of polyether with short alternating conjugated and nonconjugated blocks containing oxadiazole units

AbstractA polyether, poly[(2,5‐dimethylene‐1,3,4‐oxadiazole)dioxy‐1,4‐phenylene‐1,2‐ethenylene‐1,4‐phenylene‐1,2‐ethenylene‐1,4‐phenylene], based on short alternating conjugated oxadiazole units, has been synthesized, which is a kind of PPV derivative that emits blue light. The resulting polymer is fairly soluble in chloroform. The synthesized polymer shows a UV–visible absorbency maximum wavelength around 310 nm in solution. The photoluminescence maximum wavelength for the resulting polymer appears around 470 nm. The polymer also exhibits good thermal stability up to 300°C under N2 atmosphere. It is also observed that the onset temperature of thermal decomposition is as high as 355°C. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2682–2686, 2002

Structure−Property Relationships in Light-Emitting Polymers: Optical, Electrochemical, and Thermal Studies

A series of new PPV-based alternating copolymers containing conjugated and nonconjugated blocks have been synthesized with the objective of raising the glass transition temperature (Tg) relative to...

Synthesis and characterization of a cyano-substituted electroluminescent polymer with well-defined conjugation length

A novel copolymer, poly[1,6–hexanedioxy–1,4–phenylene–1,2–ethenylene–(2,5–dicyano–1,4–phenylene)–1,2–ethenylene–1,4–phenylene] (HCN-PPV), with alternating conjugated and non-conjugated blocks is synthesized by the Wittig reaction. The conjugated block containing one unit of 2,5-dicyano-1,4-phenylene vinylene is linked to a flexible linear hexyl chain spacer by an ether group in the backbone. The polymer is soluble in common organic solvents such as CHCl 3 and CH 2Cl 2. The number average molecular weight ( M n) is 2517 with a polydispersity index of 1.75. The optical bandgap of HCN-PPV is 2.48 eV. λ max of the PL spectrum of the polymer film sample lies at 533 nm, corresponding to green light emission. The polymer also shows good thermal stability with a decomposition temperature of 304°C under air atmosphere. Cyclic voltammetric investigation of the polymer reveals a reversible n-doping process and an irreversible p-doping process. The onset for the n-doping process occurs at −1.48 V (vs. SCE), while the onset for the p-doping process occurs at 1.19 V (vs. SCE). The results imply that HCN-PPV is an n-dope type electroluminescent polymer.

Light-emitting copolymers of cyano-containing PPV-based chromophores and a flexible spacer

Conjugated–non-conjugated alternating block copolymers containing cyano-modified distirylbenzene units as emitting chromophores were synthesized via Knoevenagel and Wittig reactions; the non-conjugated block was an alkylene spacer. The polymers were differentiated with respect to the location of the cyano groups in the chromophoric block (double bond or phenylene ring). When photoluminescence and electroluminescence spectra of these materials were compared with those of a similar structure without the cyano group, a strong bathochromic effect was observed. A more pronounced red shift and a higher electroluminescence efficiency was observed in the polymer with the cyano group attached to the aromatic ring.

Contributions of Nonconjugated Spacers to Properties of Electroluminescent Block Copolymers

ABSTRACT Six electro-optically active alternating block copolymers consisting of short substituted phenylene vinylene sequences interspersed with optically inert oligomethylene blocks were synthesized using a Wittig condensation scheme. In this study, the focus was on the effect of the aliphatic spacer length, which was systematically varied from two to twelve methylene units. The copolymers were soluble in standard organic reagents and were characterized by NMR, FTIR, elemental analysis, GPC, UV-vis, and DSC. The vinylene groups in the conjugated blocks were isomerized into the all-trans form by refluxing in toluene with catalytic iodine. The electroluminescent spectra of the six copolymers were similar and displayed a maximum around 465 nm. The EL efficiency of the single-layer LEDs fabricated with the copolymer chromophores was a function of the length of the nonconjugated spacer blocks; copolymers with longer spacers yielded higher-efficiency devices.

Segmented conjugated polymer based on poly(?-thiophenediyl)benzylidine

Segmented polymers with random sequences of conjugated and non-conjugated blocks are produced by partial dehydrogenation of poly(α-thiophenediyl)benzylidene. The fraction of conjugated block was controlled by the elimination time. The polymers gave photoluminescence. Absorption coefficients of the polymers increased with the rate of dehydrogenation, and the highest fluorescence intensity were achieved around 9 % conjugation conversion. The polymers were characterized by 1H- and 13C-n.m.r. and infrared spectra, and were soluble in common organic solvents, which suggests a good processability.

Synthesis of electrically conducting copolymers with short alternating conjugated and non-conjugated blocks

Six copolymers with short alternating conjugated and non-conjugated blocks were synthesized using the Wittig reaction. The conjugated block consists of two and one-half units of p-phenylene vinylene with methoxy groups substituted at the 3 and 5 positions of the flanking phenyl rings. These rigid blocks are linked to flexible linear alkyl chain spacers by ester groups. The polymers, characterized by 13C n.m.r., FTi.r., elemental analysis, g.p.c. and d.s.c. are soluble and fusible. Free-standing films were cast from chloroform solutions and showed electrical conductivities ranging from 10 −3 to 10 −2 S cm −1 upon doping by iodine vapour at room temperature.