Polymer-based Devices Research Articles

In vivo biocompatibility and in vitro characterization of poly-lactide-co-glycolide structures containing levetiracetam, for the treatment of epilepsy

Epilepsy is a chronic neurological disorder characterized by recurrent seizures, and is highly resistant to medication with up to 40% of patients continuing to experience seizures whilst taking oral antiepileptic drugs. Recent research suggests that this may be due to abnormalities in the blood-brain barrier, which prevent the passage of therapeutic substances into the brain. We sought to develop a drug delivery material that could be implanted within the brain at the origin of the seizures to release antiepileptic drugs locally and avoid the blood brain barrier. We produced poly-lactide-co-glycolide drop-cast films and wet-spun fibers loaded with the novel antiepileptic drug Levetiracetam, and investigated their morphology, in vitro drug release characteristics, and brain biocompatibility in adult rats. The best performing structures released Levetiracetam constantly for at least 5 months in vitro, and were found to be highly brain biocompatible following month-long implantations in the motor cortex of adult rats. These results demonstrate the potential of polymer-based drug delivery devices in the treatment of epilepsy and warrant their investigation in animal models of focal epilepsy.

In situ current voltage measurements for optimization of a novel fullerene acceptor in bulk heterojunction photovoltaics

The evaluation of the power conversion efficiency (PCE) of new materials for organic bulk heterojunction (BHJ) photovoltaics is difficult due to the large number of processing parameters possible. An efficient procedure to determine the optimum conditions for thermal treatment of polymer-based bulk heterojunction photovoltaic devices using in situ current-voltage measurements is presented. The performance of a new fullerene derivative, 1,9-dihydro-64,65-dihexyloxy-1,9-(methano[1,2] benzomethano)fullerene[60], in BHJ photovolatics with poly(3-hexylthiophene) (P3HT) was evaluated using this methodology. The device characteristics of BHJs obtained from the in situ method were found to be in good agreement with those from BHJs annealed using a conventional process. This fullerene has similar performance to 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methano fullerene in BHJs with P3HT after thermal annealing. For devices with thickness of 70 nm, the short circuit current was 6.24 mA/cm2 with a fill factor of 0.53 and open circuit voltage of 0.65 V. The changes in the current-voltage measurements during thermal annealing suggest that the ordering process in P3HT dominates the improvement in power conversion efficiency. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

RIR-MAPLE deposition of conjugated polymers for application to optoelectronic devices

Resonant-infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) is a promising deposition technology for the fabrication of conjugated polymer-based optoelectronic devices for two primary reasons: (i) the ability to control film morphology, and (ii) the ability to deposit multi-layered heterostructures. This article reviews a variation of RIR-MAPLE that uses emulsified targets of organic solvents and water such that the incident laser wavelength (Er:YAG at 2.9 μm) is resonant with hydroxyl (O–H) bonds in the host matrix, which are absent from the guest material. The novelty of the approach lies in the fact that while most polymers of interest and many compatible solvents do not resonantly absorb the laser energy at 2.9 μm, the emulsion with water enables high-quality, thin-film deposition with minimal photochemical and structural degradation for almost any polymer of interest. In addition, the advantages of emulsion-based RIR-MAPLE for conjugated polymer-based optoelectronic devices are demonstrated by two important studies. First, conjugated polymer films deposited by RIR-MAPLE are shown to have higher hole drift mobilities than films deposited using traditional drop-casting and spin-casting techniques. Second, the unique capability of RIR-MAPLE to enable conjugated polymer-based optical heterostructures is demonstrated by the fabrication and characterization of a multi-layer, polymer distributed Bragg reflector.

Synthesis of Poly(fluorene-alt-thieno[3,4-b]thiophene) for Photovoltaic Devices

Polymer-based bulk heterojunction solar cells have been fabricated using blended films composed of an electrondonating conjugated polymer and an electron-accepting material such as a fullerene derivative; they are promising low-cost, flexible renewable energy sources. A solar cell’s power conversion efficiency (PCE) is the product of the short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) divided by the incident light power density. The Voc of polymer solar cells is generally proportional to the energy difference between the highest occupied molecular orbital (HOMO) of the electron donor and the lowest unoccupied molecular orbital (LUMO) of the electron acceptor, suggesting that polymers with deep HOMO levels are advantageous if conditions are otherwise similar. Photovoltaic cells have had PCEs of ca. 5% reported when a mixture of poly(3-hexylthiophene) (P3HT) and a fullerene derivative were respectively used as the electron donor and acceptor. Interesting low band gap polymers consisting of alternating ester or ketone-substituted thieno[3,4-b]thiophene and alkoxybenzo[1,2-b:4,5-b']dithiophene units have been developed. Among them, a polymer consisting of a fluorinated ketone-substituted thieno[3,4-b]thiophene and alkoxybenzo[1,2-b:4,5-b']dithiophene units, whose band gap and HOMO level were 1.77 eV and −5.22 eV, respectively, exhibited a maximum PCE of 7.73%. We have also synthesized conjugated polymers for photovoltaic applications. Especially, a polymer consisting of alternating carbazole and ester group-substituted thieno[3,4b]thiophene units (PCTT1) showed a band gap of 2.1 eV and HOMO level of −5.52 eV. This polymer, blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), when used as the photoactive layer of solar cells, yielded a maximum PCE of 2.53%. Polyfluorenes have advantages such as thermal and oxidation stabilities. They can also form films easily and have good hole-transporting properties. The band gaps and HOMO levels of fluorene-containing polymers have been reported to be similar to those of carbazole-containing counterpart polymers, though different PCEs were shown by resulting polymer-based solar cells. Thus poly(fluorenealt-thieno[3,4-b]thiophene polymer) (PFTT) was synthesized as a counterpart of PCTT1. This work describes the synthesis and characterization of the polymer along with preliminary tests of resulting polymer-based photovoltaic devices. Experimental Section

Hydrophobic–hydrophilic conversion in microfluidic polymer channels with TiO2 thin films

In this study, recoverable and maintainable hydrophilic micropatterns on polymer-based microfluidic devices were developed through liquid phase deposition of a photocatalytic TiO2. The TiO2-coated region in the microchannel could be hydrophilized through brief UV irradiation upon request. A titanium oxide film was deposited either on bare polymer substrates or on a titanium-sputtered surface using an aqueous TiCl3 solution at 80 °C. A cyclic olefin copolymer (COC) coated with the TiO2 film displayed effective photocatalytic activity, which reduced the water contact angle from 50° to less than 5°. The Ti seed layer was facilitated to pave the substrate with TiO2 grains during the liquid phase deposition. Thus, the photocatalytic effect could be achieved with less number of LPD cycles. Even after contamination from an organic solvent or photoresist, the surface became hydrophilic upon 5 min of UV irradiation. The photocatalytic effect was so stable that super-hydrophilicity of the surface could be promptly recovered after a month of storage under atmospheric conditions. A COC microchannel was fabricated with TiO2 patterns to allow for controlled delivery of the aqueous liquid without the need for external pumps or valves. A hydrogel pattern self-organized along the TiO2 pattern demonstrated a practical application of the recoverable hydrophilicity in microchannels. Functional hydrogels can easily and stably be integrated into a polymer microchannel using this approach.

Large Enhancements in Optoelectronic Efficiencies of Nano-plastically Stressed Conjugated Polymer Strands

The photoluminescence (PL) of well dispersed molecules of a conjugated polymer, poly[2-methoxy-5-((2'-ethylhexyl)oxy)-1,4-phenylene-vinylene] (MEH-PPV), in an optically inert matrix manifested dramatic increases when the individual molecular strands were fully stretched. The PL increase rose with stretching and may reach several folds when the mechanical strain of the matrix polymer went beyond 550%. Strong polarization effects indicate that stretching individual polymer chains was responsible for the PL enhancement. This effect was attributed to suppression of electron-phonon interactions in the stress-rigidified polymer chain segments and may be useful for efficiency-enhanced polymer-based optoelectronic devices.

Molecular dynamics simulations of the effects of carbon dioxide on the interfacial bonding of polystyrene thin films

Fabrication of nanoscale polymer-based devices, especially in biomedical applications, is a challenging process due to requirements of precise dimensions. Methods that involve elevated temperature or chemical adhesives are not practicable due to the fragility of the device components and associated deformation. To effectively fabricate devices for lab-on-a-chip or drug delivery applications, a process is required that permits bonding at low temperatures. The use of carbon dioxide (CO2) to assist the bonding process shows promise in reaching this goal. It is now well established that CO2 can be used to depress the glass transition temperature (Tg) of a polymer, allowing bonding to occur at lower temperatures. Furthermore, it has been shown that CO2 can preferentially soften a polymer surface, which should allow for effective bonding at temperatures even below the bulk Tg. However, the impact of this effect on bonding has not been quantified, and the optimal bonding temperature and CO2 pressure conditions are unknown. In this study, a molecular dynamics model is used to examine the atomic scale behavior of polystyrene in an effort to develop understanding of the physical mechanisms of bonding and to quantify how the process is impacted by CO2. The final result is the identification of a range of CO2 pressure conditions which produce the strongest bonding between PS thin films at room temperature. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Electrochemical properties of a polypyrrole-polyimide composite

The electrochemical properties of a polypyrrole-polyimide composite are studied from the point of view of its prospective use as a material in polymer-based charge-storage devices, that is, rechargeable batteries and supercapacitors. The content of polypyrrole and the polarization potential of the composite are varied in the experiments. The charge behavior of the composite is characterized via potential step amperometry and electrochemical impedance spectroscopy. The results are discussed in terms of various physical models that take into account the formation of charge-transfer complexes and the storage of electric energy in the double electric layer. A relationship is found between the type of doping (cationic or anionic) of the matrix and the filler and the origin of the charge-storage ability: the capacity due to doping or energy storage in the double electric layer.

Organic−Inorganic Nanocomposites by Placing Conjugated Polymers in Intimate Contact with Quantum Rods

Recent advances in the synthesis [ 1 ] and assembly [ 2 ] of nanocrystals (NCs) provide unique opportunities to exploit NCs for the development of next generation organic/inorganic hybrid solar cells as one of the most promising alternatives to Si solar cells to deliver effi cient energy conversion with inexpensive fabrication. [ 1 , 2 ] These conjugated polymer-based photovoltaic devices capitalize on the advantages peculiar to conjugated polymers (CPs), such as light weight, fl exibility, processability, roll-to-roll production, low cost, and large area, in conjunction with the high electron mobility and tunable optical properties of inorganic NCs. Of the organic/inorganic hybrids, poly(3-hexylthiopene) (P3HT) is one of the most extensively utilized CPs due to its excellent solution processability, environmental stability, high charge carrier mobility, and tailorable electrochemical properties. [ 3 , 4 ] CdSe quantum dots (QDs) are the most commonly investigated NCs because of their quantum-confi ned nature and well-matched energy level with P3HT. [ 5–11 ]

TRANSITION OF BIPOLARON TO POLARONS INDUCED BY MAGNETIC IMPURITIES IN CONJUGATED POLYMERS

Both polarons and bipolarons are composite particles with internal structures in nondegenerate conjugated polymers. A bipolaron is a spinless species with charge ±2|e|, while a polaron is a spin-bearing one (spin 1/2, charge ±|e|). Serving as charge carriers, they play an important role in the transport properties of polymer-based optoelectronic devices. By using a nonadiabatic dynamic method, the motion of a bipolaron under an external electric field is theoretically investigated in a conjugated polymer with magnetic impurities. Our results show that a bipolaron can pass through the magnetic impurities, or break down into two polarons with different spins, or be trapped by the magnetic impurities, depending on the width of the impurity region. When the width of the impurity region is comparable to the polaron width, the bipolaron is transformed into two polarons easily. As a polaron and a bipolaron possess different spin characteristics, the decomposition of bipolarons induced by the magnetic impurities may have important effect on the spin polarized transportation.

Fabrication of polymer-based micro devices: Formulation and study of the paste

We investigate in the present work the fabrication of polymer-based micro-fluidic and micro-mechanical devices using sacrificial, resistive/conductive pastes, through a screen-printing process. An organic sacrificial paste is first screen-printed onto a substrate, building the future empty space (channels, cavities…) where it lies. Then, a resistive paste based on thermosetting polymer resin and graphite is deposited onto the previous layer. Finally, the sacrificial paste is removed at 150°C by sublimation through the composite, thus yielding the desired pattern. For such applications, the key is naturally the formulation of the pastes. In a previous work, we already proposed a potential formulation for the organic sacrificial paste, based on polyols [1]. In the present paper, we will focus on the formulation of the cover paste, which must be adapted rheologically for the screen-printing process, but also chemically due to strong potential interactions between the sacrificial layer and the over-layer. Finally, micro-devices such as simple fluidic channels and suspended structures were produced and shown to be operational, demonstrating the high potential of our process.

Complete plastic nanofluidic devices for DNA analysis via direct imprinting with polymer stamps

Development of all polymer-based nanofluidic devices using replication technologies, which is a prerequisite for providing devices for a larger user base, is hampered by undesired substrate deformation associated with the replication of multi-scale structures. Therefore, most nanofluidic devices have been fabricated in glass-like substrates or in a polymer resist layer coated on a substrate. This letter presents a rapid, high fidelity direct imprinting process to build polymer nanofluidic devices in a single step. Undesired substrate deformation during imprinting was significantly reduced through the use of a polymer stamp made from a UV-curable resin. The integrity of the enclosed all polymer-based nanofluidic system was verified by a fluorescein filling experiment and translocation/stretching of λ-DNA molecules through the nanochannels. It was also found that the funnel-like design of the nanochannel inlet significantly improved the entrance of DNA molecules into nanochannels compared to an abrupt nanochannel/microfluidic network interface.

P‐ and n‐Type Bipolar Redox‐Active Radical Polymer: Toward Totally Organic Polymer‐Based Rechargeable Devices with Variable Configuration

The p- and n-type bipolar redox-active radical polymer with a nitronylnitroxyl group, poly(nitronylnitroxylstyrene), expands its utilization as a cathode- or anode-active material in organic polymer-based rechargeable devices with two unprecedented configurations: a poleless battery and a “rocking-chair type” battery. These batteries exhibit remarkably high power rate capability (chargeable within 20 s) and high cycle performance.

Synthesis of poly(3-hexylthiophene-alt-2,3-bis(4-fluorophenyl)thieno[3,4-b]pyrazine) for polymer photovoltaic applications

In this study, 2,3-bis(4-fluorophenyl)thieno[3,4-b]pyrazine and 3-hexylthiophene units were used as electronreleasing and electron-withdrawing moieties, respectively, to further decrease the band-gap, as compared to the previous thienopyrazine-carbazole copolymers because carbazole units may be more twisted from thieno[3,4-b]pyrazine units than from thiophene units, due to two hydrogen atoms of carbazole unit that are at ortho positions from to the main chain. This paper describes the synthesis, optical and electrochemical properties, and preliminary results of the polymer-based photovoltaic devices. Experimental

Evaluating the chemio-physio properties of novel zinc oxide–polyaniline nanocomposite polymer films

Recently, polymer matrix-based nanocomposites have become a prominent area of research and development in optics, as well as in optoelectronic, biomedical, electrical and electronic applications. Organic polymer-based electronic devices have the potential to be lower in cost and more flexible in the manner in which they are manufactured. However, they need significant improvement in both efficiency and long-term stability. Therefore, we made an attempt to synthesize zinc oxide (ZnO)–polyaniline (PANI) using chemical and emulsion polymerization techniques. The properties of ZnO–PANI films were then systematically characterized with several physical techniques. Electrochemical investigations revealed that the individual redox properties of ZnO and PANI can be maintained in a nanocomposite ZnO–PANI system. Furthermore, our results indicated that ZnO–PANI films can exhibit a wide redox potential window. Moreover, we observed the formation of a single-layer nano-Schottky junction in ZnO–PANI films, and interesting positive temperature coefficient properties in ZnO–PANI-embedded polystyrene films. Recently, polymer matrix-based nanocomposites have become a prominent area of current research and development in optics, optoelectronic, biomedical, electrical and electronic applications. Organic polymer-based electronic devices offer the potential for lower cost and more flexible manufacture but need significant improvements in both efficiency and long-term stability. Therefore, we have made an attempt to synthesize polyaniline (PANI)–zinc oxide (ZnO) using chemical and emulsion polymerization techniques. The properties of ZnO–PANI films were then systematically characterized with several physical techniques. The electrochemical investigations revealed that the individual redox properties of ZnO and PANI can be maintained in a nanocomposite ZnO–PANI system. Further, results indicated that ZnO–PANI films could exhibit a wide potential window. Moreover, we have observed the formation of a single-layer nano-Schottky junction in the ZnO–PANI film, and interesting positive temperature coefficient properties in ZnO–PANI.

Thermal-Actuated Optoelectronic Memory Medium Based on Carbon Nanotube− and Nickel−Poly(dimethylsiloxane) Composites

Employing the strong capability of absorbing near-infrared (NIR) light and converting the heat of single-walled carbon nanotubes (SWNTs), and the fast thermal actuation of carbon nanotube−poly(dimethylsiloxane) (PDMS) nanocomposite, in this work we proposed a novel NIR optoelectronic memory (OEM) medium based on a SWNT−PDMS/Ni−PDMS composite bilayer, which was actuated by a low-power-density NIR light illumination. The demo device has shown high performance, including a low operating NIR intensity, a high switching ratio, and a rapid photoresponse with no relaxation. In general, the performance has remarkably outperformed state-of-the-art polymer-based OEM devices. Moreover, the device can be performed reversibly using electric pulses. This study provides a new mechanism to design OEMs and other NIR optic-related devices.

Photovoltaic and Spectral Properties of Conjugated Polymer Poly(3-octylthiophene) Doped with Various Acceptor Materials

We propose a novel approach for enhancement of efficiency of polymer-based photovoltaic devices by means of a low-molar-mass liquid crystal as a dopent. The active layers of the photovoltaic cells are primarily fabricated from the conjugated polymer poly(3-octylthiophene) as well as various acceptor substances including single-wall carbon nanotubes, buckminsterfullerene and the nematic liquid crystal E7 for comparison. The current–voltage characteristics are measured in dark and under green light illumination at wavelength of 514.5 nm. The power conversion efficiency for the active layer doped with E7 is the best among all cells in the simplest monolayer cell structure. Owing to the superior miscibility and orientational order of the nematic liquid crystal, E7 molecules may have promoted a composite film with fewer defects and locally helped the macromolecules align more orderly and consequently improved the charge-carrier transport, leading to the higher photocurrent and power conversion efficiency.

Batteries and charge storage devices based on electronically conducting polymers

Strong interest in energy generation and storage has yielded excellent progress on organic based solar cells, and there is also a strong desire for equivalent advancement in polymer-based charge storage devices such as batteries and super-capacitors. Despite extensive research on electronically conducting polymers including polypyrrole, polythiophene, and polyaniline, limitations to the maximum doping density and chemical stability had been considered a significant restriction on the development of polymer batteries. Recent work appears to show a meaningful increase in the upper bound of the maximum density from 0.5 to 1.0 electrons per monomer depending on the structure, processing, and ionic species used in charging and discharging of the polymers. Several recent examples have also implied that more stable, reversible charge-discharge cycling is being observed in n-doped polymers. These observations suggest that the performance metrics of this class of electronically conducting polymer may ultimately reach the levels required for practical battery applications. Further efforts are essential to perfect practical large-scale electrode fabrication to move toward greater compatibility in the methods used for solar cells and those used in producing batteries. A better understanding must also be developed to elucidate the effects of molecular structure and polymer architecture on these materials.

Electrical fatigue in ferroelectric P(VDF-TrFE) copolymer films

In recent years, ferroelectric polymer-based nonvolatile memory devices have attracted much attention due to their advantages over inorganic ferroelectrics. Polarization fatigue is a major problem in ferroelectric memories and seldom studies have been reported in this field. Here we will summarize our recent work on electric fatigue in polymers as well as some of work from the other researchers. Our experimental observations indicated that the injection and the trap of space charges acted a crucial role in the origination of polarization fatigue in ferroelectric polymers.

Size and shape dependent second order nonlinear optical properties of nanomaterials and their application in biological and chemical sensing.

The development of nonlinear optical (NLO) materials has been driven by a multitude of important technological applications that can be realized if suitable materials are available 1–15. Future generations of optoelectronic devices for telecommunications, information storage, optical switching, and signal processing are predicted to a large degree on the development of materials with exceptional NLO responses 1–15. A large number of organic π-conjugated molecules have been investigated in the last thirty years for suitability to function as components in hypothetical NLO materials 1–19. Several books and reviews have appeared dealing with theory of nonlinear optics and the structural characteristics and applications of nonlinear optical molecules and materials 1–19. Truly, all-optical NLO effects were not discovered until the discovery of lasers. Second-harmonic generation (SHG) was first observed in a single crystal of quartz by Franken et.al. 20 in 1961. Parametric amplification was observed in lithium niobate (LiNbO3) by two-wave mixing in temperature-tuned single crystals 21. Rentzepis and Pao 22 made the first observation of SHG in an organic material, benzpyrene, in 1964. Heilmeir examined hexamethylenetetramine single crystal SHG in the same year 24. Two other organic materials followed rapidly: hippuric acid and benzil 25. Benzil was the first material that proved relatively easy to grow into large single crystals. Over the last two decades the study of nonlinear optical process in organic and polymer systems has enjoyed rapid and sustained growth 1–19, 25–39. One indication of the growth is the increase in the number of articles published in refereed society journals, as one can find from web of science 25, SCIFINDER 26 and Scopus 27 search. The four years period 1980–1983 saw the publication of 124 such articles. In the next four years period 1984–1987, the production of articles increased to 736 (nearly six times). From 1988–1992, the number of articles increased to more than 4000 25–27. In the last decade, academia, industry and government laboratories have been working in this field to replace electronics by photonics and as a result, the number of publications has reached more than 70,000 25–27. The rapid growth of the field is mainly due to the technological promise of these materials 1–19, 28–37. Traditionally, the materials used to measure second-order NLO behavior were inorganic crystals, such as lithium niobate (LiNbO3) and potassium dihydrogen phosphate (KDP). The optical nonlinearity in these materials is to a large extent caused by the nuclear displacement in an applied electric field, and to a smaller extent by the movement of the electrons 1–10. This limits the bandwidth of the modulator. Organic materials have a number of advantages over inorganic materials for NLO applications 28–35. The ease of modification of organic molecular structures makes it possible to synthesize tailor-made molecules and to fine-tune the properties for the desired application 28–35. Unfortunately, not all organic materials display second-order NLO properties. At the molecular level, they need to be non-centrosymmetric. A large number of organic π-conjugated molecules have been investigated 1–9, 28–35 in the last twenty years. The outcome of the results has helped to establish certain guidelines for molecular design to get good second order NLO materials. However, roughly more than 80% of all π-conjugated organic molecules crystallize in centro-symmetric space groups 1–19, therefore producing materials with no second order bulk susceptibility. To overcome this limitation, organic NLO material doped or covalently attached in polymers, have been introduced by Dalton et. al 5,6,16,38–39. A few of these chromophores have served as components of functioning polymer-based optoelectronic devices; the physical properties of all these prototype materials possess one or more critical deficiencies that render commercialization of these systems impractical 28–39. These facts suggest that new types of molecular design are necessary if significant advances are to be realized. From 1998 onwards, researchers started effort on developing various nanomaterials, with high second order NLO properties and seeking for their applications in photonics as well as chemical and biological detection 40–106. The surface-enhanced phenomenon is predicted to have a particularly important impact in nonlinear optical NLO applications, since the generally weak nonlinear effects can be significantly increased via strong electromagnetic fields at the surfaces of metallic nanostructures 60–129. NLO based sensing have provided great potentials and opportunities for detecting different environmental toxins that exhibit some specific advantages, compared to other conventional and nanomaterial based techniques. Aim of this review is mainly to summarize and evaluate the achievements in development of nanoparticle based second order NLO materials with different sizes and shapes and it will focus on the following three major issues: (i) design of novel NLO active materials using nanoparticles (ii) nonlinear optical properties of single nanoparticle, nanoparticle aggregates and self assembly, and (iii) applications in chemical and biological sensing.