Phase-separated Polymer Research Articles

Achieving a High Fill Factor and Stability in Perylene Diimide–Based Polymer Solar Cells Using the Molecular Lock Effect between 4,4′‐Bipyridine and a Tri(8‐hydroxyquinoline)aluminum(III) Core

AbstractTwo novel perylene diimide (PDI)–based derivatives, Alq3‐PDI and Alq3‐PDI2, are synthesized by flanking a 3D tri(8‐hydroxyquinoline)aluminum(III) (Alq3) core with PDI and a helical PDI dimer (PDI2) to construct high‐performance small molecular nonfullerene acceptors (SMAs). The 3D Alq3 core significantly suppresses the molecular aggregation of the resulting SMAs, leading to a well‐mixed blend with a PTTEA donor polymer and weak phase separation. Compared with Alq3‐PDI, the extended π‐conjugation of Alq3‐PDI2 results in higher‐order molecular packing, which improves the absorption and phase separation behavior. Thus, the Alq3‐PDI2 devices have higher Jsc and FF values and better device performance, which are further enhanced by a small amount of 4,4′‐bipyridine (Bipy) as an additive. The coordination between Bipy and the Alq3 core promotes molecular packing and phase separation, which lower charge recombination and enhanced charge collection in the resulting devices. Therefore, a largely improved Jsc of 15.74 mA cm−2 and very high FF of 71.27% are obtained in the Alq3‐PDI2 devices, resulting in a power conversion efficiency of 9.54%, which is the best value reported for PDI‐based polymer solar cells. The coordination can also serve as a “molecular lock,” which prevents molecular motion and thus improves device stability.

Pyridine-based additive optimized P3HT:PC61BM nanomorphology for improved performance and stability in polymer solar cells

Photovoltaic (PV) performance of bulk heterojunction polymer solar cells (BHJ PSCs) highly depends on the optoelectronic properties of the photoactive layer. In this regard, additives (solvent or solid) are considered as one of the versatile methods and key morphology directing agents for solution processed PV devices. Herein this contribution, we rationally utilize a new kind of pyridine-based solid additives (2-hydroxypyridine (2-DHP) and 2,4-dihydroxypyridine (2,4-DHP)) to manipulate and engineer the nanomorphology of the poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) photoactive system, that can influence the optoelectronic properties for enhanced performance in BHJ PSCs. The optical, electrical, morphological, surface, structural characteristics of the fabricated BHJ PSCs were systematically examined. The atomic force microscopy and X-ray photoelectron spectra measurements confirmed the structural reorganization, phase separation of polymer and fullerene domains resulting in the formation of well-ordered blend in the additive based devices. BHJ PSCs fabricated with P3HT:PC61BM:2-DHP and P3HT:PC61BM:2,4-DHP yielded a best power conversion efficiency (PCE) of 4.35% and 3.58%, respectively, which in contrast outperformed than the PCE obtained from P3HT:PC61BM device (3.01%). The remarkably improved performance for P3HT:PC61BM:2-DHP can be attributed to the enhanced short-circuit current density induced by the morphology optimization. Additionally, the devices fabricated with the solid additives (2-DHP and 2,4-DHP) exhibited improved air-stability due to the oxygen insensitivity of the functional groups present in the chosen additives. The pyridine and hydroxyl groups in 2-DHP and 2,4-DHP possibly form the intermolecular interactions with the photoactive components (P3HT/PC61BM), contribute to the morphology optimization and subsequent PV performance enhancement in BHJ PSCs. It is believed that our findings can inspire and trigger further advanced research toward the design and development of stable and highly efficient BHJ PSCs by rationally choosing low-cost solid additives.

Precision cell delivery in biphasic polymer systems enhances growth of keratinocytes in culture and promotes their attachment on acellular dermal matrices.

Current approaches for precision deposition of cells are not optimized for moist environments or for substrates with complex surface topographic features, for example, the surface of dermal matrices and other biomaterials. To overcome these challenges, an approach is presented that utilizes cell confinement in phase-separating polymer solutions of polyethylene glycol and dextran to precisely deliver keratinocytes in well-defined colonies. Using this approach, keratinocyte colonies are produced with superior viability, proliferative capacity, and barrier formation compared with the same number of cells dispersedly seeded across substrate surfaces. It is further demonstrated that keratinocytes delivered in colonies to the surface of acellular dermal matrices form an intact epidermal basal layer more rapidly and more completely than cells delivered by conventional dispersed seeding. These findings demonstrate that delivery of keratinocytes in phase-separating polymer solutions holds potential for enhancing growth of keratinocytes in culture and production of functional skin equivalents.

Poly(3-alkylthiophene)- block-poly(3-alkylselenophene)s: Conjugated Diblock Co-polymers with Atypical Self-Assembly Behavior.

Understanding self-assembly behavior and resulting morphologies in block co-polymer films is an essential aspect of chemistry and materials science. Although the self-assembly of amorphous coil-coil block co-polymers is relatively well understood, that of semicrystalline block co-polymers where each block has distinct crystallization properties remains unclear. Here, we report a detailed study to elucidate the rich self-assembly behavior of conjugated thiophene-selenophene (P3AT- b-P3AS) block co-polymers. Using a combination of microscopy and synchrotron-based X-ray techniques, we show that three different film morphologies, denoted as lamellae, co-crystallized fibers, and patchy fibers, arise from the self-assembly of these block co-polymers over a relatively narrow range of overall degrees of polymerization (30 < N < 90). Crystallization-driven phase separation occurs at a very low N (<35), and lamellar films are formed. Conversely, at medium N (50-60) and high N (>80), the thiophene and selenophene blocks co-crystallize into nanofibers, where medium N leads to much more mixing than high N. The overall tendency for phase separation in these systems follows rather different trends than phase separation in amorphous polymers in that we observe the greatest degree of phase separation at the lowest N. Finally, we demonstrate how each morphology influences transport properties in organic thin-film transistors comprised of these conjugated polymers.

Phase field simulation and microscopic observation of phase separation and thermal stability of polymer modified asphalt

Phase separation and thermal stability of polymer modified asphalt are the most challenging issues that significantly affect the performance of asphalt in engineering applications. This paper employed phase field method to study phase separation behavior of styrene-butadiene-styrene (SBS) modified asphalt, aiming to reveal the key parameter controlling the phase separation process and evaluate the influences of various styrene-butadiene structures on phase behavior. Combined with the observation from fluorescence microscopy (FM), phase field model related to polymer structure was established for each SBS modified asphalt. The model parameters shed lights on interaction and swelling among SBS and asphalt. Furthermore, the coupled Navier-Stokes equations phase field model (NSPFM) was developed to simulate thermal stability of SBS modified asphalt. The simulated stability was compared with stability observed from experimental test. The results indicated that the block ratio of SBS significantly affects the phase behavior of modified asphalt, which explains the changes of morphology. The diffusion and swelling of SBS molecule as well as the interaction among polystyrene blocks, polybutadiene blocks, and asphalt component mainly control the phase separation process. By comparison with microscopy images, it was proved that NSPFM is an effective approach to track the phase separation process during storage. The simulated stability results were consistent with experimental results of tube tests.

Rigidochromic conjugated polymers carrying main-chain molecular rotors.

We design and prepare rigidochromic conjugated polymers that carry molecular rotors in the main chain. We show how a suitable design maintains the mechanosensitivity of the rotors upon incorporation into an extended π-conjugated system. Construction of donor-acceptor polymers enables their use as ratiometric probes for polymer micromechanics, which we evidence through micromechanical imaging of a phase-separated polymer blend.

Effects of different molecular chain lengths and side groups on structure and property of UV-curable waterborne cathodic electrophoretic deposition coatings

A series of UV-curable waterborne cathodic electrophoretic deposition (CED) coatings with various structures were successfully developed by introducing modified acrylic monomer with different molecular chain lengths and side groups. The length of linear chain of the modified monomer had little effect on phase separation of acrylic polymer and easily formed homogeneous structure. Owing to the longer molecular chains, the pencil hardness decreased from 2H to H and the glass transition temperature reduced to 77.7 °C. The hard side groups of the modified monomer in CED coatings exhibited opposite characteristics resulting from the steric hindrance of the rigid groups and the crosslinking network, which were more likely to exhibit partial phase separation. Therefore, the stronger rigidity of the molecular structure improved the particle size to 122.42 nm, the smoothness of the CED cured film and surface contact angle decreased. And the morphology of the films became rough and surface contact angle decreased. Besides, the adhesive force and flexibility of the films could be excellent. The surface rust of the modified CED film is not remarkable under salt spray test. The photopolymerization rate (Rp) and the double bond conversion were evaluated. It was found that the Rp and the conversion obviously decreased with increasing rigid groups in acrylate molecular chains. However, the maximum decomposition temperature of it was higher than the other samples. The residue char yield (Yc) was 5.24 wt%. The stronger rigidity of the molecular structure had better thermal stability and mechanical properties.

Selective location and migration of poly(methyl methacrylate)- grafted clay nanosheets with low grafting density in poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) blends

The location and migration of poly (methyl methacrylate)-grafted clay (clay-g-PMMA) nanosheets with low grafting density in a phase-separated polymer blend of poly (methyl methacrylate)/poly (styrene-co-acrylonitrile) (PMMA/SAN) was investigated. The grafting density and molecular weight for grafted PMMA chains on the surface of clay nanosheets was about 0.04 chains/nm2 and 7.8 × 104, respectively, indicating that the grafted chains might exhibit the mushroom-like distribution. With phase separation of PMMA/SAN blend matrix, the well-dispersed clay-g-PMMA nanosheets in the blend matrix were first located in the SAN-rich phase and then gradually migrated to the interface between PMMA and SAN. The first abnormal location of modified clay in the blend matrix might be attributed to the favorable interaction between clay-g-PMMA and SAN induced by the miscibility of grafted PMMA/surrounding SAN chains and the unfavorable interaction between clay-g-PMMA and PMMA without any interpenetration of grafted and matrix chains. The enhanced miscibility between grafted PMMA and surrounding SAN just began to be destroyed after being annealed for a long time, resulting in the final migration of clay-g-PMMA to the interface of PMMA/SAN (thermodynamic equilibrium state).

Analysis of Interaction Between Interfacial Structure and Fibrinogen at Blood-Compatible Polymer/Water Interface.

The correlation between the interfacial structure and protein adsorption at a polymer/water interface was investigated. Poly(2-methoxyethyl acrylate)(PMEA), which is one of the best blood compatible polymers available, was employed. Nanometer-scale structures generated through the phase separation of polymer and water were observed at the PMEA/phosphate buffered saline interface. The interaction between the interfacial structures and fibrinogen (FNG) was measured using atomic force microscopy. Attraction was observed in the polymer-rich domains as well as in the non-blood compatible polymer. In contrast, no attractive interactions were observed, and only a repulsion occurred in the water-rich domains. The non-adsorption of FNG into the water rich domains was also clarified through topographic and phase image analyses. Furthermore, the FNG molecules adsorbed on the surface of PMEA were easily desorbed, even in the polymer-rich domains. Water molecules in the water-rich domains are anticipated to be the dominant factor in preventing FNG adsorption and thrombogenesis on a PMEA interface.

Field effect in amorphous carbon nanomesh directly synthesized from phase-separated polymer blends

Nano engineering of semi-metallic graphene can create graphene nanostructures such as nanoribbon and nanomesh for applications in electronic devices. Graphene materials manipulated by delicate patterning technology have exhibited semiconducting behavior, but the complex processes and degradation of the graphene during processing can be a bottleneck for the practical applications. Herein, a direct patterning approach to fabricate mesh-type carbon materials with a nanohole array was developed using phase-separated polyacrylonitrile (PAN)/poly (methyl methacrylate) (PMMA) blends. Carbon nanomeshes (CNMs) with various neck widths less than 100 nm were obtained by controlling the mixing ratio of PAN to PMMA. Gate dependent ambipolar transport behavior was observed even though the CNM exhibits imperfect crystallinity due to the catalyst-free preparation. Based on the systematic investigation of the temperature dependent electrical transport of the CNM-based field effect transistor, it was demonstrated that the charge conduction in CNMs is dominated by variable range hopping between localization states.

Improving the Colloidal Stability of Temperature-Sensitive Poly(N-isopropylacrylamide) Solutions Using Low Molecular Weight Hydrophobic Additives.

Poly(N-isopropylacrylamide) (PNIPAM) is an important polymer with stimuli-responsive properties, making it suitable for various uses. Phase behavior of the temperature-sensitive PNIPAM polymer in the presence of four low-molecular weight additives tert-butylamine (t-BuAM), tert-butyl alcohol (t-BuOH), tert-butyl methyl ether (t-BuME), and tert-butyl methyl ketone (t-BuMK) was studied in water (D2O) using high-resolution nuclear magnetic resonance (NMR) spectroscopy and dynamic light scattering. Phase separation was thermodynamically modeled as a two-state process which resulted in a simple curve which can be used for fitting of NMR data and obtaining all important thermodynamic parameters using simple formulas presented in this paper. The model is based on a modified van’t Hoff equation. Phase separation temperatures Tp and thermodynamic parameters (enthalpy and entropy change) connected with the phase separation of PNIPAM were obtained using this method. It was determined that Tp is dependent on additives in the following order: Tp(t-BuAM) > Tp(t-BuOH) > Tp(t-BuME) > Tp(t-BuMK). Also, either increasing the additive concentration or increasing pKa of the additive leads to depression of Tp. Time-resolved 1H NMR spin–spin relaxation experiments (T2) performed above the phase separation temperature of PNIPAM revealed high colloidal stability of the phase-separated polymer induced by the additives (relative to the neat PNIPAM/D2O system). Small quantities of selected suitable additives can be used to optimize the properties of PNIPAM preparations including their phase separation temperatures, colloidal stabilities, and morphologies, thus improving the prospects for the application.

Importance of Nucleation during Morphology Evolution of the Blade-Cast PffBT4T-2OD-Based Organic Solar Cells

Different temperatures, solvents, and additives are used as influencing parameters to drive molecular packing and phase separation for the development of the controlled evolution of nanostructures for organic solar cells (OSCs). The temperature-dependent aggregation (TDA) features of polymers are explored by investigating aggregation in solution and solid thin-film states using solution small-angle neutron scattering (SANS), soft/hard X-ray scattering, and transmission electron microscopy (TEM) characterizations. In situ grazing incidence wide angle X-ray scattering (GIWAXS) reveals that the nucleation process is highly significant and responsible for the ultimate film morphology. Processing conditions such as temperatures, solvents, and additives were used to influence the nucleation and evolution of film morphology. The nucleation process may improve the polymer packing and phase separation. It may translate into optimized multilength scale domains and efficient charge percolation pathways, a strong imp...

Polynorbornene-Based Anion Exchange Membranes for Alkaline Fuel Cells

Fuel cells based on alkaline exchange membranes (AEM) have garnered increased interest because of their inherent advantages over traditional proton exchange membranes (PEM). High pH operating conditions allow for more facile reaction kinetics, reduced platinum-group metal catalyst, and reduced fuel crossover issues. Moreover, these materials can be made from inexpensive precursors which can further drive down the cost of the membrane electrode assemblies (MEA) for fuel cells. In this study, a variety of block copolymer AEMs based on based on vinyl addition and ring-opening metathesis polymerization of norbornenes was synthesized and characterized. Correlations were found between the ionic conductivity and mechanical properties and the polymer structure, channel size and phase separation. The AEMs were also used to construct MEAs for alkaline fuel cells and their performance was analyzed using electrochemical impedance spectroscopy. Financial support provided by the ARPA-E IONICS program for G.H. and M.M. is gratefully acknowledged.

Multi-block sulfonated poly(arylene ether nitrile) polymers bearing oligomeric benzotriazole pendants with exceptionally high H2/O2 fuel cell performance

Aromatic ionomers have emerged as promising alternatives to perfluorosulfonic acid polymers to be applied as proton exchange membranes in fuel cells. However, the paradox between the ion conductivity and stability is still a challenge precluding the commercialization of aromatic ionomers. In this paper, we report the design of a novel sulfonated poly(arylene ether nitrile) multi-block structure by introducing a key component, alkyl benzotriazole (Bt) side chains, into the hydrophobic segments. The modified structure could facilitate polymer phase-separation and generate self-standing films with excellent mechanical properties, and it effectively suppresses the excessive swelling of the membrane owing to strong electrostatic interactions between the Bt chains and sulfonic acid groups. Moreover, the Bt unit could act as both a proton acceptor and proton donor, causing a dramatic increase in the ion conductivity of the membrane. The most optimal membrane possesses an ion-exchange capacity of 2.15 meq g−1 and exhibits a weaker relative humidity (RH) dependence and higher proton conductivity than the commercial Nafion 212 over the entire RH range. Remarkably, the maximum power output of the fuel cell based on the most optimal membrane reaches 1090, 856, and 451 mW cm−2 at 95%, 70%, and 30% RH, respectively, which are more than 2 times higher than those of the non-Bt analogue. Further, the current densities (I0.6) ranging up to 1500 and 1000 mA cm−2 (0.6 V) at 95% and 70% RH are both much higher than those of the Nafion. Our study provides a novel methodology for the design of aromatic ionomer structures with excellent performances for practical fuel cell application.

Flash pyrolysis of polymer-derived SiOC ceramics

For the first time, flash pyrolysis was carried out to fabricate polymer derived silicon oxycarbide (SiOC) ceramics. With the application of a DC electric field at a furnace temperature of only 780 °C, the SiOC ceramics exhibit characteristics that usually have to be pyrolyzed at ∼1300 °C. Both electric field and current density accelerate the SiOC microstructure development, causing carbon and SiC phases to form at >520 °C lower pyrolysis temperatures than conventional within the SiOC matrix. With higher electric fields, the samples experience greater mass loss and linear shrinkage, while also forming more SiC and a more ordered carbon phase. The SiC formation inversely impacts the carbon content, causing a decrease in electrical conductivity. Further, reducing current density results in significant carbon precipitation without SiC formation. The fundamentals can be explained based on increased nucleation rate by the electrical field, accompanied by Joule heating and electromigration. This work is the first to demonstrate the great potential of flash pyrolysis on accelerated phase separation of polymer derived SiOC.

Leaf-Inspired Self-Healing Polymers

Summary Hierarchical multiphase fibrous morphologies provide strength and elasticity for biological species, facilitating responses to environmental changes. Wound closure of leaves is one example. If polymers can be formed in a similar manner by introducing multiphase-separated morphologies, self-healing in a variety of commodity materials can be achieved. In these studies, we demonstrate the role of phase morphologies, interphases, and viscoelasticity-driven shape memory effects on self-healing. We synthesized phase-separated polycaprolactone-polyurethane fibrous thermoplastic polymers in which microphase separation facilitates the formation of stable interfacial regions between hard and soft segments. Self-healing can be repeated many times. This behavior is attributed to the shape memory effect, given that micron-scale interphase reduces chain slippage, enabling entropic energy storage during damage. Chemically identical but nanophase-separated copolymers do not exhibit this behavior. These studies show that self-healing can be achieved by morphology control and facilitated by thermal or other volume-induced transitions.

Coarse-grained molecular dynamics modeling of reaction-induced phase separation

We develop a model to describe reaction-induced phase separation in thermoplastic-toughened thermoset polymers at molecular scales. To achieve the temporal and spatial scales required for phase separation we use coarse-grained molecular dynamics where beads represent bi-functional epoxy, a di-amine crosslinker and monomers in the thermoplastic. The model describes the curing of the thermoset using a distance criterion to identify chemical reactions on the fly during a molecular dynamics simulation. We characterize how composition, curing temperature and conversion degree affect the onset of phase separation and domain growth. The onset of phase separation in terms of degree of cure depends on cure temperature and the subsequent growth of domain size during cure can be described with two power laws. Interestingly, the domain size vs. time following quenching after cure also follows power-law behavior but with exponent of approximately ¼, lower than those observed in simple binary mixtures and linear chain polymers.

Nanoscale "Noise-Source Switching" during the Optoelectronic Switching of Phase-Separated Polymer Nanocomposites.

A method is developed to directly map nanoscale "noise-source switching" phenomena during the optoelectronic switching of phase-separated polymer nanocomposites of tetrathiafulvalene (TTF) and phenyl-C61 -butyric acid methyl ester (PCBM) molecules dispersed in a polystyrene (PS) matrix. In the method, electrical current and noise maps of the nanocomposite film are recorded using a conducting nanoprobe, enabling the mapping of a conductivity and a noise-source density. The results provide evidence for a repeated modulation in noise sources, a "noise-source switching," in each stage of a switching cycle. Interestingly, when the nanocomposite is "set" by a high bias, insulating PS-rich phases shows a drastic decrease in a noise-source density which becomes lower than that of conducting TTF-PCBM-rich phases. This can be attributed to a trap filling by charge carriers generated from a TTF (donor)-PCBM (acceptor) complex. In addition, when the film is exposed to UV, an optical switching occurs due to chemical reactions which lead to irreversible changes on the noise-source density and conductivity. The method provides a new insight on noise-source activities during the optoelectronic switching of polymer nanocomposites and thus can be a powerful tool for basic noise research and applications in organic memory devices.

Investigating the nanostructures and proton transfer properties of Nafion-GO hybrid membranes

This study is aimed at exploiting the potential of graphene oxide (GO) as inorganic filler of hybrid membranes for efficient proton transfer. A series of GO nanosheets with different lateral size and oxidation degree are incorporated into Nafion. Atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS) characterization of the obtained membranes demonstrates the effects of the size and oxidation degree of GO on the nanophase separation of Nafion: (i) large GO sheets can strongly disturb the nanophase separation, while the thoroughly sonicated ones produce moderate interruption; (ii) increasing the oxidation degree of GO can also restrict nanophase separation, but the effect is not pronounced as that of GO size. Furthermore, GO sheets with small sheet size and high oxidation degree can effectively increase the water uptake and proton conductivity of Nafion membrane. For instance, the incorporation of 5 wt% optimal GO sheets into Nafion results in about 1.5-fold increase of proton conductivity under 40–80% RH and 80 °C. By comparison, embedding the same GO sheets into sulfonated poly (ether ether ketone) (SPEEK) leads to comparable proportion of proton conductivity increment under the same conditions, indicating that GO in these membranes is actually a structure modifier rather than a proton accelerator. This feature is beneficial to pushing the limits of phase-separated polymers with intrinsically fast proton transfer characteristics.

Bio‐inspired Highly Scattering Networks via Polymer Phase Separation

AbstractA common strategy to optimize whiteness in living organisms consists in using 3D random networks with dense and polydisperse scattering elements constituted by relatively low refractive index materials. Inspired by these natural architectures, a fast and scalable method to produce highly scattering porous polymer films via phase separation is developed. By varying the molecular weight of the polymer, the morphology of the porous films is modified, and therefore their scattering properties are tuned. The achieved transport mean free paths are in the micrometer range, improving the scattering strength of analogous low refractive index systems, e.g., standard white paper, by an order of magnitude. The produced porous films show a broadband reflectivity of ≈75% while only 4 µm thick. In addition, the films are flexible and can be readily index‐matched with water (i.e., they become transparent when wet), allowing for various applications such as coatings with tunable transmittance and responsive paints.