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Combining the acceptor–donor–acceptor-type fused ring-based molecular architecture into a polymeric backbone is a promising strategy to design polymer acceptors for high-performance all-polymer sol...

D–A conjugated polymers have played critical roles in recently reported nonfullerene acceptors-based polymer solar cells (NF-PSCs) with high performance. Although the molecular design of the D–A po...

Two polymers J75 and J76 with selenophene instead of thiophene on the conjugated side chain of benzodithiophene (BDT) unit or π bridges of polymer J71 were designed and synthesized, for investigating the effect of selenophene substitution on the photovoltaic performance of the conjugated polymer donors in comparison with J71. The selenophene π bridges in J76 can narrow optical band gap and red-shift absorption of the polymer film by ca. 25 nm, but the highest occupied molecular orbital (HOMO) energy level (EHOMO) of J76 is up-shifted slightly by 0.04 eV. Two typical electron acceptors of fullerene derivative PC71BM and the nonfullerene acceptor m-ITIC were used to investigate photovoltaic performance of the polymer donors. For the PC71BM-based polymer solar cells (PSCs), J76 with selenophene π bridges shows the best power-conversion efficiency (PCE) of 8.40% in comparison with the J71-based device (PCE = 6.79%), benefitted from the red-shifted absorption, larger coherence length, purer average domains, an...

Side-chain engineering of conjugated polymer donor materials is an important way for improving photovoltaic performances of polymer solar cells (PSCs). On the basis of the polymer J61 synthesized in our group, here, we design and synthesize three new 2D-conjugated polymers J62, J63, and J64 with different types of side chains to further investigate the effect of side chain on their physicochemical and photovoltaic properties. With the narrow bandgap n-type organic semiconductor (n-OS) ITIC as acceptor, the optimized PSCs based on polymer donor of J62 with linear octyl, J63 with linear unsaturated hexylene, and J64 with cyclohexane side chains display power conversion efficiency (PCE) of 10.81%, 8.13%, and 8.59%, respectively. After thermal treatment at 200 °C for 2 h on the active layer,the PCE of the PSC based on J63 still keeps 92% of the original value, which verifies that the cross-linking of the polymer can improve the thermal stability of PSCs. Morphological studies show that the active layer based ...

Organic solar cells based on multinary components are promising to further boost the device performance. The complex interplay of the morphology and functionality needs further investigations. Here, we report on a systematic study on the morphology evolution of prototype ternary systems upon adding sensitizers featuring similar chemical structures but dramatically different crystallinity, namely poly(3-hexylthiophene) (P3HT) and indene-C60-bis-adduct (ICBA) blends with poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadi-azole)-5,5′-diyl] (Si-PCPDTBT) and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (C-PCPDTBT), employing energy-filtered transmission electron microscopy (EFTEM) and resonant soft X-ray scattering (RSoXS). In addition, a combined density functional theory (DFT) and artificial neuronal network (ANN) computational approach has been utilized to calculate the solubility paramet...

All polymer photovoltaic cells offer unique potentials owing to the chemical and electronic tunability of both polymer donors and polymer acceptors. Compared with the numerous π-conjugated polymer donors, choices of π-conjugated polymer acceptors are limited for photovoltaic applications. Although 2D-conjugated benzo[1,2-b:4,5-b′]dithiophene (BDT) units are widely used as building blocks in highly efficient donor polymers in recent years, polymer acceptors based on these units have not been reported yet. Herein, a novel 2D-conjugated polymer acceptor (PBDTNDI-T) based on naphthalene diimide (NDI) and alkylthiothiophene-substituted BDT was designed, synthesized, and in-depth characterized. The polymers’ photophysical, electrical, crystallinity, and morphological properties are addressed in homopolymer and blend films and well correlated with device performance. Under the weight ratio of 1.5:1 and 3 vol % of 1-chloronaphthalene, the PBDTNDI-T-based all polymer photovoltaic device exhibited a desirable PCE o...

A high fullerene molecular miscibility of over 40 wt % is found in the copolymer poly((4,4-octyldithieno(3,2-b:2′,3′-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl) (PSBTBT-08)—a member of the PSBTBT low-bandgap polymer family that have produced power conversion efficiencies as high as 5.9% in solar cells. This observation suggests molecular miscibility plays a key role in the photovoltaic effect in this system. The level of miscibility is additionally measured to be highly dependent on the fullerene species with significant differences between C60- and C70-based fullerenes, highlighting a new parameter to be monitored and controlled when considering different fullerene moieties and species in organic solar cells. Surprisingly, a wide-angle X-ray scattering study reveals no significant crystallinity in the PSBTBT with octyl side chains, potentially the cause of low mobilities and in stark contrast to dodecyl and ethylhexyl PSBTBTs, which demonstrates the importance of the side chain in device mo...

The chain and the crystal unit cell orientation of linear low density polyethylene (LLDPE) were measured with near edge X-ray absorption fine structure (NEXAFS) spectroscopy. A strongly attractive substrate, silicon, and a weakly attractive substrate, mica, were used. For a 100 nm thick LLDPE film on the silicon substrate, the crystals exhibit an edge-on lamellar morphology, with the chains predominantly parallel to the substrate, and the orthorhombic unit cell ⟨a, b, c⟩ in the following approximate orientation: b and c are in the film plane with b along the crystal fibril direction and c perpendicular to the fibril direction and a perpendicular to the film plane. On the mica substrates, LLDPE films with thickness below 180 nm completely dewet the surface and form isolated droplets, while a film 366 nm thick crystallizes as spherulites with most of the chains perpendicular to the substrate before annealing and with a twisted lamellar structure after isothermal crystallization at 60 °C. The results demonst...

The intercalation capability of poly(styrene-b-ethylene butylene-b-styrene) (SEBS) in nanocomposites of isotactic polypropylene (PP) with 5 wt % of organically modified montmorillonite (C20A), prepared by melt blending, has been investigated. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies have shown the presence of intercalated structures in the nanocomposite. In a previous research, we studied the intercalation capability of a commercial compatibilizer.(1) Those results, with the study we present in this work, allow us a better understanding of the mechanism of compatibilization and a deeper characterization of the structure and morphology of the nanocomposite. Scanning transmission X-ray microscopy (STXM) has been used. Because of the excellent chemical sensitivity and the high spatial resolution (∼40 nm) of this technique, we have proved that C20A is not in direct contact with the PP phase because the clay is always located inside the elastomer domains. The elastomer is surr...

Scanning transmission X-ray microscopy has been used to quantitatively map the composition of P3HT:PCBM blend films in the vicinity of PCBM crystals formed during annealing at 140 °C. The observed PCBM concentration profiles around these crystals have been fitted to Fick’s second law of diffusion and the diffusion constant found to be 2.5 × 10−14 m2 s−1. The PCBM concentration at the crystal boundary was found to be 19% (v/v) and is interpreted, together with the annealing temperature of 140 °C, as a point on the bimodal line of the composition−temperature phase diagram. The diffusion of PCBM through P3HT is observed to be bulk-dominated, in contrast to the surface/interface-dominated diffusion observed in MDMO-PPV:PCBM blend films by Yang et al.(1, 2)

The morphological evolution of laterally phase-separated polyfluorene blends composed of poly(9,9′-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylenediamine) (PFB) and poly(9...

We present results of scanning transmission X-ray microscopy (STXM) measurements of bulk heterojunction organic solar cells built from a ternary blend of poly(3-hexylthiophene) (P3HT), [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and [2,3,12,13-tetracyano-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrinato]copper(II) (Cu(CN)4P) porphyrin. These results show evidence of solvent trapping due to porphyrin in the film. Upon annealing, submicrometer depressions are observed in the ternary blend films, corresponding to the evolution of solvent that is associated with the small porphyrin aggregates that phase segregate in the middle of the depressions. The areal density and size of the depressions change systematically with porphyrin concentration in the ternary blend. The relationship of the observed morphologies to the previously measured device performance is discussed.

The composition of blend films of poly(9,9‘-dioctylfluorene-co-bis-N,N‘-(4-butylphenyl)-bis-N,N‘-phenyl-1,4-phenylenediamine) (PFB) and poly(9,9‘-dioctylfluorene-co-benzothiadiazole) (F8BT) used in prototype polymer solar cells has been quantitatively mapped using scanning transmission X-ray microscopy (STXM). The resolution of the STXM technique is 50 nm or better, allowing the first nanoscale lateral chemical mapping of this blend system. For 1:1 blend films spin-coated from xylene we find that the F8BT-rich domain is over 90% pure (by weight) and the PFB-rich domain contains 70% PFB. For 5:1 and 1:5 blend films processed from xylene, the minority phases are found to be intermixed, containing as much as 50% by weight of the majority polymer. Films prepared from chloroform with a 1:1 weight ratio have also been imaged but show no features on the length scale of 50 nm or greater. Additionally, the performance of photovoltaic devices fabricated using films prepared in an identical fashion to those prepared...

Cyclohexane has been frequently used as a selective solvent to remove PS layers or domains from polystyrene:poly(methyl methacrylate) (PS:PMMA) blends and for reorganization or self-assembly of polymer brushes and block copolymers. We have found that cyclohexane is not efficient at PS removal, observing significant residual PS at PMMA surfaces. In contrast, 1-chloropentane was found to be a far greater selective solvent (i.e., residual PS was essentially nonexistent). These results were compared to PMMA surfaces after PS was allowed to adsorb to the surface from a dilute theta solution in cyclohexane. Using near-edge X-ray absorption fine structure spectroscopy and inverse gas chromatography, coupled with self-consistent mean-field theory calculations, we have demonstrated that selectively washing a polymer from a polymer blend is nearly identical to adsorption of a polymer to a "soft" surface from a dilute solution. Improved knowledge about the effects of selective solvents will improve experimental analysis of washed systems as well as manipulation of block copolymers and polymer brushes for reorganization or self-assembly.

We have studied the morphology of blends of PS/PMMA, PC/SAN24, and PMMA/EVA and compared the morphologies with and without modified organoclay Cloisite 20A or Cloisite 6A clays. In each case we found a large reduction in domains size and the localization of the clay platelets along the interfaces of the components. The increased miscibility was accompanied in some cases, with the reduction of the system from multiple values of the glass transition temperatures to one. In addition, the modulus of all the systems increased significantly. A model was proposed where it was proposed that in-situ grafts were forming on the clay surfaces during blending and the grafts then had to be localized at the interfaces. This blending mechanism reflects the composition of the blend and is fairly nonspecific. As a result, this may be a promising technology for use in processing recycled blends where the composition is often uncertain and price is of general concern.

Isotopic labeling (deuteration) is known to affect the phase behavior of polystyrene (PS) and poly- (methyl methacrylate) (PMMA) blends, but little is known regarding the changes in the interfacial properties at the PS/PMMA interface due to deuteration of PS and/or PMMA. To investigate these potential changes, secondary ion mass spectrometry (SIMS) was used to measure real-space depth profiles of dPS in hPS:dPS/hPMMA bilayers, with the hPS:dPS blend being well within the single-phase region of the phase diagram. Profound changes in the thermodynamic behavior of this system at the polymer/polymer interface are observed in the form of significant segregation of dPS to the hPS:dPS/hPMMA interface. The observation of a depletion hole during the formation of an equilibrium excess of dPS implies that the energetic gain at the interface per dPS chain has to be >kT. These results cannot be described, even qualitatively, using previously reported changes in for PS/PMMA due to isotopic labeling. The previously reported values of for dPS/hPMMA and hPS/hPMMA actually predict a depletion of dPS at the hPS:dPS/hPMMA interface rather than the observed segregation. The observed interfacial excess is quantified by generating theoretical profiles, using self-consistent mean-field theory (SCMF), and fitting an effective interaction energy parameter ¢ p as a function of temperature. This parameter represents the asymmetry in dPS/hPMMA and hPS/PMMA interactions. The temperature dependency of ¢ p was found to be a factor of 3-4 greater than any of those reported for of PS/PMMA. It was also found that SCMF theory accurately describes the concentration dependency of dPS segregation at a constant dPS molecular weight using a concentration-independent ¢ p; however, ¢ p was found to be dependent on dPS molecular weight.

Poly(cyclohexyl methacrylate) (PCHMA) and polystyrene (PS) are miscible with each other, but each is highly immiscible with PMMA. Identifiable by the asymmetries in the binary mean-field interaction parameters χ, PS preferentially segregates to the PCHMA/PMMA interface. Secondary ion mass spectrometry was used to provide real-space depth profiles of deuterated PS (dPS) in a miscible blend with PCHMA. The initial dPS concentration was varied from 5 to 20% (v/v), and the blend film was annealed at 150 °C on a film of PMMA for 42 h. X-ray reflectometry was used to determine the interfacial width between PCHMA and PMMA at 150 °C. Using self-consistent mean-field theory, good agreement was found between the experimental and theoretical interfacial excess Z* of dPS at each concentration. Because of their similar glass transition temperatures (∼100 °C for PS and PCHMA) and the ability of PS and PCHMA to be controllably synthesized with low polydispersities, we anticipate this blend to be a model system for futur...

The crystallization of poly(ethylene−vinyl acetate) and linear low-density polyethylene (LLDPE) films spun-cast from the polymer/toluene solutions with as-cast thickness from 460 to 10 nm was studied. The lamellar thickness was measured using small-angle X-ray scattering (SAXS) and found to increase from 14 to 21 nm for films thinner than 100 nm. The morphology of LLDPE measured by scanning probe microscopy (SPM) showed an edge-on lamellae for the films thicker than 30 nm and flat-on lamellae for the films thinner than 15 nm. A pseudo-“shish-kebab” tiny crystal structure was observed in between the larger lamellae. Crystallinity was confirmed using attenuated total reflectance−Fourier transformed infrared spectroscopy (ATR-FTIR) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The shear modulation force microscopy technique (SMFM) was used to measure the melting point, Tm, which was found to decrease for films thinner than 100 nm. The rate of decrease was a function of the annealing pr...

Thin films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) were spun-cast onto silicon substrates, annealed, and analyzed by atomic force microscopy (AFM), total electron yield (TEY), and partial electron yield (PEY) near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in order to resolve conflicting prior literature regarding the tendency of PS to form a wetting layer or overlayer on top of PMMA. From the comparison of the three methods of analysis and on the basis of the extraordinary surface sensitivity of PEY NEXAFS, we conclude that PS does not form an overlayer in samples with morphologies near thermodynamic equilibrium. The PS forms droplets of a large size range on top of a PMMA layer that wets the hydrophilic SiOx substrate. From our results, the maximum thickness of a continuous PS wetting layer would be about 0.25 nm. This is in contrast to recent experiments that imply an equivalent PS wetting layer of about 5−10 nm is forming during annealing.

Random copolymers of methyl methacrylate with polyhedral oligomeric silsesquioxane (POSS) were synthesized and blended with PS and PMMA homopolymer thin films. The effects of the POSS on phase segregation were studied using a variety of complementary techniques. The results showed that these copolymers were efficient at compatibilizing immiscible polymer blends. Compatibilization occurred when the POSS was grafted onto the backbone and a favorable interaction existed between the POSS functional groups and the PS homopolymers. The consequences of this compatibilization were studied using a comprehensive array of characterization methods and found to be as follows: reduced domain size, increased interfacial width, and greatly improved fracture toughness. This compatibilization is due to the increased site functionality provided by the POSS molecule without the entropic penalty associated with introducing functionalities via grafting directly onto the polymer chains.

Scanning transmission X-ray microscopy (STXM) and atomic force microscopy have been used to study the morphology and chemical composition of macrophase-segregated block copolymers in plaque formulations based on water-blown flexible polyurethane foams. Although there has been a large body of indirect evidence indicating that the observed macrophase-segregated features in water-rich polyurethane foams are due principally to urea components, this work provides the first direct, spatially resolved spectroscopic proof to support this hypothesis. The STXM results are consistent with a segregation model where urea segments segregate, forming enriched phases with the majority of the polyether- polyol and urethane groups at the chain ends of the urea hard segments. Chemical mapping of the urea, urethane, and polyether distribution about the urea-rich segregated phases showed that the urea concentration changes gradually (across several hundred nanometers) in a butylene oxide-based foam. This mapping also showed the urea-rich segregated phases present as a partial network in an ethylene oxide/propylene oxide sample.

We report the visualization and quantitative analysis of the cross-link structure in model core/shell hydrogel polymers on the microscopic scale, in a fully swollen state, using soft X-ray microscopy. The cross-link density in these materials and their microscopic or even nanoscopic variation critically influence materials characteristics, yet the cross-link density is difficult to characterize by conventional methods. By the use of soft X-ray microscopy, one can investigate these materials in a fully swollen state and thus directly visualize and quantitatively determine the cross-link structure on a microscopic scale. Materials that were cross-linked by different methods were shown to give rise to differently shaped profiles. Abrupt and gradient cross-link density profiles have been investigated, and the spatial variation in their cross-link density has been determined quantitatively.

Mechanical alloying is performed at cryogenic temperatures to incorporate polyisoprene (PI) or its hydrogenated analogue poly(ethylene-alt-propylene) (PEP) into poly(methyl methacrylate) (PMMA) as an example of high-energy solid-state blending. Morphological characterization of the blends by X-ray and electron microscopies confirms that the degree of dispersion of the constituent polymers improves with increasing milling time. Such dispersion in the PEP/PMMA blends is, however, ultimately compromised by phase coarsening when the materials are postprocessed above the PMMA glass transition temperature in the melt. Milling-induced PI cross-linking serves to suppress phase coarsening in PI/ PMMA blends, which remain relatively well-dispersed even after postprocessing. These blends are generally less fracture-resistant than the as-received PMMA due mainly to the accompanying reduction in PMMA molecular weight. Their optical transparency is observed to decrease dramatically with increasing PEP or PI concentration until they appear opaque. An overall improvement in blend properties by mechanical alloying is, however, anticipated upon judicious selection of more degradation-resistant polymers.

Cryogenic mechanical alloying is used to incorporate a poly(methyl methacrylate-b-isoprene) (MI) diblock copolymer into blends of poly(methyl methacrylate) (PMMA) and polyisoprene (PI). Mechanical milling of the copolymer promotes a reduction in the molar mass of the M block, as discerned from glass transition temperature measurements performed by thermal calorimetry, and induces chemical cross- linking of the I block, as determined from sol-gel analysis. These effects become more pronounced with increasing milling time. Morphological characterization of PMMA-rich PI/MI/PMMA blends by X-ray and electron microscopies reveals that the characteristic size scale of the minority phase decreases with increasing MI content, as well as milling time. The nanostructural features observed in such blends are retained at relatively high MI concentrations during subsequent melt-pressing. Impact testing demon- strates that the blends become tougher upon addition of the MI copolymer, even at relatively low copolymer concentrations. Blend toughness likewise increases with increasing milling time up to a point, beyond which phase inversion occurs within the ternary blends (the PI becomes continuous) and impact strength sharply decreases.

Morphological characterization of bulk polymer blends or compatibilized alloys at submicron spatial resolution has almost exclusively relied upon electron microscopy techniques. Since, however, most organic polymers are composed of carbon and light elements, an arsenal of methods to enhance phase contrast has been developed to facilitate discrimination and analysis of propertygoverning morphological features.1 For transmission electron microscopy (TEM), chemical modification of polymeric specimens through, for example, functionality-specific incorporation of heavy-metal staining agents (typically OsO4 or RuO4) may result in ambiguous results due to an inadequate understanding of the staining reaction or competing reaction kinetics. While microanalytical techniques such as light-element energydispersive X-ray mapping2 and electron spectroscopic (energy-filtered) imaging3-5 can potentially eliminate the need for chemical modification in TEM and have proven highly valuable in identifying morphological characteristics in unmodified multiphase polymer systems, they are typically only capable of distinguishing among constituent elements. In addition, appropriate steps must be exercised during data acquisition to minimize specimen damage due to electron beam irradiation. Another microscopy technique recently applied to the morphological characterization of multiphase polymers is X-ray microscopy (XRM),6-9 in which chemical sensitivity is based on the principles of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy.10 This technique utilizes a diffractive optical element (zone plate) to focus highly monochromatic soft X-rays in the energy range of 200-600 eV generated by a synchrotron radiation source onto an ultrathin polymer specimen (typically 100-200 nm in thickness). The transmitted flux is detected as a function of specimen position as the specimen is raster-scanned by piezoelectric transducers. A detailed description of the microscope employed in this work is provided elsewhere.11 The practical spatial resolution of XRM is dictated by zone plate technology, with the smallest available probe presently limited to a full width at half-maximum of about 55 nm, resulting in a minimum feature resolution of about 35 nm. Recent XRM studies of multiphase polymers have successfully elucidated the morphological characteristics of a binary blend composed of polypropylene (PP) and poly(styrene-r-acrylonitrile) (SAN).6,8 Such blends are expected a priori to be highly demixed due to the chemical dissimilarity of the constituent polymers. Moreover, SAN (unlike PP) contains nitrogen, in which case the SAN spatial distribution can be discerned by either XRM or the element-specific techniques mentioned earlier. In this work, we explore the sensitivity of XRM by examining a polymer blend in which the constituent materials are identical in terms of the elements present (C and H only) and, moreover, differ only in terms of their fraction of the same chemical moiety (polystyrene, PS). As a consequence, blends of this type have been found12,13 to exhibit complex phase behavior that is highly dependent on both blend composition and molecular weight considerations. Results obtained here from XRM are compared with TEM micrographs of the same blend after functionalityspecific OsO4 staining. The polymers employed in this work were PS with Mh n ) 120 000 andMh w/Mh n≈ 1.04 and a “random” diblock copolymer (RBC) synthesized by living anionic polymerization in the presence of sec-butyllithium and a potassium alkoxide. A detailed description of the copolymer synthesis and molecular characterization (in terms of monomer sequencing) is provided elsewhere14-16 and is not addressed, for the sake of brevity, in this communication. The RBC was a poly[(styrene-r-isoprene)′-b-(styrene-r-isoprene)′′], (S/I)′-b-(S/I)′′, copolymer, where the ′ and ′′ denote different block compositions, namely, 75/25 and 50/50 (wt %) S/I. The overall composition of the RBC was 68 wt % S, as measured by 1H NMR, and the block lengths were approximately 40 000 each. Two RBC/PS blends, one consisting of 20 wt % RBC and 80 wt % PS and the other 80 wt % RBC and 20 wt % PS, were prepared by solution casting from toluene, as described elsewhere.13 Upon slow solvent evaporation and subsequent annealing, the resultant films were sectioned in a Reichert-Jung Ultracut-S cryoultramicrotome maintained at -100 °C. Sections for TEM analysis, nominally 100-120 nm in thickness and stained with the vapor of OsO4(aq) for 90 min, were imaged on a Zeiss EM902 electron spectroscopic microscope operated at 80 kV and ∆E ) 50 eV. The XRM analysis was performed on the X-1A Beamline of the National Synchrotron Light Source at Brookhaven National Laboratory.