Spin-cast Films Research Articles

Substrate-Dependent Physical Aging of Confined Nafion Thin Films.

The humidity-induced physical aging, or structural relaxation, of spin-cast Nafion thin films on gold, carbon, and native oxide silicon (n-SiO2) substrates was examined using spectroscopic ellipsometry (SE). Physical aging rates, β, were calculated from the change in measured sample thickness, h, upon exposure to controlled humidity. Three Nafion films, h = 188, 57, and 27 nm, deposited on gold substrates demonstrated an increased β with decreasing thickness due to confinement. The Nafion film on n-SiO2, h = 165 nm, also showed a humidity-induced aging, while a Nafion film deposited on carbon, h = 190 nm, exhibited no measurable humidity-induced aging. The reported rate of aging was related to the strength of the polymer/substrate interactions during film formation. Strong interactions between Nafion and the gold and n-SiO2 substrates anchored the thin film to the substrate during film formation, resulting in a nonequilibrium as-cast film and subsequent relaxation upon exposure to water vapor until complete plasticization. Weak interactions between the carbon substrate and Nafion resulted in fully relaxed as-cast films which displayed no relaxation upon hydration.

Morphology‐Driven Sensitivity Enhancement of MEH‐PPV Langmuir‐Blodgett Films on Plastic Substrates for NO2 Gas

Abstract3D ordered Langmuir–Blodgett (LB) films of poly[2‐methoxy‐5‐ (2‐ethylhexyloxy)‐1, 4‐phenylenevinylene] (MEH‐PPV) were prepared via thermal annealing. Surface properties of MEH‐PPV prepared by using different solvents, through both LB and Spin Casting techniques, were examined at the air‐water interface by measuring surface pressure‐area isotherms and Brewster Angle Microscopy. Dramatically different surface morphologies were obtained for spin cast and LB films, both prepared by using different solvents. The annealed LB films showed enhanced NO2 gas sensing as compared to spin cast films. Spectroscopy and morphological analyses reveal that enhanced response of LB films toward NO2 gas is due to their 3D ordered morphology, preferred molecular orientation for in‐plane transport as well as higher surface area.

Elongated phase separation domains in spin-cast polymer blend thin films characterized using a panoramic image.

Polymer thin films with micro/nano-structures can be prepared by a solvent evaporation induced phase separation process via spin-casting a polymer blend, where the elongated phase separation domains are always inevitable. The striation defect, as a thickness nonunifomity in spin-cast films, is generally coexistent with the elongated domains. Herein, the morphologies of polymer blend thin films are recorded from the spin-cast center to the edge in a panoramic view. The elongated domains are inclined to appear at the ridge regions of striations with increasing radial distance and align radially, exhibiting a coupling between the phase separation morphology and the striation defect that may exist. We demonstrate that the formation of elongated domains is not attributed to shape deformation, but is accomplished in situ. A possible model to describe the initiation and evolution of the polymer blend phase separation morphology during spin-casting is proposed.

Spotlight on Excitonic Coupling in Polymorphic and Textured Anilino Squaraine Thin Films

Structural diffraction analysis of an anilino squaraine with branched isobutyl side chains shows crystallization into two polymorphic structures in the bulk and in spin-casted thin films. We observe multipeaked and pleochroic absorption spectra being blue-(red)-shifted for the monoclinic (orthorhombic) polymorph. We understand the packing as Coulombic molecular H-(J)-aggregates supporting Davydov splitting. Pictures of projected Davydov components in oriented thin films fit well to polarization resolved spectro-microscopy and crossed-polarized light microscopy investigations. By comparison with literature on anilino squaraines with linear alkyl side chains, we point out a general trend for steering the thin film excitonic properties by simple side chain and/or processing condition variation. Combined with the ability to locally probe the direction of transition dipole moments, this adds value to the rational design of functional thin films for optoelectronic applications, especially envisioning ultrastron...

Disentangling "Bright" and "Dark" Interactions in Ordered Assemblies of Organic Semiconductors.

We report on spatially correlated wavelength-resolved photoluminescence and Kelvin probe force microscopy to probe ground state charge-transfer coupling and its correlation with pi-stacking order in nanoscale assemblies of a small molecule n-type organic semiconductor, tetraazaterrylene (TAT). We find a distinct upshift in surface potential contrast (SPC) corresponding to a decrease in work function in TAT in the transition from disordered spun-cast films to ordered crystalline nanowire assemblies, accompanied by a nanowire size dependence in the SPC shift suggesting that the shift depends on both ground state charge transfer interaction and a size (volume)-dependent intrinsic doping associated with the nitrogen substitutions. For the smallest nanowires studied (surface height ≈ 10-15 nm), the SPC shift with respect to disordered films is +110 meV, in close agreement with recent theoretical calculations. These results illustrate how "dark" (ground-state) interactions in organic semiconductors can be distinguished from "bright" (excited-state) exciton coupling typically assessed by spectral measurements alone.

Thickness dependence of structural relaxation in spin‐cast polynorbornene films with high glass transition temperatures (>613 K)

ABSTRACTThe isothermal structural relaxation (densification) of a family of glassy polynorbornene films with high glass transition temperatures (Tg > 613 K) is assessed via spectroscopic ellipsometry. Three polymers were examined: poly(butylnorbornene) (BuNB), poly(hydroxyhexafluoroisopropyl norbornene) (HFANB), and their random copolymer, BuNB‐r‐HFANB. The effective aging rate, β(T), of thick (∼1.2 μm) spun cast films of BuNB‐r‐HFANB is approximately 10−3 over a wide temperature window (0.49 < T/Tg < 0.68). At higher temperatures, these polymers undergo reactions that more dramatically decrease the film thickness, which prohibits erasing the process history by annealing above Tg. The aging rate for thick BuNB‐r‐HFANB films is independent of the casting solvent, which infers that rapid aging is not associated with residual solvent. β (at 373 K) decreases for films thinner than ∼500 nm. However, the isothermal structural relaxation of thin films of BuNB‐r‐HFANB exhibits nonmonotonic temporal evolution in thickness for films thinner than 115 nm film. The thickness after 18 h of aging at 373 K can be greater than the initial thickness. The rapid aging of these polynorbornene films is attributed to the unusual rapid local dynamics of this class of polymers and demonstrates the potential for unexpected structural relaxations in membranes and thin films of high‐Tg polymers that could impact their performance. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 53–61

Controlling Morphology and Aggregation in Semiconducting Polymers: The Role of Solvents on Lasing Emission in Poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene-vinylene

Systematic experiments were performed to investigate solvent-dependent morphology and aggregation of the semiconducting polymer film poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV), which was span-cast from nonaromatic strong polarity solvents tetrahydrofuran (THF), trichloromethane (TCM) and aromatic weak polarity solvents chlorobenzene (CB), toluene, and p-xylene. The results indicated that the conformation of the spin-cast MEH-PPV films with weak aggregation such as THF and TCM demonstrated excellent lasing emission performances because of inhibiting the fluorescence quenching induced by bi-molecule process. The Atomic Force Microscope (AFM) images confirmed the distinct morphologies of the spin-cast MEH-PPV films. The amplified spontaneous emission (ASE) was investigated in a simple asymmetric slab planar waveguide structure by methods of variable stripe length (VSL) and shifting excitation stripe (SES). The amplified spontaneous emission (ASE) experiments confirmed the distinct polymer chain conformation. The conformation, which preserved from the spin-cast process, indicated the distinct interactions between solvents and MEH-PPV polymer chains. The pure film spectra were performed to confirm the effect of distinct conformation on the material energy level. This work provides insights into the morphology and aggregation effect of the spin-cast polymer films on the performances of lasers.

Tunable Anisotropic Photon Emission from Self-Organized CsPbBr3 Perovskite Nanocrystals.

We report controllable anisotropic light emission of photons originating from vertically aligned transition dipole moments in spun-cast films of CsPbBr3 nanocubes. By depositing films of nanocrystals on precoated substrates we can control the packing density and resultant radiation pattern of the emitted photons. We develop a technical framework to calculate the average orientation of light emitters, i.e., the angle between the transition dipole moment vector (TDM) and the substrate. This model is applicable to any emissive material with a known refractive index. Theoretical modeling indicates that oriented emission originates from an anisotropic alignment of the valence band and conduction band edge states on the ionic crystal lattice and demonstrates a general path to model the experimentally less accessible internal electric field of a nanosystem from the photoluminescent anisotropy. The uniquely accessible surface of the perovskite nanoparticles allows for perturbation of the normally isotropic emissive transition. The reported sensitive and tunable TDM orientation and control of emitted light will allow for applications of perovskite nanocrystals in a wide range of photonic technologies inaccessible to traditional light emitters.

Detection of Ultralow Concentrations of Non-emissive Conjugated Polymer Aggregates via Fluorescence Correlation Spectroscopy.

The aggregation of conjugated polymers in common organic solvents is investigated using fluorescence correlation spectroscopy (FCS), burst analysis, and microscopy. Poly(3-hexylthiophene) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] are both shown to form weakly bonded non-emissive aggregates in toluene that persist even at picomolar concentrations. These aggregates decrease the bulk emission intensity in solution but do not affect the fluorescence spectra or lifetimes, consistent with a static quenching mechanism. Passing the solutions through a syringe filter causes an increase in the number of emitters as measured by FCS, indicating that this process dissociates the aggregates. Films cast from solutions that have been filtered are more uniform and significantly more emissive than those made from unfiltered solutions. These results show that FCS is a highly sensitive probe of non-emissive aggregates in solution that have a deleterious effect on the emission properties and overall quality of spin-cast thin films, even at sub-nanomolar concentrations.

Short chains for a smart film.

Nanomaterials Polymer films capable of changing their structural properties at the nanoscale in response to physiological temperature changes could have a number of biological applications. To achieve periodic structures with feature sizes below 10 nm, an amphiphilic polymer must join highly immiscible polymer blocks with short chains. Nowak et al. synthesized an amphiphile that joined a disaccharide and a polypropylene oligomer with a triazole linkage. Spin-cast ultrathin films of this material formed “fingerprint” lamellar domains of the blocks that were perpendicular to the surface with a spacing of ∼6 nm at room temperature. However, heating to 38°C for 72 hours led to a nanostructure in which the chains were stacked parallel to the surface in head-tail bilayers. J. Am. Chem. Soc. 10.1021/jacs.6b13285 (2017).

Semifluorinated Synergistic Nonfouling/Fouling-Release Surface.

The preparation of a fluorine-containing synergistic nonfouling/fouling-release surface, using a b-PFMA-PEO asymmetric molecular brush possessing both poly(ethylene glycol) (PEO) and poly(2,2,2-trifluoroethyl methacrylate) (PFMA) side chains densely distributed on the same repeat unit along the polymeric backbone, is reported. On the basis of the poly(Br-acrylate-alkyne) macroagent comprising two functionalities (alkynyl and 2-bromopropionate), which is prepared by reversible addition-fragmentation chain transfer homopolymerization of a new trifunctional acrylate monomer of Br-acrylate-alkyne, b-PFMA-PEO asymmetric molecular brushes are obtained by concurrent atom transfer radical polymerization and Cu-catalyzed azide/alkyne cycloaddition "click" reaction in a one-shot system. A spin-cast thin film of the b-PFMA-PEO asymmetric molecular brush exhibits a synergistic antifouling property, in which PEO side chains endow the surface with a nonfouling characteristic, whereas PFMA side chains display the fouling-release functionality because of their low surface energy. Both protein adsorption and cell adhesion tests provided estimates of the antifouling activity of the asymmetric molecular brush surfaces, which was demonstrated to be influenced by the degree of polymerization of the backbone and the length of the PEO and PFMA side chains. With compositional heterogeneities, all asymmetric molecular brush surfaces show considerable antifouling performance with much less protein adsorption (at least 45% off, up to 75% off) and cell adhesion (at least 70% off, up to 90% off) in comparison with a bare surface.

Cellulose nanofiber/cationic conjugated polymer hybrid aerogel sensor for nitroaromatic vapors detection

A novel porous aerogel of cellulose nanofiber (CNF)/cationic water-soluble poly[9,9-bis[3′-(N,N-dimethy)-N-ethylammonium)propyl)-2,7-fluorene-alt-1,4-phenylene]dibromide (CPFD) is prepared by freeze drying. CNF can effectively prevent aggregation of the conjugated polymer CPFD backbones. The CNF/CPFD hybrid aerogel is use for the detection of nitroaromatic (NAC) vapors. Due to the porous structure, the CNF/CPFD hybrid aerogel possesses a large number of accessible cavities, which could be sufficiently large to allow the diffusion of NAC vapors into the aerogel. As a result, the CNF/CPFD aerogel sensor shows high sensitivity toward NAC vapors. For 120-s exposure, the fluorescence quenching efficiency of the CNF/CPFD aerogel sensor toward 2,4-dinitrotoluene vapor is up to 85.9%, which is much larger than that of the spin-cast CPFD film. Furthermore, the sensing performance of the CNF/CPFD hybrid aerogel is not heavily dependent on the thickness, and the sensing process of the CNF/CPFD aerogel sensor is reversible.

Supramolecular Polymer of Near-Infrared Luminescent Porphyrin Glass

A comprehensive study of supramolecular polymerization of ditopic zinc (2-pyridylethynyl)porphyrin dimer 1 in toluene and thin films was performed. A glass-forming porphyrin bearing 3,4,5-tri((S)-3,7-dimethyloctyloxy)phenyl groups, named “porphyrin glass”, was introduced with the 2-pyridylethynyl group as a supramolecular organizing unit; two zinc (2-pyridylethynyl)porphyrins were held together by self-complementary pyridyl-to-zinc coordination bonds to form a slipped-cofacial stack. Then, ditopic zinc (2-pyridylethynyl)porphyrin could be extended to a linear supramolecular polymer. The small binding constant limited the degree of supramolecular polymerization of 1 in toluene solution. In spin-cast film, on the other hand, 1 adopted a form of supramolecular polymer of porphyrin glass, which was effective enough to display a large bathochromic shift of the absorption bands exceeding the narrowest limit of the optical band gap extrapolated from the electronic structures in solution. The supramolecularly pol...

Air-Liquid Interfacial Self-Assembly of Non-Amphiphilic Poly(3-hexylthiophene) Homopolymers.

Here, we demonstrate that the self-assembly of poly(3-hexylthiophene) (P3HT) at the air-water interface can lead to free-standing films of densely packed P3HT nanowires. Interfacial self-assembly on various liquid subphases, such as water, diethylene glycol, and glycerol, indicates that the viscosity of the subphase is an important factor for the formation of well-ordered nanostructures. The thin-film morphology is also sensitive to the concentration of P3HT, its molecular weight (MW), and the presence of oxidative defects. The densely packed nanowire films can be easily transferred to solid substrates for device applications. The ultrathin films of P3HT prepared by the interfacial assembly showed significantly higher hole mobility (∼3.6 × 10-2 cm2/V s) in a field-effect transistor than comparably thin spin-cast films. This work demonstrates that the air-liquid interfacial assembly is not limited to amphiphilic polymers and can, under optimized conditions, be applied to fabricate ultrathin films of widely used conjugated polymers with controlled morphologies.

High-Performance Organic Field-Effect Transistors Fabricated Based on a Novel Ternary π-Conjugated Copolymer.

In this study, we developed a ternary conjugated polymer, IFBT-TT, consisting of centrosymmetric indaceno[1,2-b:5,6-b']dithiophene and thieno[3,2-b]thiophene as the electron-donating units and an asymmetric 5-fluorobenzo[c][1,2,5]thiadiazole as the electron-accepting unit. The target copolymer was synthesized using an acceptor-donor-acceptor (A-D-A) type of macromonomer, which gave the target copolymer a precisely defined D1-A-D2-A architecture. Theoretical simulation revealed that the IFBT-TT features C-H···N and F···S nonbonding interactions, leading to a highly rigid and planar molecular backbone. Although the spin-cast IFBT-TT films exhibited an amorphous morphology lacking in ordered structures, the fabricated field-effect transistors presented remarkable p-type transport properties with high mobility of up to 5.0 cm2 V-1 s-1 and excellent ambient stability. These observations highlight that the integration of a three-component D1-A-D2-A-type backbone framework is an effective molecular design strategy for high-mobility conjugated polymers.

アゾベンゼン側鎖を有するポリマーブラシ膜の液晶相転移挙動

High density liquid crystalline (LC) polymer brushes with side chain LC azobenzene were synthesized via surface-initiated atom transfer radical polymerization. LC phase transition behavior was investigated by in situ UV-visible absorption spectroscopy and grazing incidence small angle X-ray scattering (GI-SAXS). The free LC polymer chains in a spincast film exhibited three distinct thermal transitions, glass phase-smectic C (SmC)-smectic A (SmA)-isotropic, which correspond to the phase change behavior in the bulk state. On the other hand, the brush film only exhibited the SmA phase in the LC temperature range and the SmA-SmC phase transition was inhibited. This fact indicates that the LC phase transition behavior in the LC polymer brush is different from that in the free LC polymer of the spincast film. The high density grafted polymer brush structure constrains the mobility of the main chains, which impedes the tilt of mesogenic side chains.

Voltage-induced Infrared Absorption from a Spin-cast Thin Film of Ferroelectric Poly(vinvlidene fhioride-co-trifluoroethvlene) (P(VDF-TrFE))

Voltage-induced infrared spectra of annealed spin-cast thin films of ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) (molar ratio, 3:1) were measured in a stepwise cyclic external electric field. Most of the observed infrared bands originated from the β ferroelectric crystalline phase. The voltage-induced spectral changes were decomposed into zeroth- (original), first-, and second-derivative spectra, and were attributed to the rotational motions of the polymer chains and the vibrational Stark effect. The values of the original spectral absorbance change ratios, ΔA/A, for the 849-cm-1 band (CF2 symmetric stretching, a1) and the 884-cm-1 band (CH2 rocking, b2) of the film exhibited double minimum and maximum peak hysteresis loops, respectively. The intensity of each band increased or decreased suddenly near a coercive field of ±0.6 MV/cm. These sudden intensity changes were attributed to the rotational inversion of the polymer chains that are associated with ferroelectricity.

Poly(3-hexylthiophene)/gold nanoparticle nanocomposites: relationship between morphology and electrical conductivity

The morphology and electrical properties of gold nanoparticles (AuNP) layer vacuum-deposited onto spin-cast thin films of poly(3-hexylthiophene), P3HT, were studied. The electrical conductivity was measured during temperature cycling and related to the morphology of the same composite structures, which was monitored by transmission electron microscopy (TEM) and extra-high resolution scanning electron microscopy (XHR SEM). Comparison to the analogous polystyrene/AuNP layers was made to distinguish the role of the polymer support on the morphology and electrical properties of the nanoparticles assembly. Gold deposited in a very thin layer formed a nanoparticles-like island structure with the morphology depending on the effective thickness of the deposited layer and on its subsequent thermal treatment. A stabilizing effect of the thiophene–gold interaction on the nanoparticles morphology was observed.

Elastomeric Light Emitting Polymer Enhanced by Interpenetrating Networks.

Conjugated polymers containing long-chain alkyl side groups for solubility are generally unstretchable: large strain induces crack formation, fracture, or plastic deformation. When the polymers are stretched to reorient the conjugated chains along the stretching direction, high dichroic ratio is observed both for absorption of the ground state and radiative decay of the excited state. Here an interpenetrating polymer network (IPN) approach is reported to impart elastomeric deformability to a conjugated polymer. A soluble alkyloxy phenyl substituted poly(1,4-phenylenevinylene) (SY-PPV) with bright yellow fluorescent emission was admixed with an ionically conductive medium containing poly(ethylene oxide), exoxylated trimethylolpropanetriacrylate, and lithium trifluoromethanesulfonate. The spin-cast blend film formed an IPN morphology wherein SY-PPV forms a porous network with pores filled by the ionic medium. PeakForce quantitative nanomechanical mapping showed that the local Young's modulus was high in the SY-PPV phase, while the ionic phase was two-times softer. No global polarization of the SY-PPV chains was observed at strains up to 100% as the dichroic ratio remains close to 1. Light-emitting devices based on the blend sandwiched between two stretchable transparent composite electrodes could be stretched by up to 140% strain. No electroluminescence polarization was observed.

Role of Solution Structure in Self-Assembly of Conjugated Block Copolymer Thin Films

Conjugated block copolymers provide a pathway to achieve thermally stable nanostructured thin films for organic solar cells. We characterized the structural evolution of poly(3-hexylthiophene)-block-poly(diketopyrrolopyrrole–terthiophene) (P3HT-b-DPPT-T) from solution to nanostructured thin films. Aggregation of the DPPT-T block of P3HT-b-DPPT-T was found in solution by small-angle X-ray scattering with the P3HT block remaining well-solvated. The nanostructure in thin films was determined using a combination of wide and small-angle X-ray scattering techniques as a function of processing conditions. The structure in solution controlled the initial nanostructure in spin-cast thin films, allowing subsequent thermal annealing processes to further improve the ordering. In contrast to the results for thin films, nanostructural ordering was not observed in the bulk samples by small-angle X-ray scattering. These results suggest the importance of controlling solvent induced aggregation in forming nanostructured th...