Spin-cast Films Research Articles

Breath Figures in Polymer and Polymer Blend Films Spin-Coated in Dry and Humid Ambience

We investigate effects of two spin-coating parameters, relative humidity (5% < or = RH < or = 80%) in ambient atmosphere and water content (3 wt % < or = f(H2O) < or = 20 wt %) in solution (rich in tetrahydrofuran), on the structure of breath figures (BF) formed in spin-cast films of polar poly(methyl methacrylate) (PMMA) and PMMA mixed with nonpolar polystyrene (PS). Film morphologies, examined with atomic and lateral force microscopy, are analyzed with integral geometry analysis to yield morphological BF measures. In PMMA, water added to solution has much stronger impact than that from moisture on formed BFs, which could be ordered (with conformational entropy S approximately 0.9-1.0). In PMMA/PS, BFs decorate exclusively polar PMMA domains, resulting in morphologies with two length scales (sub-micrometer BFs and domains >10 microm). This suggests a novel strategy for herarchic structure formation in multicomponent polymer films. In PS/PMMA, BFs are better developed than in pure PMMA spin-coated in identical conditions. These observations show that the air boundary layer facing the spin-cast polymer film (region) is more important than the ambient atmosphere.

Equilibrium Pathway of Spin-Coated Polymer Films

Spin-coating is a common method of making thin polymer films. Recent experiments show that polymer films produced by this method are highly nonequilibrated. By monitoring the temporal evolution of the surface structure of freshly spin-cast polystyrene films on Si with molecular weights, 2.3 ≤ Mw ≤ 393 kg/mol, we find that the relaxations can be fully accounted for by thermal excitations of surface capillary waves on the film surface. Modeling of the data based on this relaxation scheme leads to excellent agreement between the viscosity of the films and that of the bulk polymers. Our results provide compelling evidence that thickness uniformity is the major cause of the nonequilibration of the films.

Use of the β-Phase of Poly(9,9-dioctylfluorene) as a Probe into the Interfacial Interplay for the Mixed Bilayer Films Formed by Sequential Spin-Coating

Spin-coating one polymer solution on another spin-cast polymer film is believed to result in unfavorable interfacial mixing. Here, we show some results to demonstrate that some interesting properties will be obtained in such mixed bilayer films formed by sequential spin-coating. Poly(9-vinylcarbazole) and poly(9,9-dioctylfluorene-2,7-diyl) were chosen as the first and second polymer layers, respectively. By varying the initial thickness of the first layer, some interesting variations were observed. The spectroscopic features of beta-phase polyfluorene were utilized to reflect the variations at the complicated interface. Morphologies were also presented to illustrate that the variations of spectroscopic features were accompanied with some interesting morphological changes. On the basis of these results, a schematic model was proposed to gain insight into the mixed interface formed by sequential spin-coating. Polymer light-emitting devices based on such films were also investigated.

Poly(3-hexythiophene)/multi-walled carbon nanotube composites: electrochemical and optical characterization

ABSTRACTCarbon nanotubes (CNT) / conjugated polymer composites have been used for the preparation of thin film-modified electrodes. Thin, short, multi-walled CNT (MWNT) (purity higher than 95%) was completely coated by regioregular poly(3-hexylthiophene) (P3HT) through precipitation of a mixture of both in the non-solvent methanol. This work reports the electrochemical and optical characterization of P3HT and composites after dissolution in xylene and spin casting of films with thickness &lt;1 micrometer. Cyclic voltammetry showed that all samples were characterized by a semi-reversible behavior. The samples with 0% and 2% of MWNT, for instance, present anodic peak potential at +0.67 and +0.68V respectively when cycling at 100 mV/s. Optical properties, such as optical absorption and photoluminescence (PL), have been investigated. The samples showed maximum of absorption at ˜530 nm and intense PL in the orange. PL spectra of nanocomposites exhibited a new band at lower energy characteristic of a structure with higher conjugation length when compared to P3HT.

Micropatterning of thin P3HT films via plasma enhanced polymer transfer printing

We have developed a simple but robust process, plasma enhanced polymer Transfer printing (PEPTP), for fabricating micropatterns of semi-conducting poly(3-hexyl thiophene) (P3HT) thin films. The method is based on transferring thin P3HT film spin cast directly on a pre-patterned elastomeric poly(dimethyl siloxane) (PDMS) mold. Printing is accomplished by the application of oxygen plasma on both P3HT film and substrate for surface energy modulation at ambient conditions without additional pressure. The control of the relative interfacial surface energy by the plasma enables us to micro/nanopattern a wide range of polymers from P3HT to conventional insulating ones on various substrates including Si, glass and polymers over large areas. A bottom contact organic thin film transistor with the printed P3HT patterns using PEPTP exhibits a carrier mobility of approximately 0.02 cm2V−1 s−1 with a relatively high on/off current ratio of 6 × 103.

Directly Imprinted Surface-Emitting Distributed Feedback Structure Polymer Sensor Laser Devices for Enhanced Oxygen Sensitivity

We report on a gas detection enhancement by means of amplified spontaneous emission (ASE) as well as laser emission from the conjugated polymer poly[(2-methoxy-5(2-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV). Upon photo-pumping of spin-cast films, ASE was achieved. Periodic distributed feedback (DFB) structures were fabricated directly in the active polymer layer using the soft-lithographic technique of liquid imprinting. When the structured DFB grating was photo-pumped, laser emission was observed perpendicular to the substrate surface. For both, ASE as well as lasing, we monitored the output emission intensities under the influence of argon and ambient atmosphere resulting in a change in their threshold behavior. We clearly show that these changes are by virtue of a sensitivity enhancement resulting from oxygen acting as reversible quenching defect.

Synthesis, Characterization, and Surface Morphology of Pendant Polyhedral Oligomeric Silsesquioxane Perfluorocyclobutyl Aryl Ether Copolymers

The synthesis and characterization of solution processable, semi-fluorinated perfluorocyclobutyl (PFCB) aryl ether polymers possessing covalently bound pendant polyhedral oligomeric silsesquioxanes (POSS) cages is reported. The synthesis of POSS aryl trifluorovinyl ether (TFVE) monomers was accomplished by the condensation of commercial monosilanolalkyl-POSS with a TFVE-functionalized chlorosilane. POSS TFVE monomers were elucidated by 1H, 19F, 13C, and 29Si NMR spectroscopy, ATR-FTIR analysis, and elemental (C, H, and F) combustion analysis. Bulk thermal copolymerization of POSS TFVE monomers afforded random and block PFCB aryl ether copolymers functionalized with various POSS loadings. Quantitative monomer conversion was monitored by 19F NMR, which produced copolymer number-average molecular weights (Mn) of (19.5−24.9) × 103 (in CHCl3 using polystyrene as standard) determined by gel permeation chromatography (GPC). Thermal properties of POSS PFCB aryl ether copolymers were evaluated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Transmission electron microscope (TEM) analysis of spin cast optically transparent, flexible POSS PFCB aryl ether copolymer films revealed the presence of 5−20 nm-sized POSS agglomerates. Optical profilometer analysis of spin and drop cast film surfaces exhibited a modest increase in surface roughness of POSS PFCB aryl ether copolymers as compared to PFCB aryl ether homopolymers without POSS inclusion. The POSS copolymers exhibited a modest increase in hydrophobicity as measured by static water contact angle analysis. Synthesis, characterization, thermal analysis, and unique surface features of POSS PFCB aryl ether copolymers are discussed.

Site-Selective Doping of Dyes into Polystyrene-block-Poly(4-vinyl pyridine) Diblock Copolymer Films and Selective Laser Ablation of the Dye-Doped Films

The site-selective doping of dyes into spin-cast thin films with different phase separation structures of polystyrene-block-poly(4-vinyl pyridine) (PS-b-P4VP) diblock copolymers and the dopant-induced laser ablation of the dye-doped films fabricated were investigated. The selective doping of dyes into P4VP parts was carried out by immersing the films in a methanol solution containing the dyes, in which the driving force was induced by hydrogen bonds between the nitrogen atoms of pyridyl groups and the carboxylic groups of the dyes. Schematic models for the adsorption process of the dyes in the methanol solution were discussed. Dopant-induced laser ablation was carried out on the dye-doped block copolymer films, resulting in the ablation of dye-doped P4VP parts only. Factors affecting selective ablation, such as dyes, the morphologies of phase separation structures, the number-average molecular weights of the diblock copolymers, and irradiation environments, were discussed.

Morphology and Phase Segregation of Spin-Casted Films of Polyfluorene/PCBM Blends

In this study the morphology of spin-casted films of polymers blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM) has been studied. It was found that the lateral structure formation in the films is favored by rapid solvent evaporation and strong polymer−PCBM repulsion. The formation of homogeneous films is favored by slow evaporation and weak polymer−PCBM repulsion. The effect of solvent evaporation rate is the opposite of what is found for spin-casting polymer−polymer blends. The results can be explained by the kinetics of phase separation and the phase behavior involving limited solubility and crystallization of PCBM.

Formation of Nanowire Striations Driven by Marangoni Instability in Spin-Cast Polymer Thin Films

Parallel striations made of silver nanowires were formed through the Marangoni instability induced during spin casting of poly(2-vinyl pyridine)/silver nanowire/chloroform solutions. The striation patterns of the silver nanowires resembled those obtained from spin casting of the corresponding neat polymer solutions, indicating essentially the same driving mechanism (i.e., the Marangoni instability). The silver nanowires were found to concentrate in the valleys of the striation pattern to balance the nonuniform surface tension distribution in the polymer thin film. The resulting nanowire striation patterns were found to depend on polymer concentration, rotational speed, and nanowire loading. Interestingly, this nanowire striation phenomenon was found to be independent of the substrate characteristics, hydrophobic or hydrophilic.

New Thermally Cross-Linkable Polymer and Its Application as a Hole-Transporting Layer for Solution Processed Multilayer Organic Light Emitting Diodes

A new thermally cross-linkable copolymer containing reactive benzocyclobutene (BCB) units and the well-known hole-transporting moiety N,N‘-bis(3-methylphenyl)-N,N‘-diphenylbenzidine) (TPD) was synthesized and characterized. Thermal annealing of spun-cast films of the copolymer, followed by cross-linking at 200 °C, led to insoluble polymer films with a smooth surface. Green emitting fluorescent OLEDs were fabricated using the new cross-linkable polymer and compared with conventionally prepared devices. Using the thermally cross-linked copolymer as a hole-transporting layer, solution processed multilayer light emitting diodes were prepared that exhibited high performance with 10.4% external quantum efficiency at a brightness of 350 cd/m2.

Luminescent properties of rare earth complexes of Gd1 − xEux(TTA)3Dipy in nano-TiO2

Some kinds of rare earth complexes were synthesized using the ligands of thenoyltrifluoroacetone (HTTA), 2, 2/-dipyridyl(Dipy) and different metal ions (Eu3+ and Gd3+). Their contents, structures and optoelectronic property were monitored by elemental analysis, DTA-TGA, IR and fluorescence spectroscopy. This result showed luminescence of Eu3+ complex was enhanced after doped with Gd3+ , the thermal stability of rare earth complexes was also reported. In addition, the organic–inorganic combined device ITO/PVK: Gd1−xEux(TTA)3Dipy: TiO2/Al was fabricated, the nano-TiO2 has been used in the luminescence layer to change the electroluminescence property of Gd1−xEux(TTA)3Dipy. We have monitored the spectra of Gd1−xEux(TTA)3Dipy with different ratios for spin-cast film r min−1. It was found that there was an efficient energy transfer process between the ligands and metal ions.

Large-area patterning of a solution-processable organic semiconductor to reduce parasitic leakage and off currents in thin-film transistors

We describe two techniques for patterning spin-cast thin films of a solution-processable organic semiconductor, triethylsilylethylnyl anthradithiophene (TES ADT), to eliminate parasitic leakage currents and to lower off currents in thin-film transistors. One technique utilizes UV light in the presence of solvent vapors to simultaneously define the active channel and to crystallize TES ADT. The second technique selectively removes TES ADT from the nonchannel regions of the thin-film transistors through direct contact with a poly(dimethylsiloxane) stamp. Both patterning techniques yield thin-film transistors with high charge-carrier mobility (⩾0.1cm2∕Vs), low off currents (10−10–10−11A), and minimal parasitic leakage.

Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols

High charge-separation efficiency combined with the reduced fabrication costs associated with solution processing and the potential for implementation on flexible substrates make 'plastic' solar cells a compelling option for tomorrow's photovoltaics. Attempts to control the donor/acceptor morphology in bulk heterojunction materials as required for achieving high power-conversion efficiency have, however, met with limited success. By incorporating a few volume per cent of alkanedithiols in the solution used to spin-cast films comprising a low-bandgap polymer and a fullerene derivative, the power-conversion efficiency of photovoltaic cells (air-mass 1.5 global conditions) is increased from 2.8% to 5.5% through altering the bulk heterojunction morphology. This discovery can potentially enable morphological control in bulk heterojunction materials where thermal annealing is either undesirable or ineffective.

Thin film morphology and ion interaction behaviour of functional polymers for iontronic applications

Based on comprehensive studies by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) we report on the morphology and the ion–ion as well as the ion–polymer interaction properties of thin spin-cast films comprising an oligo(ethylene oxide) (OEO) grafted poly( p-phenylene) blended with either lithium- or tetrabutylammonium trifluoromethanesulfonate. Assuming that the ionic species intercalate within the OEO side-chain matrix, the covalent attachment of OEO appears to be an appropriate strategy to overcome the generally observed phenomenon of large-scale phase-separations between conjugated polymers and solid-state electrolytes. XPS turns out to be a suitable approach to study the basics of the ion–ion and the ion–polymer interactions of these model systems directly for the thin, spin-cast films.

Unique Molecular Orientation in a Smectic Liquid Crystalline Polymer Film Attained by Surface-Initiated Graft Polymerization

Liquid crystalline (LC) polymer brushes containing a mesogenic azobenzene (Az) moiety are synthesized on a quartz or silicon substrate by surface-initiated atom transfer radical polymerization. The molecular orientation of the Az units and the LC properties in the grafted chains are evaluated by UV-vis spectroscopy, polarized optical microscopy, and grazing incidence X-ray diffraction measurements. The Az side chains of the grafted chains exhibited a smectic LC phase in which the smectic layers are oriented perpendicular to the substrate with a parallel orientation of the mesogens. In contrast, a spincast film of the identical LC polymer without grafting to the surface shows layer structures parallel to the substrate. A drastic effect of tethering one end to the substrate on the LC orientation is demonstrated.

Compositional Mismatch between Chemical Patterns on a Substrate and Polymer Blends Yielding Spin-Cast Films with Subpattern Periodicity

Two blends (1:1 and 5:3 w:w) of poly(2-vinylpyridine) (PVP) and partly brominated polystyrene were spin-cast (with constant inherent domain scale 2R = 4.2 ± 0.5 μm) onto a gold substrate which had been microcontact printed with stripes of hexadecanethiol SAM. Two chemical patterns on the substrate were used, one with symmetrical stripes (4 μm Au/4 μm hexadecane SAM) and the other with narrow gold stripes and wider SAM stripes (3 μm/5 μm). The resulting film morphologies were mapped with atomic force microscopy. Secondary ion mass spectrometry shows that raised regions on the film surface correspond to PVP-rich domains, and the surface polymer composition extends all the way down to the substrate. Fourier analysis reveals structural modes (λ/n) smaller than the pattern periodicity λ = 8 μm. PVP preferentially adsorbs to the gold regions of the pattern. For the asymmetric pattern, which has a relatively small Au area interacting with the PVP, the fundamental mode (n = 1) is extinguished and higher-order substructures (n > 1) are seen. Use of a PVP-rich blend increases the intensity of the n = 3 mode and reduces that of the n = 2 mode. When both blend and pattern were asymmetric, both λ/2- and λ/3-substructures were observed, and the fundamental pattern periodicity was extinguished.

Study of poly(vinylidene fluoride-trifluoroethylene) as a potential organic high K gate dielectric

Spin cast films of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer deposited on bare Si and SiO2 coated Si substrates were vacuum annealed and treated in supercritical carbon dioxide (scCO2) and the dielectric properties of the films before and after treatments were studied using capacitance-voltage and conductance-voltage techniques on thin film capacitor structures. After annealing, the static dielectric constant (K) for P(VDF-TrFE) was found to be more than double that for SiO2 films and to increase with film thickness and annealing time, and reached a maximum of nearly 10 for thick films on SiO2 or Si substrates. Treating annealed P(VDF-TrFE) films in scCO2 initially decreased K and the refractive index n to as-deposited values that increased and stabilized after reannealing. The changes in P(VDF-TrFE) were attributed to densification and ordering. No systematic differences in interface charges and states were found between treated and untreated samples and with various substrates. The leakage current for P(VDF-TrFE) films was found to be higher than that for SiO2.

Processing Methods of Ultrathin Poly(ε-caprolactone) Films for Tissue Engineering Applications

Ultrathin poly(epsilon-caprolactone) (PCL) films were fabricated through biaxially drawn films made from three different methods, namely, spin casting, 2-roll milling, and solution casting. Biaxial drawn spin cast films yield thickness of 1.2 microm which is 9 and 12 times thinner that 2-roll mill and solvent cast films, respectively. The films fabricated were found to exhibit different drawing ratios. 2-roll mill film exhibits the highest drawing ratio of 4 x 4 while spin cast films can only draw up to a ratio of 2 x 2. The morphology of the films, studied using a polarized microscope and atomic force microscope, showed fine fibrillar networks of different thicknesses. Biaxially drawn 2-roll mill and solvent cast films showed thicker fibrils as compared to those for the spin cast films. Such a difference can be attributed to larger spherulites caused by slower cooling rates during melt pressing for both 2-roll mill and solvent cast films and smaller spherulites because of fast cooling during evaporation for spin cast films. Thermal analysis through differential scanning calorimetry revealed a slight increase in the peak-melting temperature after biaxial drawing. A drop in percentage crystallinity was also noted. The result of the water vapor transmission rate (WVTR) was found to be dependent on fabrication techniques that determine the spherulites formation. It was also found that the WVTR was inversely proportional to the thickness of the films. Tensile strength and modulus of the films showed significant improvements after biaxial stretching. By identifying the unique strengths of each individual PCL film produced via different techniques, it is possible to apply to different areas of membrane tissue engineering such as dermatology, ophthalmology, vascular graft engineering, and soft tissue regeneration.

Poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) conjugated polymer domains in a thermoplastic polyurethane matrix

Self-sustained and spin-casting films formed from blends of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) conjugated polymer and thermoplastic poly-urethane (TPU) at different concentrations have been investigated. Scanning electron microscopy images of self-sustained films show the formation of circularlike domains of MEH-PPV in the TPU matrix, characterizing the blend material as heterogeneous. The circularlike domains in the spin-casting films were also observed by electric force microscopy (EFM) and by atomic force microscopy. The EFM, in particular, has allowed the differentiation of the MEH-PPV domains from the TPU matrix due to the electrical properties of the conjugated polymer. The MEH-PPV domains in the spin-casting films are much smaller in size than in the case of self-sustained films. The glass transition temperatures for the TPU and for the self-sustained blended films were obtained by temperature modulated differential scanning calorimetry. A very small variation in the glass transition temperature (Tg) has been observed for the blends. This fact further corroborates that MEH-PPV and TPU form immiscible blends. Intermolecular interactions between the MEH-PPV and the TPU materials were proposed to occur at the interfaces of the domains. These interfacial interactions have not affected the Raman modes, which pointed out that they are relatively weak interactions. The formation of small conjugation segments of the MEH-PPV molecules due to these interfacial interactions with the TPU molecules at the interfaces of the domains was assigned to be responsible by the appearance of a large tail and clear shoulders to the high energy side of the photoluminescence (PL) spectra of self-sustained samples. This was also assigned to be responsible by the blueshift observed in the PL spectra of spin-casting films. The redshift of the pure electronic transition peak and the first vibronic band (which presents an inhomogeneous broadening) were observed in the self-sustained blended films and were related, respectively, to the increase of the internal intermolecular and intramolecular interactions due to the conformational disorder of the MEH-PPV molecules imposed by the casting spread technique.