Periodic porous stripe patterning in a polymer blend film induced by phase separation during spin-casting

Water vapor induced self-assembly of islands/honeycomb structure by secondary phase separation in polystyrene solution with bimodal molecular weight distribution

The formation of complex structures in thin films is of interest in many fields. Segregation of polymer chains of different molecular weights is a well-known process. However, here, polystyrene with bimodal molecular weight distribution, but no additional chemical modification was used. It was proven that at certain conditions, the phase separation occurred between two fractions of bimodal polystyrene/methyl ethyl ketone solution. The films were prepared by spin-coating, and the segregation between polystyrene phases was investigated by force spectroscopy. Next, water vapour induced secondary phase separation was investigated. The introduction of moist airflow induced the self-assembly of the lower molecular weight into islands and the heavier fraction into a honeycomb. As a result, an easy, fast, and effective method of obtaining island/honeycomb morphologies was demonstrated. The possible mechanisms of the formation of such structures were discussed.

Hierarchical Morphology of Polymer Blend Films Induced by Convection-Driven Solvent Evaporation

Homogeneous thin films of polymer blends with a desired morphology are necessary because of their applications in the fields such as optoelectronics, sensors, biomedicine, and so on. The frequently employed approach for the thin film preparation, spin coating is only able to achieve a homogeneous film for a small area because of the overwhelming spin-driven solvent evaporation with increased size. Here, a convection-guided morphology formation for polystyrene:poly(methyl methacrylate) blend films is reported. In situ observation shows that the morphology changed from homogeneous deposition with a scale less than 10 μm to a self-organized cellular pattern with a scale of more than 100 μm after the fluid flow is involved. Selective dissolution of the hierarchical films reveals that the cellular morphology is attributed to the flow-field-guided deposition of sequentially generated precipitates. The coupling of phase separation and fluid convection results in the hierarchical morphology that includes Voronoi cellular division as the primary structure and the detailed heterogeneous inner-cell features as the secondary structure. Isolated modulation of either micro- or mesoscale in the hierarchical morphology could be carried out via adjusting phase interaction or the convection disturbance correspondingly, providing a flexible and straightforward strategy to construct designed hierarchical structures for polymer thin films toward desired function or property.

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.

Oxygen and relative humidity monitoring with films tailored for enhanced photoluminescence

Approaches to generate porous or doped sensing films, which significantly enhance the photoluminescence (PL) of oxygen optical sensors, and thus improve the signal-to-noise (S/N) ratio, are presented. Tailored films, which enable monitoring the relative humidity (RH) as well, are also presented. Effective porous structures, in which the O2-sensitive dye Pt octaethylporphyrin (PtOEP) or the Pd analog PdOEP was embedded, were realized by first generating blend films of polyethylene glycol (PEG) with polystyrene (PS) or with ethyl cellulose (EC), and then immersing the dried films in water to remove the water-soluble PEG. This approach creates pores (voids) in the sensing films. The dielectric contrast between the films' constituents and the voids increases photon scattering, which in turn increases the optical path of the excitation light within the film, and hence light absorption by the dye, and its PL. Optimized sensing films with a PEG:PS ratio of 1:4 (PEG's molecular weight Mw ∼8000) led to ∼4.4× enhancement in the PL (in comparison to PS films). Lower Mw ∼200 PEG with a PEG:EC ratio of 1:1 led to a PL enhancement of ∼4.7×. Film-dependent PL enhancements were observed at all oxygen concentrations. The strong PL enhancement enables (i) using lower dye (luminophore) concentrations, (ii) reducing power consumption and enhancing the sensor's operational lifetime when using organic light emitting diodes (OLEDs) as excitation sources, (iii) improving performance when using compact photodetectors with no internal gain, and (iv) reliably extending the dynamic range. The effect of RH on O2 sensing is also presented. Dye:EC films are sensitive to the RH, as shown by the change of the dye's PL decay time with RH at a given O2 concentration. Surprisingly, this RH sensitivity vanishes by adding PEG to EC, including by washing PEG off. In contrast, doping EC with TiO2 nanoparticles maintains the RH effect with the advantage of significant PL enhancement. This enhancement enables differentiation of <10% changes in the RH, which is unattained with the dye:EC sensing films. The results are discussed in terms of the composition, thickness, and microstructure, whether porous or nanoparticle doped, of the composite films.

Spontaneous formation of linearly arranged microcraters on sol-gel-derived silica-poly(vinylpyrrolidone) hybrid films induced by Bénard-Marangoni convection

Complex, sophisticated surface patterns on micrometer and nanometer scales are obtained when solvent evaporates from solutions containing nonvolatile solutes dropped on a solid substrate. Such evaporation-driven pattern formation has been utilized as a fabrication process of highly ordered patterns in thin films. Here, we suggested the spontaneous pattern formation induced by Bénard-Marangoni convection triggered by solvent evaporation as a novel patterning process of sol-gel-derived organic-inorganic hybrid films. Microcraters of 1.0-1.5 μm in height and of 100-200 μm in width were spontaneously formed on the surface of silica-poly(vinylpyrrolidone) hybrid films prepared via temperature-controlled dip-coating process, where the surface patterns were linearly arranged parallel to the substrate withdrawal direction. Such highly ordered micropatterns were achieved by Bénard-Marangoni convection activated at high temperatures and the unidirectional flow of the coating solution on the substrate during dip-coating.

Microporous phase-separated films of polymer blends for enhanced outcoupling of light from OLEDs

Thin microporous films were formed by dropcasting a toluene solution containing various ratios of polystyrene:polyethylene glycol blends on a glass substrate, with OLEDs on the ITO that coated the opposite side of that substrate. We demonstrate for the first time that such easily-fabricated films with surface and bulk micropores in the index-matching polystyrene can serve as random microlens-like arrays to improve forward OLED light extraction by up to ~60%. A theoretical interpretation of the angular emission profile of the device, considering the geometrical change at the substrate/air interface and the scattering by the pores within the films, was established in excellent agreement with the experiments. The use of such blended thin films provides an economical method, independent of the OLED fabrication technique, for improving the outcoupling efficiency.