Phase separation induced ordered patterns in thin polymer blend films

Strategies for the utilization of phase separation to generate ordered pattern in polymer thin film are reviewed. First, the fundamental theory and factors influencing phase separation in polymer thin film are discussed. That is followed by a discussion of the formation of ordered patterns induced by phase separation in polymer thin films under the influence of a chemical heterogeneous substrate, convection or breath figures. The mechanisms and the conditions for well-ordered structures generated by phase separation are then discussed to show that multi-scaled/multi-component patterns, stimuli-responsive patterns may be developed by controlling the preparation conditions or exposing the sample to different environments more complex structures. Finally, applications of fabricated patterns in pattern generation and reproduction, antireflecting coating, catalysis, bio-chips and optoelectronics are also discussed.     

Distribution of particles during solvent evaporation from films

We examine films, subject to evaporation, that contain particles. The volume fraction of particles increases as solvent is removed up to a maximum value of close packing. The governing equation for evolution of particle volume fraction is derived and numerical solutions obtained for various Peclet numbers. For large Peclet numbers an asymptotic solution gives the position and functional form of a front of close packed particles which passes through the film. For intermediate Peclet numbers the scaling for the spatial gradient in volume fraction is found to be Pe^1/2. This is different from conventional sedimentation, where convection rather than diffusion dominates.     

Ordered porous polymer films via phase separation in humidity environment

A transition of morphology from island-like structure to disordered and ordered holes on the surface of polystyrene (PS) and poly(2-vinylpyridine) (PVP) blend films were observed with the increase of humidity. At appropriate weight ratio of PS/PVP and PS molecular weight, when humidity reached to a critical value, the hexagonal arrays of holes formed for PS/PVP blend films due to ‘breath figures’ stabilized by PVP with its strong hygroscopic characteristics during phase separation.     

Pattern formation in polymer films under the mask

This paper presents a straightforward method for patterning thin films of polymers, i.e. a prepatterned mask is used to induce self-assembly of polymers and the resulting pattern is the same as the lateral structures in the mask on a submicrometre length scale. The patterns can be formed at above T_g+30 °C in a short time and the external electric field is not crucial. Electrostatic force is assumed to be the driving force for the pattern transfer. Viscous fingering and novel stress-relief lateral morphology induced under the featureless mask are also observed and the formation mechanisms are discussed.     

Salami pattern formation during phase separation induced by polymerization of 2-chlorostyrene in the presence of polystyrene

Development of morphological structures during phase separation induced by radical polymerization of 2-chlorostyrene in the presence of polystyrene was studied with an electron microscope. No stirring of the mixture was made. Domain structure changed significantly with initial monomer composition within a narrow range. Salami domains similar to those observed in high-impact polystyrene (HIPS) formed at certain initial compositions, notwithstanding that production of graft polymers, which was known to play an essential role of the salami structure formation in HIPS, was negligible in this system. An explanation for the mechanism of salami pattern formation was proposed.     

Patterning thin polymer films by surface-directed dewetting and pattern transfer

The pattern evolution processes of thin polystyrene (PS) film on chemically patterned substrates during dewetting have been investigated experimentally. The substrates have patterns of self-assembly monolayers produced by microcontact printing with octadecyltrichlorosilane. Optical microscopy and atomic force microscopy images reveal that ordered micrometer scale pattern can be created by surface direct dewetting. Various pattern sizes and pattern complexities can be achieved by controlling the experimental parameters. The dewetting pattern has been transferred to form PDMS stamp for soft lithography.     

热诱导相分离法制备高分子微孔膜的原理与进展

简述了热诱导相分离 (TIPS)法制备高分子微孔膜的相平衡热力学及相分离动力学原理 ,并对国内外研究进展进行了评述     

Drying-induced reduction in electrical resistivity of carbon black-polyamideimide nanocomposite films

We provide evidence that the electrical resistivity of carbon black (CB)/polyamideimide (PAI) composites are tunable by drying below the glass transition temperature of the polymer matrix. Solution casting experiments of N-methyl-2-pyrrolidone (NMP)/CB/PAI systems reveal that with increasing drying temperature, the measured volume resistivity reduces by four orders of magnitude in an intermediate range of particle volume fractions, which is between 12% and 15%. However, no temperature-dependence was observed for particle-free polymeric films and nanocomposites with high particle volume fractions, which is above 23%. The measured values obey a single master curve, independent of the primary size and surface pH of the nanoparticles. This suggests that the desorption of PAI molecules from the nanoparticle surface enhances the evolution of conductive pathway formation in the course of drying.     

Phase separation in polymer blend thin films studied by differential AC chip calorimetry

AC chip calorimetry is used to study the phase separation behavior of 100 nm thin poly(vinyl methyl ether)/poly(styrene) (PVME/PS) blend films. Using the on-chip heaters, very short (10 ms-10 s) temperature jumps into the temperature window of phase separation are applied, simulating laser heating induced patterning. These temperature pulses produce a measurable shift in the glass transition temperature, evidencing phase separation. The effect of pulse length and height on phase separation can be studied. The thus phase separated PVME/PS thin films remix rapidly, in contrast with measurements in bulk. AC chip calorimetry seems to be a more sensitive technique than atomic force microscopy to detect the early stages of phase separation in polymer blend thin films.     

Effect of polymer density on polyethylene hollow fiber membrane formation via thermally induced phase separation

Microporous high‐density polyethylene (HDPE) and low‐density polyethylene (LDPE) hollow fiber membranes were prepared from polyethylene–diisodecyl phthalate solution via thermally induced phase separation. Effect of the polyethylene density on the membrane structure and performance was investigated. The HDPE membrane showed about five times higher water permeability than the LDPE membrane because it had the larger pore and the higher porosity at the outer membrane surface. The formation of the larger pore was owing to both the initial larger structure formed by spinodal decomposition and the suppression of the diluent evaporation from the outer membrane surface due to the higher solution viscosity. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 471–474, 2004     

Phase dynamics and wetting layer formation mechanisms of pattern-directed phase separation in binary polymer mixture films with asymmetry compositions

Focusing on the binary polymer mixture films under the off-critical condition, the phase dynamics and wetting layer formation mechanisms of pattern-directed phase separation are numerically investigated. The simulated results demonstrate that, for different compositions, the polymer mixtures on the strip patterned surface can exhibit various phase morphologies in the strips of the bulk, which can be used to tailor the microscopic structures of films. The evolutions of these phase structures in the strips of the bulk obey almost the same power law with an exponent of 1/3, i.e., the Lifshitz-Slyozov growth law for the films with various off-critical degrees. It is found that the wetting layer thickness near the patterned surface grows logarithmically at the initial stages, just like the wetting layer formation mechanism of the polymer mixture near the surface with an isotropic potential. This revels that only patterning the surface potential may not change the growth law of the wetting layer. The simulated results also indicate that the diffusion of the component in the direction parallel to the surface originates from the edge of the strips.     

Novel nanofiltration membranes with tunable permselectivity by polymer mediated phase separation in polyamide selective layer

Microstructure in selective layer has played a decisive role in permselectivity of nanofiltration (NF) membranes, and nanomaterials were well-known additives that had been applied to mediate the microstructure and permeability of polyamide NF membranes. However, nanoadditives generally displayed a poor dispersion in membranes or in fabrication process. To solve this problem, we showed an interesting concept that novel NF membranes with hybrid selective layer consisting of flexible polyisobutylene (PIB) and rigid polyamide could be fabricated from wel-defined interfacial polymerization. The hydrophobic polymer mediated phase separation and microdomains formation in polyamide layer were found. The immiscibility between the rigid polyamide and flexible PIB as well as the resultant interface effect was interpreted as the reason for the polymer enhanced permselectivity, which was similar with the well-known thin film nanocomposite (TFN) membranes that nanoparticles incorporated contributed significantly to membrane permeability and rejection performance. Our results have demonstrated that novel NF membranes with enhanced performance can be prepared from im-miscible polymers, which is a new area that has not been extensively studied before.     

Janus-like polymer particles prepared via internal phase separation from emulsified polymer/oil droplets

Poly(methylmethacrylate) (PMMA) and polystyrene(PS)/PMMA particles with Janus-like morphology were prepared via the internal phase separation followed by extraction of hexadecane (HD) template. The internal phase separation was triggered by evaporation of dichloromethane (DCM) from the polymer/HD/DCM-in-water emulsion droplets, which led to the formation of HD/PMMA or HD/PMMA/PS microparticles. After extraction of HD with hexane, PMMA or PS/PMMA particles with different morphologies were produced. Poly(vinyl pyrrolidone) (PVP), sodium dodecyl sulfonate (SDS) or sodium dodecyl benzylsulfate (SDBS) was chosen as the emulsifier. The morphology depended on the HD/polymer ratio and the interfacial tensions, which were adjusted by changing the type of the emulsifier and its concentration. With poly(vinyl pyrrolidone) (PVP) emulsifier, PMMA hollow spheres were observed; while with SDS emulsifier, the particles changed from bowl-like particles to hemispheres and truncated spheres with the increase of SDS content. The morphology of PS/PMMA composite particles depended on the ratio of the two polymers. Scanning electron microscopy observation, selective etching and X-ray photoelectron spectroscopy results confirmed that PMMA tended to engulf PS component. With the increase of PMMA/PS ratio, the particles changed the morphology from capped acorn to ‘ball in bowl’ morphology. Furthermore, the particle morphology was simulated via a theoretical model based on the minimum interfacial energy of the system. The simulation results agreed with the experimental observations. Our results indicate that internal phase separation is an effective method to obtain Janus-like microparticles. Via adjusting the composition of the system and the corresponding interfacial tensions, we could tailor the polymer particles with different morphologies.     

Macromolecular effect on crystal pattern formation in ultra-thin films: Molecular segregation in a binary blend of PEO fractions

In this work we report our investigation on the crystal patterns of a 50/50 blend of two polyethylene oxide (PEO) fractions with different molecular weights in ultra-thin films. Using AFM with a hot stage the samples on the surface of silicon wafer were isothermally crystallized at 20.0 ≤ T_c ≤ 60.0 °C. The crystal patterns are different from those of the two pure fractions. Co-crystallization, partial segregation and full segregation have been observed. Especially, within 47.0 ≤ T_c ≤ 54.0 °C dual thickness crystals formed. The thickness of the middle part of the crystal corresponds to the lamella thickness of 35k-PEO fraction with multiple folds, while the thickness of the edge part is nearly equal to an extended-chain lamella thickness of 5k-PEO fraction. We suppose that the appearance of the dual thickness crystals is due to molecular partial segregation. Utilizing in-situ AFM, the growth of the crystal with dual thicknesses as a function of time was monitored at T_c = 54.0 °C. Two growth processes were observed and a pattern formation mechanism was suggested on the basis of specified molecular motion and chain-fold crystallization of polymers.     

Light emission in phase separated conjugated and non-conjugated polymer blends

Phase separations in conjugated polymer (MEH-PPV) and non-conjugated polymer (PMMA) blended films are studied by photoluminescent (PL) near-field scanning optical microscope and spectrometer. The morphologies of blended films changed from stripe patterns to isolated islands as PMMA concentrations increased. The PL spectra in isolated micron dots are the same as MEH-PPV. Nevertheless, we found new PL spectra, peaked at 500 nm wavelength, in the PMMA matrix. Since, PMMA does not emit light, the new spectra indicate some MEH-PPV diluted in the PMMA rich regions. The excitons of MEH-PPV in the PMMA matrix are localized and emit light as PPV oligomers.     

Effect of diluents on membrane formation via thermally induced phase separation

The effect of diluents on isotactic polypropylene (iPP) membrane formation via thermally induced phase separation was investigated. The diluents were methyl salicylate (MS), diphenyl ether (DPE), and diphenylmethane (DPM). The cloud-point curve was shifted to a lower temperature in the order iPP–MS, iPP–DPE, and iPP–DPM, whereas the crystallization temperature was not influenced so much by diluent type. Droplet-growth processes were investigated under two conditions: quenching the polymer solution at the desired temperature and cooling at a constant rate. Although droplet sizes were in the order iPP–MS, iPP–DPE, and iPP–DPM in both cases, the difference was more pronounced with the constant cooling rate condition. Scanning electron microscopy indicated that interconnected structures were obtained when the polymer solution was quenched in ice water. The effect of the diluents on these structures was observed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 169–177, 2001     

Gel-size dependence of temperature-induced microphase separation in weakly-charged polymer gels

The gel-size dependence of microphase separation in weakly-charged gels of N-isopropylacrylamide (NIPA) and 1-vinylimidazole (VI) copolymers has been investigated using swelling measurement, small-angle neutron scattering (SANS), and dynamic and static light scattering (DLS/SLS). It is known that weakly-charged polymer gels undergo microphase separation in a poor solvent as a result of competing interactions involving hydrophobic attraction versus electrostatic repulsion. The microphase separation is characterized by a scattering maximum in SANS intensity functions of which Bragg spacing, Λ, is around 20-30 nm. However, when gel size was reduced to the order of Λ, no microphase separation was observed. Instead, a typical scattering of isolated spherical particles was clearly observed. On the basis of the experimental evidence, we conclude that microphase separation has its own wavelength independent of gel size, and nanometer-order gels, i.e., nanogels, do not undergo microphase separation.     

Buoyancy-driven pattern formation in reactive immiscible two-layer systems

Buoyancy-driven convection can be induced by concentration and temperature gradients near the interface between two immiscible fluids filling a vertical Hele–Shaw cell, each of them containing a reactant of an exothermic A+B→C reaction taking place in the bulk of the lower layer. A chemo-hydrodynamical pattern appears then due to the combined action of Rayleigh–Taylor, diffusive layer convection and Rayleigh–Bénard instabilities occurring either below or above the interface. The mathematical model we develop to describe such dynamics consists in a set of reaction–diffusion–advection equations ruling the evolution of concentrations and temperature coupled to Navier–Stokes equation, written in a Hele–Shaw approximation. We perform numerical simulations of the non-linear system and study the influence on pattern formation of changes in the Damköhler number of the problem and in the ratio of initial reactant concentrations.     

Formation mechanism of asymmetric porous polymer films by photoinduced phase separation in the presence of solvent

Photoinduced phase separation in the presence of a nonreactive volatile solvent provides a simple, light-tunable, and cost-effective route for fabrications of porous polymer films. Here, we show that the short-time (<10 s) ultraviolet irradiation of a photoreactive solution film promotes particular asymmetries in the composition and porosity across the thickness. Confocal Raman spectroscopy revealed that the compositional asymmetry is maximum at a certain critical light intensity. We discuss the film formation mechanism based on the light-driven diffusion of the solvent and monomer. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47867.     

Control of mesh pattern of surface corrugation via rate of solvent evaporation in solution casting of polymer film in the presence of convection

A polygonal mesh pattern was observed on an as-cast polymer film when cast from a dilute solution on a hot plate. It was found that the pattern was ascribed to surface corrugation. In the dilute solution, a similar polygonal mesh pattern due to convection was present. A possible imprinting mechanism of the convection pattern on the as-cast polymer film as the surface corrugation was investigated by real time observation using the so-called shadowgraph technique. For this purpose poly(styrene-ran-butadiene) random copolymer was used as a solute and toluene as a solvent. Furthermore, control of the mesh size via the rate of solvent evaporation was examined. It was found that the mesh size was increased with an increase of the rate in the regime of the lower evaporation-rate (regime I) and that it was decreased tremendously with an increase of the rate in the regime of the higher evaporation-rate (regime II). These regimes can be referred to as a kinetic control regime and a temperature-gradient control regime, respectively.