In Search of a Green Process: Polymeric Films with Ordered Arrays via a Water Droplet Technique

As an efficient technique for the preparation of polymeric hexagonal orderly arrays, the breath figure (BF) process has opened a modern avenue for a bottom-up fabrication method for more than two decades. Through the use of the water vapor condensation on the solution surface, the water droplets will hexagonally pack into ordered arrays, acting as a template for controlling the regular micro patterns of polymeric films. Comparing to the top-down techniques, such as lithography or chemical etching, the use of water vapor as the template provides a simple fabrication process with sustainability. However, using highly hazardous solvents such as chloroform, carbon disulfide (CS2), benzene, dichloromethane, etc., to dissolve polymers might hinder the development toward green processes based on this technique. In this review, we will touch upon the contemporary techniques of the BF process, including its up-to-date applications first. More importantly, the search of greener processes along with less hazardous solvents for the possibility of a more sustainable BF process is the focal point of this review.

Dynamic effects and adhesion of water droplet impact on hydrophobic surfaces: bouncing or sticking

This work reported the dynamic effects of water droplet impact on flat, porous and pincushion structure films of star shaped polyhedral oligomeric silsesquioxane (POSS) fluorinated acrylates, POSS-poly(trifluoroethyl methacrylate)₈ (POSS-(PTFEMA)₈) and POSS-(poly(trifluoroethyl methacrylate)-b-poly(methyl methacrylate))₈ (POSS-(PTFEMA-b-PMMA)₈), using the breath figure method. The porous and pincushion structure films with different surface chemical compositions were obtained by controlling the copolymer structure and temperature and by stripping of the surface. The water contact angles on the different films were measured, and the water droplets on the pincushion structure films when reversed at 45°, 90°, 135° and 180° were also studied. It was found that the pincushion structure films revealed a water adhesion ability. Furthermore, the water droplet impact behavior on these films was investigated. The morphology variations of water droplets, spreading diameter of the droplets, energy conversion, restitution coefficient and adhesion force were examined. Finally, the schematic illustration of water droplets under the static and dynamic states in contact with the pincushion and porous structure surfaces was proposed. It is critical to materialize various applications such as microdroplet transportation, soil erosion, spray painting, anti-icing surface and antifouling agents for textiles.

Au Nanoparticles Immobilized on Honeycomb-Like Polymeric Films for Surface-Enhanced Raman Scattering (SERS) Detection

We have successfully developed novel surface-enhanced Raman scattering (SERS) substrates with three-dimensional (3D) porous structures for effectively improving the sensitivity and reproducibility of SERS, which can rapidly detect small molecules (rhodamine 6G as an example). Periodical arrays of the honeycomb-like substrates were fabricated by self-assembling polyurethane-co-azetidine-2,4-dione (PU-PAZ) polymers. PU-PAZ comprising amphiphilic dendrons could stabilize the phase separation between the water droplets and polymer solution, and then organize into regular porous structures during the breath figure method. Subsequently, SERS substrates were fabricated by immobilizing gold nanoparticles (AuNPs) onto the honeycomb-like films with various 3D porous structures, controlled by the different PU-PAZ concentrations and relative humidities. Results show that surface enhancement factors of honeycomb-like substrates were 20 times higher than that of flat-film substrates (control group) due to enormous hot-spots resonance effects by the 3D porous structure, verified through Raman mapping at various positions of the z-axis. Furthermore, the particle size effects were evaluated by immobilized 12 and 67 nm of AuNPs on the honeycomb-like substrates, indicating larger AuNPs could induce more pronounced hot-spots effects. The generation of hot-spots resonance to enhance Raman intensity is strongly dependent on the diameter of AuNPs and the pore size of the honeycomb-like and 3D porous substrates for label-free and rapid SERS detection.

Tailored honeycomb-like polymeric films based on amphiphilic poly(urea/malonamide) dendrons

A series of hydrogen bond-rich poly(urea/malonamide) dendrons were utilized as surfactants to facilitate the formation of honeycomb-like porous structures from the breath figure (BF) process.

Multiple interfaces in self-assembled breath figures

Progress in the breath figure method is reviewed by emphasizing the role of the multiple interfaces and the applications of honeycomb films in separation, biocatalysis, biosensing, templating, stimuli-responsive surfaces and adhesive surfaces.

Honeycomb-structured films by multifunctional amphiphilic biodegradable copolymers: surface morphology control and biomedical application as scaffolds for cell growth

Recently, fabrication of functional porous polymer films with patterned surface structures at the scale from nanometer to micrometer has been attracting increasing interests in material science and nanobiotechnology. In this work, we present new preparation of two series of multifunctional amphiphilic copolymers and preparation of their microporous thin films on solid substrates. First, diblock dendritic poly(l-lysine)-b-poly(l-lactide)s and triblock dendritic poly(l-lysine)-b-poly(l-lactide)-b-dendritic poly(l-lysine)s (C1-C6) were synthesized through 4-dimethylaminopyridine (DMAP)-catalyzed living ring-opening polymerization of (l-)-lactide with (l-)-lysine dendron initiators, and their structures were characterized by nuclear magnetic resonance spectrometer (NMR), gel permeation chromatography (GPC) and matrix-assisted laser desorption/ionization Fourier-transformed mass spectra (MALDI-FTMS). Employing the breath-figure (BF) fabrication strategy, thin films of the synthesized amphiphiles (C1-C6) were drop-cast, and their surface topologies were examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM), and the effects of new amphiphile structure and drop-casting parameters of amphiphile concentration, humidity and temperature on self-assembly of ordered porous surface were studied. Furthermore, the influence of surface energy of drop-casting substrates was additionally investigated. With a human cervical epithelial carcinoma cell line (HeLa), cytotoxicity of the prepared honeycomb-structured films by new amphiphile C6 was evaluated by thiazoyl-blue-tetrazolium-bromide (MTT) assay, and HeLa cell growth behavior with microporous amphiphile films as the scaffolds was also examined. It was found that tunable micropore diameter sizes and well ordered surface topologies of BF films could be achieved for the new prepared amphiphiles, and utilization of the honeycomb-like microporous films as scaffolds indicated favorable enhancement in cell proliferation. Therefore, the honeycomb-structured films by these biocompatible multifunctional amphiphiles may provide new materials as 3D-scaffold materials for potential application in tissue engineering and regeneration.

Poly(urethane/malonamide) dendritic structures featuring blocked/deblocked isocyanate units

Novel reactive dendritic structures possessing terminal blocked isocyanates were synthesized, which allows further replacement of reactive exterior groups with desired functionality.

Facilitated and site-specific assembly of functional polystyrene microspheres on patterned porous films

Functional patterned materials have received considerable attention because of their potential applications in biochips, sensors, and optical or electronic materials. Here, we report a versatile approach to functional patterned films based on facilitated and site-specific assembly of microspheres. This method includes the hierarchical formation of honeycomb-patterned porous films from amphiphilic block copolymers and the assembly of functional polystyrene microspheres driven by the gravity and the electrostatic interaction. Polystyrene microspheres containing carboxyl groups with a narrow size distribution were synthesized by dispersion polymerization. Honeycomb-patterned porous films were prepared from polystyrene-block-poly(N,N-dimethylaminoethyl methacrylate) (PS-b-PDMAEMA) by the breath figure method and then quaternized. We found that direct deposition of the microspheres on the patterned films reaches high filling ratio only when using ethanol dispersions that can wet the film pores. Plasma treatment of the films improves the hydrophilicity and introduces charged species to the external surface as well as the pore surface, leading to nonspecific assembly of microspheres. Negatively charged microspheres dispersed in buffer solution show a facilitated and site-specific assembly on the quaternized film. The electrostatic interaction as well as the gravity facilitates the assembly and the suborder arrangement of the hydrophilic PDMAEMA block around the pores is responsible for the site-specific assembly. In addition, we demonstrate the applicability of this method in preparing photoluminescent patterns by the assembly of porphyrinated microspheres, which is useful in various fields such as intelligent sensing.

Preparation and characterisation of super-hydrophobic surfaces

The interest in highly water-repellent surfaces has grown in recent years due to the desire for self-cleaning surfaces. A super-hydrophobic surface is one that achieves a water contact angle of 150 degrees or greater. This article explores the different approaches used to construct super-hydrophobic surfaces and identifies the key properties of each surface that contribute to its hydrophobicity. The models used to describe surface interaction with water are considered, with attention directed to the methods of contact angle analysis. A summary describing the different routes to hydrophobicity is also given.

Orderly arranged NLO materials based on chromophore-containing dendrons on exfoliated layered templates

Three chromophore-containing dendrons were intercalated into montmorillonite layered silicates via an ion-exchange process. Enlarged d spacings ranging from 50 to 126 A were achieved for these novel organoclays. After the organoclays were blended with a polyimide, the steric bulkiness of the dendrons and the interaction between dendron and polyimide resulted in an ordered morphology. The orderly arranged nanocomposites were characterized by a UV-visible spectrophotometer, a variable-temperature infrared spectrometer, and electro-optical modulation. The dendrons in layered silicates were capable of undergoing a critical conformational change into an ordered structure, indicated by the drastic changes of interlayer distances at certain packing densities. Electro-optical coefficients increased sharply from 0 to 6 pm/V while the conformational change occurred. Furthermore, the addition of a polyimide capable of interaction-induced orientation was found to exert an enhancing effect on the degree of the noncentrosymmetric alignment.