Liang-Wei Zhu

Multiple interfaces in self-assembled breath figures

This feature article describes the multiple interfaces in the breath figure (BF) method toward functional honeycomb films with ordered pores. If a drop of polymer solution in a volatile solvent such as carbon disulphide is placed in a humid environment, evaporative cooling leads to self-assembled arrays of condensed water droplets. After evaporation of the solvent and water, patterned pores can be formed. During this BF process, the interfaces between the solution and the substrate, the solution and water droplets, and the film surface and air play extremely important roles in determining both the structures and functions of the honeycomb films. Progress in the BF method is reviewed by emphasizing the roles of the interfacial interactions. The applications of hierarchical and functional honeycomb films in separation, biocatalysis, biosensing, templating, stimuli-responsive surfaces and adhesive surfaces are also discussed.

Polystyrenes with hydrophilic end groups: synthesis, characterization, and effects on the self-assembly of breath figure arrays.

We report the synthesis and characterization of a series of hydroxyl-end-functionalized polystyrenes (PS-OH) and the formation of patterned porous films. The polymers were synthesized by chain end reaction of polystyrene having a bromide end group (PS-Br) with hydramines including ethanolamine, diethanol amine, 2-amino-1,3-propanediol, and 2-(2-aminoethoxy) ethanol. The polymers were characterized by gel permeation chromatography (GPC), nuclear magnetic resonance (NMR), matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS), and differential scanning calorimetry (DSC). It was found that the end groups can influence the glass transition temperature (T(g)) of the polystyrenes. The polymers with different end groups were then used to prepare honeycomb-patterned porous films by the breath figure method. Results reveal that the subtle chain-end modification leads to a dramatic change in the morphology of the films. Honeycomb films with large area ordered structure can be easily prepared from PS-OH. Effects of the end groups as well as blending PS-OH with PS-Br on the surface pore diameter, pore center distance, and the hierarchical structure were studied in detail. As supported by the results of polymer hydrophilicity, in situ observation of the film formation process, as well as the chain mobility, the film structure is supposed to be mainly determined by the precipitation of polystyrene at the solution/water droplet interface and the interfacial activity enhanced by the end groups.

Synthesis of polystyrene with cyclic, ionized and neutralized end groups and the self-assemblies templated by breath figures

Polymers with functional end groups were synthesized via atom transfer radical polymerization (ATRP) using a novel cyclic lactone initiator, which can be facilely converted into ionized and neutralized groups by hydrolysis and acidification processes, respectively. Results from Fourier transform infrared (FTIR) and 1H and 13C nuclear magnetic resonance (NMR) spectroscopies indicate that the cyclic lactone end group can be fully converted into sodium carboxylate in alkaline solution whereas the acidification process induces both neutralization and esterification. Gel permeation chromatography (GPC) curves reveal intramolecular esterification instead of intermolecular esterification during the acidification process. The polymers with various end groups that show different hydrophilicities were then utilized to fabricate honeycomb-patterned porous films by the breath figure method. Polystyrene with an ionized or neutralized end group forms highly ordered self-assembled films in an easy and reproducible way, whereas that with a less hydrophilic lactone end group generates irregular films. Moreover, polystyrene with an ionized end group, which is the most hydrophilic, results in porous films with a multilayered structure and a much smaller surface pore size (it decreases to ∼610 nm from 1.8 μm for films prepared from the neutralized polymer). In addition, we found that the end-functionalized polystyrene with a very low molecular weight (∼2960 g mol−1) is able to form highly ordered honeycomb films. It is speculated that the ionized end group endows polystyrene with high interfacial activity, leading to the unique surface morphologies. This is evidenced from the results of water contact angles on film surfaces with a pincushion structure obtained by removing the top surface layer. The proposed approach to well-controlled end-functionalized polymers is useful in the fabrication of self-assemblies with adjustable morphologies.

Polystyrene with hydrophobic end groups: synthesis, kinetics, interfacial activity, and self-assemblies templated by breath figures

The breath figure method has emerged as a new self-assembly technique to fabricate ordered porous materials, which show potential applications in many fields, such as size-selective separation membranes. However, it is challenging to customize the structures, especially to precisely tune the pore size in a wide range. Moreover, the relationship between polymer structure and film morphologies is still unknown. In this paper, we report a facile, effective, and controllable way to manipulate the evolution of morphologies of honeycomb films, which is based on the blends of an amphiphilic block copolymer and polystyrenes with hydrophobic end groups. A series of atom transfer radical polymerization (ATRP) initiators with alkyl or fluorinated groups were synthesized for polystyrenes with hydrophobic end groups. Polymerization kinetics confirmed the viability of these ATRP initiators. Surface segregation behaviors of the hydrophobic end groups were demonstrated by measuring the surface chemical composition and surface free energies. We found that the polystyrenes with hydrophobic end groups form ordered films only at high polymer concentration (40–60 mg mL−1); moreover, the use of blends of two types of polystyrenes, one of which has a hydrophobic end group whereas the other has a hydrophilic block, can greatly increase the regularity of the honeycomb films and provide the possibility to fine-tune the pore diameter. Moreover, the evolution of surface morphologies of the films can be ideally correlated with the surface free energies of the end-functionalized polymers.

Synthesis of core cross-linked star polystyrene with functional end groups and self-assemblies templated by breath figures

In this paper, we report the synthesis of core cross-linked star (CCS) polymers with functional end groups for self-assembled films, which show monolayer, bilayer, or multilayer structures, depending on arm numbers, arm length, and end groups. The stars were synthesized via the “arm-first” method using a linear polystyrene as an atom transfer radical polymerization (ATRP) macroinitiator that is end-capped with a hydrolysable cyclic lactone end group. The relatively hydrophobic cyclic group can be converted into hydrophilic neutralized carboxyl and hydroxyl groups via alkaline hydrolysis and acidification processes. The polymers were characterized by gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance spectroscopy (NMR). The conversion of the macroinitiators was evaluated by fitting the GPC curves. We found that the star polymers are easier to form ordered honeycomb films than the corresponding linear analogue. Moreover, the end groups of the stars show an obvious impact on the film surface structures only at relatively low concentration. More importantly, stars with lower arm numbers or longer arms tend to form bilayer or multilayer structured films. On the other hand, stars with hydrophilic end groups are much easier to form bilayer or multilayer structured films. Considering the importance of mono- or multilayer structures of honeycomb films in various fields such as separation membranes, templating materials, and optical materials, these results will be valuable in tailoring film-forming materials toward films with designed structures.