Hongkun He

Design and Preparation of Porous Polymers

Dingcai Wu,*,† Fei Xu,† Bin Sun,† Ruowen Fu,† Hongkun He,‡ and Krzysztof Matyjaszewski*,‡ †Materials Science Institute, Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, Peoples Republic of China ‡Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States

A Simple and Universal Gel Permeation Chromatography Technique for Precise Molecular Weight Characterization of Well-Defined Poly(ionic liquid)s

Poly(ionic liquid)s (PILs) are an important class of technologically relevant materials. However, characterization of well-defined polyionic materials remains a challenge. Herein, we have developed a simple and versatile gel permeation chromatography (GPC) methodology for molecular weight (MW) characterization of PILs with a variety of anions. PILs with narrow MW distributions were synthesized via atom transfer radical polymerization, and the MWs obtained from GPC were further confirmed via nuclear magnetic resonance end group analysis.

Preparation of Polymeric Nanoscale Networks from Cylindrical Molecular Bottlebrushes

The design and control of polymeric nanoscale network structures at the molecular level remains a challenging issue. Here we construct a novel type of polymeric nanoscale networks with a unique microporous nanofiber unit employing the intra/interbrush carbonyl cross-linking of polystyrene side chains for well-defined cylindrical polystyrene molecular bottlebrushes. The size of the side chains plays a vital role in the tuning of nanostructure of networks at the molecular level. We also show that the as-prepared polymeric nanoscale networks exhibit high specific adsorption capacity per unit surface area because of the synergistic effect of their unique hierarchical porous structures. Our strategy represents a new avenue for the network unit topology and provides a new application for molecular bottlebrushes in nanotechnology.

Reversible CO2 capture with porous polymers using the humidity swing

Several polymeric materials were prepared for reversible CO2 capture. These materials contain quaternary ammonium ions and hydroxide counter ions, including polymers grafted from carbon black, crosslinked porous polymers templated by ordered colloidal crystals, and high internal phase emulsion systems. The porous polymers displayed an order of magnitude improvement in the kinetics of the absorption and desorption processes and a significant improvement in the swing sizes compared to a commercially available material with similar functional groups. This work suggests a new direction for the design of porous polymeric materials for CO2 air capture.

Biologically Derived Soft Conducting Hydrogels Using Heparin-Doped Polymer Networks

The emergence of flexible and stretchable electronic components expands the range of applications of electronic devices. Flexible devices are ideally suited for electronic biointerfaces because of mechanically permissive structures that conform to curvilinear structures found in native tissue. Most electronic materials used in these applications exhibit elastic moduli on the order of 0.1–1 MPa. However, many electronically excitable tissues exhibit elasticities in the range of 1–10 kPa, several orders of magnitude smaller than existing components used in flexible devices. This work describes the use of biologically derived heparins as scaffold materials for fabricating networks with hybrid electronic/ionic conductivity and ultracompliant mechanical properties. Photo-cross-linkable heparin–methacrylate hydrogels serve as templates to control the microstructure and doping of in situ polymerized polyaniline structures. Macroscopic heparin-doped polyaniline hydrogel dual networks exhibit impedances as low as Z = 4.17 Ω at 1 kHz and storage moduli of G′ = 900 ± 100 Pa. The conductivity of heparin/polyaniline networks depends on the oxidation state and microstructure of secondary polyaniline networks. Furthermore, heparin/polyaniline networks support the attachment, proliferation, and differentiation of murine myoblasts without any surface treatments. Taken together, these results suggest that heparin/polyaniline hydrogel networks exhibit suitable physical properties as an electronically active biointerface material that can match the mechanical properties of soft tissues composed of excitable cells.

Modular polymerized ionic liquid block copolymer membranes for CO2/N2 separation

The continuing discovery of broad classes of materials, such as ionic liquids, zeolites, metal–organic frameworks, and block copolymers, presents an enormous opportunity in developing materials for new applications. Polymerized ionic liquid block copolymers (PIL-BCPs) fall at the union of two already large sets of materials, and are an emerging class of materials useful in gas separation membranes, ion and electron conducting materials, and as mechanical actuators. A wide range of ionic liquid moieties can be used as pendant groups along the polymer backbone, potentially allowing for a wide variation in the resulting material properties; however in practice the range of ionic liquids explored is hindered by the need to optimize polymerization conditions for each new monomer. Here, we present a modular approach to PIL-BCP synthesis where a variety of olefin bearing cations are readily conjugated to polymers using thiol-Michael click chemistry. This approach allowed for the rapid development of a diverse material library including phase separated thin films, ion-gels, and liquid PIL-BCPs, with a reduced investment in synthetic time. Finally, we demonstrate that this approach identified PIL-BCPs with increased CO2 permeability relative to PILs, which could find use in carbon capture from flue gas.

Carbon black functionalized with hyperbranched polymers: synthesis, characterization, and application in reversible CO2 capture

The functionalization of carbon black by grafting of hyperbranched polymers from the surface via self-condensing atom transfer radical polymerization (SC-ATRP) is reported. The surfaces of the pristine carbon black were modified with ATRP initiators using two different methods. The first method uses acid oxidation to place COOH groups on the carbon black surface, followed by the esterification to give ATRP initiators bound to the carbon black surface. Alternatively, ATRP initiators bearing both azido and bromine groups were placed directly on the carbon black surface by nitrene chemistry. The hyperbranched poly(p-chloromethylstyrene) (PCMS) was grafted from the ATRP initiator modified carbon black surfaces using the inimer of p-chloromethylstyrene, and CuCl/CuCl2/N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) as the catalytic system in N,N-dimethylformamide (DMF). In addition, a one-pot two-step method was developed to graft crosslinked polymers from the carbon black surfaces. The polymerization process was well controlled and the fraction of the grafted polymers on carbon black could be adjusted by changing the polymerization time. The resulting samples were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The hyperbranched polymers on the carbon black surfaces were quaternized to give an ammonium group on the polymer, and a chloride counterion was subsequently exchanged to give a hydroxide counterion. Under these basic conditions, the ATRP initiators should be attached to the surface using nitrene chemistry rather than acid oxidation, to avoid the hydrolysis of the ester groups that link the grafted polymers to the carbon black surface. The ammonium hydroxide functionalized carbon black materials were utilized for the reversible absorption and desorption of CO2 from ambient air, providing improved absorption/desorption kinetics compared with the commercially available Excellion membrane.

Multifunctional photo-crosslinked polymeric ionic hydrogel films

A facile approach was developed to prepare crosslinked ionic polymer hydrogel films by photo-crosslinking utilizing p-vinylbenzyl trimethylammonium chloride (VBTMACl) or p-vinylbenzyl trimethylammonium hydroxide (VBTMAOH) as the monomer and poly(ethylene oxide) dimethacrylate (PEODMA, Mn = 750) as the crosslinker. The films with different crosslinking degrees (20%, 40%, 60%, 80%, and 100%) were prepared and characterized by swelling measurements, scanning electron microscopy (SEM), UV-visible spectroscopy, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), and small-angle X-ray scattering (SAXS). It was found that the mechanical and thermal properties of the films were largely influenced by the contents of the crosslinker in the films. By ion-exchange of the anions in the films with various other anions, the hydrophobicity/hydrophilicity of the films was changed. In addition, fluorescent films were prepared by treatment with fluorescein, and paramagnetic films with FeCl4− as a counter anion showed catalytic activity for Friedel–Crafts alkylation. The ionic films with quaternary ammonium chloride groups displayed antimicrobial activity against Escherichia coli (E. coli) with almost 100% killing efficiency. Multifunctional films with various tunable properties have significant potential for a wide range of applications.

Cubosomes from hierarchical self-assembly of poly(ionic liquid) block copolymers.

Cubosomes are micro- and nanoparticles with a bicontinuous cubic two-phase structure, reported for the self-assembly of low molecular weight surfactants, for example, lipids, but rarely formed by polymers. These objects are characterized by a maximum continuous interface and high interface to volume ratio, which makes them promising candidates for efficient adsorbents and host-guest applications. Here we demonstrate self-assembly to nanoscale cuboidal particles with a bicontinuous cubic structure by amphiphilic poly(ionic liquid) diblock copolymers, poly(acrylic acid)-block-poly(4-vinylbenzyl)-3-butyl imidazolium bis(trifluoromethylsulfonyl)imide, in a mixture of tetrahydrofuran and water under optimized conditions. Structure determining parameters include polymer composition and concentration, temperature, and the variation of the solvent mixture. The formation of the cubosomes can be explained by the hierarchical interactions of the constituent components. The lattice structure of the block copolymers can be transferred to the shape of the particle as it is common for atomic and molecular faceted crystals.

Multifunctional Hydrogels with Reversible 3D Ordered Macroporous Structures

Three‐dimensionally ordered macroporous (3DOM) hydrogels prepared by colloidal crystals templating display highly reversible shape memory properties, as confirmed by indirect electron microscopy imaging of their inverse replicas and direct nanoscale resolution X‐ray microscopy imaging of the hydrated hydrogels. Modifications of functional groups in the 3DOM hydrogels result in various materials with programmed properties for a wide range of applications.