Wangqing Zhang

Synthesis of Hierarchical Hollow Silica Microspheres Containing Surface Nanoparticles Employing the Quasi-Hard Template of Poly(4-vinylpyridine) Microspheres

A facile method of preparing hierarchical hollow silica microspheres containing surface silica nanoparticles (HHSMs) through the sol-gel process of tetraethylorthosilicate employing a quasi-hard template of non-cross-linking poly(4-vinylpyridine) microspheres is proposed. The quasi-hard template contains the inherent catalyst of the basic pyridine group, and a few of the polymer chains can escape from the template matrix into the aqueous phase, which initiates the sol-gel process spontaneously both on the surface of the template used to prepare the hollow silica shell and in the aqueous phase to produce the surface silica nanoparticles. By tuning the weight ratio of the silica precursor to the quasi-hard template, HHSMs with a size of about 180 nm and a shell thickness ranging from 14 to 32 nm and surface silica nanoparticles ranging from 17 to 36 nm are produced initially through the deposition of surface silica nanoparticles onto the silica shell, followed by template removal either by calcination or solvent extraction. The synthesized HHSMs are characterized, and a possible mechanism for the synthesis of HHSMs is proposed.

Synthesis of porous poly(styrene-co-acrylic acid) microspheres through one-step soap-free emulsion polymerization: Whys and wherefores

Synthesis of porous poly(styrene-co-acrylic acid) (PS-co-PAA) microspheres through one-step soap-free emulsion polymerization is reported. Various porous PS-co-PAA microspheres with the particle size ranging from 150 to 240 nm and with the pore size ranging from 4 to 25 nm are fabricated. The porous structure of the microspheres is confirmed by the transmission electron microscopy measurement and Brunauer-Emmett-Teller (BET) analysis. The reason for synthesis of the porous PS-co-PAA microspheres is discussed, and the phase separation between the encapsulated hydrophilic poly(acrylic acid) segment and the hydrophobic polystyrene domain within the PS-co-PAA microspheres is ascribed to the pore formation. The present synthesis of the porous PS-co-PAA microspheres is anticipated to be a new and convenient way to fabricate porous polymeric particles.

In situ synthesis of block copolymer nano-assemblies by polymerization-induced self-assembly under heterogeneous condition

Controlled synthesis of amphiphilic block copolymer nanoparticles in a convenient way is an important and interest topic in polymer science. In this review, three formulations of polymerization-induced self-assembly to in situ synthesize block copolymer nanoparticles are briefly introduced, which perform by reversible addition-fragmentation chain transfer (RAFT) polymerization under heterogeneous conditions, e.g., aqueous emulsion RAFT polymerization, dispersion RAFT polymerization and especially the recently proposed seeded RAFT polymerization. The latest developments in several selected areas on the synthesis of block copolymer nano-assemblies are highlighted.

Synthesis and micellization of a multi-stimuli responsive block copolymer based on spiropyran

The multi-stimuli responsive block copolymer of poly(N-iso-propylacrylamide)-b-poly(N-acryloylglycine-co-N-acryloylglycine spiropyran ester) containing a LCST-type thermo-responsive poly(N-isopropylacrylamide) block and a pH-, light- and UCST-type multi-responsive poly(N-acryloylglycine-co-N-acryloylglycine spiropyran ester) block was synthesized by post-polymerization modification, and micellization modulated by UV light irradiation, temperature and pH was demonstrated.

One-pot preparation of BAB triblock copolymer nano-objects through bifunctional macromolecular RAFT agent mediated dispersion polymerization

One-pot preparation of the BAB triblock copolymer nano-objects of polystyrene-block-poly(4-vinylpyridine)-block-polystyrene (PS-b-P4VP-b-PS) containing a central solvophilic A block and two outer solvophobic B blocks through dispersion RAFT polymerization is proposed. Ascribed to the bifunctional macro-RAFT agent of bis(trithiocarbonate)-terminated poly(4-vinylpyridine) (TTC-P4VP-TTC), which contains two functional RAFT moieties at each terminal of the polymer backbone, the bifunctional TTC-P4VP-TTC macro-RAFT agent mediated dispersion polymerization affords the in situ synthesis of the PS-b-P4VP-b-PS triblock copolymer nano-objects. The parameters affecting the morphology of the PS-b-P4VP-b-PS triblock copolymer nano-objects are investigated, and it is found that the morphology of the PS-b-P4VP-b-PS nano-objects undergoes a transition from nanospheres, to worms, to vesicles and finally to lacunal nanospheres with the extension of the PS block during the dispersion RAFT polymerization. The bifunctional macro-RAFT agent mediated dispersion polymerization is demonstrated to be a valid method to prepare BAB triblock copolymer nano-objects with interesting morphology.

RAFT synthesis of triply responsive poly[N-[2-(dialkylamino)ethyl]acrylamide]s and their N-substitute determined response

The thermo- and pH/CO2-responsive poly[N-[2-(dialkylamino)ethyl]acrylamide]s containing a polyacrylamide backbone but different N-substitutes of dialkylamine were synthesized and their solution properties were comparatively checked. A controllable RAFT synthesis of poly[N-[2-(dialkylamino)ethyl]acrylamide]s was achieved when a typical trithiocarbonate containing an easily cleavable R group was employed. The RAFT polymerization rate decreases with the increasing C-number in the N-substitutes. The thermo- and pH/CO2-responsive property of poly[N-[2-(dialkylamino)ethyl]acrylamide]s is firmly correlative to the N-substitutes. With the C-number in the R-substitute increasing, the solution properties of poly(N-[2-(dialkylamino)ethyl]acrylamide)s undergo a soluble-to-thermoresponsive-to-insoluble evolution, and the critical pH of poly[N-[2-(dialkylamino)ethyl]acrylamide]s gradually decreases. The dialkylamine moieties in the poly[N-[2-(dialkylamino)ethyl]acrylamide]s lead to a characteristic CO2-response during CO2/N2 bubbling. The present study reveals the structure-dependent solution properties of the thermo- and pH/CO2-responsive poly[N-[2-(dialkylamino)ethyl]acrylamide]s, and these multistimuli-responsive polymers are believed to be very useful due to the controllable RAFT synthesis and tunable solution properties.

A new thermoresponsive polymer of poly(N-acryloylsarcosine methyl ester) with a tunable LCST

Poly(N-acryloylsarcosine methyl ester) (PNASME), a thermoresponsive N-ester substituted polyacrylamide that has a well-defined molecular weight and a low Đ, was synthesized by solution RAFT polymerization. The synthesized PNASME exhibited a cloud point temperature (Tcp) or lower critical solution temperature (LCST) of 44.0 °C. It was found that various factors could affect the Tcp of PNASME, e.g., the polymer molecular weight, the polymer concentration, the salt effect on Tcp following the order of the Hofmeister series, urea induced increasing Tcp, and phenol induced decreasing Tcp. The mechanism of urea and phenol inducing the phase transition of PNASME was revealed by 2D NOESY spectroscopy, and how the hydrogen bonding between PNASME and the solutes of urea and phenol affected the phase transition of PNASME was clarified. The contrary effect on the Tcp of PNASME induced by urea and phenol makes it easy to adjust the thermoresponse of PNASME, and the tunable thermoresponse of the diblock copolymer of poly(N-acryloylsarcosine methyl ester)-block-poly(N-isopropylacrylamide) was demonstrated. The present study is believed to offer a new route to tune the solution behavior of thermoresponsive polymers.

In situ synthesis of a self-assembled AB/B blend of poly(ethylene glycol)-b-polystyrene/polystyrene by dispersion RAFT polymerization

A diblock-copolymer/homopolymer self-assembled blend of poly(ethylene glycol)-block-polystyrene/polystyrene (PEG-b-PS/PS) was synthesized through dispersion RAFT polymerization. In this dispersion RAFT polymerization reaction, a macro-RAFT agent and a small RAFT agent containing the same RAFT moiety were simultaneously used. By tuning the molar ratio of the macro-RAFT agent and the small RAFT agent fed into the dispersion RAFT polymerization mixture, PEG-b-PS/PS self-assembled blends with different morphologies could be prepared. It is found that the PEG-b-PS/PS self-assembled blend synthesized by dispersion RAFT polymerization is very different from the blend prepared via micellization of a pre-synthesized PEG-b-PS/PS mixture, and its morphology changes from vesicles, to compartmentalized vesicles and finally to porous nanospheres with a decreasing ratio of PEG-b-PS/PS in the blends. It is believed that the present dispersion RAFT polymerization technique, which simultaneously uses a macro-RAFT agent and a small RAFT agent, is a valid method to synthesize self-assembled block copolymer blends.

Fabrication of hierarchical microparticles by depositing the in situ synthesized surface nanoparticles on microspheres during the seed emulsion polymerization

A general strategy for the synthesis of polymeric hierarchical microparticles containing surface nanoparticles through modified seed emulsion polymerization is proposed. This modified seed emulsion polymerization has a character that suitable amount of monomer miniemulsion is added during the polymerization. The in situ synthesized surface nanoparticles which are resulted from the monomer miniemulsion as well as the shell-forming polymer coagulate on the seed particles and therefore hierarchical microparticles are fabricated. Various polymeric hierarchical microparticles containing 20-36 nm poly(styrene-co-acrylamide), poly(styrene-co-acrylic acid), and polystyrene surface nanoparticles are synthesized following the proposed method. The advantages in the present synthesis including both the well controls in the size, the composition, and the number of the surface nanoparticles and the convenience are demonstrated. The proposed strategy is anticipated to be a general method to fabricate hierarchical microparticles and is believed to have promising application in particle surface modification.

Dual-responsive supramolecular colloidal microcapsules from cucurbit[8]uril molecular recognition in microfluidic droplets

The macrocyclic host, cucurbit[8]uril, is used to facilitate cross-linking of colloidal particles and polymers in microdroplets resulting in thermo- and photo-responsive supramolecular colloidal microcapsules. Methyl viologen-bearing colloidal particles were prepared using template polymerisation and combined with cucurbit[8]uril and an azobenzene-functionalised polymer within microfluidic droplets. The colloidal particles self-assembled at the droplet interface, whereupon polymeric cross-links formed via ternary host–guest complexation with cucurbit[8]uril. The resultant supramolecular colloidal microcapsules were uniform in size and were able to retain a macromolecular cargo. It is shown that the capsule skin porosity, and consequently the rate of release of encapsulated cargo, can be remotely controlled via either temperature or light triggers. This simple and versatile method could be extended to other polymer or colloidal derivatives for the fabrication of nano- and microcapsules with dual stimuli response for controlled release.