Hanbin Liu

CO2-Responsive polymer materials

CO2-Responsive polymer materials have received enormous attention in recent years, since CO2 as a new trigger has many advantages such as abundant availability, low cost, energy-saving, environment-friendly, non-toxic, good reversibility as well as great biocompatibility. In this review, we first discuss the virtues of CO2-responsiveness by comparing with traditional stimuli-sensitive materials that respond to pH, light, or redox stimuli. Then, the chemical fundamentals of CO2-responsive polymer materials are revealed including recently discovered “unexpected” CO2-sensitive features. Recent progress of CO2-responsive polymer materials is highlighted followed by various CO2-responsive “smart” polymer systems. Finally, challenges and outlooks in this area are discussed.

CO2-driven vesicle to micelle regulation of amphiphilic copolymer: random versus block strategy

Precise morphological control over self-assemblies is attractive due to their promising applications, especially in biotherapy. Block copolymer is a common choice for morphological control, since the geometry of self-assemblies can be easily predicted by the hydrophilic volume fraction. However, random copolymers are rarely taken into consideration. Here, starting from the same hydrophilic segment poly(ethylene oxide) (PEO), and using CO2-responsive 2-(diethylamino)ethyl methacrylate (DEAEMA) and hydrophobic styrene (St), we designed and synthesized a random and an entire block copolymer with similar polymerization degrees but different monomer sequences: PEO45-b-(DEAEMA90-r-St66) (Pr) and PEO45-b-DEAEMA93-b-St66 (Pb). In aqueous solution, the two polymers both aggregate into vesicles. Upon CO2-stimulus, however, the vesicle of the random copolymer Pr transforms into a spherical micelle, whereas that of the triblock copolymer Pb shows an expansion instead of a morphological transition. The restricted hydration in the random structure of Pr accounts for such a morphological transition, and the random strategy in polymer design might be useful in self-assembly regulation.

CO2-induced reversible morphology transition from giant worms to polymersomes assembled from a block-random segmented copolymer

Well-defined block copolymers represent “stars” among amphiphilic compounds for self-assembly. However, few studies have been directed to their block-random “hybrid” counterparts. In this work, a segmented diblock copolymer containing one random block, PEO113-b-P(4VP90-r-DEAEMA30), was prepared via the RAFT technique from the hydrophilic poly(ethylene oxide) block and a random hydrophobic block copolymerized from 2-(diethylamino)ethyl methacrylate (DEAEMA) and 4-vinyl pyridine (4VP). It was found that first the copolymer in aqueous media could self-assemble into vesicles, which then fuse hierarchically into giant worm-like micelles similar to shish kebab, with length and diameter of ca. 15 μm and 215 nm, respectively. After bubbling CO2 into the copolymer solution up to saturation (pH 5.43), the giant worms transform into polymersomes with a diameter about 75 nm, which is considerably larger than that of the spherical micelles assembled from the same polymer treated with HCl (pH 3.32). The vesicles obtained could revert to worm-like aggregates after depleting CO2 by bubbling N2. Protonation–deprotonation of the PDEAEMA unit, the intensive steric hindrance effect from the adjacent 4VP groups and hydrogen bonding between different 4VP units and free H2O in the interior of polymersomes accounted for such a CO2-driven reversible morphology transition.

Solvent-Driven Formation of Worm-Like Micelles Assembled from a CO2-Responsive Triblock Copolymer

Polymer worm-like micelles (WLMs) are difficult to target due to the narrow composition window. In this work, we report polymer WLMs self-assembled from a linear ABC triblock copolymer consisting of an intermediate fluorinated block of poly(2,2,3,4,4,4-hexafluorobutyl methacrylate) (F), a hydrophilic segment of poly(ethylene oxide) (O) and a CO2-responsive flank of poly(2-(diethylamino)ethyl methacrylate) (E). In the mixed solvent of water and ethanol, the polymer aggregates evolve from spheres to short rods, then long cylinders and finally WLMs when the volume ratio of water increases from 0 to 50%. Upon the stimulus of CO2, the E block is protonated, thus transforms from hydrophobic to hydrophilic. However, the WLMs just partially return back to spheres even the protonation degree of E block is up to 95%. The closely packed arrangement of fluorinated block caused by the increasing interfacial tension of the fluorinated blocks and solvent could account for the formation of WLMs and its shape alternation under CO2 stimulus.

CO2-Induced Reversible Dispersion of Graphene by a Melamine Derivative

Smart graphene with stimuli-responsive dispersity has great potential for applications in medical and biochemical fields. Nevertheless, reversible dispersion/aggregation of graphene in water with biocompatible and removable trigger still represents a crucial challenge. Here, we report CO2-induced reversible graphene dispersion by noncovalent functionalization of reduced graphene oxide with N(2),N(4),N(6)-tris(3-(dimethylamino)propyl)-1,3,5-triazine-2,4,6-triamine (MET). It was demonstrated that MET can be strongly adsorbed on graphene surface through van der Waals interaction to facilitate dispersing graphene in water. Moreover, reversible aggregation/dispersion of graphene can be achieved simply by alternately bubbling CO2 and N2 to control the desorption/adsorption of MET on graphene surface.

Synthesis and self-assembly of ABC linear triblock copolymers to target CO2-responsive multicompartment micelles

Multicompartment micelles (MCMs) have advantage in medical applications because of their capability of transporting and releasing two incompatible compounds at one time. However, the achievement of stimulus-responsive MCMs, which is a necessary condition for controlled delivery, is rarely assessed. In this study, a series of ABC linear triblock copolymers were synthesized by a stepwise RAFT polymerization method starting from a macromolecular chain transfer agent containing poly(ethylene oxide) and using monomers of 2,2,3,4,4,4-hexafluorobutyl methacrylate and 2-(diethylamino)ethyl methacrylate to construct other two segments. The block lengths were tailored in order to achieve hierarchical CO2-responsive MCMs. A morphological transition from spheres to MCMs under stimulation of CO2 is finally observed with two copolymers among this series. The volume fraction of each block in the triblock copolymer was calculated and then depicted in a ternary phase diagram, in which a narrow composition window for the CO2-responsive MCMs was suggested. These findings will guide the future design and fabrication of MCMs.

A CO2-switchable amidine monomer: synthesis and characterization

Abstract Smart system employed CO2 gas as new trigger has been attracting enormous attention in recent years, but few monomers that are capable of switching their hydrophobicity/hydrophility upon CO2 stimulation have been reported. A novel CO2 responsive monomer, 4-vinylbenzyl amidine, is designed and synthesized in this work with N,N-dimethylacetamide dimethyl acetal and 4-vinylbenzyl amine that is prepared through the Gabriel reaction. In bi-phase solvent of n-hexane and water, the monomer dissolves in n-hexane first and then transforms into water upon the CO2 treatment, indicating a hydrophobic to hydrophilic transition. This transformation is demonstrated as reversible by monitoring the conductivity variation of its wet dimethyl formamide solution during alternate bubbling/removing CO2. The protonation of 4-vinylbenzyl amidine upon CO2 treatment is demonstrated by 1H NMR which also accounts for the dissolubility change. The reversible addition-fragmentation chain-transfer polymerization of this monomer is also performed, finding the reaction only occurs in glacial acetic acid. The reason can be ascribed to the different radical structure produced in different solvent.