The ability of amphiphilic block copolymers to self‐assemble in selective solvents has been widely studied in academia and utilized for various commercial products. The self‐assembled polymer vesicle ...is at the forefront of this nanotechnological revolution with seemingly endless possible uses, ranging from biomedical to nanometer‐scale enzymatic reactors. This review is focused on the inherent advantages in using polymer vesicles over their small molecule lipid counterparts and the potential applications in biology for both drug delivery and synthetic cellular reactors.
Synthesis of diblock copolymer nano‐objects: alcohol is a good idea! RAFT dispersion polymerization of benzyl methacrylate in alcohol using weak polyelectrolyte‐based chain transfer agents allows the ...facile synthesis of sterically stabilized diblock copolymer nano‐objects with very high monomer conversions. Such syntheses are usually problematic when conducted in water due to electrostatic repulsion between highly charged stabilizer chains, which impedes in situ self‐assembly. Construction of a detailed phase diagram facilitates reproducible syntheses of well‐defined diblock copolymer spheres, worms or vesicles, since it allows mixed phase regions to be avoided. Aqueous electrophoresis studies confirm that these nano‐objects can acquire substantial surface charge when transferred to aqueous solution due to ionization (or protonation) of the polyacid (or polybase) stabilizer chains.
Polymerization-induced self-assembly (PISA) is used for the highly convenient and efficient preparation of ampholytic diblock copolymer nanoparticles directly in acidic aqueous solution. Cationic ...nanoparticles comprising a protonated polyamine stabilizer block and a hydrophobic polyacid core-forming block are formed at pH 2. Micelle inversion occurs at pH 10 to produce anionic nanoparticles with an ionized polyacid stabilizer block and a hydrophobic polyamine core-forming block. Macroscopic precipitation occurs at around pH 6–7, which lies close to the isoelectric point of this ampholytic diblock copolymer. Incorporation of fluorescein and rhodamine dye labels into the acid and amine blocks, respectively, leads to dual-color bifluorescent self-reporting pH-responsive nanoparticles.
Benzyl methacrylate (BzMA) is polymerized via reversible addition–fragmentation chain transfer (RAFT) chemistry under alcoholic dispersion polymerization conditions in ethanol using a ...poly(2-(dimethylamino)ethyl methacrylate) (PDMA) chain transfer agent (CTA) at 70 °C. In principle, polymerization-induced self-assembly can lead to the formation of either spherical micelles, worm-like micelles, or vesicles, with the preferred morphology being dictated by the hydrophilic–hydrophobic balance of the PDMA–PBzMA diblock copolymer chains. Very high monomer conversions (>99%) are routinely obtained within 24 h as judged by 1H NMR studies. Moreover, THF GPC analyses confirmed that relatively low polydispersities (M w/M n < 1.30) are achieved, indicating reasonably good pseudoliving character. A detailed phase diagram was constructed using a PDMA31 macro-CTA by systematically varying both the target degree of polymerization of the PBzMA block and the total solids concentration of the reaction solution. This phase diagram can be used to reliably predict the synthesis conditions required to produce pure phases, rather than merely mixed phases (e.g., spheres plus worms or worms plus vesicles). Finally, these PDMA–PBzMA diblock copolymer nanoparticles remain colloidally stable when transferred from ethanol into water; aqueous electrophoresis studies confirmed that the particles acquire appreciable cationic character below pH 7 due to protonation of the PDMA stabilizer chains.
Polymerization-induced self-assembly (PISA) is a powerful platform technology for the rational and efficient synthesis of a wide range of block copolymer nano-objects (e.g., spheres, worms or ...vesicles) in various media. In situ small-angle X-ray scattering (SAXS) studies of reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization have previously provided detailed structural information during self-assembly (see M. J. Derry et al., Chem. Sci. 2016, 7, 5078–5090 ). However, conducting the analogous in situ SAXS studies during RAFT aqueous emulsion polymerizations poses a formidable technical challenge because the inherently heterogeneous nature of such PISA formulations requires efficient stirring to generate sufficiently small monomer droplets. In the present study, the RAFT aqueous emulsion polymerization of 2-methoxyethyl methacrylate (MOEMA) has been explored for the first time. Chain extension of a relatively short non-ionic poly(glycerol monomethacrylate) (PGMA) precursor block leads to the formation of sterically-stabilized PGMA-PMOEMA spheres, worms or vesicles, depending on the precise reaction conditions. Construction of a suitable phase diagram enables each of these three morphologies to be reproducibly targeted at copolymer concentrations ranging from 10 to 30% w/w solids. High MOEMA conversions are achieved within 2 h at 70 °C, which makes this new PISA formulation well-suited for in situ SAXS studies using a new reaction cell. This bespoke cell enables efficient stirring and hence allows in situ monitoring during RAFT emulsion polymerization for the first time. For example, the onset of micellization and subsequent evolution in particle size can be studied when preparing PGMA29-PMOEMA30 spheres at 10% w/w solids. When targeting PGMA29-PMOEMA70 vesicles under the same conditions, both the micellar nucleation event and the subsequent evolution in the diblock copolymer morphology from spheres to worms to vesicles are observed. These new insights significantly enhance our understanding of the PISA mechanism during RAFT aqueous emulsion polymerization.
Benzyl methacrylate (BzMA) is polymerized using a poly(lauryl methacrylate) macromolecular chain transfer agent (PLMA macro-CTA) using reversible addition–fragmentation chain transfer (RAFT) ...polymerization at 70 °C in n-dodecane. This choice of solvent leads to an efficient dispersion polymerization, with polymerization-induced self-assembly (PISA) occurring via the growing PBzMA block to produce a range of PLMA–PBzMA diblock copolymer nano-objects, including spheres, worms, and vesicles. In the present study, particular attention is paid to the worm phase, which forms soft free-standing gels at 20 °C due to multiple inter-worm contacts. Such worm gels exhibit thermo-responsive behavior: heating above 50 °C causes degelation due to the onset of a worm-to-sphere transition. Degelation occurs because isotropic spheres interact with each other much less efficiently than the highly anisotropic worms. This worm-to-sphere thermal transition is essentially irreversible on heating a dilute solution (0.10% w/w) but is more or less reversible on heating a more concentrated dispersion (20% w/w). The relatively low volatility of n-dodecane facilitates variable-temperature rheological studies, which are consistent with eventual reconstitution of the worm phase on cooling to 20 °C. Variable-temperature 1H NMR studies conducted in d 26-dodecane confirm partial solvation of the PBzMA block at elevated temperature: surface plasticization of the worm cores is invoked to account for the observed change in morphology, because this is sufficient to increase the copolymer curvature and hence induce a worm-to-sphere transition. Small-angle X-ray scattering and TEM are used to investigate the structural changes that occur during the worm-to-sphere-to-worm thermal cycle; experiments conducted at 1.0 and 5.0% w/w demonstrate the concentration-dependent (ir)reversibility of these morphological transitions.
A carboxylic acid based reversible additionfragmentation transfer (RAFT) agent is used to prepare gels composed of worm‐like diblock copolymers using two non‐ionic monomers, glycerol monomethacrylate ...(GMA) and 2‐hydroxypropyl methacrylate (HPMA). Ionization of the carboxylic acid end‐group on the PGMA stabilizer block induces a worm‐to‐sphere transition, which in turn causes immediate degelation. This morphological transition is fully reversible as determined by TEM and rheology studies and occurs because of a subtle change in the packing parameter for the copolymer chains. A control experiment where the methyl ester derivative of the RAFT agent is used to prepare the same diblock copolymer confirms that no pH‐responsive behavior occurs in this case. This end‐group ionization approach is important for the design of new pH‐responsive copolymer nano‐objects as, unlike polyacids or polybases, only a minimal amount of added base (or acid) is required to drive the morphological transition.
From worms to spheres: Ionization of a single terminal carboxylic acid on each stabilizer chain of gels containing non‐ionic diblock copolymer worms confers unexpected pH‐sensitivity; this subtle change in the packing parameter induces a reversible worm‐to‐sphere transformation with concomitant degelation.
Stimulus-Responsive Liquid Marbles Dupin, Damien; Armes, Steven P; Fujii, Syuji
Journal of the American Chemical Society,
04/2009, Letnik:
131, Številka:
15
Journal Article
Recenzirano
Millimeter-sized “liquid marbles” are usually prepared using highly hydrophobic particles such as fluorosilane-treated lycopodium powder or alkylated silica sols. In the present work it is shown that ...“liquid marbles” can be prepared using sterically stabilized polystyrene latex; remarkably, such latex particles can be readily prepared by aqueous emulsion polymerization using a well-defined styrene-functionalized poly(2-(diethylamino)ethyl methacrylate) macromonomer as a reactive steric stabilizer. The macromonomer stabilizer chains are water-soluble when partially protonated below their pK a of ∼7.0 but become sufficiently hydrophobic in their deprotonated form to allow formation of robust “liquid marbles” that remain stable when placed at the air/water interface. Moreover, the stabilizer chains confer pH-responsive behavior on the “liquid marbles”; addition of acid to the aqueous subphase causes immediate destruction of the “liquid marble”.
We investigate the shear and extensional flow behavior of dispersions composed of two types of worm-like nanoparticles (WLNPs) with comparable cross-sectional diameters, similar persistence lengths ...but differing contour lengths, and thus differing flexibility. By measuring the flow-induced birefringence (FIB) of WLNP dispersions in two contrasting microfluidic devices, we obtain an experimental quantification of the role of shearing and planar extensional flows at aligning a short and stiff WLNP (S-WLNP) and a relatively long and flexible WLNP (L-WLNP). We show that shear and extensional flows induce the alignment of both types of WLNPs. However, extensional deformations are more effective than shear deformations at triggering the onset of alignment of the WLNP. The difference between shear and extensional deformations for WLNP alignment is explained based on the ratio of extensional and shear viscosity of the solvent fluid (Trouton ratio of the solvent) and a structural parameter related to the WLNP extensibility and flexibility. Under shear flow, these WLNP dispersions display shear-thinning behavior, with an exponential reduction in viscosity with increasing alignment. Under extensional flow, the WLNP alignment leads to extensional thinning, making WLNP ideal additives for industrial and biotechnology formulations exposed to extensional dominated flows (e.g., jetting, spraying, and printing processes).
Core cross-linked poly(stearyl methacrylate)–poly(benzyl methacrylate)–poly(ethylene glycol dimethacrylate) S31–B200–E20 triblock copolymer nanoparticles were synthesized directly in an industrial ...mineral oil via polymerization-induced self-assembly (PISA). Gel permeation chromatography analysis of the S31–B200 diblock copolymer precursor chains indicated a well-controlled reversible addition–fragmentation chain transfer dispersion polymerization, while transmission electron microscopy, dynamic light-scattering (DLS), and small-angle X-ray scattering studies indicated the formation of well-defined spheres. Moreover, DLS studies performed in THF, which is a common solvent for the S and B blocks, confirmed successful covalent stabilization because well-defined solvent-swollen spheres were obtained under such conditions. Tribology experiments using a mini-traction machine (MTM) indicated that 0.50% w/w dispersions of S31–B200–E20 spheres dramatically reduce the friction coefficient of base oil within the boundary lubrication regime. Given their efficient and straightforward PISA synthesis at high solids, such nanoparticles offer new opportunities for the formulation of next-generation ultralow-viscosity automotive engine oils.
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