Peculiar Behavior of Methyl Methacrylate Emulsion Polymerization-Induced Self-Assembly Mediated by RAFT Using Poly(Methacrylic Acid) Macromolecular Chain Transfer Agent

Reversible addition-fragmentation chain transfer (RAFT) emulsion polymerization of methyl methacrylate (MMA) is successfully performed in water in the presence of a poly(methacrylic acid) (PMAA) macromolecular chain transfer agent (macroCTA) leading to the formation of self-stabilized PMAA-b-PMMA amphiphilic block copolymer particles. At pH 3.7, the reactions are well-controlled with narrow molar mass distributions. Increasing the initial pH, particularly above 5.6, results in a partial loss of reactivity of the PMAA macroCTA. The effect of the degree of polymerization (DPn) of the PMMA block, the solids content, the nature of the hydrophobic segment, and the pH on the morphology of the obtained diblock copolymer particles is then investigated. Worm-like micelles are formed for a DPn of PMMA of 20 (PMMA20), while "onion-like" particles and spherical vesicles are obtained for PMMA30 and PMMA50, respectively. In contrast, spherical particles are obtained for the DPns higher than 150. This unusual evolution of particle morphologies upon increasing the DPn of the PMMA block seems to be related to hydrogen bonds between hydrophilic MAA and hydrophobic MMA units.

Effect of Temperature, Oil Type, and Copolymer Concentration on the Long-Term Stability of Oil-in-Water Pickering Nanoemulsions Prepared Using Diblock Copolymer Nanoparticles

A poly(glycerol monomethacrylate) (PGMA) precursor was chain-extended with 2,2,2-trifluoroethyl methacrylate (TFEMA) via reversible addition-fragmentation chain transfer (RAFT) aqueous emulsion polymerization. Transmission electron microscopy (TEM) studies confirmed the formation of well-defined PGMA52-PTFEMA50 spherical nanoparticles, while dynamic light scattering (DLS) studies indicated a z-average diameter of 26 ± 6 nm. These sterically stabilized diblock copolymer nanoparticles were used as emulsifiers to prepare oil-in-water Pickering nanoemulsions: either n-dodecane or squalane was added to an aqueous dispersion of nanoparticles, followed by high-shear homogenization and high-pressure microfluidization. The Pickering nature of such nanoemulsion droplets was confirmed via cryo-transmission electron microscopy (cryo-TEM). The long-term stability of such Pickering nanoemulsions was evaluated by analytical centrifugation over a four-week period. The n-dodecane droplets grew in size significantly faster than squalane droplets: this is attributed to the higher aqueous solubility of the former oil, which promotes Ostwald ripening. The effect of adding various amounts of squalane to the n-dodecane droplet phase prior to emulsification was also explored. The addition of up to 40% (v/v) squalane led to more stable nanoemulsions, as judged by analytical centrifugation. The nanoparticle adsorption efficiency at the n-dodecane-water interface was assessed by gel permeation chromatography when using nanoparticle concentrations of 4.0, 7.0, or 10% w/w. Increasing the nanoparticle concentration not only produced smaller droplets but also reduced the adsorption efficiency, as confirmed by TEM studies. Furthermore, the effect of varying the nanoparticle concentration (2.5, 5.0, or 10% w/w) on the long-term stability of n-dodecane-in-water Pickering nanoemulsions was explored over a four-week period. Nanoemulsions prepared at higher nanoparticle concentrations were more unstable and exhibited a faster rate of Ostwald ripening. The nanoparticle adsorption efficiency was monitored for an aging nanoemulsion prepared at a copolymer concentration of 2.5% w/w. As the droplets ripened over time, the adsorption efficiency remained constant (∼97%). This suggests that nanoparticles desorbed from the shrinking smaller droplets and then readsorbed onto larger droplets over time. Finally, the effect of temperature on the stability of Pickering nanoemulsions was examined. Storing these Pickering nanoemulsions at elevated temperatures led to faster rates of Ostwald ripening, as expected.

Effect of the Addition of Diblock Copolymer Nanoparticles on the Evaporation Kinetics and Final Particle Morphology for Drying Aqueous Aerosol Droplets

A deeper understanding of the key processes that determine the particle morphologies generated during aerosol droplet drying is highly desirable for spray-drying of powdered pharmaceuticals and foods, predicting the properties of atmospheric particles, and monitoring disease transmission. Particle morphologies are affected by the drying kinetics of the evaporating droplets, which are in turn influenced by the composition of the initial droplet as well as the drying conditions. Herein, we use polymerization-induced self-assembly (PISA) to prepare three types of sterically stabilized diblock copolymer nanoparticles comprising the same steric stabilizer block and differing core blocks with z-average diameters ranging from 32 to 238 nm. These well-defined nanoparticles enable a systematic investigation of the effect of the nanoparticle size and composition on the drying kinetics of aqueous aerosol droplets (20-28 μm radius) and the final morphology of the resulting microparticles. A comparative kinetics electrodynamic balance was used to obtain evaporation profiles for 10 examples of nanoparticles at a relative humidity (RH) of 0, 45, or 65%. Nanoparticles comprising the same core block with mean diameters of 32, 79, and 214 nm were used to produce microparticles, which were dried under different RH conditions in a falling droplet column. Scanning electron microscopy was used to examine how the drying kinetics influenced the final microparticle morphology. For dilute droplets, the chemical composition of the nanoparticles had no effect on the evaporation rate. However, employing smaller nanoparticles led to the formation of dried microparticles with a greater degree of buckling.

Microwave Irradiation-Assisted Reversible Addition-Fragmentation Chain Transfer Polymerization-Induced Self-Assembly of pH-Responsive Diblock Copolymer Nanoparticles

Herein, we present a versatile platform for the synthesis of pH-responsive poly([N-(2-hydroxypropyl)]methacrylamide)-b-poly[2-(diisopropylamino)ethyl methacrylate] diblock copolymer (PHPMA-b-PDPA) nanoparticles (NPs) obtained via microwave-assisted reversible addition-fragmentation chain transfer polymerization-induced self-assembly (MWI-PISA). The N-(2-hydroxypropyl) methacrylamide (HPMA) monomer was first polymerized to obtain a macrochain transfer agent with polymerization degrees (DPs) of 23 and 51. Subsequently, using mCTA and 2-(diisopropylamino)ethyl methacrylate (DPA) as monomers, we successfully conducted MWI-PISA emulsion polymerization in aqueous solution with a solid content of 10 wt %. The NPs were obtained with high monomer conversion and polymerization rates. The resulting diblock copolymer NPs were analyzed by dynamic light scattering (DLS) and cryogenic-transmission electron microscopy (cryo-TEM). cryo-TEM studies reveal the presence of only NPs with spherical morphology such as micelles and polymer vesicles known as polymersomes. Under the selected conditions, we were able to fine-tune the morphology from micelles to polymersomes, which may attract considerable attention in the drug-delivery field. The capability for drug encapsulation using the obtained in situ pH-responsive NPs, the polymersomes based on PHPMA₂₃-b-PDPA₁₀₀, and the micelles based on PHPMA₅₁-b-PDPA₁₀₀ was demonstrated using the hydrophobic agent and fluorescent dye as Nile red (NR). In addition, the NP disassembly in slightly acidic environments enables fast NR release.

Morphology Control via RAFT Emulsion Polymerization-Induced Self-Assembly: Systematic Investigation of Core-Forming Blocks

Polymerization-induced self-assembly (PISA) is a useful formulation for readily obtaining nanoparticles from block copolymers in situ. Reversible addition-fragmentation chain-transfer (RAFT) emulsion polymerization is utilized as one of the PISA formulations. Various factors have so far been investigated for obtaining nonspherical particles via RAFT emulsion polymerization, such as the steric structure of the shell, the glass-transition temperature (T _g) of the core-forming block, and the water solubility of the core-forming monomer. This study focuses on core-forming blocks without changing the structure of the shell-forming block. In particular, we elucidate the balance between T _g for the core-forming block and the water solubility of the core monomer. A series of alkyl methacrylates, such as methyl methacrylate (MMA), ethyl methacrylate (EMA), and n-propyl methacrylate (PrMA), are emulsion-polymerized in the presence of a poly[poly(ethylene glycol) methyl ether methacrylate] (PPEGMA) macromolecular chain-transfer agent via the RAFT process. The resulting in situ morphology changes to form shapes such as spheres, worms (toroids), and vesicles are systematically investigated. The properties of the core that determine whether a morphological change occurs from spheres are (i) the solubility of the core-forming monomer in water, (ii) the relationship between T _g for the core-forming block and the polymerization temperature, and (iii) the hydrophobic core volume, which changes the packing parameter. These factors allow prediction of the block copolymer morphology produced during RAFT emulsion polymerization of other methacrylates such as n-butyl methacrylate (BuMA), tetrahydrofurfuryl methacrylate (THFMA) with physical properties of the homopolymer (poly(tetrahydrofurfuryl methacrylate) (PTHFMA)) between those for poly(MMA) (PMMA) and PBuMA, and 1-adamantyl methacrylate (ADMA) with low monomer solubility in water and high T _g of the homopolymer (PADMA).

Sterically Stabilized Diblock Copolymer Nanoparticles Enable Convenient Preparation of Suspension Concentrates Comprising Various Agrochemical Actives

It is well known that sterically stabilized diblock copolymer nanoparticles can be readily prepared using polymerization-induced self-assembly. Recently, we reported that such nanoparticles can be employed as a dispersant to prepare micron-sized particles of a widely used fungicide (azoxystrobin) via ball milling. In the present study, we examine the effect of varying the nature of the steric stabilizer block, the mean nanoparticle diameter, and the glass transition temperature (Tg) of the core-forming block on the particle size and colloidal stability of such azoxystrobin microparticles. In addition, the effect of crosslinking the nanoparticle cores is also investigated. Laser diffraction studies indicated the formation of azoxystrobin microparticles of approximately 2 μm diameter after milling for between 15 and 30 min at 6000 rpm. Diblock copolymer nanoparticles comprising a non-ionic steric stabilizer, rather than a cationic or anionic steric stabilizer, were determined to be more effective dispersants. Furthermore, nanoparticles of up to 51 nm diameter enabled efficient milling and ensured overall suspension concentrate stability. Moreover, crosslinking the nanoparticle cores and adjusting the Tg of the core-forming block had little effect on the milling of azoxystrobin. Finally, we show that this versatile approach is also applicable to five other organic crystalline agrochemicals, namely pinoxaden, cyproconazole, difenoconazole, isopyrazam and tebuconazole. TEM studies confirmed the adsorption of sterically stabilized nanoparticles at the surface of such agrochemical microparticles. The nanoparticles are characterized using TEM, DLS, aqueous electrophoresis and 1H NMR spectroscopy, while the final aqueous' suspension concentrates comprising microparticles of the above six agrochemical actives are characterized using optical microscopy, laser diffraction and electron microscopy.

Crosslinked p(MMA) Particles by RAFT Emulsion Polymerisation: Tuning Size and Stability

The controlled synthesis of amphiphilic di-block copolymers allows a large array of nanostructures to be created, including block copolymer particles, which have proved valuable for biomedical applications. Despite progress in...