Block Copolymer Nanoparticles are Effective Dispersants for Micrometer-Sized Organic Crystalline Particles

Abstract

Well-defined sterically stabilized diblock copolymer nanoparticles of 29 nm diameter are prepared by RAFT aqueous emulsion polymerization of methyl methacrylate using a dithiobenzoate-capped poly(glycerol monomethacrylate) precursor. These nanoparticles are evaluated as a dispersant for the preparation of organic crystalline microparticles via ball milling. This is exemplified for azoxystrobin, which is a broad-spectrum fungicide that is widely used to protect various food crops. Laser diffraction and optical microscopy studies indicate the formation of azoxystrobin microparticles of approximately 2 μm diameter after ball milling for 10 min at 400 rpm. Nanoparticle adsorption at the surface of these azoxystrobin microparticles is confirmed by electron microscopy studies. The extent of nanoparticle adsorption on the azoxystrobin microparticles can be quantified using a supernatant assay based on solution densitometry. This technique indicates an adsorbed amount of approximately 5.5 mg m –2 , which is sufficient to significantly reduce the negative zeta potential exhibited by azoxystrobin. Moreover, this adsorbed amount appears to be essentially independent of the nature of the core-forming block, with similar data being obtained for both poly(methyl methacrylate)- and poly(2,2,2-trifluoroethyl methacrylate)-based nanoparticles. Finally, X-ray photoelectron spectroscopy studies confirm attenuation of the underlying N1s signal arising from the azoxystrobin microparticles by the former adsorbed nanoparticles, suggesting a fractional surface coverage of approximately 0.24. This value is consistent with a theoretical surface coverage of 0.25 calculated from the adsorption isotherm data. Overall, this study suggests that sterically stabilized diblock copolymer nanoparticles may offer a useful alternative approach to traditional soluble copolymer dispersants for the preparation of suspension concentrates affecting the context of agrochemical applications.

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

“…At first sight, this is significantly higher than that estimated by XPS studies for azoxystrobin microparticles coated with the same nanoparticles (24% surface coverage). 39 However, we found that the grayscale adjustment within ImageJ software is rather subjective, so this relatively high fractional surface coverage ideally requires corroboration by XPS. Unfortunately, this is beyond the scope of the current study.…”

“…Initially, we sought to extend our prior study by examining how adjusting various synthesis parameters affected the preparation of aqueous SCs comprising azoxystrobin, a widely used fungicide. 39 Preparation of SC formulations involves milling relatively coarse (20–76 μm diameter) hydrophobic organic crystals in the presence of a suitable polymeric dispersant ( Figure 1 b). It is perhaps worth mentioning that a control experiment performed in the absence of any dispersant resulted in poor milling efficiency (ca.…”

Synthesis and Aqueous Solution Properties of Shape-Shifting Stimulus-Responsive Diblock Copolymer Nano-Objects

“…PISA involves chain extension of a soluble homopolymer precursor using a suitable second monomer in an appropriate solvent, with the latter being chosen so that the growing second block becomes insoluble at some critical chain length, thus leading to the formation of diblock copolymer nano-objects. 14−19 In principle, such nanoparticles offer a wide range of potential applications, including new biocompatible hydrogels for cell culture and long-term storage, 10,20,21 microencapsulation of proteins and enzymes within vesicles, 22−27 bespoke Pickering emulsifiers, 28 novel flocculants, 29 dispersants for agrochemical actives such as azoxystrobin, 30 and new lubricants for the formulation of ultralow-viscosity automotive engine oils. 31 The majority of the PISA literature involves reversible addition−fragmentation chain transfer (RAFT) polymerization.…”

“…In contrast, polymerization-induced self-assembly (PISA) enables the efficient synthesis of block copolymer nano-objects of controllable size, morphology, and surface composition directly in the form of concentrated dispersions in a wide range of polar ,− or non-polar , solvents. PISA involves chain extension of a soluble homopolymer precursor using a suitable second monomer in an appropriate solvent, with the latter being chosen so that the growing second block becomes insoluble at some critical chain length, thus leading to the formation of diblock copolymer nano-objects. − In principle, such nanoparticles offer a wide range of potential applications, including new biocompatible hydrogels for cell culture and long-term storage, ,, microencapsulation of proteins and enzymes within vesicles, − bespoke Pickering emulsifiers, novel flocculants, dispersants for agrochemical actives such as azoxystrobin, and new lubricants for the formulation of ultralow-viscosity automotive engine oils . The majority of the PISA literature involves reversible addition–fragmentation chain transfer (RAFT) polymerization. − This chemistry is based on the principle of rapid reversible chain transfer: an organosulfur-based CTA is used to produce well-defined block copolymers with low dispersities and predictable mean DPs. − …”

Synthesis of Thermoresponsive Diblock Copolymer Nano-Objects via RAFT Aqueous Emulsion Polymerization of Hydroxybutyl Methacrylate

“…In striking contrast, the RAFT aqueous emulsion polymerization of relatively hydrophobic vinyl monomers such as styrene, − methyl methacrylate, − benzyl methacrylate, , n -butyl acrylate, ,, phenyl acrylate, vinyl acetate, − or 2,2,2-trifluoroethyl methacrylate − invariably leads to the formation of block copolymer nano-objects that do not exhibit any thermoresponsive behavior. Moreover, such formulations often lead solely to kinetically trapped spheres, although there are a few well-known counter-examples to this morphological limitation. ,,,− Recently, Armes and co-workers explored the RAFT aqueous emulsion polymerization of vinyl monomers such as glycidyl methacrylate, − 2-methoxyethyl methacrylate, or hydroxybutyl methacrylate (HBMA) , which exhibit moderate aqueous solubilities (e.g., 15–20 g dm –3 ) at 20 °C.…”