Determining the Effective Density and Stabilizer Layer Thickness of Sterically Stabilized Nanoparticles

Abstract

A series of model sterically stabilized diblock copolymer nanoparticles has been designed to aid the development of analytical protocols in order to determine two key parameters: the effective particle density and the steric stabilizer layer thickness. The former parameter is essential for high resolution particle size analysis based on analytical (ultra)centrifugation techniques (e.g., disk centrifuge photosedimentometry, DCP), whereas the latter parameter is of fundamental importance in determining the effectiveness of steric stabilization as a colloid stability mechanism. The diblock copolymer nanoparticles were prepared via polymerization-induced self-assembly (PISA) using RAFT aqueous emulsion polymerization: this approach affords relatively narrow particle size distributions and enables the mean particle diameter and the stabilizer layer thickness to be adjusted independently via systematic variation of the mean degree of polymerization of the hydrophobic and hydrophilic blocks, respectively. The hydrophobic core-forming block was poly(2,2,2-trifluoroethyl methacrylate) [PTFEMA], which was selected for its relatively high density. The hydrophilic stabilizer block was poly(glycerol monomethacrylate) [PGMA], which is a well-known non-ionic polymer that remains water-soluble over a wide range of temperatures. Four series of PGMAx–PTFEMAy nanoparticles were prepared (x = 28, 43, 63, and 98, y = 100–1400) and characterized via transmission electron microscopy (TEM), dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS). It was found that the degree of polymerization of both the PGMA stabilizer and core-forming PTFEMA had a strong influence on the mean particle diameter, which ranged from 20 to 250 nm. Furthermore, SAXS was used to determine radii of gyration of 1.46 to 2.69 nm for the solvated PGMA stabilizer blocks. Thus, the mean effective density of these sterically stabilized particles was calculated and determined to lie between 1.19 g cm–3 for the smaller particles and 1.41 g cm–3 for the larger particles; these values are significantly lower than the solid-state density of PTFEMA (1.47 g cm–3). Since analytical centrifugation requires the density difference between the particles and the aqueous phase, determining the effective particle density is clearly vital for obtaining reliable particle size distributions. Furthermore, selected DCP data were recalculated by taking into account the inherent density distribution superimposed on the particle size distribution. Consequently, the true particle size distributions were found to be somewhat narrower than those calculated using an erroneous single density value, with smaller particles being particularly sensitive to this artifact.

Synthesis of well-defined epoxy-functional spherical nanoparticles by RAFT aqueous emulsion polymerization

“…57 However, with only a few exceptions, [62][63][64][65][66][67][68] RAFT aqueous emulsion polymerization formulations usually result in kineticallytrapped spheres. 40,[42][43][44][45][46][47][48][49][50]69,70 Nevertheless, it was considered prudent to explore higher copolymer concentrations, if only to establish the upper limit for the production of well-defined epoxy-functional spheres. Thus PGMA 45 -PGlyMA 100 syntheses were conducted at 50°C at up to 40% w/w solids, see Table 2.…”

“…36,41 In contrast, RAFT aqueous emulsion polymerizations often result in the formation of kinetically-trapped spheres, particularly for methacrylic core-forming blocks based on methyl methacrylate, [42][43][44][45] n-butyl methacrylate, 46,47 benzyl methacrylate 48 or 2,2,2-trifluoroethyl methacrylate. 49,50 Nevertheless, the latter PISA formulations offer a potentially important route for the preparation of functional diblock copolymer nano-objects at up to 50% w/w solids, which represents a substantial improvement over traditional postpolymerization self-assembly techniques. 30 We have previously reported using poly(glycerol monomethacrylate) (PGMA) as a steric stabilizer block for the preparation of diblock copolymer nano-objects via RAFT aqueous dispersion polymerization, 30 RAFT alcoholic dispersion polymerization, 36 and RAFT aqueous emulsion polymerization.…”

“…30 We have previously reported using poly(glycerol monomethacrylate) (PGMA) as a steric stabilizer block for the preparation of diblock copolymer nano-objects via RAFT aqueous dispersion polymerization, 30 RAFT alcoholic dispersion polymerization, 36 and RAFT aqueous emulsion polymerization. 48,49 In principle, PGMA can be obtained via reaction of PGlyMA with one equivalent of water, but this derivatization suffers from a cross-linking side reaction and in practice GMA is commercially available as a specialty monomer. However, the conversion of water-immiscible GlyMA into water-soluble GMA can be readily achieved simply by heating an initial aqueous GlyMA emulsion at 80°C for 8-9 h. 51 Remarkably, if this reaction is conducted at around neutral pH in the presence of air then no background polymerization occurs and no hydrolysis of the ester bond is detected.…”

RAFT‐Mediated Polymerization‐Induced Self‐Assembly

“…The monomer‐swollen spherical block copolymer particles initially formed during the process fuse together according to a polymerization‐induced reorganization mechanism, as visualized in transmission electron microscopy (TEM) studies by Armes and co‐workers . Although, the same process could take place in emulsion polymerization, kinetically trapped spheres are obtained most of the time . (see section 5).…”

Epoxy‐Functional Sterically Stabilized Diblock Copolymer Nanoparticles via RAFT Aqueous Emulsion Polymerization: Comparison of Two Synthetic Strategies

“…Over the past decade or so, polymerization‐induced self‐assembly (PISA) has become widely recognized as a powerful platform technology for the rational design of block copolymer nanoparticles of various morphologies, including spheres, worms, vesicles, framboidal vesicles, and lamellae . In the specific context of RAFT aqueous emulsion polymerization, PISA typically involves chain extension of a water‐soluble polymer precursor using a water‐immiscible monomer such as styrene, benzyl methacrylate, 2,2,2‐trifluoroethyl methacrylate, methyl methacrylate, or n ‐butyl acrylate . At a certain critical degree of polymerization (DP), the growing hydrophobic block becomes insoluble in the aqueous phase, which drives in situ self‐assembly to form sterically stabilized diblock copolymer nano‐objects.…”