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

Bernice Akpinar,Yin Ning,Steven P Armes,Oleksandr O Mykhaylyk,Patrick W Fowler,Victoria J Cunningham,Lee A Fielding

doi:10.1021/acs.macromol.6b00987

Bernice Akpinar, Yin Ning + Show 5 more

Open Access

https://doi.org/10.1021/acs.macromol.6b00987

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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.

Highlights

For a fixed degree of polymerization (DP) of the hydrophilic PGMA stabilizer, a monotonic increase in particle diameter was observed on increasing the DP of the core-forming PTFEMA block
A substantial reduction in particle diameter was observed for PGMAx−PTFEMA400 nanoparticles on increasing the PGMA stabilizer DP
The radius of gyration, Rg, of the PGMAx precursor chains in aqueous solution was calculated theoretically and determined experimentally via small-angle X-ray scattering (SAXS). The latter value was subsequently used as a fixed parameter when modeling SAXS patterns recorded for PGMAx−PTFEMAy diblock copolymer nanoparticles in aqueous solution

Summary

■ INTRODUCTION

Steric stabilization is widely recognized to be the most important mechanism for achieving long-term colloidal stability.[1,2] Unlike charge stabilization,[3] it confers thermodynamic stability at relatively high solids, is tolerant of added salt in aqueous formulations,[4] and can be designed for a wide range of media, including both polar solvents[5−11] and non-polar solvents[12−21] as well as more exotic solvents such as supercritical carbon dioxide[22−27] or ionic liquids.[28,29] In view of these many advantages, steric stabilization is used on an industrial scale across a wide range of commercial sectors. The hydrophilic stabilizer block was chosen to be a well-known non-ionic water-soluble polymer, namely poly(glycerol monomethacrylate) [PGMA], while poly(2,2,2trifluoroethyl methacrylate) [PTFEMA] was selected as the hydrophobic core-forming block, mainly because of its relatively high solid-state density (1.47 g cm−3, see Figure 1) This model system was designed to enable the determination of the effective particle density (ρparticle) and stabilizer shell thickness (Tshell) for sterically stabilized diblock copolymer nanoparticles. RAFT Aqueous Emulsion Polymerization of PGMAx− PTFEMAy. A typical protocol for the synthesis of PGMA63− PTFEMA400 diblock copolymer nanoparticles was as follows: PGMA63 macro-CTA (0.140 g), ACVA (0.600 mg, 2.14 μmol; macro-CTA/ACVA molar ratio = 3.0), and water (4.58 g, 10% w/w) were weighed into a 14 mL sample vial, sealed with a rubber septum, and degassed with nitrogen for 30 min. Instruments, Seagate Lane, Stuart, FL), followed by injection of 100 μL of PGMAx−PTFEMAy diblock copolymer nanoparticles in the form of a 1−5% w/w aqueous dispersion

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES