title: 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-PTFEMAynanoparticles 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-3for the smaller particles and 1.41 g cm-3for 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 densitydifferencebetween 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 densitydistributionsuperimposed 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.
authors.affiliation: Department of Chemistry, University of Sheffield , Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.
authors.forename: Bernice
authors.initials: B
authors.lastname: Akpinar
authors.affiliation: Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.; School of Materials, The University of Manchester, Oxford Road, Manchester, M13 9PL, U.K.
authors.forename: Lee A
authors.initials: LA
authors.lastname: Fielding
authors.affiliation: Department of Chemistry, University of Sheffield , Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.
authors.forename: Victoria J
authors.initials: VJ
authors.lastname: Cunningham
authors.affiliation: Department of Chemistry, University of Sheffield , Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.
authors.forename: Yin
authors.initials: Y
authors.lastname: Ning
authors.affiliation: Department of Chemistry, University of Sheffield , Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.
authors.forename: Oleksandr O
authors.initials: OO
authors.lastname: Mykhaylyk
authors.affiliation: Department of Chemistry, University of Sheffield , Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.
authors.forename: Patrick W
authors.initials: PW
authors.lastname: Fowler
authors.affiliation: Department of Chemistry, University of Sheffield , Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.
authors.forename: Steven P
authors.initials: SP
authors.lastname: Armes