Determining the Effective Density and Stabilizer LayerThickness of Sterically Stabilized Nanoparticles

摘要

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], whichwas selected for its relatively high density. The hydrophilic stabilizerblock was poly(glycerol monomethacrylate) [PGMA], which is a well-knownnon-ionic polymer that remains water-soluble over a wide range oftemperatures. Four series of PGMAx–PTFEMAy nanoparticles were prepared (x = 28, 43, 63, and 98, y = 100–1400) andcharacterized via transmission electron microscopy (TEM), dynamiclight scattering (DLS), and small-angle X-ray scattering (SAXS). Itwas found that the degree of polymerization of both the PGMA stabilizerand core-forming PTFEMA had a strong influence on the mean particlediameter, which ranged from 20 to 250 nm. Furthermore, SAXS was usedto determine radii of gyration of 1.46 to 2.69 nm for the solvatedPGMA stabilizer blocks. Thus, the mean effective density of thesesterically stabilized particles was calculated and determined to liebetween 1.19 g cm^–3 for the smaller particles and1.41 g cm^–3 for the larger particles; these valuesare significantly lower than the solid-state density of PTFEMA (1.47g cm^–3). Since analytical centrifugation requiresthe density difference between the particles andthe aqueous phase, determining the effective particle density is clearlyvital for obtaining reliable particle size distributions. Furthermore,selected DCP data were recalculated by taking into account the inherentdensity distribution superimposed on the particlesize distribution. Consequently, the true particle size distributionswere found to be somewhat narrower than those calculated using anerroneous single density value, with smaller particles being particularlysensitive to this artifact.