Adsorption of Small Cationic Nanoparticles onto Large Anionic Particles from Aqueous Solution: A Model System for Understanding Pigment Dispersion and the Problem of Effective Particle Density.

S M North,G N Smith,O O Mykhaylyk,S P Armes,E R Jones,T Annable

doi:10.1021/acs.langmuir.6b04541

Abstract

The present study focuses on the use of copolymer nanoparticles as a dispersant for a model pigment (silica). Reversible addition–fragmentation chain transfer (RAFT) alcoholic dispersion polymerization was used to synthesize sterically stabilized diblock copolymer nanoparticles. The steric stabilizer block was poly(2-(dimethylamino)ethyl methacrylate) (PDMA) and the core-forming block was poly(benzyl methacrylate) (PBzMA). The mean degrees of polymerization for the PDMA and PBzMA blocks were 71 and 100, respectively. Transmission electron microscopy (TEM) studies confirmed a near-monodisperse spherical morphology, while dynamic light scattering (DLS) studies indicated an intensity-average diameter of 30 nm. Small-angle X-ray scattering (SAXS) reported a volume-average diameter of 29 ± 0.5 nm and a mean aggregation number of 154. Aqueous electrophoresis measurements confirmed that these PDMA71–PBzMA100 nanoparticles acquired cationic character when transferred from ethanol to water as a result of protonation of the weakly basic PDMA chains. Electrostatic adsorption of these nanoparticles from aqueous solution onto 470 nm silica particles led to either flocculation at submonolayer coverage or steric stabilization at or above monolayer coverage, as judged by DLS. This technique indicated that saturation coverage was achieved on addition of approximately 465 copolymer nanoparticles per silica particle, which corresponds to a fractional surface coverage of around 0.42. These adsorption data were corroborated using thermogravimetry, UV spectroscopy and X-ray photoelectron spectroscopy. TEM studies indicated that the cationic nanoparticles remained intact on the silica surface after electrostatic adsorption, while aqueous electrophoresis confirmed that surface charge reversal occurred below pH 7. The relatively thick layer of adsorbed nanoparticles led to a significant reduction in the effective particle density of the silica particles from 1.99 g cm–3 to approximately 1.74 g cm–3, as judged by disk centrifuge photosedimentometry (DCP). Combining the DCP and SAXS data suggests that essentially no deformation of the PBzMA cores occurs during nanoparticle adsorption onto the silica particles.

Highlights

Depending on the choice of monomers, colloidally stable nanoparticle dispersions can be obtained in water, ethanol, or n-alkanes.[20,35−39] Of particular relevance to the present work, poly(2-(dimethylamino)ethyl methacrylate)−poly(benzyl methacrylate) (PDMA−PBzMA) copolymer nanoparticles can be prepared directly in either ethanol or ethanol/water mixtures using reversible addition− fragmentation chain transfer (RAFT) dispersion polymerization.[40−43]
BzMA via RAFT dispersion polymerization in an 85:15 ethanol/water mixture. 1H NMR confirmed that almost complete BzMA polymerization occurred after 8 h at 70 °C; THF Gel Permeation Chromatography (GPC) analysis showed an increase in the molecular weight relative to that of the PDMA macro-CTA, and the relatively low final copolymer polydispersity (Mw/Mn = 1.28) indicated a well-controlled polymerization
PDMA−PBzMA diblock copolymer nanoparticles were synthesized at 15 w/w % solids and diluted to 1.0 w/w % in water before mixing with silica particles, which serve as a model pigment

Summary

■ INTRODUCTION

AB diblock copolymers can undergo spontaneous self-assembly in solution.[1−4] Various copolymer morphologies can be obtained depending on the diblock composition, including spherical micelles, worm-like particles, or vesicles.[5−20] It is well-known that a range of diblock copolymers can be used as dispersants for numerous inorganic and organic pigments.[21−27]. A typical protocol for the synthesis of PDMA71−PBzMA100 diblock copolymer nanoparticles at 15% w/w solids is as follows: benzyl methacrylate (BzMA, 5.0 g), PDMA71 macro-CTA (3.26 g), and azobisisobutyronitrile (AIBN) (9.9 mg; macro-CTA/AIBN molar ratio = 5.0) were dissolved in a 85:15 w/w ethanol/water mixture This reaction mixture was sealed in a round-bottomed flask, purged with nitrogen gas for 20 min, and placed in a preheated oil bath at 70 °C for 8 h. Analyses were conducted using a Q500 TGA instrument (TA Instruments) on dried sediments of aqueous dispersions of nanoparticle-coated silica particles obtained after centrifugation for 25 min at 5000 rpm, followed by drying under vacuum. This value was used to correct the mass losses obtained for the nanoparticle-coated silica particles

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES