Controlling Adsorption of Diblock Copolymer Nanoparticles onto an Aldehyde-Functionalized Hydrophilic Polymer Brush via pH Modulation

Abstract

Sterically stabilized diblock copolymer nanoparticles with a well-defined spherical morphology and tunable diameter were prepared by RAFT aqueous emulsion polymerization of benzyl methacrylate at 70 °C. The steric stabilizer precursor used for these syntheses contained pendent cis-diol groups, which means that such nanoparticles can react with a suitable aldehyde-functional surface via acetal bond formation. This principle is examined herein by growing an aldehyde-functionalized polymer brush from a planar silicon wafer and studying the extent of nanoparticle adsorption onto this model substrate from aqueous solution at 25 °C using a quartz crystal microbalance (QCM). The adsorbed amount, Γ, depends on both the nanoparticle diameter and the solution pH, with minimal adsorption observed at pH 7 or 10 and substantial adsorption achieved at pH 4. Variable-temperature QCM studies provide strong evidence for chemical adsorption, while scanning electron microscopy images recorded for the nanoparticle-coated brush surface after drying indicate mean surface coverages of up to 62%. This fundamental study extends our understanding of the chemical adsorption of nanoparticles on soft substrates.

Scheme 1. Synthesis of PGMA₅₁–PBzMA_x Diblock Copolymer Nanoparticles via RAFT Aqueous Emulsion Polymerization of BzMA to Afford Spherical Nanoparticles. Increasing the Target DP of the PBzMA Block (y) Leads to a Systematic Increase in the Mean Nanoparticle Diameter

Figure 1

PGMA₅₁–PBzMA_y diblock copolymer nanoparticles (y = 200, 400 or 800) prepared via RAFT aqueous emulsion polymerization of BzMA at 70 °C: (a) DLS particle size distributions (where “G” denotes PGMA and “B” denotes PBzMA) and (b−d) representative TEM images recorded for the three types of nanoparticles, illustrating their well-defined spherical morphology.

Scheme 2. Growth of a PGEO5MA Brush from a Planar Silicon Wafer via SI-ARGET ATRP Followed by Selective Oxidation of the Pendent cis-Diol Groups with NaIO₄ to Produce an Aldehyde-Functionalized PAGEO5MA Brush. Schematic Representation of the Attempted Adsorption of PGMA₅₁–PBzMA_y Nanoparticles (y = 200, 400 or 800; See Scheme 1) onto the Aldehyde-Functionalized Brush via Acetal Bond Formation

Scheme 3. Acid-Catalyzed Acetal Bond Formation When Reacting an Aldehyde with a cis-Diol Compound

Figure 2

SEM images recorded for a series of PGMA₅₁–PBzMA_y nanoparticles (where y = 200, 400, or 800) adsorbed onto PAGEO5MA brushes (∼32 nm dry brush thickness) grown from planar silicon wafers. Nanoparticle adsorption experiments were conducted at 25 °C using aqueous dispersions adjusted to pH 4, 7, or 10.

Figure 3

QCM experiments and SEM studies conducted at 25 °C for the adsorption of PGMA₅₁–PBzMA₈₀₀ nanoparticles onto either PAGEO5MA or PGEO5MA brushes (≈32 nm dry brush thickness) grown from either a silica QCM sensor or a planar silicon wafer (SEM). Adsorbed amounts are calculated from QCM experiments using the Sauerbrey equation.

Figure 4

QCM experiments conducted at 25 °C for the adsorption of PGMA₅₁–PBzMA_y nanoparticles [for y = 200 (upper curve), y = 400 (middle curve), or y = 800 (lower curve)] onto PAGEO5MA brushes at pH 4.

Figure 5

Temperature dependence of the adsorbed amount for PGMA₅₁–PBzMA₈₀₀ nanoparticles bound to a PAGEO5MA brush at pH 4 for the 25–40 °C interval as indicated by QCM studies. The adsorbed amount is arbitrarily determined after 2500 s (25 min) in each case. Greater adsorption at higher temperature provides strong evidence for chemical adsorption of the nanoparticles via acetal bond formation.