(363e) Effect of Dispersity on pH-Response of Spherical Weak Polyelectrolyte Brushes

Spherical weak polyelectrolyte brushes have been widely applied in drug delivery systems, sensors, and water purification systems due to their rapid response to environmental conditions such as pH and salt concentration. The functions of weak polyelectrolyte brushes are governed by their stimulus response. Brush properties such as grafting density and brush length have been examined as parameters for tuning the stimuli-responsive brush behavior, but the effect of dispersity (i.e. breadth in the molecular weight distribution) on the degree of response remains questionable. Experimental studies have shown that brush length is independent of pH for spherical weak polyelectrolyte brushes of high grafting density and low molecular weight.¹ However, simulations demonstrate that increasing dispersity releases crowding effect in brushes, thereby changing their pH-responsive behavior.² The relationship between dispersity and pH-responsive behavior thus remains underexplored. Here, we quantify the brush length of weak polyelectrolyte brushes grafted from nanoparticles and show that dispersity affects their pH-responsive behavior. We synthesized poly(tert-butyl acrylate) (PtBA)-grafted nanoparticles, with nearly constant grafting density but varying dispersities, using atom transfer radical polymerization in the presence of a chain terminating agent. Next, weak polyelectrolyte PAA brushes were generated via hydrolysis of the PtBA brushes grafted on nanoparticles. The effect of dispersity on the brush length of PtBA-grafted nanoparticles was discovered to be different for brushes of low weight-average degree of polymerization (N_w) as compared to brushes with higher N_w. Hence, the pH-responsive lengths of PAA-grafted nanoparticles with varying dispersities were investigated at low and high N_w. At low N_w, the length of the low-dispersity PAA brushes was independent of pH, whereas the high-dispersity PAA brush length increased with pH. At high N_w, however, the lengths of PAA brushes with different dispersities showed the same response to varying pH. Additionally, the zeta potential and degree of dissociation of the PAA brushes varied with pH, regardless of dispersity and N_w. These results suggest that dispersity has distinct effects on crowding of spherical polymer brushes at low and high N_w, leading to differing pH-responsive behaviors. This tunable pH-responsive brush conformation can be utilized to design multi-functional smart materials.

References:

1. Zhang, C.; Carlson, T.; Yang, S.; Akcora, P. Ordering pH-Responsive Polyelectrolyte-Grafted Nanoparticles in a Flow Coating Process. Advanced Materials Interfaces 2018, 5 (5), 1701318 DOI: 10.1002/admi.201701318.
2. Dodd, P. M.; Jayaraman, A. Monte Carlo simulations of polydisperse polymers grafted on spherical surfaces. J. Polym. Sci. B Polym. Phys. 2012, 50 (10), 694-705 DOI: 10.1002/polb.23057.