Thermoresponsive Nanoparticles with Cyclic-Polymer-Grafted Shells Are More Stable than with Linear-Polymer-Grafted Shells: Effect of Polymer Topology, Molecular Weight, and Core Size

Abstract

Polymer brush-grafted superparamagnetic iron oxide nanoparticles can change their aggregation state in response to temperature and are potential smart materials for many applications. Recently, the shell morphology imposed by grafting to a nanoparticle core was shown to strongly influence the thermoresponsiveness through a coupling of intrashell solubility transitions and nanoparticle aggregation. We investigate how a change from linear to cyclic polymer topology affects the thermoresponsiveness of poly(2-isopropyl-2-oxazoline) brush-grafted superparamagnetic iron oxide nanoparticles. Linear and cyclic polymers with three different molecular weights (7, 18, and 24.5 kg mol –1 ) on two different core sizes (3.7 and 9.2 nm) and as free polymer were investigated. We observed the critical flocculation temperature (CFT) during temperature cycling dynamic light scattering experiments, the critical solution temperature (CST), and the transition enthalpy per monomer during differential scanning calorimetry measurements. When all conditions are identical, cyclic polymers increase the colloidal stability and the critical flocculation temperature compared to their linear counterparts. Furthermore, the cyclic polymer shows only one uniform transition, while we observe multiple transitions for the linear polymer shells. We link the single transition and higher colloidal stability to the absence in cyclic PiPrOx shells of a dilute outer part where the particle shells can interdigitate.

Linear and Ring Polymer Brushes: A Density Functional Theory Study

“…Except for the hightemperature regime, both the brush-brush overlap and the attraction strength are weaker for the ring brushes compared to their equivalent linear counterparts. This DFT result is in agreement with recent experiments, [29] dissipative particle dynamics simulations, [39] and lattice-based SCF theory. [41]…”

“…Having confirmed that the trends in DFT density profiles of linear and ring brushes agree with the earlier simulation [33,39] and mean-field studies, [41] we next focus on two opposing parallel brushes, their overlap, and their interaction, which are the keys for analyzing colloidal steric stabilization. [1,29] To this end, Figure 1b shows representative DFT results for total monomer density profiles 𝜌 1 (z) and 𝜌 2 (z) of two opposing linear (upper panel) and ring (lower panel) brushes at T = 2 at their respective equilibrium separations. For the linear brush, we set N = 32 and 𝜎 g = 0.06, while an "equivalent" ring brush is obtained 3b below).…”

“…In particular, both MD and DFT methods predicted that (non-grafted) ring polymers act as stronger depletion agents compared to linear chains. Recall that the situation is reversed in the case of ring and linear brushes, as indicated both by experimental [29] and theoretical [39,41] results.…”

“…[25][26][27] Yet another useful control variable involves the grafted polymer topology. [28] Thus, a recent experimental study [29] has shown that thermoresponsive nanoparticles grafted with ring polymers are better protected (via steric stabilization) against coagulation compared to nanoparticles grafted with linear chains. In view of this finding, it would be of interest to perform a detailed direct theoretical comparison between ring and linear brushes, focusing specifically on the brush-brush interactions.…”

“…Ring polymer brushes have already received substantial experimental [28][29][30][31][32] and theoretical [33][34][35][36][37][38][39][40][41] attention. The latter studies can be broadly subdivided into two groups: approaches based on computer simulations [33][34][35][36][37][38][39] and those based on mean-field theories.…”