Impact of the Solvent Quality on the Local Dynamics of Soft and Swollen Polymer Nanoparticles Functionalized with Polymer Chains

Grafting polymer chains on the surface of nanoparticles (NPs) is a strategy used to control the interaction between the NPs and their environment. The fate of the resulting particles in a given environment is strongly influenced by the solvent–polymer interaction. The solvent quality affects the behavior, conformation, and dynamics of the grafted polymer chains. However, when this polymer grafting strategy is used to functionalized polymer particles, the influence of solvent quality becomes even more complex; when the grafted polymer chains and the polymer nanoparticles are tethered together, the effect of the solvent quality on the behavior and dynamics of the system depends on the solvent interaction with both polymer components. To explore the relationship between the solvent quality and the dynamics of polymer-functionalized soft polymer NPs, we designed a system based on cross-linked polystyrene (PS) NPs grafted with a canopy of poly(methyl acrylate) (PMA). PS and PMA, two immiscible polymers, can be selectively solvated by using binary mixtures of solvents. NMR spectroscopy was used to address the effect of those selective solvents on the local mobility of the PS–PMA core-canopy NPs and revealed an interplay between the local mobility of the core and the local mobility of the canopy. A selective reduction of the solvent quality for the PMA canopy resulted in the expected reduction of the local mobility of the PMA chains, but also in the slower dynamics of the PS core. Similarly, a selective reduction of the solvent quality for the PS core resulted in a slower dynamics for both the PS core and the PMA canopy.

The organic phase was dried over MgSO4. After evaporating the solvent, the product was purified by a silica filled column chromatography (hexane:ethyl acetate = 6:4, Rf = 0.2). To remove the remaining impurities, the mixture was purified with second column chromatography (hexane:diethyl ether = 9:1, Rf = 0.46) resulting in the pure product. 1

Synthesis of polystyrene core (PS NPs)
For the preparation of PS NPs, an organic phase containing 28.6 mmol of St, 0.1 mmol of DVB, 0.1 mmol of V-59 and 1.1 mmol of hexadecane was mixed with 24 mL of 0.3 wt% aqueous solution of SDS. The mixture was pre-emulsified by mechanical agitation for 10 min and then emulsified by ultrasonication (Branson Sonifier W 450 Digital equipped with a titanium solid extender tip, diameter of 1/2"). The solution was sonicated in an ice bath at 70% amplitude for 2 minutes with a sequence of 10-seconds pulse-on and 2-seconds pulse-off. The emulsified mixture was placed in an oil bath at 80 °C to initiate free radical polymerization. When the double-bond conversion reached 90%, 2 mL of 7.6 wt% SDS aqueous solution was added in order to increase the SDS concentration to 0.5 wt%. Afterward, a mixture of 5.8 mmol of St, 0.06 mmol of DVB and 0.03 mmol of V-59 and 0.3 mmol of ATRP inimer was added to the reaction vessel at a rate of 1.5 mL/h. The reaction continued overnight under argon. The resulting macroinitiatorfunctionalized NPs were purified by precipitation (3X in methanol) followed by redispersion in THF and by (3X in n-hexane) followed by redispersion in THF and finally dried.

Synthesis of polystyrene nanoparticles functionalized with a canopy of poly(methyl acrylate) chains (PS-PMA NPs)
A stock solution of Cu(II)/PMDETA was prepared by mixing CuBr2 (5 mg, 0.022 mmol) and PMDETA (48 μL, 0.22 mmol) with DMF (2.5 mL); a stock solution of ascorbic acid was made by dissolving 0.23 mmol of ascorbic acid in DMF. A suspension of initiator functionalized PS NPs (0.1 g, 0.0078 mmol of initiator) in anisole (2 mL) was placed in a Schlenk tube with methyl acrylate (0.5 mL, 5.5 mmol). The mixture was purged with argon for 20 min. Then, 87.6 μL and 42.6 μL of Cu(II)/PMDETA and ascorbic acid stock solution were added to the Schlenk tube and the reaction was carried out in an oil bath at 70 °C. After different polymerization times, the polymerization was quenched, and the polymer functionalized PS nanoparticles were precipitated in hexane and washed by centrifugation followed by redispersion in DCM (3X).

Synthesis of free poly(methyl acrylate) (PMA)
Free PMA chains were synthesized in a free initiator (ethyl α-bromoisobutyrate). The initiator (15.3 mg, 0.07 mmol) was dissolved in anisole (20 mL) and injected into a Schlenk tube with methyl acrylate (1.05 mL, 0.01 mol). The mixed solution was degassed with argon for 20 min, and the stock solutions of Cu(II)/PMDETA (0.88 mL) and of ascorbic acid (0.43 mL) were added. The reaction mixture was placed in an oil bath at 70 °C and stirred for 1 hour. The reaction was cooled down, exposed to the air, and washed with hexane and DCM 3 times.

Control of the solvent quality
The Flory-Huggins polymer-solvent interaction parameters (χ 12 ) were calculated using the Hansen solubility parameters (Eq. S1). 4 where, is a constant, 1 is the molar volume of solvent, R is the universal gas constant, T is absolute temperature, 1, is the Hansen dispersion parameter for the solvent, 2, is the Hansen dispersion parameter for the polymer, 1, is the Hansen polarity parameter for the solvent, 2, is the Hansen polarity parameter for the polymer, 1,ℎ is the Hansen hydrogen bonding parameter for the solvent, 2,ℎ is the Hansen hydrogen-bonding parameter for the polymer.

Characterizations
Determination of the molecular weight of the PMA chains.
The monomer conversion was followed by NMR spectroscopy (Bruker Avance 300) and used to calculate the degree of polymerization and the molecular weight (Mn,NMR) (Table S1).
Furthermore, the molecular weight of the PMA chains was also confirmed by GPC (Mn,GPC) (Table   S1). To characterize the end-tethered PMA, the disulfide bonds present in the ATRP initiator and tethering the PMA chains to the PS NPs were cleaved by reduction with dithiothreitol.  (Table S4). Determination of the size of the particles The NPs were also analyzed by transmission electron microscopy (TEM) using an FEI Tecnai

Mesurement of the Spin-Spin relaxation (T2)
The