Effects of cosolvent partitioning on conformational transitions and chain flexibility of thermoresponsive microgels

The conformational collapse of polymers in mixtures of two individually good solvents is an intriguing yet puzzling phenomenon termed cononsolvency. In this paper, the concept of the preferential adsorption of the cosolvent is combined with mean-field approaches to elaborate the cononsolvency effect of dimethylformamide (DMF) on the thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) microgels in aqueous solutions. We give a quantitative description concerning the effects of DMF preferential adsorption and partitioning on the reentrant transition of PNIPAM microgels below the lower critical solution temperature (LCST) of PNIPAM. While the DMF cononsolvency incurs the conformational collapse, the affinity of DMF molecules to PNIPAM chains becomes increasingly stronger, which reveals that the conformational collapse is decoupled from the solvent quality of DMF-water mixtures. Considering the chain elasticity, spatial constraints, and surface charge of microgels, we explore the cononsolvency effect on the persistence length quantifying the PNIPAM flexibility. Our analysis elucidates that, depending on chain length and temperature, the DMF cononsolvency-induced collapse of PNIPAM microgels leads to a remarkable increase in the persistent length below LCST, which is comparable to the experimental data regarding suspension mechanical properties of PNIPAM microgels in water above LCST.

Figure 1

Schematic of the cononsolvency effect on polymer networks and corresponding cosolvent- and solvent-included collapsed states. A microgel is composed of $N_{\mathrm{ch}}/p$ unit cells of $p$ chains. Assume that the unit cell is a bcc of eight semiflexible chains ($p=8$) [31].

Figure 2

(a) Hydrodynamic radius $R_h$ and polymer volume fraction ${\varphi }_p$ with molar ratio $f=0.034$ as a function of temperature in pure water. (b) Temperature and interaction parameter χ of PNIPAM microgel as a function of ${\varphi }_p$ in pure water.

Figure 3

Hydrodynamic radius $R_h$ as a function of $x_c$. Shadow areas are two-phase regions of linear PNIPAM at indicated temperatures [10]. Insets show polymer volume fraction ${\varphi }_p$ as a function of $x_c$. Error bars of ${\varphi }_p$ are smaller than the size of symbols.

Figure 4

Adsorbed mole fractions per chain ${\varphi }_i$ as a function of $x_c$: (a) mole fraction of cosolvent bridge adsorption ${\varphi }_B$, (b) mole fraction of cosolvent nonbridge adsorption ${\varphi }_A$, and (c) mole fraction of solvent or water adsorption ${\varphi }_S$.

Figure 5

Absorbed number of cosolvent or solvent per chain as a function of $x_c$.

Figure 6

Local-bulk partition coefficient $K_p$ (a) and normalized preferential binding parameter ${\nu }_{pc}/N_l$ (b) as a function of $x_c$.

Figure 7

Relative persistence length $l_p/l_p(x_c=0)$ and persistence length $l_p$ (insets) as a function of $x_c$.

Figure 8

(a) $l_p/L_C$ as a function of $x_c$. (b) $l_p/L_C$ as a function of DMF bridge number per chain $n_C^B$.