Interplay of network architecture and ionic environment in dictating pNIPAM microgel thermoresponsiveness

Abstract

The utility of non functionalized poly(N-isopropylacrylamide) (pNIPAM) microgels in physiological and environmental applications is strictly dependent on their reversible thermoresponsiveness and stability in saline media. Despite their importance, a unified understanding of how network topology specifically crosslinker concentration and distribution regulates ionic sensitivity remains fragmented in the literature. This work systematically investigates the interplay between network topology and ionic strength (0 to 100 mM NaCl) across eight distinct microgel architectures, ranging from ultra-low crosslinked (ULC) to core-corona and homogeneously crosslinked (HC) variants. Utilizing dynamic light scattering across 22 batches, we analyzed critical thermoresponsive properties, including volume phase transition temperature (VPTT) shifts, salt tolerance thresholds, hysteresis indices, and flocculation kinetics (only at extreme salinity, 1000 mM NaCl and at 25 deg C). This comprehensive investigation enables a multidimensional analysis of how ionic strength, the presence or absence of crosslinkers (MBA), spatial crosslinking distribution, and thermodynamic states dictate microgel behavior across varying temperatures. Finally, we evaluate the applicability of this experimental library to established theoretical frameworks, specifically the Flory Rehner and Flory Rehner Donnan models, addressing ongoing debates regarding their validity in describing complex microgel systems.

Figures (17)

• Figure 1: Influence of crosslinker (MBA) concentration and crosslinking distribution (ULC, homogeneous, core-corona). Symbols represent different $NaCl$ concentrations: Circle (0 $mM$, pure water, violet), square (0.1 $mM$, light blue), triangle (1 $mM$,blue), diamond (10 $mM$, navy blue), and star (100 $mM$,black). Closed symbols represent core-corona microgels ([SDS/NIPAM] mole ratio = 0.004, [KPS/NIPAM] mole ratio = 0.016), and open symbols represent homogeneous crosslinking density microgels ([SDS/NIPAM] mole ratio = 0, [KPS/NIPAM] mole ratio = 0.019).
• Figure 2: Hydrodynamic diameter as a function of temperature for microgels with varying formulations and $NaCl$ concentrations. Top panel: Full temperature range (20–50$^{\circ}C$); Bottom panel: Zoomed-in view near the transition region. (a,d) ULC microgels, (b,e) High-crosslinking density core-corona microgels, (c,f) Homogeneously crosslinked (HC) microgels. Symbols represent different NaCl concentrations: open circle (0 $mM$, pure water, violet), square (0.1 $mM$, light blue), open triangle (1 $mM$,blue), diamond (10 $mM$, navy blue), and star (100 $mM$,black)
• Figure 3: Normalized Size ($\Lambda$) as a metric for salt tolerance across three thermodynamic states: (a) swollen state, (b) transition state, and (c) collapsed state. $\Lambda$ = 1.0 indicates the ideal case, the highest salt resistance; $\Lambda$$<$ 1.0 indicates salt-induced deswelling. Soft architectures (ULC, homogeneous) show significant osmotic deswelling, while rigid core-corona networks maintain structural integrity. Symbols: open square (homogeneous crosslinking-HC microgel), triangle (low initiator concentration), inverted triangle (high initiator concentration), open circle with dotted line (ULC microgel), closed dark yellow circle (highest crosslinking densidensity), and green circle (lowest crosslinking density).
• Figure 4: The Hysteresis Index (HI), derived from heating–cooling cycles across varying ionic strengths ($NaCl$ concentration), is significantly influenced by both the MBA crosslinker concentration and its specific spatial distribution within the microgel (ULC, homogeneous, core-corona). The x-axis is logarithmic, with "0*" plotted as a nominal notation for pure water (0 $mM$$NaCl$). HI = 10% can be considered as the the operational threshold between the reversible regime (HI $<$ 5%) and the irreversible aggregation regime (HI $>$ 5%). Symbols: closed violet diamonds (ULC), closed blue squares ([MBA/NIPAM] = 0.007), open blue circles (HC), and black stars ([MBA/NIPAM] = 0.097).
• Figure 5: Lag time ($t$) versus normalized autocorrelation function ($g_{2}/g_{0}$) for different microgel formulations at 1000 $mM$$NaCl$ at 25°C. Panels a--d represent microgels with varying crosslinking densities and distributions: (a) ULC, (b) [MBA/NIPAM] = 0.007, (c) HC [MBA/NIPAM] = 0.020, and (d) [MBA/NIPAM] = 0.097. Data represent experimental times from 0 to 990 seconds, with colors transitioning from yellow to dark blue