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We produce thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) colloidalhydrogel particles of different stiffnesses by controlling the concentration ofcrosslinker in a one-pot synthesis method. We employ oscillatory rheology andcryogenic scanning electron microscopy to correlate the temperature andstiffness-induced mechanical properties and microscopic structures of denseaqueous suspensions of the synthesized PNIPAM colloids. Using Fourier transforminfrared spectroscopy, we show that particle hydrophobicity increases withincreasing suspension temperature and decreasing particle stiffness. Ourfrequency and strain amplitude sweep rheology experiments reveal that thelinear viscoelastic moduli and yield stresses of dense PNIPAM suspensions,constituted by particles of varying stiffnesses, increase when temperature israised above the lower consolute solution temperature (LCST). Combining ourcryogenic scanning electron microscopy and rheology data, we report that densesuspensions of the softest PNIPAM hydrogels show a gel-liquid-gel transitionwhen temperature is increased across the LCST. Suspensions of stiff particles,in contrast, exhibit a glass-glass transition across the LCST without goingthrough an intermediate liquid state. Our observations reveal that thetemperature-induced phase transformations in dense PNIPAM suspensions dependsensitively on the stiffness of the constituent particles and can be explainedby considering hydrophobicity-driven morphological changes in the suspensionmicrostructures over the experimental temperature range. We can thereforeachieve a variety of suspension phases with unique properties by tuning justthe particle stiffness and suspension temperature.

Hydrogel stiffness determines particle self-assembly and suspension rheology across the lower consolute solution temperature