  We synthesize thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) colloidal
microgel particles of different stiffnesses by controlling the concentration of
crosslinker in a one-pot synthesis method. We employ oscillatory rheology and
cryogenic scanning electron microscopy to study the temperature and
stiffness-induced mechanical properties and microscopic structures of dense
aqueous suspensions of the synthesized PNIPAM microgels. Using Fourier
transform infrared (FTIR) spectroscopy, we show that particle hydrophobicity
increases with increasing suspension temperature and decreasing particle
stiffness. Our zeta potential measurements of soft PNIPAM particles and those
of intermediate stiffnesses demonstrate that these particles are
electrostatically unstable and prone to aggregation even at temperatures below
the lower consolute solution temperature (LCST). In contrast, stiff PNIPAM
particles in dilute aqueous suspensions are electrostatically stabilized at all
temperatures explored in this study. Interestingly, our frequency and strain
amplitude sweep rheology experiments reveal that the linear viscoelastic moduli
and yield stresses of all the PNIPAM suspensions increase when the temperature
is raised above the LCST. Combining cryogenic scanning electron microscopy
(cryo-SEM) and rheology, we demonstrate that dense suspensions of soft PNIPAM
microgels show a gel-liquid-gel transition with increase in temperature across
the LCST. Suspensions of stiff particles, in contrast, exhibit a glass-glass
transition under the same temperature sweep conditions and do not pass through
an intermediate liquid state.
