  Thermo-responsive microgel particles can exhibit a drastic volume shrinkage
upon increasing the solvent temperature. Recently we found that the spreading
of poly(N-isopropylacrylamide)(PNiPAm) microgels at a liquid interface under
the influence of surface tension hinders the temperature-induced volume phase
transition. In addition, we observed a hysteresis behavior upon temperature
cycling, i.e. a different evolution in microgel size and shape depending on
whether the microgel was initially adsorbed to the interface in expanded or
collapsed state. Here, we model the volume phase transition of such microgels
at an air/water interface by monomer-resolved Brownian dynamics simulations and
compare the observed behavior with experiments. We reproduce the experimentally
observed hysteresis in the microgel dimensions upon temperature variation. Our
simulations did not observe any hysteresis for microgels dispersed in the bulk
liquid, suggesting that it results from the distinct interfacial morphology of
the microgel adsorbed at the liquid interface. An initially collapsed microgel
brought to the interface and subjected to subsequent swelling and collapsing
(resp. cooling and heating) will end up in a larger size than it had in the
original collapsed state. Further temperature cycling, however, only shows a
much reduced hysteresis, in agreement with our experimental observations. We
attribute the hysteretic behavior to a kinetically trapped initial collapsed
configuration, which relaxes upon expanding in the swollen state. We find a
similar behavior for linear PNiPAm chains adsorbed to an interface. Our
combined experimental - simulation investigation provides new insights into the
volume phase transition of PNiPAm materials adsorbed to liquid interfaces.
