  Responsive microgels based on poly(N-isopropylacrylamide) (PNIPAM) exhibit
peculiar behaviours due to the competition between the hydrophilic and
hydrophobic interactions of the constituent networks. The interpenetration of
poly-acrilic acid (PAAc), a pH-sensitive polymer, within the PNIPAM network, to
form Interpenetrated Polymer Network (IPN) microgels, affects this delicate
balance and the typical Volume-Phase Transition (VPT) leading to complex
behaviours whose molecular nature is still completely unexplored. Here we
investigate the molecular mechanism driving the VPT and its influence on
particle aggregation for PNIPAM/PAAc IPN microgels by the joint use of Dynamic
Light Scattering and Raman Spectroscopy. Our results highlight that PNIPAM
hydrophobicity is enhanced by the interpenetration of PAAc promoting
interparticle interactions, a crossover concentration is found above which
aggregation phenomena become relevant. Moreover we find that, at variance with
PNIPAM, for IPN microgels a double-step molecular mechanisms occurs upon
crossing the VPT, the first involving the coil-to-globule transition typical of
PNIPAM and the latter associated to PAAc steric hindrance.
