Microglobule formation and a microscopic order parameter monitoring the phase transition of aqueous poly(N-isopropylacrylamide) solution

The coil-to-globule transition of poly(N-isopropylacrylamide) (pNIPAm) in water is generally believed to be driven by hydrophobic interaction between the isopropyl groups of its side chains. However, it is still unclear how dehydration and critical fluctuations of the polymer chains are correlated. Here, we use small- and wide-angle x-ray scattering and dielectric relaxation spectroscopy to cover a wide range of the relevant length and time scales, enabling us to grasp an overall picture of this phase transition. We find that the hydration number of pNIPAm decreases only moderately with temperature up to about 6 K below its spinodal temperature TS, but then drops steeply on approaching TS. This rapid dehydration is coupled to a mean-field-like power-law divergence of the correlation length ξ, representing fluctuations of the density order parameter. Real-space decoding of an observed interference peak reveals partial-globule formation even far below TS and demonstrates that the polymer-rich phase above TS can be understood as a high-density assembly of the microglobules. Strikingly, condensation of the microglobules and the divergence of ξ do not run parallel. Instead, the condensation occurs only above TS and is completed about 6 K above TS. The local number density of the microglobules, exhibiting a steplike increase just above TS, should be identified as an additional microscopic order parameter governing the phase transition of pNIPAm.