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

Abstract

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 T S , but then drops steeply on approaching T S . 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 T S and demonstrates that the polymer-rich phase above T S 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 T S and is completed about 6 K above T S . The local number density of the microglobules, exhibiting a steplike increase just above T S , should be identified as an additional microscopic order parameter governing the phase transition of pNIPAm.

SANS quantification of bound water in water-soluble polymers across multiple concentration regimes

“…This second characteristic length scale has been attributed to the formation of PNIPAM microglobules at high concentration, which are also observable by small and wide-angle X-ray scattering (SWAXS). 78 Fitting the PNIPAM scattering intensities taken at 250 and 300 mg mL À1 to a sum of the Ornstein-Zernike and pseudo-Voight Lorentzian functions (eqn (S3) and (S4), ESI †) allows for the estimation of both blob correlation length and microglobule correlation length (Tables S5 and S6, ESI †). 67,78 These microglobules are dispersed in the matrix of polymer chains and have been hypothesized to weaken the structure of the hydration water network when the PNIPAM undergoes its thermally driven phase transition.…”

“…78 Fitting the PNIPAM scattering intensities taken at 250 and 300 mg mL À1 to a sum of the Ornstein-Zernike and pseudo-Voight Lorentzian functions (eqn (S3) and (S4), ESI †) allows for the estimation of both blob correlation length and microglobule correlation length (Tables S5 and S6, ESI †). 67,78 These microglobules are dispersed in the matrix of polymer chains and have been hypothesized to weaken the structure of the hydration water network when the PNIPAM undergoes its thermally driven phase transition. 78,79 The appearance of these microglobules at high concentration even below the PNIPAM LCST may explain the trend in hydration number.…”

“…67,78 These microglobules are dispersed in the matrix of polymer chains and have been hypothesized to weaken the structure of the hydration water network when the PNIPAM undergoes its thermally driven phase transition. 78,79 The appearance of these microglobules at high concentration even below the PNIPAM LCST may explain the trend in hydration number. As the solution transitions from semidilute to concentrated, microglobules begin to form, with the distance between microglobules decreasing as concentration increases (Table S6, ESI †), crowding out the hydration water network and leaving only the amide H-bonded water molecules bound to the monomer units.…”

Structural Insights into Polymethacrylamide-Based LCST Polymers in Solution: A Small-Angle Neutron Scattering Study

“…The Flory–Huggins solution theory describes the transformation from a favored interaction of polymer with solvent to a state where polymer–solvent interactions are unfavorable and, thus, polymer–polymer interactions increase. , On the macromolecular level, the phase transition can be described by a solution of well-solvated polymer coils below the LCST and a dispersion of (aggregated) globules above the LCST under bad solvent conditions. This coil-to-globule transition was predicted by the theories of Flory in the middle of the past century and since then was described by various groups. , Coil-to-globule transitions were studied and reported for many polymers including polystyrene, poly­(methyl methacrylate), and poly- N -acrylamides. − Researchers put a lot of effort in the investigation of the phase transition process, such as the observation of the phase transition process from a single polymer chain, the detection of microglobule formation close to the cloud point, and the phase transition mechanism with respect to the water–polymer interaction. − Furthermore, it was shown that the thermoresponsive behavior of polymers can be influenced by side groups introduced to the polymer by copolymerization. This can be used, for example, to control whether polymers possess LCST- or UCST-like behavior. , …”

“…To obtain information on the polymer structure at the nanoscale level, techniques such as small-angle neutron or X-ray scattering (SANS, SAXS) are typically required. − By using SAXS, Yanase et al have recently shown that semidilute PNIPAM solutions feature microglobules already far below the transition temperature . This example demonstrates how complex solution structures of polymers under different solvent conditions can be and that the phase transition behavior can be complex, involving phenomena on different length scales.…”