Pressure-Dependence of Poly(N-isopropylacrylamide) Mesoglobule Formation in Aqueous Solution

Abstract

Above their cloud point, aqueous solutions of the thermoresponsive polymer poly­(N-isopropylacrylamide) (PNIPAM) form large mesoglobules. We have carried out very small-angle neutron scattering (VSANS with q = 0.21–2.3 × 10–3 Å–1) and Raman spectroscopy experiments on a 3 wt % PNIPAM solution in D2O at atmospheric and elevated pressures (up to 113 MPa). Raman spectroscopy reveals that, at high pressure, the polymer is less dehydrated upon crossing the cloud point. VSANS shows that the mesoglobules are significantly larger and contain more D2O than at atmospheric pressure. We conclude that the size of the mesoglobules and thus their growth process are closely related to the hydration state of PNIPAM.

Nanoscale disintegration kinetics of mesoglobules in aqueous poly(N-isopropylacrylamide) solutions revealed by small-angle neutron scattering and pressure jumps

“…32 Except for a sharp increase 3.5 s after the jump, which we attribute to the change of fitting model, ξ OZ continues the decreasing trend of R g , until it becomes constant after ∼30 s and reaches a value of 17.6 nm, which is the value of the final semi-dilute solution. 30 I OZ decreases as well and becomes constant at the same time, i.e., the solution becomes homogeneous, and the released polymers increasingly dominate the scattering.…”

“…25,26 The collapsed PNIPAM chains form mesoglobules, i.e., long-lived aggregates typically having sizes between tens of nanometers to several micrometers. [27][28][29][30] The origin of their long lifetime is their rigid shell which hinders their coalescence. 30 In turbidity and dynamic light scattering experiments during slow cooling of an aqueous dispersion of PNIPAM mesoglobules, two disintegration steps were identified: an initial swelling process, which was related to the entanglements of the polymer, and the subsequent release of single polymers.…”

“…[27][28][29][30] The origin of their long lifetime is their rigid shell which hinders their coalescence. 30 In turbidity and dynamic light scattering experiments during slow cooling of an aqueous dispersion of PNIPAM mesoglobules, two disintegration steps were identified: an initial swelling process, which was related to the entanglements of the polymer, and the subsequent release of single polymers. 31 When the number of polymer entanglements in the mesoglobules was reduced, only the release of single polymers was observed.…”

Temperature-Controlled Interactions between Poly(N-isopropylacrylamide) Mesoglobules Probed by Fluorescence

“…[9][10][11][12] The disruption of hydrophobic assemblies as PNIPAM chains dehydrate and collapse around Tc has been studied experimentally and discussed from a theoretical view point to a great extent. [13][14][15][16][17][18][19][20][21] Fluorescence spectroscopy is particularly useful to study how hydrophobes interact with each other and with the PNIPAM chain during the heat-induced phase transition of PNIPAM in water. Pyrene, a hydrophobic fluorophore, stands out as a powerful tool to study hydrophobically modified water-soluble polymers in aqueous solution.…”

Mobility of bound water in PNIPAM microgels

“…attracts unrelenting interest to these systems. [1][2][3][4][5][6] Due to the possibility to change the properties of PNIPAM based systems by variation of temperature, pressure or pH, such polymer systems are promising for an application in medicine, bioengineering or smart polymer coatings. [7][8][9][10][11] However, the numerous applications are strongly dependent on the polymer-solvent interaction.…”