Solvent Dynamics in Solutions of PNIPAM in Water/Methanol Mixtures—A Quasi-Elastic Neutron Scattering Study

Abstract

The solvent dynamics of concentrated solutions of poly(N-isopropylacrylamide) (PNIPAM, 25 wt %) in water/methanol mixtures (85:15 v/v) are measured with the aim of shedding light onto the cononsolvency effect. Quasi-elastic neutron scattering (QENS) with contrast variation has been carried out at temperatures below and above the cloud point by using in the first set of experiments the mixture H2O:d-MeOD (d-MeOD denotes fully deuterated methanol) as a solvent and in the second set of experiments the mixture D2O:MeOH (MeOH denotes methanol). As a reference, bulk H2O, bulk MeOH and the mixtures H2O:d-MeOD and D2O:MeOH (both 85:15 v/v) have been investigated as well. In the PNIPAM solution in H2O:d-MeOD, two water populations are identified, namely strongly and less strongly arrested water. At the cloud point, the former is partially released from PNIPAM. The diffusion coefficient of the latter one is similar to the one in the water/methanol mixture, and its residence time decreases at the cloud point. The PNIPAM solution in D2O:MeOH reveals similar dynamics to the one in H2O:d-MeOD which may reflect that the dynamics of MeOH near the PNIPAM chain is similar to the one of H2O. The similarity may, however, partially be due to H/D exchange between D2O and MeOH. In both PNIPAM solutions, the mean-square displacement of the PNIPAM chain decreases gradually above the cloud point.

Synthesis of Polymer Single‐Chain Nanoparticle with High Compactness in Cosolvent Condition: A Computer Simulation Study

“…What is surprising is that some polymers can undergo a "swell-collapseswell" transition in two mixed good solvents, and a "collapseswell-collapse" transition is experienced in the two mixed poor solvents. These two unexpected and puzzling behaviors are called co-nonsolvency [37][38][39][40][41][42][43][44][45][46][47] and cosolvency. [45,[48][49][50][51][52] For the co-nonsolvency effect, polymer dissolves well in a good solvent and with the addition of another good solvent the polymer undergoes a coil-globule transition.…”

Random copolymer gels of N-isopropylacrylamide and N-ethylacrylamide: effect of synthesis solvent compositions on their properties

“…1 PNIPAM undergoes phase separation when increasing the temperature beyond the LCST value, where the polymer chains change from a random coil to a globular state. [2][3][4][5] Phase separation can also happen when a specic water-miscible organic solvent is mixed with water at a certain range of compositions, for example, methanol, [6][7][8][9] ethanol, 10,11 tetrahydrofuran (THF), 12,13 dimethyl sulfoxide (DMSO), 14,15 known as cononsolvency, 8 a phenomenon where a mixture of two good solvents behaves as a poor solvent for a polymer at particular compositions. Winnik et al studied cononsolvency of linear PNIPAM chains in different methanolwater mixtures and explored different aspects of cononsolvency of PNIPAM in those solvents.…”

A cosolvent surfactant mechanism affects polymer collapse in miscible good solvents

“…By contrast, cosolvents that are depleted from the polymer surface favor the globule state over the coil state and therefore decrease polymer solubility and the LCST. Although this simple picture applies in many cases, it is not generic and a recent paradigm shift followed several intriguing observations that polymer collapse could in fact be triggered by preferential interactions with cosolvents [13][14][15][16][17][18][19] , specific salts [20][21][22][23] , or cosolutes 6,7,[24][25][26][27][28][29] that partition at the polymer surface.…”

“…The question we address herein is how these preferential interactions drive polymer collapse in water-alcohol mixtures. Several thermoresponsive polymers undergo coil-globule-coil transitions (cononsolvency) in binary mixtures of two good solvents (e.g., water and alcohol in the case of polyacrylamides) with increasing concentration of the cosolvent (alcohol) at a fixed temperature 4,5,14,17,[30][31][32] . The underlying molecular mechanism is however not understood.…”