Thermoresponsive Polymers under Pressure with a Focus on Poly(N-isopropylacrylamide) (PNIPAM)

Abstract

Pressure is a key variable in the phase behavior of responsive polymers, both for applications and from a fundamental point of view. In this feature article, we review recent developments, particularly applications of neutron techniques such as small-angle neutron scattering (SANS) and quasi-elastic neutron scattering (QENS), across the temperature−pressure phase diagram. These are complemented by kinetic SANS experiments following pressure jumps. In the prototype system poly(Nisopropylacrylamide) (PNIPAM), QENS revealed the pressuredependent characteristics of hydration water around the lower critical solution temperature transition. The size, water content, and inner structure of the mesoglobules formed in the two-phase region depend strongly on pressure, as shown by SANS. Beside these changes at the phase transition, the mesoglobule formation at low pressure is determined by kinetic factors, namely the formation of a polymer-rich, rigid shell, which hampers further growth by coalescence. At high pressure, in contrast, the growth proceeds by diffusion-limited coalescence without any kinetic hindrance. The disintegration of the mesoglobules evolves either via chain release from their surface or via swelling, depending on the osmotic pressure of the water. Moreover, we report on the profound influence of pressure on the cononsolvency effect. In the temperature−pressure frame, the one-phase region is hugely expanded upon the addition of the cosolvent methanol. SANS experiments unveil the enthalpic and entropic contributions to the effective Flory−Huggins interaction parameter between the segments and the solvent mixture. QENS experiments demonstrate an increase in polymer associated water with pressure, whereas methanol is released. Correspondingly, the solvent phase becomes enriched in methanol, providing a mechanism for the breakdown of cononsolvency at a high pressure. Finally, we outline future opportunities for high-pressure studies of thermoresponsive polymers, with a focus on neutron methods.

6-Ring Piperidine-Based Polymers with Both Upper and Lower Critical Solution Temperatures as Kinetic Hydrate Inhibitors

“…The high-pressure conditions might decrease the UCST of PNANAm in the presence of dissolved organic alkane gas components. Several studies on polyamides indicate that the pressure affects the LCST and, therefore, also the UCST. − An increase in pressure generally favors the swollen or coil-like state of the thermoresponsive polymer over the collapsed state, not only above the cloud point temperature . This effect is, at least in part, proposed to be due to a weakening of the hydrophobic interactions with increasing pressure.…”