PNIPAM Mesoglobules in Dependence on Pressure

Area of Science:

• Polymer Science
• Physical Chemistry
• Soft Matter Physics

Background:

• Poly(N-isopropylacrylamide) (PNIPAM) exhibits a lower critical solution temperature (LCST) behavior in aqueous solutions.
• Above its cloud point temperature (Tcp), PNIPAM forms mesoglobules, which change in size and water content with pressure.

Purpose of the Study:

• To investigate the pressure-induced transition of PNIPAM mesoglobules.
• To characterize the size and water content of mesoglobules under varying pressure and temperature conditions.
• To identify the transition line separating low- and high-pressure regimes.

Main Methods:

• Isothermal pressure scans using optical microscopy.
• Very small angle neutron scattering (VSANS) to determine mesoglobule size and water content.
• Analysis of mesoglobule behavior at different temperatures above Tcp.

Main Results:

• A distinct transition in mesoglobule state occurs between 35-55 MPa, dependent on temperature.
• The transition is smooth at high temperatures (far from coexistence) and abrupt at low temperatures (near coexistence).
• At high temperatures, mesoglobule swelling dominates; at low temperatures, coalescence prevails, with incomplete disintegration of aggregates upon pressure decrease.

Conclusions:

• A new transition line is identified, separating distinct low- and high-pressure regimes for PNIPAM mesoglobules.
• The pressure-induced transition exhibits temperature-dependent characteristics, influencing mesoglobule swelling and coalescence.
• Historical effects (cooling at high pressure) show limited impact on the observed transition, suggesting partial reversibility of mesoglobule aggregation.