Control of swelling of responsive nanogels by nanoconfinement

Abstract

The volume phase transition (VPT) behavior and the swelling properties of individual thermoresponsive poly(N-isopropylacrylamide) (PNIPAM)-based nanogels are investigated by in situ atomic force microscopy (AFM). Using a template-based synthesis method, cylindrical nanogels are synthesized for different polymerization times within nanopores (80 nm) of poly(ethylene terephthalate) (PET) track-etched membranes. The confinement conditions, characterized by the ratio Φ between the average chain length and the pore diameter, are varied between 0.35 and 0.8. After dissolving the membranes, the volume of individual nanogels composed of PNIPAM-g-PET diblock copolymers is numerically extracted from AFM images while varying the water temperature from 28 to 44 °C. From the measured volumes, the swelling of nanogels is investigated as a function of both the water temperature and the confinement conditions imposed during the synthesis. Contrary to the VPT, the maximum swelling of the nanogels is strongly affected by these confinement conditions. The volume of nanogels in the swollen state can reach 1.1 to 2.1 times their volume in the collapsed state for a ratio Φ of 0.8 and 0.5, respectively. These results open a new way to tune the swelling of nanogels, simply by adjusting the degree of confinement imposed during their synthesis within nanopores, which is particularly interesting for biomedical applications requiring a high degree of control over swelling properties, such as drug-delivery nanotools.