Dependence of Shell Thickness on Core Compression in Acrylic Acid Modified Poly(N-isopropylacrylamide) Core/Shell Microgels

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Abstract

The swelling properties of poly(N-isopropylacrylamide) (pNIPAm)-based core/shell microgels are investigated as a function of shell thickness. Core particles composed of cross-linked pNIPAm-co-acrylic acid serve as nuclei for subsequent polymerization of a cross-linked pNIPAm shell. The thickness of the shell component is varied by changing the amount of monomer present during the shell addition polymerization. Photon correlation spectroscopy results indicate that the thickness of the shell greatly impacts the swelling properties of the core as a function of polymer network density, solution pH, and temperature. Previously reported results on this core/shell polymer combination show that the core is restricted from swelling to its native volume in the presence of the shell below the polymer phase transition temperature of 31 °C. The shell also compresses the core above the polymer phase transition temperature in pH solutions above the acid pKa by inducing a volume change, despite the fact that the core is fully charged due to deprotonation. This investigation shows that as the thickness of the shell is increased, the impact of these two phenomena increases as well.

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