Cononsolvency of poly-N-isopropyl acrylamide (PNIPAM): Microgels versus linear chains and macrogels

• Cononsolvency of macro-and microgels is hardly dependent on cross-linker density.
• Microgels have experimental advantages over chains and macrogels.
• Theoretical studies of PNIPAM in water and water/methanol mixtures are presented.
• The different explanations for cononsolvency given in the literature are listed.

Poly-N-isopropyl acryl amide (PNIPAM) is swollen in both pure water and pure methanol but collapses in mixtures of these solvents. In this review, this cononsolvency of PNIPAM in water/methanol mixtures is discussed. Experimental studies of linear PNIPAM chains and macrogels are compared to microgels. Theoretical studies are presented based on molecular dynamics simulation and quantum mechanical calculations as well as semi-empirical models. The different explanations for the cononsolvency available in the literature are introduced. Experiments show that all PNIPAM species collapse and re-swell at comparable methanol fraction in the mixture. Cross-linker density of macrogels and microgels has only slight influence on cononsolvency, whereas chain length of linear chains has a significant influence. Microgels provide advantages to study cononsolvency by en'abling a broader experimental approach. Furthermore, multi-sensitive microgels can be prepared, which contain compartments sensitive to different stimuli.

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