Preparation and thermal response behavior of poly(N-isopropylacrylamide-co-a crylic acid) microgels via soap-free emulsion polymerization based on AIBN initiator

Poly(N-isopropylacrylamide-co-acrylic acid) (poly(NIPAM-co-AA)) microgels with different copolymer compositions were prepared through soap-free emulsion polymerization at 80°C, and 2, 2'-azobisisobutyronitrile (AIBN) was used as initiator. Scanning electron microscope (SEM) characterization shows that the prepared microgels are regular and smooth and not easy to distort. Result of 1 H-NMR characterization shows that with increasing of the initial concentration of AA (AA in feed), the AA content in polymer chains increases. The thermal response of microgels latex was investigated by UV-3010 spectrophometer through detecting the transmittance of the latex at different temperature in the range of 190-900 nm. The thermal response of the poly(NIPAM-co-AA) microgels was tested by dynamic light scattering (DLS). The results show that with the increase of AA content in polymer chains, the low critical solution temperature (LCST) of microgels latex first decreases and then increases. Still, with increasing of AA in poly(NIPAM-co-AA) microgels, the LCST of microgels first increases and then decreases. The basic reasons causing the changes of LCST of microgels latex and microgels are interpreted clearly in this article from the perspective of hydrogen bonding interaction.