Structure of doubly temperature sensitive core-shell microgels based on poly-N-isopropylacrylamide and poly-N-isopropylmethacrylamide

Swelling properties of doubly temperature sensitive core-shell microgels consisting of two thermosensitive polymers namely poly-N-isopropylacrylamide (PNIPAM) with a lower critical solution temperature (LCST) at ca. 34 degrees C and poly-N-isopropylmethacrylamide (PNIPMAM) with a LCST of ca. 44 degrees C have been investigated by small-angle neutron scattering (SANS). Two types of microgels with different core-shell composition were studied: PNIPAM-core PNIPMAM-shell microgels as well as a microgel with inverse structure, i.e. PNIPMAM-core - PNIPAM-shell. A core-shell form factor has been employed to evaluate the structure and the real space particle structure is expressed by radial density profiles. By this means the influences of composition and temperature on the internal structure have been revealed. At temperatures between the LCSTs the swelling of the PNIPMAM-shell leads to an expansion of the PNIPAM-core. At temperatures below the core LCST, the core cannot swell to its native size (i.e. in the absence of a shell) because the maximum expanded shell network prohibits further swelling. Thus depending on temperature the shell either expands or compresses the core. The inverse PNIPMAM-core PNIPAM-shell microgel displays qualitatively different behavior. At intermediate temperatures, the segment density of the shell is higher as compared to the core. Since the density ratio of shell and core depends on temperature, such core-shell microgels provide interesting opportunities for encapsulation and controlled release.