Criteria for colloidal gelation of thermo-sensitive poly(N-isopropylacrylamide) based microgels

Suspensions of the poly(N-isopropylacrylamide) (PNIPAM) based temperature(T)-sensitive microgels can undergo colloidal gelation forming a three-dimensional sparse network-like structure in the hydrophobic and shrunken state of T > T* (T*: volume transition temperature), despite their considerably low particle volume fractions (<0.2). The effective surface charge density is expected to be a key factor governing the colloidal gelation and gel modulus.

The combined analysis of the viscoelasticity and electrophoretic mobility (EPM) was performed varying systematically pH and ionic strength (I). The microgels containing the extremely small content of electrolyte (0.1 mol%) with the T* and swelling degree being insensitive to pH and I were employed to facilitate the exclusive analysis of their effects on colloidal gelation.

The results unambiguously reveal (1) that the gelation requires the adequate suppressions of the interparticle charge repulsion, and (2) that a reduction in the interparticle charge repulsion results in an increase in gel modulus by several orders of magnitude. The long-term linear creep behavior show that the colloidal gels are identified as a viscoelastic fluid with a long relaxation time and a high viscosity whereas they behave elastically at relatively short timescale in conventional oscillatory tests.