Reversible temperature-controlled gelation in mixtures of pNIPAM microgels and non-ionic polymer surfactant.

We investigate the reversible gelation of poly(N-isopropylacrylamide) (pNIPAM) microgels in the presence of triblock-copolymer (PEO-PPO-PEO type) surfactant. We demonstrate that the association of these polymers with the microgel particles at elevated temperature is responsible for the gelation, due to the temperature responsive nature of the components. This is highlighted by an increase in the apparent hydrodynamic diameter of the particles in dynamic light scattering experiments, which only occurs above the volume phase transition temperature of pNIPAM. The gels that result shrink over a time period much larger than that of the collapse of pNIPAM microgels, and retain the shape of the container they form in. We investigate the mechanism that leads to this gelation and the structure of the gels that result. Confocal microscopy experiments show that both polymers are present in the gel network, indicating that an associative mechanism is responsible for the gelation. We vary the pNIPAM particle architecture to further investigate the gelation process, and find that the cross-link distribution plays a key role in the gelation mechanism, where for uniformly cross-linked particles the gelation is not observed. This shows that the fuzzy corona of the pNIPAM microgels is involved in the association of the polymers, allowing the triblock-copolymer to penetrate the outer corona of the microgels and bridge the particles. The phase transition observed is close to physiological conditions, so these gels have the potential for use in biomedical applications, including tissue engineering.