Aggregation-induced microgelation: a new approach to prepare gels in solution

Using poly(vinyl alcohol)-b-poly(N-isopropylacrylamide) (PVA-b-PNIPAm) as an example, we demonstrate the concept of aggregation-induced gelation of PVA in dilute solution. PVA-b-PNIPAm forms aggregates in dilute solution at temperature above the transition point of PNIPAm, which significantly increases the local concentration of PVA chains. Therefore, PVA chains have a better chance to crystallize or to generate small crystalline nuclei, which function as cross-linkers in the later stages. When the solution is cooled to room temperature, PVA-b-PNIPAm gradually forms microgels at the time scale of days. We further investigated the effects of block ratio, heating rate, and urea concentration on the gelation process. It is found that larger PVA ratio, longer aging time and less urea content facilitated the gel formation. Rheological measurement at semidilute concentration also confirms gel formation after the heating/cooling treatment. The aggregation-induced gelation offers a new approach to prepare PVA gels without the introduction of toxic chemical cross-linkers.