Modifying the Properties of Microemulsion Droplets by Addition of Thermoresponsive BAB* Copolymers

Oil-in-water (O/W) microemulsions (ME) typically feature a low viscosity and exhibit ordinary viscosity reduction as a function of temperature. However, for certain applications, avoiding or even reverting the temperature trend might be required. This can be conceived by adding thermoresponsive (TR) block copolymers that induce network formation as the temperature increases. Accordingly, various ME-polymer mixtures were studied for which three different block copolymer architectures of BAB*-, B2AB*-, and B(AB*)2-types were employed. Here, B represents a permanently hydrophobic, A a permanently hydrophilic, and B* a TR block. For the TR-block, three different poly(acrylamide)s, namely poly(N-n-propylacrylamide) (pNPAm), poly(N,N-diethylacrylamide) (pDEAm), and poly(N-isopropylacrylamide) (pNiPAm), were used, which all exhibit a lower critical solution temperature. For a well-selected ME concentration, these block copolymers lead to a viscosity enhancement with increasing temperature. At a polymer concentration of about 22 g L-1, the most pronounced enhancement was observed for the pNPAm-based systems with factors up to 3, 5, and 8 for BAB*, B2AB*, and B(AB*)2, respectively. This phenomenon is caused by the formation of a transitory network mediated by TR-blocks, as evidenced by the direct correlation between the attraction strength and the viscosity enhancement. For applications requiring a high hydrophobic payload, which is attained via ME droplets, this kind of tailored temperature-dependent viscosity control of surfactant systems should therefore be advantageous.

Automated summary

Oil-in-water microemulsions usually have low viscosity and exhibit ordinary viscosity reduction with increasing temperature. However, for certain applications, it might be necessary to avoid or even reverse the temperature trend. This can be achieved by adding thermoresponsive block copolymers that induce network formation as the temperature increases. The viscosity enhancement with increasing temperature was observed for various microemulsion-polymer mixtures, and the most pronounced enhancement was observed for pNPAm-based systems.