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

Oil-in-water (O/W) microemulsions typically feature a low viscosity and exhibit the 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 temperature rises. Accordingly, various ME–polymer mixtures were studied for which three different block copolymer architectures of BAB*, B2AB*, and B(AB*)2 type 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 (LCST). For a well selected ME concentration, these block copolymers lead to a viscosity enhancement with rising 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 microemulsion droplets, this kind of tailored temperature-dependent viscosity control of surfactant systems should therefore be advantageous.