In Situ Monitoring of Droplet Behavior in Inverse Microemulsions.

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Thermoresponsive polymer assemblies are of growing interest in fields ranging from photonics to drug delivery, with their phase transitions often attributed to upper- or lower-critical solution temperatures and cloud-point behaviors. However, the direct imaging of these nanoscale transitions remains underexplored. This study addresses that gap by developing a temperature-sensitive inverse microemulsion system and elucidating its dynamic structural transitions under heating. We present a temperature-sensitive inverse microemulsion system composed of the nonionic surfactants Brij 010 and Span 80. Upon heating within a stable microemulsion temperature range, the decrease in hydrogen bonding between the hydrophilic surfactant head and the dispersed phase results in an initial droplet contraction. Above a critical destabilization temperature, the droplets expand and destabilize as the affinity of the surfactant for the continuous phase increases. This intriguing behavior was observed via dynamic light scattering and liquid-phase transmission electron microscopy, which revealed a rapid and reversible droplet transformation during heating cycles. This versatile inverse microemulsion system also serves as a modular nanoreactor for polymerizations, demonstrated through both conventional radical and photoiniferter polymerization. Our research contributes to the understanding of inverse microemulsions, which offer a platform for precise nanoparticle synthesis.