Multicore expandable microbubbles: Controlling density and expansion temperature

Stable microbubbles can be prepared by evaporation of a liquid core inside an expandable polymeric shell. To control the expansion temperature and the size of resulting microbubbles, we prepared polymeric microcapsules that contain multiple liquids in their core. One-step suspension polymerization allowed for encapsulation of at least two different liquids with near quantitative yield. The liquids may be miscible or immiscible, which directly affects the vapor pressure inside the capsule according to either Raoult's or Dalton's law, respectively. In the case of miscible liquids, e.g., perfluorohexane and perfluoropentane, both the vapor pressure and the expansion temperature vary within a range bounded by the properties of the neat liquids. For immiscible liquids, e.g. perfluoropentane and isopentane, the total vapor pressure is above this range as it equals to a sum of the individual pressures. Due to increased vapor pressure, encapsulation of two immiscible liquids significantly lowers the expansion temperature below the corresponding temperatures observed during expansion of microcapsules with neat perfluoropentane and isopentane cores. The microbubble diameter and shell thickness were controlled within ca. 10-50 mm and 0.1-3 mm, respectively, by varying the fractions and vapor pressure of the core fluids. We also showed that the effect of core composition on the expansion temperature was more significant than the effect of the shell thickness. Furthermore, one of the encapsulated materials may carry additional functions including imaging contrast, catalysis, and density compensation. To that end, these designer microcapsules may find applications in the fields of drug delivery, acoustic imaging, drilling, and self-healing materials. (C) 2016 Published by Elsevier Ltd.