The Effect of Core T_g on the Air–Water Surface Mechanical Properties of Water–Spread Block Copolymer Micelles

In the pursuit of the development of a novel polymer lung surfactant therapeutic, proper design and understanding of the air–water surface mechanical properties of the polymer surfactant particles must be achieved. To this end, the effect of the core T_g of amphiphilic block copolymer micelles on their corresponding air-water surface mechanical behavior of various hydrophobic core chemistries, all having poly(ethylene glycol) as the hydrophilic block, was studied. At temperatures below T_g, the solid core domains are kinetically ‘frozen’ and prevented from rearranging at the interface. X-ray reflectivity data suggests the core domain remains largely submerged in the water subphase. The rise in surface pressure upon compression is a result of the osmotic pressure build up in the PEG corona. At temperatures above Tg, a transition in the surface-mechanical behavior occurs which is dependent upon the interfacial properties of the core domain. For core chemistries which have a high interfacial tension for both air and water, like poly(styrene), the micelle structure transitions to form surface micelles. For core chemistries which have lower interfacial tension with air than water such as poly(tert butyl methacrylate), the core domain is able to spread on the water surface to form a continuous film. In both cases, transitions are shown to be irreversible, and the temperature of the transition in the surface-mechanical behavior agrees well with core T_g measured using a previously disclosed T₂ NMR method.