Multiresponsive Star-Graft Quarterpolymer Monolayers


Multifunctional star-graft quarterpolymers PS<sub><i>n</i></sub>[P2VP-<i>b</i>-(PAA-<i>g</i>-PNIPAM)]<sub><i>n</i></sub> with two different arm types, shorter PS arms and longer P2VP-<i>b</i>-PAA block copolymer arms with grafted PNIPAM chains, were studied in terms of their ability to form micellar structures at the air/water and air/solid interfaces. Because of the pH-dependent ionization of P2VP and PAA blocks, as well as thermoresponsiveness of PNIPAM chains, these multifunctional stars have multiple responsive properties to pH, temperature, and ionic strength. We observed that the molecular surface area of the stars is the largest at basic pH, when the PAA blocks are strongly charged and extended, and PNIPAM chains are spread at the interface. At acidic conditions, the molecular surface area is the smallest because the P2VP blocks submerge into the water subphase and the PAA blocks are contracted and form hydrogen bonding with grafted PNIPAM chains. The molecular surface area of the stars at the air/water interface gradually increases at elevated temperature. We suggest that the transition across lower critical solution temperature (LCST) results in the emerging of PNIPAM chains from the water subphase to the interface due to the hydrophilic to hydrophobic transition. Moreover, at higher surface pressure, the stars tend to form intermolecular micellar aggregates above LCST. The graft density of PNIPAM chains as well as the arm number was also found to have strong effects on the thermo- and pH-response. Overall, this study demonstrates that the star block copolymer conformation and aggregation are strongly dependent on the intramolecular interactions between different blocks and spatial distribution of the arms, which can be controlled by the external conditions, including pH, temperature, ionic strength, and surface pressure

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