Thermo-responsiveness of poly(N,N-diethylacrylamide) polymers at the air–water interface: The effect of a hydrophobic block

The poly(N,N-diethylacrylamide) (h-PDEA) homopolymer and the poly(N-decylacrylamide)-b-PDEA (PDcA₁₁-b-PDEA₂₃₁) diblock copolymer were studied in the range of 10 to 40 °C, at the air–water interface. The π–A isotherms of h-PDEA appear nearly invariant with temperature while the π–A isotherms of PDcA₁₁-b-PDEA₂₃₁ deviate significantly to lower areas with the temperature increase evidencing the thermo-responsiveness of this diblock copolymer at the interface. For the copolymer, the limiting area per segment versus temperature shows a break point around 29 °C, slightly lower than the lower critical solution temperature (LCST) of h-PDEA in water (31–33 °C). AFM images of LB monolayers transferred at 40 °C revealed for both polymers the presence of hydrophobic aggregates due to the conformational changes (collapse) of chains that occur at the LCST. Differences in the morphology of these aggregates, flat irregular structures for h-PDEA and round-shaped domains for PDcA₁₁-b-PDEA₂₃₁, were related with the condensing effect of the hydrophobic block. The PDcA₁₁ block, anchoring the polymer to the interface, ensures a better stability and cohesion to the film and preserves the thermo-sensitivity of the h-PDEA at the interface.