Adsorbed and Spread Films of Amphiphilic Triblock Copolymers Based on Poly(2,3-dihydroxypropyl methacrylate) and Poly(propylene oxide) at the Air−Water Interface

• Published in: Langmuir 2010-04, Vol.26 (8), p.5507-5519
• Main Authors: Amado, Elkin, Blume, Alfred, Kressler, Jörg
• Format: Article
• Language: English
• Subjects: Interfaces: Adsorption, Reactions, Films, Forces
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la903748z

The adsorption behavior of the novel type of water-soluble amphiphilic triblock copolymers PGMA-b-PPO-b-PGMA at the air−water interface is studied by tensiometry and monolayer techniques. In particular, (PGMA14)2−PPO34 is found to be strongly surface active (Πmax ∼ 38.1 mN/m, cmc ∼ 50 μM), in spite of having a relatively short hydrophobic PPO middle block. Time-dependent adsorption measurements exhibit two different types of adsorption kinetics depending on concentration. Monolayers deposited by spreading form pseudo-Langmuir films, in spite of (PGMA14)2−PPO34 high water solubility. The transition from a dilute to a semidilute regime during compression of the monolayer occurs at a mean molecular area around 4424 Å2/molecule (Π = 0.03 mN/m). Above Π = 2.1 mN/m (1291 Å2/molecule) PGMA segments begin to change from a flat two-dimensional conformation to loops and tails protruding into the subphase. The onset of the conformational change for PO segments takes place at a mean molecular area of ∼625 Å2/molecule (Π = 15.5 mN/m). In the range Π ∼ 21.4−23.8 mN/m the PPO blocks adopt a three-dimensional conformation. A new methodology for the estimation of the amount of polymer absorbed at the interface (Γ) as a function of the solution bulk concentration from tensiometry measurements in pseudo-Langmuir spread films is proposed. The obtained adsorption isotherm suggests the occurrence of a conformational change of the PPO block segments toward conformations having smaller molar areas for actual bulk concentrations above 6.0 × 10−8 M. The surface tension versus polymer bulk concentration curve (γ vs ln C) exhibits a sharp break at low concentrations, which is probably due to a conformational change within the adsorbed layer. A theoretical multiple conformation model reproduces satisfactorily the experimental dependency of surface pressure on polymer adsorption (Π vs Γ) at low and intermediate ranges of polymer concentrations in the solution bulk. A ratio of approximately eight between the average molecular areas at low coverage and at full coverage confirms that (PGMA14)2−PPO34 chains are highly flexible and able to adopt very different conformations during the transition of the adsorbed polymer film from a highly diluted to a nearly saturated state. There are some experimental indications that nonequilibrium effects might determine a bimodal conformational distribution within the adsorbed (PGMA14)2−PPO34 layer, which departs substantially from the equilibrium adsorp