Aggregation behavior of symmetric poly(n‐butyl acrylate)‐block‐poly(acrylic acid) on subphases of different ionic strengths

Abstract

Influences of subphase pH value and ionic strength (NaCl concentration) on the aggregation behavior of a symmetric diblock polyelectrolyte poly(n-butyl acrylate)-block-poly(acrylic acid) (PnBA-b-PAA) at the air/water interface and its Langmuir-Blodgett (LB) films were studied. With the increase of subphase pH value, surface pressure-molecular area isotherms of PnBA-b-PAA monolayers shift toward smaller area due to the gradually enhanced ionization and underwater solubility of hydrophilic PAA chains. All the initial LB films transferred from different subphases with/without NaCl (before monolayer compression) exhibit the densely mixed structures of worm-like aggregates and circular micelles, which indicates that the aggregate formation belongs to the spontaneous mechanism. The micelle core diameters in the LB films increase with the increase of subphase pH value, owing to the enhanced PAA chain solubility and PnBA block aggregation. Under acidic condition, micelle cores are slightly larger upon addition of NaCl due to the enhanced underwater solubility of the uncharged PAA chains triggered by their possible interaction with Cl À . Under neutral/alkaline conditions, micelle cores are also larger upon addition of NaCl, which is attributed to the electrostatic shielding effect of Na + . Upon compression, the LB films prepared under neutral/alkaline conditions exhibit the predominantly large circular micelles.

Aggregation behavior of triblock terpolymer poly[n‐butyl acrylate‐block‐N‐isopropylacrylamide‐block‐2‐(dimethylamino)ethyl acrylate] at the air/water interface

“…The LB films were obtained when the sliding barriers were at their initial positions, and then at 2 mN/m. In the latter case, surface pressure was maintained for 20 min, and the silicon wafer was pulled up through the interface at a speed of 2 mm/min 1 . The transfer ratios of the initial LB films could not be determined without the movement of the barriers.…”

“…In the latter case, surface pressure was maintained for 20 min, and the silicon wafer was pulled up through the interface at a speed of 2 mm/min. 1 The transfer ratios of the initial LB films could not be determined without the movement of the barriers. At 2 mN/m, the transfer ratios of the LB films were about 0.8-3.6.…”

“…Compared with diblock copolymers, the interfacial aggregation behavior of the PnBA-b-PNIPAM-b-PDMAEA triblock terpolymer is more complicated, which is mainly represented by the π-A isotherms and hysteresis curves of the copolymer monolayers. 11,23 At the interface, it is expected that the terpolymer would form surface micelles composed of hydrophobic PnBA cores and hydrophilic PNIPAM-PDMAEA coronas, which is some different from the common core-corona structures of amphiphilic diblock copolymers 1,11 and triblock copolymers with middle hydrophobic blocks. 17,18 Surfaces micelles are usually formed by spontaneous aggregation during the process of solution spreading and solvent evaporation, which mainly depends on the attractive forces between hydrophilic blocks and water molecules, and the repulsive forces between the hydrophobic blocks and water molecules/ hydrophilic blocks.…”

“…Amphiphilic diblock copolymers tend to form surface aggregates at the air/water interface due to their unique self-assembly properties. 1,2 Their interfacial behavior is affected by several factors such as copolymer composition, spreading solution concentration and volume, and subphase pH and temperature. [3][4][5][6][7][8][9] Especially with the increase of the hydrophobic block content, their Langmuir-Blodgett (LB) films transferred on solid substrates usually exhibit circular, rod-like, and planar aggregates.…”

“…Amphiphilic diblock copolymers tend to form surface aggregates at the air/water interface due to their unique self‐assembly properties 1,2 . Their interfacial behavior is affected by several factors such as copolymer composition, spreading solution concentration and volume, and subphase pH and temperature 3–9 .…”