Self-assembling characteristics of amphiphilic zwitterionic brush random copolymers at the air-water interface

Self-assembling characteristics of a series of amphiphilic zwitterionic brush random copolymers, poly(oxy(11-(3-sulfonylpropyltrimethylglycinyl)undecylesterthiomethyl)ethylene-co-oxy(n-do-decylthiomethyl)ethylene)s (PECH-DMAPS(m) where m is the mol% of DMAPS end group), were investigated at an air-water interface by using surface pressure-area isotherms, infrared spectroscopy, and X-ray reflectivity analysis. Interestingly the random polymers (m: 20-60 mol%) always formed Langmuir monolayer structures only rather than any other structures, regardless of the surface pressures. The Langmuir monolayers possessed enhanced lateral ordering together with conformational changes of the backbone and bristles through increasing the surface pressure. The monolayer structures were basically composed of a hydrophobic bristle phase in the air side and a hydrophilic backbone and bristle phase in the water side. The unique, highly ordered Langmuir monolayer structures could be realized by positive, cooperative efforts of several factors such as the compositional balance of hydrophobic and hydrophilic zwitterionic bristles, the air- and water-induced segregations of the bristles and backbone, the lateral ordering capabilities of the alkyl groups and the alkylenyl linkers in the bristles under the assistance of surface pressure, and the relatively strong anchoring power of DMAPS end groups in the water side using their high water affinity due to the hydrophilic zwitterionic nature. Overall, PECH-DMAPS(m) revealed a very unique self-assembling behavior at the air-water interface, always producing the Langmuir monolayer structure.