title: Molecular arrangement of symmetric and non-symmetric triblock copolymers of poly(ethylene oxide) and poly(isobutylene) at the air/water interface.
abstract: The behavior of a series of amphiphilic triblock copolymers of poly(ethylene oxide) (PEO) and poly(isobutylene) (PIB); including both symmetric (same degree of polymerization (DP) of the terminal PEO blocks) PEOm-b-PIBn-b-PEOm and non-symmetric (different DP of the terminal PEO blocks) PEOm-b-PIBn-b-PEOz, is investigated at the air/water interface by measuring surface pressure vs mean molecular area isotherms (π vs mmA), Langmuir-Blodgett (LB) technique, and infrared reflection-absorption spectroscopy (IRRAS). The block copolymer (PEO32-b-PIB160-b-PEO32) with longer PEO segments forms a stable monolayer and the isotherm reveals a pseudo-plateau starting at π∼5.7 mN/m, also observed in the IRRAS, which is assigned to the pancake-to-brush transition related to the PEO dissolution into the subphase and subsequent PEO brush dehydration. Another plateau is observed at π∼40 mN/m, which is attributed to the film collapse due to multilayer formation. The pancake-to-brush transition could not be observed for samples with smaller PEO chains. The isotherms for block copolymers, with short PEO chains, both symmetric (PEO3-b-PIBn-b-PEO3) and non-symmetric (PEO12-b-PIBn-b-PEO3), reveal another transition at π∼20-25 mN/m. This is interpreted to be due to the conformational transition from a folded state where the middle PIB block is anchored to the water surface at both ends by the terminal hydrophilic segments to an unfolded state with PIB anchored to the water surface at one end. It is assumed that this transition involves the removal of PEO3 chains from the water surface in case of non-symmetric PEO12-b-PIB85-b-PEO3 and in case of symmetric, probably one PEO3 of each PEO3-b-PIB85-b-PEO3 chain. Because of the weaker interaction of the short PEO3 chains with the water surface as compared with the relatively longer PEO12 chains, the film of PEO3-b-PIB85-b-PEO3 collapses at much lower surface pressure after the transition as compared with the PEO12-b-PIB85-b-PEO3. The AFM images reveal the formation of microdomains of almost uniform height (6-7 nm) in LB films of PEO3-b-PIB85-b-PEO3 and PEO12-b-PIB85-b-PEO3 after transferring onto silicon surfaces. These domains are assumed to be the mesomorphic domains of ordered and folded PIB chains.
authors.affiliation: Martin Luther University Halle-Wittenberg, Department of Chemistry, D-06099 Halle (Saale), Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Fuchs
authors.affiliation: Martin Luther University Halle-Wittenberg, Department of Chemistry, D-06099 Halle (Saale), Germany.
authors.forename: Hazrat
authors.initials: H
authors.lastname: Hussain
authors.affiliation: Martin Luther University Halle-Wittenberg, Department of Chemistry, D-06099 Halle (Saale), Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Schwieger
authors.affiliation: Martin Luther University Halle-Wittenberg, Department of Chemistry, D-06099 Halle (Saale), Germany.
authors.forename: Matthias
authors.initials: M
authors.lastname: Schulz
authors.affiliation: Martin Luther University Halle-Wittenberg, Department of Chemistry, D-06099 Halle (Saale), Germany.
authors.forename: Wolfgang H
authors.initials: WH
authors.lastname: Binder
authors.affiliation: Martin Luther University Halle-Wittenberg, Department of Chemistry, D-06099 Halle (Saale), Germany. Electronic address: joerg.kressler@chemie.uni-halle.de.
authors.forename: Jörg
authors.initials: J
authors.lastname: Kressler
mesh_terms: D000388:Air; D018625:Microscopy, Atomic Force; D015394:Molecular Structure; D011090:Polyenes / Q000737:chemistry*; D011092:Polyethylene Glycols / Q000737:chemistry*; D011108:Polymers / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared; D014867:Water
chemical_list: D011090:Polyenes; D011108:Polymers; D014867:Water; D011092:Polyethylene Glycols; C008967:polyisobutylene
keywords: Block copolymers; Infrared reflection–absorption spectroscopy; Langmuir films; Langmuir–Blodgett; Poly(ethylene oxide); Poly(isobutylene)