Conformation of Polymer Brushes at Aqueous Surfaces Determined with X-ray and Neutron Reflectometry. 2. High-Density Phase Transition of Lipopolyoxazolines

• Published in: Macromolecules 2001-02, Vol.34 (5), p.1334-1342
• Main Authors: Wurlitzer, A, Politsch, E, Huebner, S, Krüger, P, Weygand, M, Kjaer, K, Hommes, P, Nuyken, O, Cevc, G, Lösche, M
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Surface properties
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma000932n

We have investigated the molecular conformations of a lipopolymer with a polyoxazoline headgroup at air/water interfaces as a function of lateral area per molecule with X-ray and neutron reflectometry. The polymer 1,2-dioctadecanyl-sn-glycero-3-poly(2-methyl-2-oxazoline), PMO−(C18)2, forms stable surface monolayers. Pressure/area isotherms around room temperature show a plateau region, indicative of a phase transition whose origin was examined. For data evaluation, a novel approach was used that acts on explicit quasi-molecular ensemble conformations of the polymer [Politsch et al., preceding paper in this issue]. At lower surface pressure, the polymer density distribution exhibits a maximum near the interface, indicative of attractive interaction between the predominantly hydrophilic polymer chains and the hydrophobic surface. Across the plateau region of the isotherm, a change in the volume density distribution of the alkyl chains was observed which is indicative of a partial immersion of the lipid moieties into the aqueous subphase. In contrast, no major structural change across the phase transition was detected in the polymer volume density profiles which comply with scaling predictions at both sides of the phase transition if deviations due to nonidealities are neglected. We interpret these observations as an alkyl chain ordering induced by the steric interference between the PMO:  Immersion of alkyl chains into the subphase relaxes the strain on the hydrophobic anchors which derives from a reduction of the configurational entropy of the PMO chains due to their confinement to the interface.