Surface Pressure-Induced Interdiffused Structure Evidenced by Neutron Reflectometry in Cellulose Acetate/Polybutadiene Langmuir Films

Abstract

Binary blends of water-insoluble polymers are a versatile strategy to obtain nanostructured films at the air–water interface. However, there are few reported structural studies of such systems in the literature. Depending on the compatibility of the polymers and the role of the air–water interface, one can expect various morphologies. In that context, we probed Langmuir monolayers of cellulose acetate (CA), of deuterated and postoxidized polybutadiene (PBd) and three mixtures of CA/PBd at various concentrations by coupling surface pressure–area isotherms, Brewster angle microscopy (BAM), and neutron reflectometry at the air–water interface to determine their thermodynamic and structural properties. The homogeneity of the films in the vertical direction, averaged laterally over the spatial coherence length of the neutron beam (∼5 μm), was assessed by neutron reflectometry measurements using D2O/H2O subphases contrast-matched to the mixed films. At 5 mN/m, the whole mixed films can be described by a single slightly hydrated thin layer. However, at 15 mN/m, the fit of the reflectivity curves requires a two-layer model consisting of a CA/PBd blend layer in contact with the water, interdiffused with a PBd layer at the interface with air. At intermediate surface pressure (10 mN/m), the determined structure was between those obtained at 5 and 15 mN/m depending on film composition. This PBd enrichment at the air–film interface at high surface pressure, which leads to the PBd depletion in the blend monolayer at the water surface, is attributed to the hydrophobic character of this polymer compared with the predominantly hydrophilic CA.

Vertically Heterogeneous 2D Semi-Interpenetrating Networks Based on Cellulose Acetate and Cross-Linked Polybutadiene

“…The effect of PB cross-linking on the vertical structure of the mixed films is then presented using in situ neutron reflectometry (NR). As a reference, the structure of the mixed films obtained at 10 mN m –1 before UV irradiation is illustrated in Figure A, as previously reported …”

“…In situ NR measurements of the studied Langmuir films were carried out on two time-of-flight reflectometers, namely, FIGARO (Fluid Interfaces Grazing Angles ReflectOmeter) at the ILL (Institut Laue-Langevin, Grenoble, France) and HERMES at the Léon Brillouin Laboratory (LLB, CEA Saclay, France), as previously described . FIGARO allows a range of the scattering vector Q from 0.005 to 0.3 Å –1 while a HERMES reflectometer offers a Q range of up to 0.2 Å –1 . , To avoid the exchange of D 2 O with H 2 O coming from the atmosphere, sealed Langmuir troughs from Riegler & Kirstein Gmbh at LLB and Nima at ILL were used.…”

“…We have recently shown that even though these polymers have significantly different solubility parameters (δ CA ∼ 25 MPa 1/2 ; δ PB ∼ 16 MPa 1/2 ), their behavior in a 2D configuration as Langmuir films is quite unexpected . Indeed, the blend formed at the air–water interface can be considered ideal at low surface pressure and intermediate surface pressure for the PB-rich films.…”

“…As the schemes aim at illustrating the evolution of the thickness upon PB cross-linking and the structure of the interdiffused layers, the number of polymer chains stacking is not representative of the real structure. Figure A is reproduced with permission from ref . Copyright [2021] Publisher ACS.…”

Chemical Composition and Strain at Interfaces between Different Morphologies in Block Copolymer Thin Films

“…In addition to chain strain, the structure formed is the result of a delicate balance between elastic energy in polymer chains and surface energy between polymer blocks or between polymers and the underlying substrate or air interface. − The corona surrounding the valleys may lead to an energy barrier to defect annihilation or structure transition during postdeposition processes. For example, when valleys merge to become lamella, the corona may contribute to the energy barrier of the transition.…”