Confinement and Cross-Linking of 1,2-Polybutadiene in Two Dimensions at the Air-Water Interface

Revealing the Molecular Physics of Lattice Self-Assembly by Vibrational Hyperspectral Imaging

Lattice self-assemblies (LSAs), which mimic protein assemblies, were studied using a new nonlinear vibrational imaging technique called vibrational sum-frequency generation (VSFG) microscopy. This technique successfully mapped out the mesoscopic morphology, microscopic geometry, symmetry, and ultrafast dynamics of an LSA formed by β-cyclodextrin (β-CD) and sodium dodecyl sulfate (SDS). The spatial imaging also revealed correlations between these different physical properties. Such knowledge shed light on the functions and mechanical properties of LSAs. In this Feature Article, we briefly introduce the fundamental principles of the VSFG microscope and then discuss the in-depth molecular physics of the LSAs revealed by this imaging technique. The application of the VSFG microscope to the artificial LSAs also paved the way for an alternative approach to studying the structure-dynamic-function relationships of protein assemblies, which were essential for life and difficult to study because of their various and complicated interactions. We expect that the hyperspectral VSFG microscope could be broadly applied to many noncentrosymmetric soft materials.

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

This work reports the feasibility of polybutadiene (PB) cross-linking under UV irradiation in the presence of a linear polymer, cellulose acetate (CA), to form semi-interpenetrating polymer networks at the air-water interface. The thermodynamic properties and the morphology of two-dimensional (2D) CA/PB blends are investigated after UV irradiation and for a wide range of CA volume fractions. A contraction of the mixed Langmuir films is observed independent of the composition, in agreement with that recorded for the individual PB monolayer after cross-linking. The PB network formation is demonstrated by in situ sum-frequency generation spectroscopy on the equivolumic CA/PB mixed film. From Brewster angle microscopy observations, the PB network synthesis does not induce any morphology change at the mesoscopic scale, and all of the mixed films remain homogeneous laterally. In situ neutron reflectometry is used to probe the effect of PB cross-linking on the vertical structure of CA/PB mixed films. For all studied compositions, significant thickening of the films is evidenced, consistent with their contraction ratio. This thickening is accompanied by a partial expulsion of the PB toward the film-air interface, which is attributed to the hydrophobic character of the PB. This phenomenon is stronger for films rich in PB. In particular, the structure of the PB-rich film undergoes a transition from vertically homogeneous to inhomogeneous along the depth. 2D semi-interpenetrating polymer networks can thus be synthesized at the air-water interface with a morphology that is strongly influenced by the polymer-polymer and polymer-environment interactions.

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

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.

Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) Copolymer 2D Single Network at the Air-Water Interface

In this work, Langmuir monolayers based on poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) (PEG-PPG-PEG) triblock copolymer were in situ stabilized at the air-water interface in the presence of a cross-linking agent, benzene-1,3,5-tricarboxaldehyde (BTC), in the aqueous subphase. The reaction takes place through acid-catalyzed acetalization between the terminal hydroxyl groups of the copolymer and aldehyde functions of the BTC molecules. Mean area per repeat unit measurements as a function of the reaction time show a significant monolayer contraction associated with an increase in its compressibility modulus. In addition, Brewster angle microscopy observations indicate the appearance of higher-density two-dimensional domains, irreversibly formed at constant surface pressure. This is also confirmed on a smaller scale by atomic force microscopy (AFM). These arguments, consistent with copolymer monolayer cross-linking in acidic medium, are supported in situ at the air-water interface by sum-frequency generation (SFG) spectroscopy. Furthermore, PEG-PPG-PEG monolayer cross-linking is not evidenced in alkaline medium, in coherence with the interfacial acid-catalyzed acetalization.