Gasous hole closing in a polymer Langmuir monolayer

From Homogeneous to Segregated Structure of Poly(dimethylsiloxane)/Cellulose Derivative Mixed Langmuir Films Depending on Composition: An in Situ Neutron Reflectivity Study

The mixing behavior of deuterated polydimethylsiloxane (PDMSd) and cellulose acetate butyrate (CAB) spread as Langmuir films at the air-water interface was studied by means of surface pressure-area isotherms, Brewster angle microscopy (BAM) observations, and in situ neutron reflectivity. The contrast variation method was used with different D2O/H2O mixtures as subphase, allowing contrast matching to either CAB, PDMSd, or PDMSd/CAB mixed film if homogeneous. At PDMSd volume fractions Φ lower than 0.6, the mixed film is a homogeneous monolayer throughout the film compression, in agreement with the monophasic film observed by BAM and the attractive interactions between PDMSd and CAB evidenced from the isotherm measurements. In contrast, at PDMSd volume fractions Φ higher than 0.6, a vertically segregated structure of the mixed film is highlighted. Indeed, whatever the surface pressure, a bilayer structure is observed with a PDMSd layer in contact with the air over a thin CAB layer in contact with the subphase. These results show that the structure of the film is mainly driven by the PDMSd volume fraction which allows obtaining either a homogeneous membrane which composition can be tuned or a vertically segregated system. In contrast, only the thickness of the layers varies with the surface pressure while the structure of the film is not affected.

Synthesis at the air-water interface of a two-dimensional semi-interpenetrating network based on poly(dimethylsiloxane) and cellulose acetate butyrate

The UV-induced cross-linking of methacryloxypropyl-terminated poly(dimethylsiloxane) oligomers was studied at the air-water interface either in pure PDMS Langmuir monolayers or in mixed films containing cellulose acetate butyrate. Surface pressure-area isotherms, area measurement at constant surface pressure, Brewster angle microscopy observations, and infrared-visible sum frequency generation (SFG) spectroscopy were combined to follow the evolution of the monolayers upon in situ UV photoirradiation. For both systems, the mean area per repeat unit decreases with irradiation time reflecting the monolayer contraction. In addition, SFG measurements evidence the conversion of the methacrylate groups into unconjugated poly(methacrylate) ones. These results demonstrate PDMS cross-linking, leading to the formation of either a single PDMS network or a PDMS network entrapped in a CAB matrix. The network formation is accompanied by morphology changes as shown by atomic force microscopy on the transferred monolayer. Indeed, filamentous structures appear on both pure and mixed preirradiated monolayers.

25th anniversary article: what can be done with the Langmuir-Blodgett method? Recent developments and its critical role in materials science

The Langmuir-Blodgett (LB) technique is known as an elegant method for fabrication of well-defined layered structures with molecular level precision. Since its discovery the LB method has made an indispensable contribution to surface science, physical chemistry, materials chemistry and nanotechnology. However, recent trends in research might suggest the decline of the LB method as alternate methods for film fabrication such as layer-by-layer (LbL) assembly have emerged. Is LB film technology obsolete? This review is presented in order to challenge this preposterous question. In this review, we summarize recent research on LB and related methods including (i) advanced design for LB films, (ii) LB film as a medium for supramolecular chemistry, (iii) LB technique for nanofabrication and (iv) LB involving advanced nanomaterials. Finally, a comparison between LB and LbL techniques is made. The latter reveals the crucial role played by LB techniques in basic surface science, current advanced material sciences and nanotechnologies.

Polymer compatibility in two dimensions. Modeling of phase behavior of mixed polymethacrylate Langmuir films

We analyze the possibility of polymer blends undergoing phase separation in two dimensions. To this end, we investigate a model system consisting of water-supported Langmuir monolayers, obtained from binary polyalkyl-methacrylate mixtures (PXMA, where X stands for any of the type of ester side groups used: M, methyl-; E, ethyl-; B, butyl-; H, hexyl-; O, octyl-; L, lauryl-methacrylate), by means of self consistent field (SCF) calculations. In particular, we address the conditions which determine demixing and phase separation in the two-dimensional system, showing that a sufficient chain length mismatch in the ester side group moieties is able to drive the polymer demixing. When the difference in length of the alkyl chain of the ester moieties on the two types of polymers is progressively reduced, from 11 carbon atoms (PMMA/PLMA) to 4 carbons only (POMA/PLMA), the demixing tendency is also reduced. The polymer/subphase interactions affect more the distribution of the polymer coils in the POMA/PLMA blend monolayer. Mixing of the two polymers is observed, but also a partial layering along the vertical direction. We also add, to a PMMA/PLMA blended monolayer, a third component, namely, a symmetrical diblock copolymer of the type PLMA-b-PMMA. We observe adsorption of the diblock copolymer exclusively at the contact line between the two homopolymer domains, and a concomitant lowering of the line tension. The line tension varies with the chemical potential of the diblock copolymer according to Gibbs' law, which demonstrates that PLMA-b-PMMA can act as a "lineactant" (the equivalent of a surfactant in two-dimensional systems) in the binary demixed PMMA/PLMA Langmuir monolayer.

Polymers at the water/air interface, surface pressure isotherms, and molecularly detailed modeling

Surface pressure isotherms at the air/water interface are reproduced for four different polymers, poly-L-lactic acid (PLLA), poly(dimethylsiloxane) (PDMS), poly(methyl methacrylate) (PMMA), and poly(isobutylene) (PiB). The polymers have the common property that they do not dissolve in water. The four isotherms differ strongly. To unravel the underlying details that are causing these differences, we have performed molecularly detailed self-consistent field (SCF) modeling. We describe the polymers on a united atom level, taking the side groups on the monomer level into account. In line with experiments, we find that PiB spreads in a monolayer which smoothly thickens already at a very low surface pressure. PMMA has an autophobic behavior: a PMMA liquid does not spread on top of the monolayer of PMMA at the air/water interface. A thicker PMMA layer only forms after the collapse of the film at a relatively high pressure. The isotherm of PDMS has regions with extreme compressibility which are linked to a layering transition. PLLA wets the water surface and spreads homogeneously at larger areas per monomer. The classical SCF approach features only short-range nearest-neighbor interactions. For the correct positioning of the layering and for the thickening of the polymer films, we account for a power-law van der Waals contribution in the model. Two-gradient SCF computations are performed to model the interface between two coexistent PDMS films at the layering transition, and an estimation of the length of their interfacial contact is obtained, together with the associated line tension value.