Surface viscoelastic behaviour of polymeric Langmuir monolayers (original) (raw)
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Recent developments of surface light scattering as a tool for optical-rheology of polymer monolayers
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2004
Surface Quasi-Elastic Light Scattering (SQELS) is an application of dynamic light scattering to measure the dynamics of the thermal roughness of liquid surfaces. An analysis of the spectrum of thermal fluctuations provides information on surface properties like tension and elasticity. In this work we will focus particularly on its use to study polymer or polymer-like Langmuir monolayers. We review work in this area and give an up-to-date overview of the method. Important advances have very recently taken place in the theoretical understanding of this problem, and this has allowed improvements in the analysis of the experimental data. A practical method to estimate the region of physical parameters that can be reliably measured is presented.
2002
We report a detailed study of photoinduced changes in the microscopic structure of monolayers, multilayers, and superlattices of a photosensitive side chain liquid crystalline polymer, deposited by the Langmuir-Schaefer technique. We probe both out-of-plane and in-plane ordering and its changes due to optical pumping of the trans-cis photoisomerization transition of the azobenzene side chain in an azopolyacrylate. Microscopic structure was studied mainly by synchrotron radiation x-ray reflectometry and grazing incidence diffraction; we also used null-ellipsometry and atomic force microscopy. Our results provide a quantitative modeling of the structural changes and corresponding relaxation times taking place as a function of confinement, temperature and optical pumping, and in particular confirm previously reported ellipsometric results on such changes as a function of sample thickness. This allows a quantitative description of the effects of reduced dimensionality on the structural transitions in this glass-forming system.
The Journal of Chemical Physics, 2007
The equilibrium properties of monolayers of two polymers: poly͑octadecyl acrylate͒ and poly͑vinyl stearate͒ on water have been measured. The surface pressure ͑⌸͒ versus surface concentration ͑⌫͒ curves indicate that the water-air interface is a poor solvent for both polymers. The thermal expansivity shows a sharp change near room temperature. This behavior is typical of a glass transition; this is the first time that such a plot is observed for Langmuir films. The ⌸ vs ⌫ curves measured by the continuous compression method show strong anisotropy effects. They also show that the monolayer is brought into nonequilibrium states depending on the compression rate. Within the linear regime, the relaxation experiments were bimodal. The longest relaxation time strongly increases as T is decreased, which might be compatible with the high increase of viscosity in the glass transition. The oscillatory barrier experiments showed that the maximum strain of the linear regime is smaller than 3% for both monolayers. The Fourier-transform analysis of the oscillatory experiments beyond the linear regime points out the contribution of different harmonics in the response function. Oscillations in the nonlinear regime show hysteresis cycles. The results obtained indicate that some of the previously published data for these polymer monolayers correspond to nonequilibrium states.
Langmuir, 2000
A surface quasielastic light scattering (SQELS) and electrocapillary wave (ECW) study of a monolayer of 1-dodecanol, obtained by placing a reservoir drop of the alcohol on top of a water surface, has been carried out as a function of temperature at different wavevectors. This monolayer goes through a liquid-solid phase transition at 312.1 K, more than 14 K above the freezing of bulk 1-dodecanol. The fit of the SQELS spectra gives the frequency, ωc, and the temporal damping, ∆ωc, of thermally induced capillary waves. In all the temperature range ωc follows Kelvin-like behavior (i.e., ωc ∼ q 3/2 ) leading to values of the apparent dynamic surface tension lower than the static ones. In addition the damping follows, ∆ωc ∼ q 1.2-1.4 very far from the expected ∼q 2 dispersion behavior. Both wave parameters show temperature trends compatible with the expected liquid-solid 2D phase transition at around 312 K. The expansion of the frequency range by using the ECW technique, shows that our experimental results cover the regions q e qR e q, where qR is the wavevector at which capillary-dilational resonance takes place. Within the classical elastohydrodynamic theory, our results can be accounted for if the dilational viscosity, κ, takes a negative sign. In these conditions mode mixing takes place for q g qR. Alternatively the experimental results can be quantitatively explained with κ ) 0 if an extra hydrodynamic coupling is included in the dispersion equation. From a physicochemical point of view this coupling may arise from the vicinity of the phase transition and/or molecular orientational effects promoted by the dilational wave. The small values of the dynamic elasticity, (ω), found in this work can be rationalized in terms of a monolayer-subsurface material transport, within the Lucassen-Van de Tempel model.
Journal of Non-Crystalline Solids, 2011
We report the details of the construction and calibration of an ultra sensitive surface rheometer, inspired by the setup described in [C.F. Brooks et al Langmuir 15, 2450], which makes use of high resolution video tracking of the motion of a floating magnetized needle and is capable of measuring the viscoelastic response of a Langmuir monolayer with an accuracy of 10 -5 N/m. This instrument is then employed for the rheological characterization of a Langmuir monolayer of a photosensitive azobenzene polymer, which can be brought out of equilibrium by a suitable photoperturbation. The complex dynamic shear modulus G= G' + i G" is measured as a function of temperature and illumination power and wavelength. The reversible rheological ch anges induced in the film by photo-perturbation are monitored during time, observing a transition from a predominantly elastic (G' > G'') to a viscoelastic (G' ≈ G'') regime. These results are confirmed by comparison with independent measurements performed by us using other rheological techniques. Finally a discussion is made, taking into account the results of a recent x-ray photon correlation spectroscopy experiment on the same polymer in equilibrium and out of equilibrium.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1995
The surface viscoelastic properties of spread monolayers at the air/water interface of poly (l-octadecylene-co-maleic anhydride), which was fully hydrolyzed to give two carboxylic acid groups per repeating unit, and poly(vinyl acetate) were examined by the surface light scattering technique in conjunction with the Wilhelmy plate method. The first polymer was selected as an example of a polymeric amphiphile with a long hydrophobic side chain per repeating unit, while the second one was chosen as a representative of short side chain vinyl polymers. The two polymers were expected to form spread monolayers serving as prime examples of the condensed and expanded type, respectively. Thus, the viscoelastic parameters of the two monolayers deduced from the surface light scattering method, i.e. the surface longitudinal elasticity ed and the corresponding viscosity •, reflect pivotal contrasts between the two types, while their surface packing dynamics appears to be similar by virtue of the same hydrocarbon backbones. Thus, we have shown here that the longitudinal storage and loss moduli constitute sensitive probes for the lateral packing dynamics of hydrophobic side chains of polymer monolayers, which is not accessible through static surface pressure measurements alone.
The Journal of Chemical Physics, 2011
An experimental study of the equilibrium properties and of the surface rheology of Langmuir monolayers of poly(methyl methacrylate) (PMMA) at the air/water interface has been carried out as a function of polymer concentration ( ) and molecular weight (M w ). Dilational and shear complex elasticity moduli covering a frequency range from 10 −3 to 0.2 Hz have been discussed. It was found that the air/water interface behaves as a poor solvent for PMMA monolayers, thus suggesting that the polymer coils take collapsed soft-disks (pancakes) shape at the interface. The equilibrium and dynamic results suggest a fluid-to-soft-glass transition as the polymer concentration increases above a critical packing fraction at constant temperature. This two-dimensional transition is in agreement with results previously discussed for the dilational rheology of poly(4-hydroxystyrene) [F. Monroy, F. Ortega, R. G. Rubio, H. Ritacco, and D. Langevin, J. Chem. Phys. 95, 056103 (2005)]. Furthermore, the -dependence of the relaxation dynamics of the monolayers suggests that the gel state may be considered as a fragile soft glass.
Looking for the glass transition in a single molecular layer on the water surface
Journal of Physics: Condensed Matter, 2003
We present here results which indicate that polymeric monolayers confined at the air-water interface may provide interesting model systems for the study of glass transition phenomenology in a strongly confined 2D geometry. We report on photoinduced relaxation of the pressure-area isotherms in Langmuir monolayers of a photosensitive polyacrylate, using null-ellipsometry to study the evolution of the film thickness in the neighbourhood of the bulk glass transition. Furthermore, viscoelastic behaviour is investigated by a combination of surface quasielastic light scattering and expansion-compression isothermal cycles.
An experimental study of the equilibrium properties and of the surface rheology of Langmuir mono-layers of poly(methyl methacrylate) (PMMA) at the air/water interface has been carried out as a function of polymer concentration () and molecular weight (M w). Dilational and shear complex elasticity moduli covering a frequency range from 10 −3 to 0.2 Hz have been discussed. It was found that the air/water interface behaves as a poor solvent for PMMA monolayers, thus suggesting that the polymer coils take collapsed soft-disks (pancakes) shape at the interface. The equilibrium and dynamic results suggest a fluid-to-soft-glass transition as the polymer concentration increases above a critical packing fraction at constant temperature. This two-dimensional transition is in agreement with results previously discussed for the dilational rheology of poly(4-hydroxystyrene) [F. Monroy, F. Ortega, R. G. Rubio, H. Ritacco, and D. Langevin, J. Chem. Phys. 95, 056103 (2005)]. Furthermore , the-dependence of the relaxation dynamics of the monolayers suggests that the gel state may be considered as a fragile soft glass.
Thickness Dependent Dewetting in Photosensitive Azo-Polyacrylate Langmuir Blodgett Films
Molecular Crystals and Liquid Crystals, 2003
In this paper we summarize our recent results on the self organized motion of a photosensitive polymeric liquid crystal. We studied the quality of pristine Langmuir-Blodgett molecular layers and their photoinduced dewetting. In particular, we have found that the dewetting proceeds in very different ways depending on the initial film thickness: above a certain threshold a long range molecular flow is obtained, with mass transport over macroscopic distances. The resulting morphology comprises of a wide distribution of polymeric droplets, very polydispersed in size. On the contrary, below the threshold, the selforganized dewetting motion is quasi-local, resulting in a dense and uniform distribution of droplets of similar size. Finally, we discuss the quality of the molecular layers as a function of substrate pre-treatment, and of layer composition when a fraction of monomers is introduced. We discuss these results-relevant for the confinement of glass formers in monodispersed nanoparticles-also in the framework of thickness dependent melting phenomena, trying to elucidate the eventual role of the polymer mesogenicity in determining the observed phenomenology.