Evanescent wave techniques, polymer

A very specific surface structure is observed after the annealing of a PS/polybuta-diene (PB) diblock copolymer, PS-b-PB, shown in Fig. 7 b. The surface is very smooth directly after preparation of the film from solution (similar to Fig. 7 a). By annealing at 120 °C the surface structure shown in Fig. 7 b evolves, which we believe is due to the formation of layers of PS and PB parallel to the surface. The outermost layer might not be completely filled due to lack of material leading to steps at the surface. Similar behavior is observed with other diblock copolymers such as PS-b-PMMA [61]. Enrichment of one component is also observed at the surface of a polymer solution [115,116] by X-ray fluorescene and evanescent wave techniques. 

Besides crystalline order and structure, the chain conformation and segment orientation of polymer molecules in the vicinity of the surface are also expected to be modified due to the specific interaction and boundary condition at the surface between polymers and air (Fig. 1 a). According to detailed computer simulations [127, 128], the chain conformation at the free polymer surface is disturbed over a distance corresponding approximately to the radius of gyration of one chain. The chain segments in the outermost layers are expected to be oriented parallel to the surface and chain ends will be enriched at the surface. Experiments on the chain conformation in this region are not available, but might be feasible with evanescent wave techniques described previously. Surface structure on a micrometer scale is observed with IR-ATR techniques [129],... 

F. Rondelez, D. Ausserre, and H. Hervet, Experimental studies of polymer concentration profiles at solid-liquid and liquid-gas interfaces by optical and X-ray evanescent wave techniques, Annu. Rev. Phys. Chem. 38, 317-347 (1987). 

Another ammonia sensor speciHcally designed for use in bioliquids is based on the evanescent wave technique and can be applied to the vapor-phase determination of ammonia above blood and serum [136]. It utilizes the ninhydrin reaction occurring in the polymer coating of the fiber, and the resulting color change is monitored by total internal reflection. The probe is applicable to clinical determinations normally carried out in the vapor phase, but works irreversibly. A linear relationship exists between absorbance and ammonia concentration in the clinically useful range of 0-4.0 pg mL. Comparison with the reference method showed a correlation coefficient of 0.92. 

J. Stumpe, Polymer Interfaces and Ultra-thin Films Characterized by Optical Evanescent Wave Techniques, Makromoleku-lare Chemie-Macromolecular Symposia 48/49, 363 (1991)... 

Experimental work on the determination of the depletion layer thickness commenced in this period. The depletion thickness S of polystyrene at a nonadsorbing glass plate was measured using an evanescent wave technique by Aflain et al. [120]. The value found for S was indeed close to the radius of gyration of the polymer. Ausserre et al. [121] measured the depletion thickness of xanthan (a polysaccharide) at a quartz waU below and above the polymer overlap concentration. In dilute solutions, below overlap, S was close to the radius of gyration of... 

Another largely unexplored area is the change of dynamics due to the influence of the surface. The dynamic behavior of a latex suspension as a model system for Brownian particles is determined by photon correlation spectroscopy in evanescent wave geometry [130] and reported to differ strongly from the bulk. Little information is available on surface motion and relaxation phenomena of polymers [10, 131]. The softening at the surface of polymer thin films is measured by a mechanical nano-indentation technique [132], where the applied force and the path during the penetration of a thin needle into the surface is carefully determined. Thus the structure, conformation and dynamics of polymer molecules at the free surface is still very much unexplored and only few specific examples have been reported in the literature. 

A fiber-optic device has been described that can monitor chlorinated hydrocarbons in water (Gobel et al. 1994). The sensor is based on the diffusion of chlorinated hydrocarbons into a polymeric layer surrounding a silver halide optical fiber through which is passed broad-band mid-infrared radiation. The chlorinated compounds concentrated in the polymer absorb some of the radiation that escapes the liber (evanescent wave) this technique is a variant of attenuated total reflection (ATR) spectroscopy. A LOD for chloroform was stated to be 5 mg/L (5 ppm). This sensor does not have a high degree of selectivity for chloroform over other chlorinated aliphatic hydrocarbons, but appears to be useful for continuous monitoring purposes. 

Unambiguous determination of the conditions under which slippage occurs requires a technique able to measure the velocity of the fluid in the immediate vicinity of the solid wall over a thickness comparable to the size of a polymer chain, i.e. a few tens of nanometers. Classical laser Doppler velocimetry does not meet this requirement even if it allows for the determination of velocity profiles which clearly reveal a non-zero velocity within typically a few 10 pm from the wall. We have developed a new optical technique. Near Field Velocimetry (N.F.V.) [14], which combines Evanescent Wave Induced Fluorescence (E.WF.) [27] and Fringe Pattern Fluorescence Recovery After Photobleaching (F.P.F.R.A.P.) [28]. The former technique gives the spatial resolution normal to the solid wall, while the latter one enables the determination of the local velocity of the fluid. A major constraint of the technique is that it needs polymer molecules labelled with an easily photobleachable fluorescent probe. 

Negative adsorption is in most cases very small compared to positive adsorption and therefore not easily detected directly. A sophisticated optical technique (evanescent-wave-induced fluorescence, EWIF) has been used to prove the reduction in segment concentration close to a non-adsorbing surface Depletion also has an effect on the flux of polymer solutions through pores since the viscosity of the liquid near the non-adsorbing surface is lower than that of the polymer solution, the flux is then higher than would be expected on the basis of the bulk viscosity. Negative adsorption at a liquid-air interface leads to a measurable increase In surface tenslon ... 

IR-transmitting optical fibres are evanescent wave sensors using a mathematical deconvolution technique to extract the absorbances and follow the concentrations of the components as they occur in both laboratory scale and process production. The fibre-optic probe used can be placed at specific locations within the samples or at the surface. The specificity of the technique, the speed of data acquisition and the portability of equipment make this method ideal as a tool to fundamentally probe polymer reactions and processes. Chalcogenide optical fibres are used to direct IR radiation from an FUR spectrometer through an attenuated total reflection (ATR) probe immersed in a reactor and back to the spectrometer. 

The flow velocity of a fluid at submicrometric distances from a solid wall is investigated using a novel optical technique based on fluorescence recovery after photobleaching and evanescent wave induced fluorescence. For a polymer melt, three different friction regimes, all characterized by slip at the wall, are identified and modeled at the molecular level. For a simple fluid (alkane), slip at the wall is put into evidence and factors influencing the extend of slip are identified. 

Wise and co-workers used an infrared (IR) evanescent wave spectroscopy technique to follow the disappearance of a tracer of deuterated 1,4-polybutadiene (PBDE) displaced in a channel by unlabeled PBDE of the same molecular weight. Linear PBDE is a convenient polymer for experimental research because it flows at room temperature and has a viscosity that is relatively insensitive to shear rate. One face of the channel was fabricated from a ZnSe IR crystal, and the deuterated polymer was placed on the surface. The deuterated polymer was then allowed to equilibrate with hydrogenated polymer of the same molecular weight, and flow was initiated. The concentration of deuterated polymer in the wall region after the... 

Abstract This chapter describes recent breakthroughs in the instrumentation for far-ultraviolet (FUV) spectroscopy. The key technique is attenuated total reflection (ATR) that is frequently used in the infrared region. ATR technique decreases the absorbance of samples with strong absorptivity because of the penetration depth of the evanescent wave less than 100 nm. Therefore, ATR-FUV spectroscopy realizes the measurement of FUV spectra of samples in liquid and solid states. Some applications (in-line monitoring, characterization of polymers and time-resolved spectroscopy in sub-microsecond) are introduced in terms of instrumentation. This chapter explains not only the detail of the instruments but also the mathematical correction for ATR spectra to separate the absorption and refraction indices. 

The structure of polymer adsorbed layers at the air-liquid interface is similar to that at the solid-liquid interface. Adsorption at air-liquid interface has been studied by ellipsometry (72), X-ray and neutron reflectivity (73,74), surface tension measurements (75), X-ray evanescent wave-induced fluorescence (76), and Langmuir trough techniques (73). Neutron reflectivity measurements indicate that in the case of homopolymers the segment density decreases as in good agreement with scaling predictions for homopolymers at the solid-liquid interface (73). 

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