Poly concentration profiles

FIG. 5 Local concentration profiles around a hydroxyl group in poly(vinyl alcohol) of heavy atoms in a (1 1) water/ethanol mixture A = OW water oxygen, A = OE ethanol oxygen, A = CE ethanol carbon. The local atomic fractions are defined as = a( )/ ZIb where a( ) is average number of atoms... 

For a typical poly crystalline material the concentration profile is divided into three segments. Near the original surface the tracer distribution will be characteristic of volume diffusion and show a typical bell shape. The average tracer concentration... 

Solvent Concentration Profile of Poly(methyl methacrylate) Dissolving in Methyl Ethyl Ketone... 

UMMETAL. Solvent Concentration Profile of Poly (methyl methacrylate) 387... 

Figure 2. Concentration profiles of poly(2-chloroethyl acrylate) in poly-(methyl methacrylate) along the thickness (L0) dimension of the samples. (GRAD) Gradient polymer, (IPN) interpenetrating networks.

The self-consistent field theory also allows one to calculate the segment density profiles of each homopol)maer and each block of the copolymer. Forward recoil spectrometry is unable to resolve the details of these concentration profiles — the apparent finite width of the copolymer layer shown in figure 6.6 is entirely due to the instrumental resolution - but from neutron reflectivity measurements on a series of differently labelled samples one is able to extract all four segment density profiles. Figure 6.20 shows an example of this, for a styrene/methyl methacrylate copolymer at an interface between polystyrene and poly(methyl methacrylate). 

The processes of reaction and diffusion occur at the same time in a variety of systems. These issues are particularly important in the formation of blend systems and are central issues in the performance property enhancement of such systems. A study of the competitive effects of the rates of the two processes can be easily carried out using FTIR microspectroscopy. The rate of diffusion can be monitored by the time evolution of the absorbance (concentration) profiles while the rate of reaction can be monitored as a time evolution of the reactant (or product) absorbance (concentration). Reaction of a random copolymer of styrene and maleic anhydride (SMA) with bis(amine)-terminated poly(tetrahydrofuran) (PTHF) is one such studied system [73]. Temperature was varied while studying the effects of two different PTHF molecular weights. The reaction rate constants were obtained from the initial slope of conversion-time plots. In addition, it was shown that the rate of diffusion was faster as diffusion of PTHF into the SMA phase occurred prior to the imide formation. The imide was formed in the SMA phase and quantitatively estimated. A corresponding decrease in the carbonyl stretching vibration of the maleic anhydride peak was seen. 

At this point, it is instructive to examine the contact method (Fig. 6). When a solvent is brought into contact with a polymer film, it diffuses into the polymer. If the film is constrained between two IR transparent substrates and the liquid is forced into the gap by capillary action, the contact and subsequent diffusion of the liquid can be easily monitored by means of chemical imaging using light transmitted in the perpendicular direction. After contact, diffusion between the two materials starts. The absorbance (concentration) profile can be measured with accuracy to yield diffusion profiles [83]. However, for hquids, where capUlary acdion is facilitated, the examination of concentration profiles in-situ is difficult. If the concentration profiles developed at higher temperatures are examined at a temperature that prevents further diffusion, the profiles are frozen-in. The diffusion of E7 into poly(butyl methacrylate) (PBMA) w is studied [84] in this manner and compo-... 

Much more effectual and very often applied are polymer-coated electrodes. Especially electrochemical polymerization is an attractive method for the immobilization of redox enzymes at electrode surfaces, and/or accumulation of electroactive reactants. An approximative analytical treatment of the response of an amperometric enzymatic electrode leading to plots of fluxes and concentration profiles has been made in [14]. The electron transport through poly-4-vinylpyridine and polystyrene-sulfonate films (widely used for immobilization of redox centers on electrodes) has been studied in [15]. 

Information on the spatial distribution of paramagnetic molecules deduced from ESRI experiments has been used successfiilly for measurement of the translational diffusion. Diffusion coefficients of paramagnetic diffusants can be deduced from an analysis of the time dependence of the concentration profiles along a selected axis of the sample. The determination of diffusion coefficients for spin probes in liquid crystals and model membranes, and the effect of polymer and probe poly-dispersity, have been described in a series of papers by Freed and co-workers (44). These papers represent an effort to move beyond phantoms, and to extract quantitative information from ESRI experiments. 

The concentration profiles of B which had been diffused, from polycrystalline material, into underlying monocrystalline material were analyzed by means of secondary ion mass spectrometry. The co-diffusion of As and B was studied in an emitter and extrinsic base configuration. In a first poly-Si layer, the diffusivity could be described by ... 

Ahn et al. deduced the concentration profile of heat-induced radicals in a poly-imide resin. In vitro degradation of poly(ortho esters) containing 30 mol% lactic acid has been studied by ID and 2D spectral-spatial ESRI, based on pH-sensitive nitroxide spin probes. Efforts to understand the failure of ultrahigh molecular weight polyethyleue (UHMWPE) in orthopedic components has been studied by spectral-spatial ESRI in Y-irradiated UHMWPE. ... 

The water sorption proeess at 37°C for semi-IPNs of copolymers of HEMA and THFMA with poly(Ethyl methacrylate) (PEMA) was studied by direet examination of the water diffusion front using MRI of the water protons in real time. NMR imaging of the water diffusion front confirmed that the Fiekian diffusion meehanism provides an adequate representation of the diffusion of water into the eylinders eomposed of PEMA/P(HEMA)-co-THFMA semi-IPNs. The diffusion eoeffieients ealculated from the water concentration profiles were comparable, but systematieally smaller than those for P(HEMA-co-THFMA) containing a similar mole fraction of HEMA. 

SANS is not prone to this limitation. The full concentration profile has been measured for two different systems, namely poly(methyl-methacrylate) (M = 265,000) adsorbed onto y-alumina and poly(dimethy1-siloxane) adsorbed onto porous silica. In both cases the results are in support of the scaling law behavior 4>(z) o (a/zr. The profile is selfsimilar, as predicted by de Gennes, in the central region < z <... 

The second technique of choice is SANS. Small angle neutron scattering is a classical tool for studying separately. polymeric and colloidal systems. As first shown by Cosgrove et al. it can also be used to observe polymers at interfaces. These authors were able to measure for the first time the concentration profile of poly(oxyethylene) chains adsorbed onto polystyrene colloidal grains. Their data were in qualitative agreement with the train-loop-tail description of Scheutjens... 

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