Diffusion compatibility, polymer

Figure 9.4c and 9.4d represent intermediate cases, 9.4c indicates partial miscibility we see a two-phase system of AB blends with different A/B ratios. This might be the result of segregation into the binodals. Figure 9.4d is called an interphase or a multiphase blend. The system is quasi-homogeneous, but it contains all A/B ratios between cpi = 0 and concentration gradients as a result of non-completed diffusion in a combination of well-compatible polymers. 

The dissolution of a polymer in a penetrant involves two transport processes, namely penetration of the solvent into the polymer, followed by disentanglement of the macromolecular chains. When an uncrosslinked, amorphous, glassy polymer is in contact with a thermodynamically compatible liquid (solvent), the latter diffuses into the polymer. A gel-like layer is formed adjacent to the solvent-polymer interface due to plasticization of the polymer by the solvent. After an induction time, the polymer is dissolved. A schematic diagram of solvent diffusion and polymer dissolution is shown in Fig. 1. However, there also exist cases where a polymer cracks when placed in a solvent. 

With solution mixing one proceeds from two independently produced polymers. The polymers are molecularly dissolved in each solution. An intimate distribution of molecules is obtained on mixing two compatible polymers. On the other hand, incompatible polymers demix already at very small concentrations and an additional fractionation according to molar mass can also occur. All attempts to maintain the internal mixture structure of solutions of incompatible polymers by rapid quenching of the state of disequilibrium have been unsuccessful. Phase separation of incompatible polymers is, of course, accelerated on distilling off the solvent because of the increase in the rate of diffusion. The solvent must be removed by freezing and sublimation or by distillation, whereupon the domain size may be increased, especially in the case of distillation. 

Welding processes are of two main types thermal and solvent. By careful application of heat or solvent to a thermoplastic substrate, one may liquefy the surface resin and use it to form the bond. The bond strength is determined by diffusion of polymer from one snr-face into another instead of by the wetting and adsorption of an adhesive layer. It is possible to weld plastics of different types. However, for both thermal and solvent welding, the success of the process will be heavily determined by the compatibility of the polymers being joined. 

Despite macroscopic thermodynamic incompatibility (see Compatibility), an atomically sharp interface between two incompatible polymers will not be stable. Although there is an enthalpy debt to be paid if a chain of polymer A starts to diffuse into polymer B, there is an entropy gain. Helfand and Tagami - introduced a model that considered the probability that a chain of polymer A has diffused a given distance into polymer B when the interactions are characterized by the parameter x They predicted that at equilibrium the thickness , d, of the interface would depend upon the interaction parameter and the... 

There still remain many problms to be solved for developing an excellently blood-compatible polymer. At least, a direct comparison of the blood compatibility in vivo by means of a powerful evaluation method is required for each of the polymers which different research groups have synthesized according to their own hypothesis. Our diffuse surface hypothesis is only one of these hypotheses. The materials illustrated in Fig. 29 may contain a large fraction of water on their surface, leading to minimum interactions with blood. 

Radioactive tracers were utilized by Bueche (1962) to measure self diSusion coefficients for polymer systems above their glass-transition temperature, Tg. Price et al. (1978) described a novel approach that used scanning electron microscopy (SEM) and dispersive energy X-ray fluorescence analysis to measure the interdiffusion (D Kh cmVsec) of compatible polymer/polymer systems. Quasi-elastic light scattering (QELS) is an unusual technique due to its ability to measure both the mutual and self diffusion coefficients. Patterson et al. (1981) and Amis s. (1983) have demonstrated the apphcahon of this technique to polymeric gels. 

Interdiffusion between two compatible polymers, e.g., PMMA and PVF2 (polyvinylidene fluoride), has been studied by Wu et a/. (34) in this case, the two diffusion coefficients and rates are unequal, and is proportional to A -2. In addition, the interfacial thicknessx(t) grows by hence... 

One of very important practical aspects that require analysis of the data based on FRET in restricted volumes is interpolymer diffusion. In experimental measurements, two thin films of the chemically identical polymer (or two compatible polymers), one labeled by the donor and second by the acceptor, are pressed together and heated above the glass transition temperature and the one-dimensional diffusion along the z axes (let s assume the horizontal axis) is monitored by FTER measurement as a function of the aimealing time, A number of these measurements have been performed, but many of them have not been analyzed properly and it was shown by Winnik [15, 28-30] that improper analysis may yield very erroneous values of the diffusion coefficients. Correct analysis is... 

Direct evidence for interdiffusion in compatible polymers does exist. Radio-metric studies [38,39] have demonstrated the presence of macromolecular diffusion. The diffusion coefficients were found to be of the order of 10 to 10" cm Vs, which Voyutskii [40] argues is completely adequate for the formation of an intrinsically strong interface between the polymers after a contact time of only a few seconds, as discussed above. Further work [41-43], using techniques of optical microscopy, including ultraviolet light employing luminescence analysis [43], has indicated that in compatible, non-polar polymers the interphase region where interdiffusion has occurred may be about 10 jxm deep but in cases where the solubility parameters, 8s, of the two polymers were appreciably different then no interdiffusion zone could be detected. 

For polymer/additive analysis complete dissolution is not a prerequisite. Rather, the solvent should at least swell the polymer by diffusion, which allows the physically blended additives to dissolve. True dissolution occurs predominantly when polymer chain lengths are small, on the order of 5000-10 000 Da. Solvent choice for dissolution or extraction should take into account restrictions imposed by further analysis steps (compatibility with chromatographic and/or spectroscopic requirements). When microwave extraction of additives from a polymer is followed by HPLC analysis, the solvent must be compatible with the HPLC mobile phase so that solvent exchange is not required before analysis. 

In an effort to optimize the solvent-containing passive sampler design, Zabik (1988) and Huckins (1988) evaluated the organic contaminant permeability and solvent compatibility of several candidate nonporous polymeric membranes (Huckins et al., 2002a). The membranes included LDPE, polypropylene (PP), polyvinyl chloride, polyacetate, and silicone, specifically medical grade silicone (silastic). Solvents used were hexane, ethyl acetate, dichloromethane, isooctane, etc. With the exception of silastic, membranes were <120- um thick. Because silicone has the greatest free volume of all the nonporous polymers, thicker membranes were used. Although there are a number of definitions of polymer free volume based on various mathematical treatments of the diffusion process, free volume can be viewed as the free space within the polymer matrix available for solute diffusion. 

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