Diffusion in polymer films

The diffusion and the permeability are inversely related to the density, degree of crystallinity, orientation, filler concentration, and crosslink density of a polymeric film. As a general rule, the presence of plasticisers or residual solvents increases the rate of diffusion in polymers. Films cast from poor solvents have high permeability. The rate of diffusion or permeability is independent of the molecular weight of the polymer, providing the polymer has a moderately high molecular weight. 

Lu X, Manners I, Winnik MA (2001) Oxygen diffusion in polymer films for luminescence barometry applications. In Valeur B, Brochon J-C (eds) New trends in fluorescence spectroscopy Applications to chemical and life sciences. Springer series on fluorescence methods and applications, vol 1. Springer, Berlin Heidelberg New York, p 229-256... 

On the basis of this model one expects that if the spatial period of the grating pattern being recorded is increased, the achievable index modulation will drop off when the period becomes larger than the distance over which monomer can diffuse in the time before the fixing exposure. The experimental spatial-frequency response curve in Fig. 18 shows this expected low-spatial-frequency cutoff (37). Measured rates of monomer diffusion in polymer films are also consistent with the basic diffusion model (38). 

Journal of Applied Polymer Science 66, No.13, 26th Dec.1997, p.2465-73 MEASURING WATER DIFFUSION IN POLYMER FILMS ON THE SUBSTRATE BY INTERNAL REFLECTION FOURIER TRANSFORM INFRARED SPECTROSCOPY Linossier I Gaillard F Romand M Feller J F Lyon,Universite Sud Bretagne,Universite... 

X. Lu, I. Manners, and M.A. Winnik, Oxygen diffusion in polymer films for luminescence barometry applications, ... 

JA Hunt, HN Joshi, VJ Stella, EM Topp. Diffusion and drug release in polymer films prepared from ester derivatives of hyaluronic acid. J Controlled Release 12 159-169, 1990. 

Let us consider the easiest example of oxygen diffusion and reaction in polymer film having the thickness 2/. When oxygen pressure is low (see Chapter 2), the rate of chain oxidation of polymer... 

The solution-diffusion model applies to reverse osmosis, pervaporation and gas permeation in polymer films. At first glance these processes appear to be very... 

Historically most of the microscopic diffusion models were formulated for amorphous polymer structures and are based on concepts derived from diffusion in simple liquids. The amorphous polymers can often be regarded with good approximation as homogeneous and isotropic structures. The crystalline regions of the polymers are considered as impenetrable obstacles in the path of the diffusion process and sources of heterogeneous properties for the penetrant polymer system. The effect of crystallites on the mechanism of substance transport and diffusion in a semicrystalline polymer has often been analysed from the point of view of barrier property enhancement in polymer films (35,36). 

The Fickian diffusion described above is relatively easy to analyze, and demonstrates the capabilities of AW devices for monitoring transient uptakes. However, Fickian diffusion in polymers is the exception rather than the rule. A wide variety of transient responses have been observed, often due to the long time constants tequiied for relaxation of the polymer chains upon absorption of species into the film [93,95]. A detailed discussion of these trends is beyond the scope of this book, and the reader is referred to the polymer literature for these details... 

Because acoustic wave devices are sensitive and respond rapidly, they are ideally suited for real-time monitoring of chemical and physical systems. As discussed in the introduction to this chapter, thin films represent a growing industrial and technological concern for a variety of applications. The use of acoustic devices to characterize the physical properties of these films has been dealt with in the previous sections. Here we describe how these devices can be used to monitor film formation or dissolution processes, or to observe and characterize film properties as a function of time (similar to the monitoring of diffusion in polymers described in Section 4.2.2). 

Wilson [666, 667] and Bauman and Maron [47] show that it is possible to express the reaction rate as a function of film thickness, diffusion constant and solubility of oxygen in the film. When the thickness of the film is reduced to less than a certain value, the chemical reaction and not the diffusion becomes the controlling factor. The activation energy of the oxidation reaction amounts to 16—35 kcal mole-1, or even more, whereas the activation energy of the diffusion of gases in polymer films [39] is of the order of only 10 kcal mole-1. Control by diffusion is facilitated at higher temperatures by the decrease of oxygen solubility in the films. 

Practical consequences of Eg modification in polymer films include significant changes of dissolution, diffusional and etching characteristics, mechanical creep behavior, and adhesion. Figure 17.30 shows a plot of the effective diffusion coefficient of perfluorooctane sulfonate photoacid as a function of film thickness of partially protected poly(4-t-butyloxycarbonyloxstyrene). The profile shows asymptotic behavior at 600 A, below which diffusion slows down remarkably, probably due to interfacial and confinement effects. Clearly, the interaction of the first few hundred angstroms of the film with the substrate determines its adhesion and can alter its electrical and optical properties as well as its topographical and surface characteristics. ... 

When considering diffusion in polymers, the diffusion coefficient is rarely constant and is, in general, a function of concentration, position within the sample, time, and the stress level within the film. A distinc-... 

The most common method of measuring diffusion in polymers uses radiolabelled additive. Typically a thin film of the Icibelled additive is put on the surface of the polymer and the increase in activity at the opposite surface is monitored as the additive diffuses into the polymer. We have used uv microscopy to follow the diffusion of additives and of atactic polypropylene into polypropylene. Samples are exposed to a saturated solution of the additive in a non-swelling solvent such as glycerol then... 

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