Poly time dependent diffusion

To further illustrate the utility of the present time-dependent diffusion coefficient approach, data from Reference 21 for the thiamine HC1 release from initially dehydrated poly-HEMA sheets with different loading levels are analyzed with equations 3-7. The results are shown in Figure 9 and Table II. It has to be emphasized... 

A linear relationship was observed between D and M. Taking into account the polymer degradation during release, Mn can be written as a function of time, resulting in a time-dependent diffusion coefficient. The new equation for the time-dependent diffusion coefficient was incorporated into solutions of the diffusion equation that could adequately describe lyso-Z3nne release from poly(ether ester) copol3nners (50). 

Diffusion of materials out of a PSi sensor was monitored by Koh and coworkers in a proof-of-concept device for monitored drug delivery applications [29], Poly-methyl methacrylate (PMMA) containing 0.25 mg/ml caffeine was first cast onto a freestanding PSi thin film. Exposure of this composite material to a pH 7 buffer caused a time-dependent decrease in the intensity of reflection at a fixed wavelength, or alternatively a time-dependent blue shift of the peak, corresponding to diffusion of the small molecule out of the PMMA-PSi composite. These data correlated well with appearance of caffeine in the buffer solution, as measured by UV-Vis spectrophotometry. 

Figure 9 shows the self-diffusion coefficients oi propane in the poly-crystalline grains after different coking times corresponding to different amounts of coke deposited <33>. In complete agreement with the proposed model, the time dependence is determined, by the amount oi coke deposits In the ire sc specimen... 

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. 

Studied the time evolution of the interfacial tension when polyisobutylene (PIB)-b-PDMS was introduced to PIB/PDMS blend, with the copolymer added to the PIB phase in that study both homopolymers were poly disperse. The time dependence of the interfacial tension was fitted with an expression that allowed the evaluation of the characteristic times of the three components. The characteristic time of the copolymer was the longest, whereas the presence of the additive was found to delay the characteristic times of the blend components from their values in the binary system. The possible complications of slow diffusivities on the attainment of a stationary state of local equilibrium at the interface were thoroughly discussed by Chang et al. [58] within a theoretical model proposed by Morse [279]. Actually, Morse [279] suggested that the optimal system for measuring the equilibrium interfacial tension in the presence of a nearly symmetric diblock copolymer would be one in which the copolymer tracer diffusivity is much higher in the phase to which the copolymer is initially added than in the other phase because of the possibility of a quasi-steady nonequilibrium state in which the interfacial coverage is depleted below its equilibrium value by a continued diffusion into the other phase. 

Fig. 18 Time-dependent change of the diffusion coefficient of poly(2-vinyl pyridine) with the molecular weight 96,000 in gelatin gel when quenched from 65 to 5.7°C [22].

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