Multicomponent Diffusion as Studied by NMRI

Figore 6.9 Solvent front movement of (a) acetone into PC for the 7S 25,65 35, and 50 SO v/o AC/MeOH-d mixtures and (b) methimol into PC for the 75 25,65 35, and 50 50 v/o AC-dg MeOD mixtures versus the square root of time. Reprinted with permission from [26]. Copyright 1992 American Chemical Society 

We were interested in determining whether the two solvent fronts move at the same rates for a particular acetone to methanol ratio. Comparison of the rates of the solvent front movements of the 65 35 AC MeOH-d4 and the 65 35 AC-d MeOD mixtures revealed that they were equal within experimental error. There is an overlapping of the front movement when plotted as a function of square root of time. This indicates that acetone and methanol are diffusing jointly through the PC rod in a Fickian manner and do not appear to separate. Similar 

A similar effect can be seen for the cyclic diffusion of methanol into water-soaked PMMA with water contents of 0.6 and 0.75 wt%. The percentage weight gain versus time of diffusion is linear with time except for an initial lag time, which is anomalous. The lag time may be attributed to methanol desorbing a thin layer of water from the PMMA, which causes the layer to be in tension from the shrinkage back to the unswollen state. This descreases the mobility of the polymer in this layer and makes the polymer less accommodating to methanol diffusion [27]. 

For polymers which are in the glassy state, the removal of diluents or small molecules may cause an increase in sorption and mass transport properties of the polymer [32]. It has been found that the removal of liquids from a polymer below its Tf can increase the porosity within the polymer [32]. This would therefore result in an increase in the rate of diffusion of dUuents such as water and MeOH into PMMA. It should be noted that there was no reduction in dry weight with each successive absorption-desorption cycle of water and MeOH, which would 

It has been shown in previous experiments [32] that, when PMMA is exposed to solvents such as MeOH, unreacted monomer present within the material will leach out. It is expected that, after drying the polymer to remove the solvent that has diffused into PMMA, the zero weight of the sample should decrease. However, the zero weight of the samples analyzed in this experiment remained the same. This indicates that not all of the methanol that diffiis into the PMMA is removed during the drying periods between the absorption cycles. 

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