Liquid−Liquid Limited-Supply Diffusion Studies in the Polystyrene−Poly(vinyl methyl ether) Pair

Abstract

Liquid−liquid diffusion at the interphase between poly(vinyl−methyl ether) (PVME) and polystyrene (PS) was experimentally studied using confocal Raman microspectroscopy. A combination of a specially designed experimental setup and a direct and precise quantification for the corrections to be applied to the Raman measurements allowed us to measure directly the PVME concentration along the diffusion path for a wide range of diffusion times. An already proposed and tested liquid−liquid diffusion model (based on liquid dynamics controlled by monomeric friction coefficients) was used to correlate and predict the detailed shape of the PVME concentration profiles and the diffusion rates as functions of diffusion time and temperature. The results obtained allowed us to discern among several approaches previously proposed in the literature to calculate monomeric friction coefficients in this system. Only the approach that considers independent monomeric friction coefficient values for PS and PVME (obtained from tracer diffusion measurements) gave good agreement between experimental results and model calculations. Calculations performed using literature data for a common monomeric friction coefficient for both PS and PVME (obtained from estimated blend viscosity data) do not agree with experimental measurements. The success of the model used for this work clearly ruled out the need for combinations of Fickean and Case II models used previously to describe PS−PVME polymer diffusion.

Diffusion Kinetics at Liquid‐Glassy Polymer Interphases

“…This case can be thought of as diffusion between a low‐viscosity liquid and a highly viscous matrix. The asymmetry observed in the diffusion profiles can be quantitatively explained in terms of the dissimilar mobility between the components of the polymer pair, as discussed in previous work 4,12. In the experiments conducted at diffusion temperatures below the matrix T g , the matrix remains glassy during the whole process.…”

“…c) All the data scale remarkably well with t 1/2 , a typical signature of Fickean diffusion. The slight downward curvature observed in all the experiments at very long diffusion times in Figure 3 is distinctive of liquid‐liquid polymer diffusion between components with different T g and can be quantitatively explained in terms of the changes in the diffusion coefficients as the diffusion process evolves 4,12…”

“…In the depth‐profiling mode, the spatial resolution decreases as we focus the laser beam deeper into the sample; for this reason, the broadening is more pronounced in those interphases located at higher depths. This problem has been thoroughly analyzed and quantified in previous work 12,13. We have shown that the actual transition between the low‐ and high‐ T g layers is abrupt and occurs in a narrow range on the depth scale (1–2 μm), as determined by using a focusing mode with invariant and superior instrumental resolution (surface profiling, 1.5 μm resolution) 13.…”

“…They are characterized by a fairly flat region along the outer first 50–75 μm, where the PS concentration is uniform. Note that because of the limited PS supply, the concentration of liquid polymer throughout this region decreases with diffusion time 12. The profile slope then becomes increasingly higher and the PS concentration rapidly decreases, until the depths of the pure matrix are reached.…”

“…Antioxidant (200 ppm, Santonox, Ciba‐Geigy) was added to the blends to prevent degradation. Polymer bi‐layers, in the form of cylindrical specimens (20 mm diameter), were prepared by sequential vacuum molding of a thin low‐ T g layer (40–150 μm thick) on top of a thicker high‐ T g layer (500 μm thick), as explained elsewhere 12. For details about the compositions of the layers see Table 1.…”