Abstract Using attenuated total‐reflectance infrared spectroscopy (ATR/FTIR), the concentration of deuterated polybutadiene near the surface of a flat zinc selenide crystal was followed as it was replaced by ordinary polybutadiene by flow and diffusion. Experiments were performed in the melt, both below (M ∼ 1,500) and above (M ∼ 15,000) the entanglement threshold. The decay profiles agree well with a finite‐element simulation of the system. In contrast to previous investigations of C 16 's, the decay profiles of both unentangled and entangled polybutadienes are consistent with a uniform diffusivity in the near‐wall region.
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