Diffusion in Polymer Alloy Melts

Abstract

Diffusion in polymer alloys or blends can be used to extract information on the fundamentals of the dynamics of individual polymer chains in the melt and the thermodynamics of the interaction between unlike polymer species. The dynamics of individual chains are available from measurements of the tracer diffusion coefficients, D*, of the various species while the thermodynamics of interaction, represented by the Flory parameter, x, can be obtained from measurements of the mutual diffusion or interdiffusion coefficient, D. We will show that these quantities can be measured conveniently by forward recoil spectrometry (FRES), an ion beam analysis technique that can determine the concentration versus depth profile of polymers labeled with deuterium diffusing into unlabeled polymer matrices.

For high enough molecular weight of the matrix, the tracer diffusion coefficient of both species in the blend scale as D₀N⁻², where N is the number of monomer segments per diffusing chain; the constant D₀, however, can differ by more than 10⁴ for chemically different molecules diffusing in the same blend, suggesting that conventional concepts of chain dynamics in melts, such as monomer friction coefficients, need to be reexamined. The mutual diffusion coefficient is controlled by the faster species in the blend (the one with the larger D*N product) in agreement with what was found in metallic alloys (but in sharp disagreement with the “slow” theory of mutual diffusion which predicts that the slower species controls). Since the combinatorial (ideal) entropy of mixing of polymers is low, the thermodynamic driving force for diffusion is dominated by enthalpy and excess entropy of mixing (x) to a degree unprecedented for atomic or small molecule systems. This means that one can observe not only a thermodynamic “slowing down” of diffusion when x becomes positive as one nears the spinodal but also a large thermodynamic “speeding up” of diffusion when x is negative. Measurements of mutual diffusion turn out to be one of the most sensitive methods available for measuring x.