Transport Properties of Polymers

Mass transport occurs just about everywhere. The driving force is Nature striving for equalisation of the chemical potential. This transport plays important roles in a broad range of processes, including transport of species through cell membranes, migration from packaging, diffusion-limited oxidation, transport of water in plants and membrane separation. Polymers are generally poor electric and heat conductors but usually good ‘solute molecule conductors’. In contrast to metals, polymers normally consist of a relatively ‘open’ structure with a lower degree of atomic packing and high free volume, especially in the amorphous phase. The ‘loose’ connectivity of the polymer molecules leads to good electric and heat-insulating properties. At the same time, the large amount of free volume favours diffusion and solubility of gas, vapour or liquid molecules within the polymer matrix. In this chapter, the focus is on mass transport in mainly homogeneous/single component (amorphous/semicrystalline) solid polymers but also includes a small section on thermal transport in these systems. Mass transport in heterogeneous systems such as polymer blends, including interpenetrating networks and composites, is extensively described in the book Transport Properties of Polymeric Membranes by Thomas et al. (2018). It is also a good compilation of membrane processes. Mass transfer in porous systems, involving, for example, Knudsen and capillary flow, is not dealt with here, but is covered in, for example, the review by Tartakovsky and Dentz (2019) and the book by Kargel et al. (2012). For foams, refer to, for example, the papers by Kimball and Frisch (1991) and Pilon et al. (2000). For ion and charge transport, the reader is referred to Aziz et al. (2018), Weber et al. (2014), Geise et al. (2014) and Jaiswal and Menon (2006), and for diffusion coupled with reaction, some of the basic treatments are presented in Crank (1986).