(560f) The Influence of Tie-Molecules and Microstructure on the Fluid Solubility in Semi-Crystalline Polymers

Predicting the absorption of gases and liquids in semi-crystalline polymers is of critical importance for a vast number of applications, as their mechanical and transport properties can be modified by the presence of solutes dissolved in their bulk. The observed mass uptake of these materials in equilibrium with an external fluid is lower than what is predicted by treating their amorphous domains as polymer mixtures at the same temperature and pressure of the fluid. In recent literature, this observation has led to the hypothesis that the amorphous domains are subject to an additional “constraint pressure” which effectively reduces their uptake at equilibrium [1,2].

In this work a new statistical mechanics’ model of semi-crystalline polymers is presented. The theory lies on the assumption that the inter-lamellar amorphous domains are in a pseudo-equilibrium state that determines the tension of the tie-molecules as a function of other thermodynamic variables, an idea originally put forward in 1965 [3]. Constraint pressure emerges naturally from this treatment due to the entropic forces acting on the network of tie-molecules, and the variation of the lamellar thickness with temperature – a phenomenon known as premelting – is predicted.

The solubility of a range of compounds in different samples of polyethylene and polypropylene is then calculated within the new framework with the help of the SAFT- Mie equation of state. Comparison with experimental data suggests that to accurately predict absorption close to the vapour pressure of the penetrant it is essential to include “free”, unconstrained amorphous domains [4] in the description (Figure 1), resulting in a multi-scale model with two adjustable parameters that characterize the inter-lamellar topology and the microstructure of each semi-crystalline polymer sample.

References

[1] P. Memari, V. Lachet and B. Rousseau, Polymer, 51(21), 4978-4984 (2010)

[2] M. Minelli and M. G. De Angelis, Fluid Phase Equilibria, 367, 173-181 (2014)

[3] A. S. Michaels and R. W. Hausslein, Journal of Polymer Science, 10, 61-86 (1965)

[4] J. Chmelař, R. Pokorný, P. Schneider, K. Smolná, P. Bělský and J. Kosek, Polymer, 58, 189-198 (2015)