Thermodynamics of water sorption in high performance glassy thermoplastic polymers.

Giuseppe Scherillo,Michele Galizia,Giuseppe Mensitieri,Pellegrino Musto,Mauro Petretta,Pietro La Manna

doi:10.3389/fchem.2014.00025

Abstract

Sorption thermodynamics of water in two glassy polymers, polyetherimide (PEI) and polyetheretherketone (PEEK), is investigated by coupling gravimetry and on line FTIR spectroscopy in order to gather information on the total amount of sorbed water as well as on the different species of water molecules absorbed within the polymers, addressing the issue of cross- and self-interactions occurring in the polymer/water systems. Water sorption isotherms have been determined at temperatures ranging from 30 to 70°C while FTIR spectroscopy has been performed only at 30°C. The experimental analysis provided information on the groups present on the polymer backbones involved in hydrogen bonding interactions with absorbed water molecules. Moreover, it also supplied qualitative indications about the different “populations” of water molecules present within the PEEK and a quantitative assessment of these “populations” in the case of PEI. The results of the experimental analysis have been interpreted using an equation of state theory based on a compressible lattice fluid model for the Gibbs energy of the polymer-water mixture, developed by extending to the case of out of equilibrium glassy polymers a previous model intended for equilibrium rubbery polymers. The model accounts for the non-equilibrium nature of glassy polymers as well as for mean field and for hydrogen bonding interactions, providing a satisfactory quantitative interpretation of the experimental data.

Highlights

Hygrothermal aging of polymer matrices is related to sorption of water molecules within the material (Mensitieri and Iannone, 2008) which promotes plasticization of the polymer depressing its glass transition temperature (Tg) (Mensitieri and Iannone, 2008) and, in some cases, hydrolytic degradation
In the following we report the expression of chemical potential of low molecular weight compounds, provided by Non-Random Hydrogen Bonding (NRHB) theory, that is used to describe the phase equilibrium between a rubbery polymer-penetrant mixture and pure low m.w. compound in the form of vapor
These interactions likely consist in H-bonds between H atoms of water molecules and ether and carbonyl groups on the PEEK repeating units, that act as proton acceptors

Summary

Introduction

Hygrothermal aging of polymer matrices is related to sorption of water molecules within the material (Mensitieri and Iannone, 2008) which promotes plasticization of the polymer depressing its glass transition temperature (Tg) (Mensitieri and Iannone, 2008) and, in some cases, hydrolytic degradation. The amount of water absorbed at equilibrium heavily depends upon the chemical structure and morphology of the polymer The understanding of this phenomenon is a crucial task for the assessment of long term durability of a polymer based composite material and for the understanding of possible effects like matrix cracking, microvoid generation, outer-ply delamination or surface blistering. This information is of importance when evaluating the suitability of a polymer matrix for separation of mixtures of low molecular weight (m.w.) substances, as a barrier to low m.w. compounds or for other technological applications. Construction and validation of suitable water sorption thermodynamic models against these experimental data supply an important tool for interpretation and quantification of the behavior of a polymer matrix exposed to a humid environment and, in turn, to predict possible plasticization effects (Musto et al, 2014)

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Conclusion