(417b) Negligible Aging in Poly(ether-ether-ketone) Membranes Widens Application Range for Solvent Processing

Negligible
aging in poly(ether-ether-ketone) membranes widens
application range for solvent processing

João da Silva Burgal, Ludmila Peeva, Andrew Livingston*

Department of Chemical Engineering
and Chemical Technology, Imperial College, Exhibition Road, London SW7 2AZ, UK

E-mail: a.livingston@imperial.ac.uk

Presenting author: Ludmila Peeva (l.peeva@imperial.ac.uk)

Keywords: ageing, organic solvent nanofiltration, high-temperature operation

Conventional molecular separation processes such as
evaporation and distillation exert substantial energy demands due to the latent
heat of vaporization of liquids. Membrane technology has lower energy
consumption than conventional separation processes, requiring only one-tenth of
the energy to process an equivalent volume of liquid. In spite of this
advantage, membranes have yet to make a significant impact in industrial
organic solvent based systems, due to unresolved issues including their
stability over time. Most commercially available nanofiltration
and reverse osmosis membranes are either integrally skinned asymmetric (ISA)
membranes, or thin film composite (TFC) membranes. Both types are made from
glassy polymeric materials which age over time leading to adverse changes in
membrane performance; many OSN membranes display more than 30 % permeance decline over the first few days of operation.

This ageing phenomenon is caused by re-arrangement of the
polymer chains in the membranes from the non-equilibrium excess state created
during the membrane formation process, towards a stabilized equilibrium state,
in a time-dependent manner via structural relaxation as represented in Figure 1. Usually, ageing is manifested
through a gradual decrease in membrane volume (densification), which leads to
increased brittleness, decreased permeability, decreased enthalpy, and
alterations in other properties. For example, the permeance
of polyimide ISA Organic Solvent Nanofiltration (OSN)
membranes created by phase inversion decreases from around 1-2 L m^-2
h^-1 bar^-1 to zero after annealing at 100 °C-200 °C. This
is consistent with the negligible intrinsic solvent permeance
of solvent-cast polyimide films, in which polymer chains reach their highest
density.

Organic solvent nanofiltration membranes
have been prepared from poly(ether-ether-ketone)
(PEEK) by phase inversion. Remarkably, these membranes undergo negligible
ageing, even under extreme conditions of high temperature air annealing, and
high temperature solute filtration with agressive
solvents. This negligible ageing of PEEK membranes is contrasted with substantial
ageing of crosslinked polybenzimidazole
(PBI) and polyimide (PI) membranes. After air annealing at 120 °C, PBI and PI
membranes become brittle and lose all permeance,
whereas PEEK membranes remain flexible and retain a constant permeance of ~ 0.2 L.h^-1.m^-2.bar^-1
for tetrahydrofuran (THF). The structural
change in PBI and PI membranes is attributed to polymer transition from a
non-equilibrium glassy state towards an equilibrium state at which chain
packing precludes permeation of solvent. In contrast, fabrication of PEEK
membranes results in polymer in a quasi-equilibrium glassy state with intrinsic
permeance, which is not adversely affected by accelerated-ageing
environments. High temperature filtrations in DMF up to 140 °C for the three
polymeric membranes showed PEEK performance to be stable and consistent with
negligible ageing, whereas PBI and PI rapidly loose stability.

Figure  SEQ Figure \* ARABIC 1 – a) Schematic of the effect
of time and temperature on the polymer chains of ISA membranes. PEEK polymer
chains show no signs of ageing densification, in contrast to PI and PBI which
undergo ageing. b) Qualitative volume vs. temperature diagram for a
glass-forming polymer depicting the equilibrium rubbery (1) and equilibrium
glassy (4) states as well as the quasi (3) and non-equilibrium glassy (2)
states.