Abstract
In membrane applications, large values of permeability and selectivity are generally desired during the whole period of application. The permeability of porous polymer membranes often is reduced by the effect of compaction. Compaction of polymer membranes is a time-dependent process which is strongly determined by the viscoelastic properties of the polymer and its plasticisation caused by the feed medium (e.g., a liquid medium or a process gas in case of porous support structures). In this study, the time-dependent compaction of porous polymer membranes under pressure is modelled. The influence of viscoelastic and diffusion properties of the polymer material on the permeability of the membrane is analysed for different types of membrane morphologies. The life-time of a porous polymer membrane is associated with the time at which the glass transition is achieved in a creep experiment. Equations are derived in order to estimate the maximum life-time of polymer membranes based on compaction. The analysis reveals that the diffusion coefficient, the average retardation time in creep, the magnitude of creep compliance and the time-temperature-pressure shift factor strongly influence compaction of microporous membranes. Generally, a larger tortuosity at constant porosity yields a lower life-time of the membrane. Buckling of cell struts is the dominant failure mechanism in porous membranes with a very high porosity and allows an estimation of life-time.
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