Membranes from Glassy Polymers Physical Aging

Membranes from Classy Polymers Physical Aging 

There is a region of thin films with thicknesses between the two previously described extreme limits, ranging from 100nm to several micrometers, where volume relaxation processes - and, hence, the change in gas-permeability properties with time - are much more rapid than that expected based on observations of bulk specimens as shown below. 

The experimental results described above show that the gas-permeability properties of thin glassy polymer films (submicrometer in thickness) are more time- or history-dependent than much thicker films (the bulk state for example, 50 pm or thicker) seem to be. This is manifested in terms of physical aging over a period of 1 year and more. The observed permeability values for the current thin films are all initially greater than the reported bulk values but approach or become less than these values after a few days or weeks, depending on the thickness. After a year, the thin films may be as much as four times less permeable than the reported bulk values. Selectivity increases with aging time, as might be expected from a densification process. 

These observations have several practical consequences for membrane processes where the selective layers are as thin as or even thinner than the low end of the range studied here. First, it is clear that use of thick film data to design or select membrane materials only gives a rough approximation of the performance that might be realized in practice. Second, because the absolute permeability of a thin film may be severalfold different than the bulk permeability, use of the latter type of data to estimate skin thickness from flux observations on asymmetric or composite membranes structures is also a very approximate method. Finally, these data indicate that one could expect 

Membranes from Rubbery Polymers Enhanced C02 Selectivity 

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Membranes from Glassy Polymers Physical Aging 69... 

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