I suggest reading the book INTERPENETRATING POLYMER NETWORKS
AND RELATED MATERIALS by Sperling. An interpenetrating network
consists of two polymer phases, possibly of quite different
chemistries, that form two continuous phases. These materials
are especially recommended for RO membranes because a
single-phase material must have a loose structure to allow
passage of a significant amount of liquid through it.
But, a loose structure will make it mechanically weak,
which is bad because you want to have a pressure head to
drive the liquid through the membrane.
By using a two-phase system, you can have one phase which
is very permeable and a second phase which is mechanically
strong. This illustrates an important principle in polymer
design: don't ask one material to perform two mutually
opposed jobs well.
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