An Interpenetrating polymer network (IPN) is a polymer comprising
two or more networks which are at least partially interlaced on a
polymer scale but not covalently bonded to each other. The network
cannot be separated unless chemical bonds are broken. The two or
more networks can be envisioned to be entangled in such a way that
they are concatenated and cannot be pulled apart, but not bonded to
each other by any chemical bond.