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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.
Simply mixing two or more polymers does not create an interpenetrating polymer network (polymer blend), nor does creating a polymer network out of more than one kind of monomers which are bonded to each other to form one network (heteropolymer or copolymer).