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.

Interpenetrating polymer network (IPN): A polymer comprising two
or more networks which are at least partially interlaced on a molecular scale
but not covalently bonded to each other and cannot be separated unless chemical
bonds are broken.

Note: A mixture of two or more pre-formed polymer networks is not an IPN.

Semi-interpenetrating polymer network (SIPN): A polymer comprising one or
more networks and one or more linear or branched polymer(s) characterized by the
penetration on a molecular scale of at least one of the networks by at least some
of the linear or branched macromolecules.

Note: Semi-interpenetrating polymer networks are distinguished from
interpenetrating polymer networks because the constituent linear or branched
polymers can, in principle, be separated from the constituent polymer network(s)
without breaking chemical bonds; they are polymer blends.

Sequential interpenetrating polymer network: Interpenetrating polymer network
prepared by a process in which the second component network is formed
following the formation of the first component network.

Sequential semi-interpenetrating polymer network: Semi-interpenetrating
polymer network prepared by a process in which the linear or branched
components are formed following the completion of the reactions that lead to
the formation of the network(s) or vice versa.

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).