Sequential semi-interpenetrating polymer network

Note 2 Semi-interpenetrating polymer networks may be further described by the process by which they are synthesized. When an SIPN is prepared by a process in which the second component polymer is formed or incorporated following the completion of formation of the first component polymer, the SIPN may be referred to as a sequential SIPN. When an SIPN is prepared by a process in which both component polymers are formed concurrently, the SIPN may be referred to as a simultaneous SIPN. (This note has been changed from that which appears in ref [4] to allow for the possibility that a linear or branched polymer may be incorporated into a network by means other than polymerization, e.g., by swelling of the network and subsequent diffusion of the linear or branched chain into the network.). 

There are at least four general types of combinations of crosslinked (x) and linear (1) polymers in a two-component system both components crosslinked (xx), one or the other component crosslinked (lx or xl), and both components linear (11). Where at least one of the components has been polymerized in the presence of the other, the xx forms have often been called interpenetrating polymer networks (IPN), the lx and the xl forms termed "semi-IPNs", and the last, linear or in situ blends. There are also a number of ways in which the components can be formed and assembled into a multicomponent system. Sequential IPNs are prepared by swelling one network polymer with the precursors of the second and polymerizing. Simultaneous IPNs are formed from a mixture of the precursors of both components polymerization to form each component by independent reactions is carried out in the presence of the other precursors or products. Usually, the simultaneous IPNs that have been reported are extremes in the component formation sequence the first component is formed before the second polymerization is begun. Sequential IPNs and simultaneous IPNs of the same composition do not necessarily have the same morphology and properties. 

Interpenetrating polymer networks (IPNs) are a combination of two or more polymers in networks where a partial interlacing on the molecular scale is present in the matrix. There is no covalent bonding and therefore the polymers cannot be separated unless the chemical bonds are broken (Fig. 1.5). Thus the mixtures of two prepared polymers, networked without any interlacing, are not generally considered to be IPNs, although many researchers have treated them as such. IPNs are of different types, viz. sequential, simultaneous and semi IPNs. As their names suggest, sequential... 

An interpenetrating polymer network (IPN) is defined as a combination of two crosslinked polymers, at least one of which has been synthesised [98] and/or crosslinked in the immediate presence of the other. From the topological point of view, IPNs are closely related to pol)nner blends and to block, graft and crosslinked copolymers. From the synthesis point of view, IPNs can be classified, broadly, into two general types (a) sequential IPNs where a polymer network is formed which is then swollen by the monomer, plus a crosslinking agent and an activator, which is then polymerised in situ to form the second network and (b) simultaneous IPNs (SIPN) where the components necessary to form both networks are mixed and polymerised, at the same time, by non-competing mechanisms. If one of the two polymers is linear (uncrosslinked), a semi-IPN results. A homo-IPN results if both the network polymers are identical in chemical composition [98]. 

Commercial interpenetrating polymer networks (not including thermoplastic compositions) include artificial teeth (Dentsply) made Ifom crosslinked PMMA mixed with MMA monomer and polymerized (sequential IPN), sound and vibration damping compositions (e.g., vinyl-phenolic Hitachi) and sheet molding compositions (acryhc/urethane/polystyrene Ferro Chemical) [164]. Water-borne acrylic methane semi-IPNs are commercial Ifom several sources where acrylate monomers are polymerized in the presence of a polymethane water dispersion. A silicone/polytetrafluoroethylene composition described as an IPN is offered by Biomed Sciences under the Silon tradename. The fluoropolymer provides the mechanical strength and the silicone rubber offers the softness and oxygen and moisture permeability for applications in the wound care area. 

Simultaneous and sequential IPNs based on various polymeric systems have been prepared using polydimethylsiloxane (PDMS) as the host network (3-8). These systems include poly(ether-urethane), polystyrene, poly(2,6-dimethyl-1,4-phenyleneoxide), polyacrylic acid, PDMS, polymethylmethacrylate, polyethylene oxide (PEO)... as the guest network. Some semi-interpenetrating networks (s-IPNs) based either on a linear polymer embedded in a polysiloxane network (5,9,10) or on a linear polysiloxane combined with a PEO network (8) have also been described. In some cases, PDMS has been replaced by polyaromatic siloxanes such as polydiphenyl or polymethylphenylsiloxanes (10-12). The focus of this paper concerns the preparation and properties of IPNs and s-IPNs based on polysiloxanes and poly(diethyleneglycol bis-allylcarbonate) (13,14). 

---