Sequential IPNs, preparation

An interesting result with respect to applications obtained with the IPN hydrogels is that these are two- phase systems (two glass transition temperatures), with the hydrophilic domains behaving essentially like the pure hydrophilic component.6,7,9 Thus, the two basic functions of these IPN hydrogels with respect to applications, namely hydrophilicity and mechanical stability, are separately taken over by the two IPN components, the hydrophilic and hydrophobic domains, respectively. Figure 1 shows TSDC and DMA results for the water content dependence of the a relaxation (dynamic glass transition) of PHEA in sequential IPNS prepared from PHEA and poly(ethyl methacrylate) (PEMA) as the hydrophobic component.9 In these IPNs a porous PEMA network was prepared first, and PHEA was then polymerized in the pores. In addition to the... 

PDMS PMAA Sequential IPN prepared with crosslinked PDMS swollen with MAA + crosslinkers followed by UV induced polymerization 190... 

The first type, termed sequential IPN s, involves the preparation of a crosslinked polymer I, a subsequent swelling of monomer II components and polymerization of the monomer II in situ. The second type of synthesis yields materials known as simultaneous interpenetrating networks (SIN s), involves the mixing of all components in an early stage, followed by the formation of both networks via independent reactions proceeding in the same container (10,11). One network can be formed by a chain growth mechanism and the other by a step growth mechanism. 

Note 2 An IPN may be further described by the process by which it is synthesized. When an IPN is prepared by a process in which the second component network is formed following the completion of formation of the first component network, the IPN may be referred to as a sequential IPN. When an IPN is prepared by a process in which both component networks are formed concurrently, the IPN may be referred to as a simultaneous IPN. 

Sequential IPNs were prepared from the crosslinked PVME sheets by swelling in a mixture of styrene, DVB and benzoin. A typical recipe was lOOg styrene, 2.26g DVB and 0.41g benzoin. After swelling to equilibrium, the swollen mass was sandwiched between Mylar film and glass plates. 

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. 

Figure 6.1. Two synthetic methods for preparing interpenetrating polymer networks. A, sequential IPNs, and B, simultaneous interpenetrating networks, SlNs.

There are several interesting polymerization schemes intermediate between a sequential IPN and an SIN. For example, in in situ prepared sequential IPNs, both monomers are polymerized via free radical reaction [He et ai, 1993 Rouf et ai, 1994]. The two monomers must have quite different reactivities towards the free radicals. This situation arises with vinyl or acrylic double bonds and aUylic double bonds. The allylic double bonds react about 100 times slower than acrylic or methacrylic bonds. Often, two initiators are used, one reacting at a lower temperature, and the other at a higher temperature. In one of the systems studied, based on methyl methacrylate and diallyl carbonate of bisphenol-A (DACBA), first, crosslinked PMMA was formed at moderate temperatures. Then, by just increasing the temperature after completion of the first polymerization, the synthesis of the allylic network followed. 

Polymer comprising two or more polymer networks which are at least partially interlaced on a molecular scale (Figure 1.16) but not covalently bonded to each other and cannot be separated rmless chemical bonds are broken [206,411]. A mixture of two or more preformed polymer networks is not an IPN. An IPN may be further described by the process by which it is synthesized e. g. when an IPN is prepared by a process in which the second component network is polymerized following the completion of polymerization of the first component network, the IPN may be referred to as a sequential IPN. In contrast, a process in which both component networks are polymerized concurrently, the IPN may be referred to as a simultaneous IPN. An IPN is distinguished from other multipolymer combinations, such as polymer blends, blocks, and grafts, in two ways (1) an IPN swells, but does not dissolve in solvents and (2) creep and flow are suppressed. 

Lipatov et prepared a series of sequential IPNs, sometimes... 

Nearly all of the materials described in this monograph were prepared by some combination of free-radical and/or condensation reactions. Most recently, however, Lipatova et obtained grafted sequential IPNs on the basis of matrices from living network polymers, using anionic polymerizations for both networks I and II. 

Lipatov and co-workers synthesized sequential IPNs containing a reinforcing silica filler as follows Polyurethane samples based on oligo-ethylene glycol adipate and tolylene diisocyanate were prepared with various levels of Aerosil, a silica filler. Trimethylolpropane was used as the... 

Chemically blended thermoplastic IPNs were made by Siegfried et Compositions included the use of SEES triblock copolymer as polymer I and poly(styrene-co-methacrylic acid) (10% MMA) as polymer II. These materials were made with a swelling process similar to that employed in the sequential IPNs. The carboxylic acid groups were neutralized with sodium hydroxide solutions while the materials were in a Brabender. Siegfried et al. also prepared the corresponding mechanically blended thermoplastic IPNs. 

Fig. 2. Two basic synthetic methods of preparing IPNs, sequentially and simultaneously, (a) Sequential IPNs (b) simultaneous interpenetrating network (SIN).

Phase Continuity in Sequentiai iPNs. The question has been repeatedly raised Do IPN morphologies really exhibit dual phase continuity Evidence for dual phase continuity in sequential IPNs was examined by Widmaier and Sperling (40,41). A series of sequential IPNs were prepared from poly(ra-butyl acrylate) and polystyrene. Two cross-linkers were used, divinyl benzene (DVB), which forms ordinary covalent cross-links, and aciyUc anhydride (AA), which forms labile crosslinks. The AA cross-links were cut by soaking the samples in a 10% aqueous ammonium hydroxide solution for about 12 hours. 

Salmeron S chez and co-workers (115) prepared sequential IPNs based on reef-poly(ethyl acrylate)-fpre- reef-poly(hydroxyethyl acrylate). These two polymers differ primarily in the presence or absence of a hydroxyl group. Investigation via differential scanning calorimetry indicated three qualitatively different kinds of thermograms depending on the initial quantity of water present. The water present is primarily associated with the poly(hydroxyethyl acrylate), but the... 

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). 

Figure 5 The preparation of a sequential IPN based on oligomers and-/or monomers as the two different building blocks of the two polymeric network...

Sequential and m-situ sequential full IPNs with different weight composition (PCN/CPU = 100/0, 75/25, 50/50, 25/75) were prepared. Sequential IPNs were prepared by swelling of a film of preliminarily synthesized CPU by CPE followed by thermal polycyclotrimerization of the latter inside CPU. On the contrary, the m-sUu method (some kind of simultaneous one) consisted of mixing all the monomers together, then synthesis of CPU in the presence of CPE monomer at low temperature and next synthesis of PCN by polycyclotrimerization of CPE inside CPU at higher temperature. 

PVOH Poly(N- isopropyl acrylamide) Samples prepared by sequential IPN procedure hydrogel properties determined 189... 

PEA PHEA Sequential IPNs crosslinked PEA prepared then swollen with HEA + crosslinker water sorption characterized 194... 

Most recently, Hourston and McCluskey prepared a series of sequential IPN s based on polyurethane and poly(methyl acrylate), varying the crosslink level. The IPN s showed high values of the Oberst damping factor independent of the crosslink level of the second network. 

Dual-phase continuity, which is defined as the continuity of both polymer I and polymer II domains throughout the macroscopic sample, seems to be better described in terms of spinodal decomposition. In [170] an example of dual-phase continuity was given. PnBA/PS sequential IPNs of various compositions were prepared by UV photopolymerization. PnBA served as network I and PS as network II. Acrylic anhydride (AA) and DVB were used as labile and permanent cross-linkers. After IPN formation, the AA-containing network I was de-cross-linked and solvent extracted. Scanning electron microscopy (SEM) of network II revealed a porous but continuous structure formed by aggregates of fused spherical PS domains. It was shown that network I was continuous, since it could be quantitatively and easily extracted. The major conclusion from this paper relates to the dual-phase continuity of PnBA/PS in sequential IPNs. 

Sequential IPNs based on PMA and PMMA were prepared by UV photopolymerization [301]. The PMA/PMMA system is immiscible and so, for... 

Hyperbranched polyurethanes are constmcted using phenol-blocked trifunctional monomers in combination with 4-methylbenzyl alcohol for end capping (11). Polyurethane interpenetrating polymer networks (IPNs) are mixtures of two cross-linked polymer networks, prepared by latex blending, sequential polymerization, or simultaneous polymerization. IPNs have improved mechanical properties, as weU as thermal stabiHties, compared to the single cross-linked polymers. In pseudo-IPNs, only one of the involved polymers is cross-linked. Numerous polymers are involved in the formation of polyurethane-derived IPNs (12). 

The temperature-sensitive poly(A-isopropyl acrylamide) and pH-sensitive poly(methacrylic acid) were used as the two component networks in the IPN system. Since both A-isopropyl acrylamide (NIPAAm) (Fisher Scientific, Pittsburgh, PA) and methacrylic acid (MAA) (Aldrich, Milwaukee, Wl) react by the same polymerization mechanism, a sequential method was used to avoid the formation of a PNIPAAm/PMAA copolymer. A UV-initiated solution-polymerization technique offered a quick and convenient way to achieve the interpenetration of the networks. Polymer network I was prepared and purified before polymer network II was synthesized in the presence of network I. Figure I shows the typical IPN structure. 

IPNs can be prepared by either the "sequential" or the "simultaneous" technique. IPNs synthesized to date exhibit varying degrees of phase separation depending primarily on the compatibility of the component polymers (4-7). 

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