Sequential IPNs

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

PEMA epoxy Full and semi-IPNs (sequential) prepared 195... 

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 reduction of palladium(II) with an alcoholic solution of NaBH4 [101] or by treatment in situ of the methanol-swollen material under hydrogen [129] yielded a supported palladium catalyst, referred to as self supported by the authors [101,129]. The same co-polymerization reaction was carried out inside the nanopores of a DMF-swollen gel-type resin made by DMA and MBAA (crosslinker, 4% mol) [101,129], thus obtaining a sequential IPN [131]. Also this material was transformed into a... 

Silicone co-polymer networks and IPNs have recently been reviewed.321 The development of IPNs is briefly described, and the definitions of the main (non-exclusive) classes of the IPNs are cited. Examples of latex IPNs, simultaneous and sequential IPNs, semi-IPNs, and thermoplastic IPNs are provided. The use of silicone-silicone IPNs in studies of model silicone networks is also illustrated. Networks in which siloxane and non-siloxane components are connected via chemical bonds are considered co-polymer networks, although some other names have been applied to such networks. Today, some of the examples in this category should, perhaps, be discussed as organic-inorganic hybrids, or nanocomposites. Silicone IPNs are discussed in almost all of the major references dealing with IPNs.322-324 Silicone IPNs are also briefly discussed in some other, previously cited, reviews.291,306... 

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. 

The main path of the research employed both methods of synthesis in turn. At first, the graduate students Yenwo and Pulido explored the use of sequential IPN s based on castor oil urethanes and polystyrene (12-16). At the same time, the graduate student Devia, working in Colombia, explored an alternate synthetic route using latex technology (17). Since nothing was known about the behavior of such materials, their collective objective was to provide a map upon which further efforts could be intelligently based. This effort has now been reviewed (18). 

Figure 4. Stress-strain curves for SIN S containing 40% castor oil elastomer (21). Discontinuous curve adapted from sequential IPN synthesis (1) COPEN (2) COPEUN (3) COPUN (4) 40/60 COPEN/PSN (S) 40/60 COPEUN/PSN (6) 40/60 COPUN/PSN (7) 40/60 COPUN/PSN...

By way of conclusion, the SIN materials were quite different from the original sequential IPN compositions. While the polyester linkage could be used with the sequential mode of synthesis also, its presence in the SIN made for much better impact strengths, probably because of the lower elastomer Tg permitted. The SIN synthesis allowed a greater range of compositions to be made. 

Simultaneous Interpenetrating Networks. An interpenetrating polymer network, IPN, can be defined as a combination of two polymers in network form, at least one of which was polymerized or synthesized in the presence of the other (23). These networks are synthesized sequentially in time. A simultaneous interpenetrating network, SIN, is an IPN in which both networks are synthesized simultaneously in time, or both monomers or prepolymers mixed prior to gelation. The two polymerizations are independent and non-interfering in an SIN, so that grafting or internetwork crosslinking is minimized (23-26). 

The synthesis of an IPN is illustrated in Figure 1, which shows both types of interpenetrating polymer syntheses. First, the reaction for a sequential IPN is shown, where monomer I is polymerized together with crosslinker I to produce a network. Then monomer II and crosslinker II are swollen in and polymerized in a sequential mode to make the IPN. 

Figure 1. The synthesis of sequential IPN above and simultaneous interpenetrating networks, SIN, below. For the synthesis of SIN, two different reactions operate simultaneously such as condensation polymerization and addition polymerization. Reproduced with permission from Ref. 23. Copyright 1981, Plenum Publishing.

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. 

This is a theoretical study on the entanglement architecture and mechanical properties of an ideal two-component interpenetrating polymer network (IPN) composed of flexible chains (Fig. la). In this system molecular interaction between different polymer species is accomplished by the simultaneous or sequential polymerization of the polymeric precursors [1 ]. Chains which are thermodynamically incompatible are permanently interlocked in a composite network due to the presence of chemical crosslinks. The network structure is thus reinforced by chain entanglements trapped between permanent junctions [2,3]. It is evident that, entanglements between identical chains lie further apart in an IPN than in a one-component network (Fig. lb) and entanglements associating heterogeneous polymers are formed in between homopolymer junctions. In the present study the density of the various interchain associations in the composite network is evaluated as a function of the properties of the pure network components. This information is used to estimate the equilibrium rubber elasticity modulus of the IPN. 

Sequential IPN. The preceding analysis does not apply to the case of a sequential IPN. The formation of this system originates with the synthesis of the network (1). Then, network (1) is swollen with monomer (2) which is subsequently polymerized in situ to form a second network. Due to perturbed chain dimensions, the modulus of the first network is higher than the corresponding modulus in the unswollen state by a factor equal to v [ ] ... 

Interpenetrating polymer networks are defined in their broadest sense as an intimate mixture of two or more pol)Mners in network form [1,2]. Ideally, they can be synthesized by either swelling the first crosslinked polymer with the second monomer and crosslinker, followed by in-situ polymerization of the second component (sequential IPN s) or by reacting a pair of monomers and crosslinkers at the same time through different, non-interfering reaction mechanisms, simultaneous interpenetrating networks, SIN s. In fact, many variations of these ideas exist in both the scientific and the patent literature. In any case, at least one of the two components must have a network structure, as an IPN prerequisite. ... 

This review will examine the morphological features of sequential IPN s, starting with transmission electron microscopy (TEM) and modulus, and continuing on with SAXS, and SANS. Dual phase continuity in IPN s will be explored, with emphasis placed on splnodal decomposition in IPN systems. 

Figure 1. Morphology of sequential IPNs. (a) Crois-poly (ethyl acrylate)-m/er-crojs-polystyrene, showing typical cellular structure and a fine structure within the cell walls, (b) Cross-poly (ethyl acrylate)-/ /cr-cross-polystyrene-s/a/-(methyl methacrylate), showing smaller domain structure. PEA structure stained with OsO. (Reproduced from ref. 5. Copyright 1972 American Chemical Society.)...

Figure 2. Morphology of various cross-polybutadiene-in/er-cross-polystyrene sequential IPNs and graft copolymers via transmission electron microscopy. The double bonds in the polybutadiene phase are stained dark with osmium tetroxide. (Reproduced from ref. 15. Copyright 1976 American Chemical Society.)...

A major interest to sequential IPN s relates to dual phase continuity, defined as a region of space where each of two phases maintain some degree of connectivity. An example is an air filter with the air flowing through it. A Maxwell demon can transverse all space within both the filter phase and the air phase, both phases being continuous. 

Figure 12. Radius of poly(dimethyl slloxane) phase as a function of weight fraction In cross-poly(dimethyl slloxane)-Inter-cross-polystyrene sequential IPN s with three different crosslink densities of network I. Broken lines are theoretical values from...

Figure 18 illustrates a model of the three component phase diagram of an IPN, where poljrmer I, poljrmer II, and monomer II are chosen for generality in expressing sequential IPN formation. On poljrmerizatlon of monomer II, first phase separation is initiated, probably by nucleation and growth. However, shortly a modified spinodal decomposition mechanism sets in as the overall composition is driven deeper into the phase separation region. 

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). 

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. 

In poly(AAm-co-BMA)/PAAc IPNs (BMA is butyl methacrylate), the synthetic procedure applied was the sequential polymerization technique shown in Fig. 10. A monomer or set of comonomers is polymerized into a polymer gel. A second monomer is introduced and subsequently polymerized and crosslinked within the initial gel (AAm-co-BMA) matrix to form the IPN. 

Physical hybrids containing silica and polymer are typically interpenetrating networks (IPNs). They can be subdivided into simultaneous or sequential IPNs. The terminology of... 

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