Sequential IPNs networks

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 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.

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. ... 

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...

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. 

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

There are two principal approaches to forming sequential IPNs (1) form the first network, swell it with the second monomer, crosslinker and catalyst and then form the second network (2) blend the two monomers, crosslinkers and catalysts together and then crosslink them. Two different initiation processes (e.g. different temperatures) can be used in what is called in-situ sequential synthesis. Finally, an alternative consists in blending the monomers and then adding the catalysts and/or the crosslinkers sequentially. 

IPNs are also attractive for development of materials with enhanced mechanical properties. As PDMS acts as an elastomer, it is of interest to have a thermoplastic second network such as PMMA or polystyrene. Crosslinked PDMS have poor mechanical properties and need to be reinforced with silica. In the IPNs field, they can advantageously be replaced by a second thermoplastic network. On the other hand, if the thermoplastic network is the major component, the PDMS network can confer a partially elastomeric character to the resulting material. Huang et al. [92] studied some sequential IPNs of PDMS and polymethacrylate and varied the ester functionalities the polysiloxane network was swollen with MMA (methyl methacrylate), EMA (ethyl methacrylate) or BuMA (butyl methacrylate). Using DMA the authors determined that the more sterically hindered the substituent, the broader the damping zone of the IPN (Table 2). This damping zone broadness was also found to be dependant on the PDMS content, and atomic force microscopy (AFM) was used to observe the co-continuity of the IPN. 

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... 

There are two general methods of synthesizing IPN s, see Figure 1. For sequential IPN s, polymer network I is synthesized, and monomer II plus crosslinker and activator is swelled in and polymerized. For simultaneous interpenetrating polymer networks, SIN s, both monomers and their respective crosslinkers and activators are mixed together and polymerized, usually by separate and non-interfering kinetic methods such as stepwise and chain polymerizations. Of course, there are many... 

Since this paper will be restricted to sequential IPN s based on cross-poly butadiene-inter-cross-polystyrene. PB/PS, it is valuable to examine the range of possible compositions, see Figure 2 ( ). The PB/PS IPN polymer pair models high-impact polystyrene, and in fact, many of the combinations made are actually more impact resistant than the commercial materials. In general, with the addition of crosslinks, especially in network I, the phase domains become smaller. The impact resistance of high-impact polystyrene, upper left, is about 80 J/ra. In the same experiment, the semi-I IPN, middle left is about 160 J/m, and the full IPN, lower left, is about 265 J/m (g). Since the commercial material had perhaps dozens of man-years of development, and the IPN composition was made simply for doctoral research with substantially no optimization, it was obvious that these materials warranted further study. 

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. 

Sequential IPN. First, polymer network I is synthesized. Then, monomer II plus crosslinker and activator are swollen into network I and polymerized in situ, see Figure 6.1 A. 

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

Where a step polymerization is used, almost always it is for the first polymer synthesized in a sequential IPN. The reasons involve the slow diffusion into a pre-existing network of most monomers used in step polymerization, and the relatively high glass transition of step polymerized polymers. The latter reason is important because in order for diffusion and concomitant polymerization to occur rapidly, polymer network 1 should be above its glass transition at the temperature of polymerization of monomer mix 11. Table 6.2 presents glycerol as a simple trifunctional crosshnker for step polymerized materials, suitable for polyesters and polyurethanes. 

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. 

The domain size of sequential IPNs (such as shown in Figure 6.2) is controlled by several features the interfacial tension coefficient between the two polymers, y, the volume fraction of polymers 1 and 2, v and v, respectively, effective network concentration of the two polymers and Vj respectively, and the gas constant times the absolute temperature, RT. 

For sin s, monomer II or (prepolymer II) is added before step (b). Thus to a greater or lesser extent, the two networks are formed simultaneously. Network I chains are stretched and diluted by network II in a sequential IPN, but only diluted in an SIN, altering many morphological and physical properties. Of course it is required that the two polymerizations be non-interfering reactions, such as by stepwise and chain kinetics. 

The variation of D2 and ( 2 is illustrated in Fig. 4 for a sequential IPN and a semi-1 IPN (only polymer I crosslinked). Typical molecular parameters were assumed. In the range of 0.1 < ( )2 < 0.7, it is interesting to note that the change in D2 with < >2 is modest. Domain diameters in the range of 700 to 1000 A are predicted. Such values are commonly seen, experimentally. Equation (1) also predicts a decrease in domain size with increasing crosslink density. While both networks exert important forces. 

Let us consider polymer structure for a moment and ask In how many distinct ways can two kinds of network chains be put together so that they interpenetrate The original sequential IPN s and the lEN s have... 

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. 

Because of the special swelling and mutual dilution effects encountered in sequential IPN s, special equations were derived for their rubbery modulus and equilibrium swelling. The new equations were used to analyze polystyrene/polystyrene homo-IPN swelling and rubbery modulus data obtained by four different laboratories. In the fully swollen state, there was no evidence for IPN related physical crosslinks, but some data supported the concept of network I domination. In the bulk state, network I clearly dominates network II because of its greater continuity in space. The analysis of the data concerning the possible presence of added physical crosslinks in the bulk state yielded inconclusive results, but this latter is of special interest for modern network theories. 

Let us consider the Young s modulus, E, of a sequential IPN having both polymers above their respective glass transition temperatures. A simple numerical average of the two network properties results in (21) ... 

An equation for a sequential IPN begins with a consideration of the front factor, r 2 / rf2 (24,25), where r represents the actual end-to-end distance of a chain segment between crosslink sites in the network, and rf represents the equivalent free chain end-to-end distance. For a single network. 

A word must be said about the use of equations (8) and (9). In each case, the modulus of the single networks determines Nj and N2 for equation (8) and the Flory-Rehner equation determines N and N2 for equation (9) from the equivalent single network. In this manner, several effects existing in the single networks are minimized, such as physical crosslinks, incomplete crosslinking, and branching. Thus, differences from theory will emphasize new effects due to sequential IPN formation. 

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]. 

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