Sequential IPNs synthesis

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...

The important points developed in this section are that sequential IPN synthesis tends to make dual phase continuous materials. Eor both sequential and simultaneous syntheses, a metastable phase diagram can be developed to study the kinetics of phase separation and gelation, so that better control of the morphology can be attained. The thermoplastic IPNs depend on equal volume and viscosity ratios to attain the dual phase continuity. 

Figure 7.14. Stress-strain curves for SINs containing 40% castor oil elastomer. Discontinuous curve adapted from the sequential IPN synthesis. ...

L. H. Sperling, J. A. Manson, G. M. Yenwo, N. Devia-Manjarres, J. Pulido, and A. Conde, Novel Plastics and Elastomers from Castor Oil Based IPN s A Review of an International Program, in Polymer Alloys, D. Klempner and K. C. Frisch, eds.. Plenum, New York (1977). Castor oil-urethane/polystyrene sequential IPNs. Synthesis, morphology, and mechanical behavior. Fatigue behavior. 

G. M. Yenwo, Synthesis, characterization, and Behavior of Interpenetrating Polymer Networks and Solution Graft Copolymers Based on Castor Oil and Polystyrene, Diss. Abstr. Int. B 37(11), 5788, (1977). Castor oil-urethane/PS sequential IPNs. Synthesis, morphology, glass transitions, mechanical properties. Ph.D. thesis. 

Over the years, people have discovered many different ways of synthesizing IFN s. Figure 2 Illustrates the sequential IPN synthesis, top, and the simultaneous Interpenetrating network, SIN, synthesis, bottom. In the sequential synthesis, polymer network I Is swollen with monomer II plus crosslinker and activator, and polymerized In situ. The SIN synthesis begins with a mutual solution of both monomers or prepolymers and their respective crosslinkers, which are then polymerized simultaneously by noninterfering modes, such as stepwise and chain polymerizations. These methods have been used In the bulk, suspension, and latex states. Each will yield a distinguishable composition, even for the same polymer pair. 

Can Figures 13.20 and 13.21 be generalized to analyze a sequential IPN synthesis Assume that the urethane is polymerized first, then methyl methacrylate is swollen in, and then polymerized. What path is followed What are the requirements to cause phase separation first, and then gelation of the poly(methyl methacrylate) ... 

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). 

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. 

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 [ ] ... 

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. 

As can be seen from all of these works, sequential IPNs have been extensively used with silicone with various crosslinking reactions and polymers. As a result, attention must be paid to the influence of processing conditions, of the chemical nature of components (silicone and the other polymer), of the evolution of the material at each step of the IPN synthesis etc. Depending on the mastering of these parameters, reactivity, morphology and entanglement can be controlled and consequently targeted properties can be reached. 

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 previous section showed how IPNs and related materials can be synthesized. The several synthetic methods, such as sequential, simultaneous, latex, and thermoplastic IPN formation, will result in different morphologies. One of the main advantages of IPN synthesis relates to the ease of promoting dual phase continuity, i.e., for a... 

The tetrahedron construction represents the synthesis of sequential IPNs as well as the SINs. It must be noted that in real SIN syntheses, it is almost impossible to have the kinetics of polymerization of both polymers proceeding at identical rates. Sequential IPN polymerization represents the other extreme, where first one polymer is completely synthesized, and then the other follows sequentially. 

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]. 

For sequential IPNs the effects of compatibility and crosslinking have already been described. When the order of polymerization is reversed in sequence, however, the new morphology is again controlled principally by the first network. In the synthesis of SINs, both networks form at the same time, but not necessarily at the same rate (see Section 5.5). 

Further electron micrographs will be found elsewhere in this book, particularly in Chapter 2. Certainly not all of the possible IPN morphologies have been discovered yet. While equation (6.36) describes the phase domain size in sequential IPNs, the correlation between synthetic detail and morphology is still in its infancy. Nonetheless, the experimentally known morphologies provide an important link between synthesis and properties. 

L. H. Sperling and D. W. Friedman, Synthesis and Mechanical Behavior of Interpenetrating Polymer Networks Poly(ethyl acrylate) and Polystyrene, J. Polym. Sci. A-2 7, 425 (1969). Synthesis of sequential IPNs. Modulus-temperature behavior. Modulus-composition behavior. 

Latex IPNs are often made by sequential polymerization of two (or more) cross-linked polymers utilizing emulsion polymerization (71). For core-shell latex IPN synthesis, first a cross-linked seed latex of polymer 1 is synthesized (2). Then, a second monomer and cross-linker are added to the system, usually with no added surfactant. Often, a starved polymerization route is employed, ie, the rate of polymerization equals or exceeds the rate of monomer addition. This reduces the swelling of the seed latex by the monomer 2 mix, producing a two-layer latex having a spherical core, and an overlaying shell. Obviously, multiple shells can be added for different purposes. 

According to the mode of synthesis, IPNs are distinguished into five different types (i) sequential IPNs, (ii) simultaneous IPNs, (iii) interpenetrating elastomeric networks, (iv) thermoplastic IPNs and (v) gradient IPNs ... 

Das and coworkers studied the morphology and mechanical properties of NR/PMMA IPNs. Mathew studied solid NR and PS, intimately mixed by the sequential method of IPN synthesis. Semi-IPNs based on NR/PS where only the NR phase is crosslinked offer a binary polymer system network having the elastomeric properties of NR imparted to the hard brittle properties of PS. Schematically, the morphology of semi-IPNs can be represented as shown... 

Figure 2. Schematic of the synthesis of sequential IPN s and sin s. On identical polymer pairs, the chemical structure may be the same, but the morphology and mechanical behavior will differ.

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. 

An interpenetrating polymer network, IPN, is defined as a combination of two polymers, both of which are crosslinked. Most IPN s are formed by synthesizing and/or crosslinking one network in the immediate presence of the other. Formed with covalent crosslinks, IPN s are thermoset, and do not flow or dissolve in ordinary solvents. Two main synthesis paths are illustrated in Figure 1 (a) polymer network I is synthesized, and monomer II plus crosslinker and activator are swollen in and polymerized in situ. This is called a sequential IPN (b) if both monomers or prepolymers are synthesized simultaneously by independent but non-interfering routes, the product is called a simultaneous interpenetrating network, SIN. ... 

Another very important class of IPNs are semi-IPNs, namely, systems in which one of the components is a linear polymer. Semi-IPNs may be characterized as sequential or simultaneous IPNs depending on the way the linear polymer is introduced. It may play the role of the host polymer for the synthesis of sequential IPNs, or that of the guest polymer for the same sequential IPNs when introduced into the system by the swelling of a network in monomers forming a linear polymer. Semi-IPNs may also be obtained by a simultaneous method via mixing the initial components of the network with linear polymer and then curing the system. Semi-IPNs may be of two types depending on what polymer is formed first linear or cross-linked. 

The dependence of the viscoelastic properties of IPNs based on PU and PUA on the segregation degree was shown in [88,256,257]. Taking into account the interconnection between the chemical kinetics and the kinetics of phase separation, various methods of IPN synthesis have been used, i.e., simultaneous and sequential methods. It is known that oHgourethane acrylate (OUA) in the presence of photoinitiators polymerizes at a high rate at room temperature, the reaction rate being much higher than that... 

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