Polymer network ideal

Gel-type supports. Gel-type supports are most often used for solid-phase synthesis and feature equal distribution of functional groups throughout a highly solvated and inert polymer network ideal for the assembly of large molecules. The support capacity can be adjusted to afford problem-free synthesis and a high yield per volume of resin. The polymer network is flexible, and the resin can expand or exclude solvent to accommodate the growing molecule within the gel. There are four types of gel resins ... 

Serious deviations of the polymer network structure from the ideal one can have several causes. One of them is the crosslinking agent involvement in intramolecular cycle formation. The contribution of this reaction grows with the system dilution as well as when the crosslinker units in the chain are close one to the other, i.e. its fraction in the copolymer increases. All this is in good agreement with the observed trend. 

The Flory principle is one of two assumptions underlying an ideal kinetic model of any process of the synthesis or chemical modification of polymers. The second assumption is associated with ignoring any reactions between reactive centers belonging to one and the same molecule. Clearly, in the absence of such intramolecular reactions, molecular graphs of all the components of a reaction system will contain no cycles. The last affirmation concerns sol molecules only. As for the gel the cyclization reaction between reactive centers of a polymer network is quite admissible in the framework of an ideal model. 

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. 

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. ... 

Even if completely homogeneous and disordered in the relaxed state, a real network differs from the ideal network, defined in Chapter I. Three types of network defects are commonly considered to be present in polymer networks unreacted functionalities, closed loops, and permanent chain entanglements. Within each group there are several possibilities dependent on the arrangement of chains the effect of defects on the elastic properties of the network is thus by no means simple, as has been stressed e.g. by Case (28). Several possible arrangements are shown in Fig. 1, where only nearest neighbour defect structures have been drawn. 

A further test of the validity of ideal network theory can be obtained through studies of the equilibrium swelling of polymer networks (Eq. 11-22). The maximum amount of information can be extracted by inducing changes in the equilibrium swelling preferably combined with unidirectional stress-strain data (Eqs. III-21, 26 and 27). 

These polymer networks have commercial applications. A number of experimental studies suggest that the network formation by this method proceeds in a highly non-ideal fashion. During copolymerization, a high fraction of pendant vinyls of a primary chain are consumed by intramolecular reactions, causing practically no increase in the molar mass of the system (see Chapter 7) ... 

With the knowledge of the concentration of different fragments along the reaction, either for the ideal system through Eqs (3.41)-(3.44) or for the system with substitution effects, Eqs (3.46)-(3.51), average statistical parameters of the polymer network may be calculated. 

The structure of precursors, the number of functional groups per precursor molecule, and the reaction path leading to the final network all play important roles in the final structure of the polymer network. Some thermosets can be considered homogeneous ideal networks relative to a reference state. It is usually the case when networks are prepared by step copolymerization of two monomers (epoxy-diamine or triol-diisocyanate reactions) at the stoichiometric ratio and at full conversion. 

It has long been a mystery why diffusion coefficients of polymer-diluent systems, especially when the diluent is a good solvent for a given polymer, exhibit so pronounced a concentration dependence that it looks extraordinary. Several proposals have been made for the interpretation of this dependence. Thus Park (1950) attempted to explain it in terms of the thermodynamic non-ideality of polymer-diluent mixtures, but it was found that such an effect was too small to account for the actual data. Fujita (1953) suggested immobilization of penetrant molecules in the polymer network, which, however, was not accepted by subsequent workers. Recently, Barrer and Fergusson (1958) reported that their diffusion coefficient data for benzene in rubber could be analyzed in terms of the zone theory of diffusion due to Barrer (1957). Examination shows, however, that their conclusion is never definitive, since it resorted to a less plausible choice of the value for a certain basic parameter. 

The imprinting of polymer supports is an exciting development in the immobilization of transition metal complexes. The process involves the copolymerization of an inorganic or an organic template into a crosslinked polymer network. In a subsequent step, the template is chemically removed leaving an imprint of molecular dimensions in the resin. Ideally, the imprint retains chemical information related to the size and shape of the template. This approach has been used to prepare chiral imprints in otherwise achiral polymer networks. The method is outlined in Scheme... 

Urethane-Based IPN Foams. Interpenetrating polymer networks (IPNs) are types of polymer alloys composed of the entanglement of at least two cross-linked components (112). An ideal IPN has essentially no covalent bonds between the polymers. The resulting morphology shows... 

Using Flory-Huggins theory it is possible to account for the equilibrium thermodynamic properties of polymer solutions, particularly the fact that polymer solutions show major deviations from ideal solution behavior, as for example, the vapor pressure of solvent above a polymer solution invariably is very much lower than predicted from Raoult s law. The theory also accounts for the phase separation and fractionation behavior of polymer solutions, melting point depressions in crystalline polymers, and swelling of polymer networks. However, the theory is only able to predict general trends and fails to achieve precise agreement with experimental data. 

For an ideal IPN, the two or more polymer networks are at least partially interlaced on a molecular scale but not covalently bonded to each other and cannot be separated unless chemical bonds are broken [Mita and Akiyama, 1997]. 

This result is shown for polyisoprene in Figure 3.5 (Akcasu et al., 1980). Direct SANS measurements of the size of chains in the melt validated Flory s original hypothesis that polymers behave ideally in the bulk. Other applications of SANS to elastomers include investigations of microscopic aspects of network deformation (Gronski et al., 1990 Boue et al., 1991 Westermann et al., 2001) and the effect of fillers, such as carbon black and silica, on network deformation (Botti et al., 2003 Westermann et al., 1999 Zhang et al., 2001). 

Simple models assume ideal networks. However, real polymer networks contain network defects, like free chain ends (dangling ends), rings, and entanglements (Fig. 4.14), which sensitively affect mechanical properties and also swelling behavior. 

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