Composite networks

The two-network theory for a composite network of Gaussian chains was originally developed by Berry, Scanlan, and Watson (18) and then further developed by Flory ( 9). The composite network is made by introducing chemical cross-links in the isotropic and subsequently in a strained state. The Helmholtz elastic free energy of a composite network of Gaussian chains with affine motion of the junction points is given by the following expression ... 

After introduction of cross-links in the strained state, the composite network retracts, upon release, to a stress-free state-of-ease (J9 ) The amount of retraction is determined by the degree of strain during cross-linking and by the ratio >i/v2. The elastic properties relative to the state-of-ease are isotropic for a Gaussian composite network ( 8, 1 9,20). 

Flory ( 9) has treated the interesting case of subsequent removal of the first stage cross-links without chain scission. Even after complete removal, i.e. =0, there is still a certain memory of the structure of the first network since the composite network strands were physically part of both networks. According to Flory s theory, the resulting network may be treated as if a certain fraction, 0, of the strands of the second network were effectively converted into strands of the first network, >ie. 

Figure 1. Effective first network modulus, Gle, after complete removal of first network cross-links plotted against second network modulus, Gi. Calculated from the composite network theory of Flory (19J for G, — 0.75 MPa.

A8. The Helmholtz elastic free energy relation of the composite network contains a separate term for each of the two networks as in eq. 5. However, the precise mathematical form of the strain dependence is not critical at small deformations. Although all the assumptions seem to be reasonably fulfilled, a simpler method, which would require fewer assumptions, would obviously be desirable. A simpler method can be used if we just want to compare the equilibrium contribution from chain engangling in the cross-linked polymer to the stress-relaxation modulus of the uncross-linked polymer. The new method is described in Part 3. 

The two-network method has been carefully examined. All the previous two-network results were obtained in simple extension for which the Gaussian composite network theory was found to be inadequate. Results obtained on composite networks of 1,2-polybutadiene for three different types of strain, namely equibiaxial extension, pure shear, and simple extension, are discussed in the present paper. The Gaussian composite network elastic free energy relation is found to be adequate in equibiaxial extension and possibly pure shear. Extrapolation to zero strain gives the same result for all three types of strain The contribution from chain entangling at elastic equilibrium is found to be approximately equal to the pseudo-equilibrium rubber plateau modulus and about three times larger than the contribution from chemical cross-links. 

The challenge is therefore to develop an experiment which allows an experimental separation of the contributions from chain entangling and cross-links. The Two-Network method developed by Ferry and coworkers (17,18) is such a method. Cross-linking of a linear polymer in the strained state creates a composite network in which the original network from chain entangling and the network created by cross-linking in the strained state have different reference states. We have simplified the Two-Network method by using such conditions that no molecular theory is needed (1,21). 

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. 

An alternative way to express the rigidity of the composite network is in terms of the rigidities of its individual components. Starting from Equation 13 it can easily be shown that ... 

It is clear that the application of Langley s method in other polymer systems is essential to settle questions about Me and g in networks satisfactorily. The Ferry composite network method (223, 296) appears to be broadly applicable as well, although requiring special care to minimize slippage prior to introduction of the permanent crosslinks. (One is also still faced with the difficult question of whether g is the same for entanglements in crosslinked networks and in the plateau region of dynamic response.) Based on the limited results of these two methods in unswelled systems, Me values deduced by equilibrium and dynamic response appear to be practically the same. 

In dry networks, three types may be recognized (i) the chains may occur in their unstrained state, i.e. they may exhibit their normal degree of coiling (ii) the chains may be supercoiled, e.g. as a result of cross-linking in the presence of a diluent, followed by removal of the diluent and (iii) the network may be of the expanded type e.g. if a crosslinked network is swollen in a mixture of monomers, which are subsequently copolymerized. In this latter case the network will be a composite network of which the first part will be expanded. 

It is the opinion of the present authors that frequently too much emphasis is placed in network studies on the influence of network defects (Chapter II, Section 2), while in reality pre-existing order, inhomogeneous crosslinking, composite network formation, or microsyneresis may play an important role in the mechanical behaviour, as well as in most other network properties. Examples of this will be given in Chapter IV. 

Several routes were investigated to improve the behavior. One route was to incorporate additional network modifiers in order to reduce the brittleness. For this reason, an additional silane (amino group containing silane, for example y-aminopropyl triethoxy silane or y-aminopropyl methyldiethoxy silane, AMDES) were introduced as a crosslinking agent for diepoxides acting as "flexible" chains between the highly crosslinked composite network. 

Destruction of polybutadiene that occurs under the action of nitric acid is caused by oxidation of a polymer macromolecule. In other words, cross-section links of a spatial composite network formed by the vulcanization process are broken. The well-known oxidization ability of sulfuric acid is responsible for a decrease in double links in the rubber molecular structure, resulting in a reduction in the RubCon strength indexes. Corrosive attack by hydrochloric acid is linked to oxidation and isomerization processes and, therefore, the durability of the composite depends on the speed of these processes. 

Wongsasulak, S., Yoovidhya, T., Bhumiratana, S., and Hongsprabhas, R. 2007. Rhysical properties of composite network of egg albnmen and cassava starch formed by a salt-induced gelation method. Food Research International 40 249-256. 

The superabsorbent composites containing SH show release of the fertilizer over 10—40 days, depending on the SH content (5 wt% to 30 wt%). The release rates into water in the initial period are higher since the SH existing on the surface or freely incorporated in the composite network are dissolved more readily in water. The SH bonded with the polymeric network needs more time to diffuse from the hydrogel granule and dissolve in water. Figure 5.72 shows schematic structures of a PAA-co-AAm/SH... 

The q toskeleton of eukaryotic cells is generally considered to be a meshwork of protein filaments that spans the space between the nucleus and the plasma membrane. In many cell types, the three-dimensional (3D) composite network of actin filaments, microtubules (MTs), and intermediate filaments (IPs) in the cytoplasm interfaces with two-dimensional networks composed largely of spectrins that line the plasma membrane and nuclear lamins that line the inner surface of the nuclear membrane. A few eukaryotic cell types contain an entirely different cytoskeleton that powers their locomotion and which is constmcted from the cationic major sperm protein instead of actin. The three cytoskeletal proteins, acdn, tubulin, and IF subunits, constitute a significant fraction of... 

FIGURE 5.2.3 Classification of soft shape-memory materials from the viewpoint of nanoaivhitectonics. (a-c) Structures and (d) molecular mechanism, (a) Chemically cross-linked polymer network, (b) supramolecular network with clay nanosheets [29], and (c) inorganic/polymer composite network system, and their shape-memory profiles [30]. (d) The nanoscale molecular mechanism for one-way and two-way SME of a cross-linked semicrystalline polymer system. 

Greene, A. Smith, K. J. Ciferri, A., Elastic Properties of Networks Formed from Oriented Chain Molecules Part-2. Composite Networks. Trans. Faraday Soc. 1965, 61, mi-riZ i. 

Another stream of the study of temporal networks concerns a model network whose history involves cross-links added at a certain stage, a part of which is subsequently removed so as not to be present in the final stage of deformation (called an addition-subtraction network). On the basis of such model composite networks, Flory [14] calculated the stress relaxation, and found that it obeys slow dynamics including a logarithmic dependence of the stress, which is closer to power law rather than exponential. 

There are other potential applications of CMPs. Owing to the massive surface area of CMPs, they can be fused with other types of conjugated polymers, which otherwise are not able to form a composite network. The pores of CMPs can be used for doping with various functional materials, e.g., TiOj or quantum dots, which could be utilized in applications of photo-voltaics and photocatalysis. A brand new microporous network can be obtained when the CMP network is deposed out. ... 

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