Network junctions, interpenetrating

Figure 1, True IPNs. The different polytner systems are cross-linked to themselves but not to each other. The polymers thus form networks that interpenetrate each other. (A) Traditional three-dhnensional depiction (B) geometrical abstraction, in which the cross-links are depicted as junctions of vertical and horizontal lines.

The use of interpenetrating network junctions is essential for the DSC. On a flat surface a monolayer of dye absorbs at most a few percent of hght because it occupies an area that is several hundred times larger than its optical cross section. 

Schematic Structure of an Interpenetrating Polymer Network Formed by Crosslinking of a Phenolic Prepolymer (Rectangles Connected by Short Lines) in Presence of Other Monomers (Network Junctions are marked with Full Squares in tiie Case of Phenolics and with Circles in Case of the Other Polymer)...

Ronca and Allegra, and independently Flory, advanced the hypothesis that real rubber networks show departures from these theoretical equations as a result of a transition between the two extreme cases of behaviour. In subsequent papers Floryl >l and Flory and Ermanl derived a theory based on this concept. At small deformations the fluctuations of the network junctions are constrained by the extensive interpenetration of neighbouring, but topologically remote chains. The severity of these constraints is characterized by the value of the parameter k (k - 0 corresponds to the phantom network, k = to the affine network). With increasing deformation these constraints become less restrictive in the direction of the principal extension. The parameter t describes the departures from affine transformation of the shape of the domains of constraints. The resulting stress-strain relation also takes the form of Eq. (7) with... 

Pig. 1. Interpenetrating network morphology of thermoplastic elastomer where A = the crystalline domain, B = the junction of crystalline lamellae, and... 

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. 

Fig. 5.3. Formation of a bulk heterojunction and subsequent photoinduced electron transfer inside such a composite formed from the interpenetrating donor/acceptor network, plotted with the device structure for such a junction (a). The diagrams showing energy levels of an MDMO-PPV/PCBM system for flat band conditions (b) and under short-circuit conditions (c) do not take into account possible interfacial layers at the metal/semiconductor interface...

The interpenetrating network in bulk hetero junction solar cells [9] helps to overcome the limitations of bilayer systems [25,95] with low mobility materials. However, less is known about the nanometer morphology of an interpenetrating network or the optimum density of donor/acceptor interfacial... 

Summarizing our experience with hypercrosslinked polystyrene networks, as well as with conventional gel-type and macroporous copolymers, we beheve that the topology of the network was undeservedly neglected by all theoretical treatments. By this we particularly mean that a network must be considered as an ensemble of interconnected and interpenetrating meshes or cycles, rather than as a combination of chains and junctions. 

Here, x and are material parameters x is proposed to be proportional to the degree of interpenetration of chains and junctions, and it defines the strength of restrictions on junction fluctuations. The value x = 0 corresponds to the free-fluctuation limit and for X - 00 the affine network is obtained. The parameter (< 1) characterizes departures of the shapes of domains from the affine transformation assumption and reflects the effect of structural inhomogeneities on the network structure Although its presence is not as critical as that of x, comparison of experiment with the theory of restricted junction fluctuations shows that it is necessary... 

Fig. 1. Hybrid organic-inorganic polymer systems can be devised for all structural paradigms of polymer chemistry including (a) homopolymer, (b) block copolymer, (c) copolymer, (d) graft copolymer, and (e) interpenetrating polymer networks, including, shown as geometrical abstractions, (f) true and (g) semi-interpenetrating versions, where crosslinks are depicted as junctions of horizontal and vertical lines.

In real polymer network the effects of excluded volume and chain entanglements should be taken into account. In 1977 Hory [26] formulated the constrained junction model of real networks. According to this theory fluctuations of junctiOTs are affected by chains interpenetration, and as the result the elastic free energy is a sum of the elastic free energy of the phantom network AAph (given by Eq. (5.78)) and the free energy of constraints AA ... 

The extent to which entanglements contribute to network elasticity is not yet fully resolved. In the model of Langley [45], Dossin and Graessley [46-49] a contribution to the equilibrium modulus is associated with the plateau modulus of viscoelasticity. On the other hand, Flory [36] and Erman [38 0] assume that interpenetration of chains is solely reflected by suppression of the fluctuations of junctions. 

Predictions for the Parameters k and The parameter f is not far from zero, which is to he expected since the surroundings of jimctions cause their deformation to be nearly affine with the macroscopic strain. The primary parameter ic is defined as the ratio of the mean-square junction fluctuations in the equivalent phantom network, ie, in the absence of constraints, to the mean-square jimction fluctuations about the centers of domains of entanglement constraints (in the absence of the network) in the isotropic state. Thus in a phantom network, the absence of constraints leads to /c = 0. In an affine one, the complete suppression of fluctuations is equivalent to /c = oo. It has been proposed that k should be proportional to the degree of interpenetration of chains and junctions (165). Since an increasing number of junctions in a volume pervaded by a chain leads to stronger constraints on these jimctions, k was taken to be... 

---