Network structure entanglements

This behaviour is thought to be due to the formation of a network structure caused by entanglement of the longchain molecules in solution. Plotting the data of viscosity measurements of pectin solutions of different concentrations reveals the same behaviour, confirming Onogi s observation, with a critical pectin concentration of about 1 % (w/w). 

Many fluids show a decrease in viscosity with increasing shear rate. This behavior is referred to as shear thinning, which means that the resistance of the material to flow decreases and the energy required to sustain flow at high shear rates is reduced. These materials are called pseudoplastic (Fig. 3a and b, curves B). At rest the material forms a network structure, which may be an agglomerate of many molecules attracted to each other or an entangled network of polymer chains. Under shear this structure is broken down, resulting in a shear... 

Equation (29) shows that the modulus is proportional to the cycle rank , and that no other structural parameters contribute to the modulus. The number of entanglements trapped in the network structure does not change the cycle rank. Possible contributions of these trapped entanglements to the modulus therefore cannot originate from the topology of the phantom network. 

One example of a relatively new technique for the non-invasive, non-destructive characterization of network structures involves pulse-propagation measurements [288,289]. In this technique, the delay Af in a pulse passing through the network is used to obtain information on the network structure, for example, the chain length between cross-links or between entanglements. The technique is illustrated schematically in Figure 12 [282]. 

Most network structures are effective host structures for small guest molecules, often the solvent. Exceptions arise when there is a high degree of interpenetration, i.e. where two or more networks are entangled [7]. This type of host-guest behaviour is not intrinsic to the molecular components themselves, but occurs in cavities or clefts created by the assembly of the network structure. Mol-... 

Since the two effects work in parallel in ordinary networks, it is necessary to know the concentration of effective cross-links and to have a molecular theory which correctly relates the modulus to the concentration of cross-links. The contribution from chain entangling is then found as the difference between the observed and the calculated modulus. This seems to be an almost hopeless task unless the network structure is very simple and the contribution from chain entangling is large. 

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. 

Note 5 Physical entanglements between network chains can lead to an increase in the concentration of elastically active network chains and, hence, increases in the shear modulus and the Young s modulus above the values expected for a perfect network structure. 

The plateau region appears when the molecular weight exceeds Mc [(Mc)soln. for solutions], and is taken to be a direct indication of chain entanglement. Indeed the presence of a plateau may be a more reliable criterion than r 0 vs M behavior, especially in solutions of moderate concentration where viscosity may exhibit quite complex concentration and molecular weight behavior. It is postulated that when M greatly exceeds Mc, a temporary network structure exists due to rope-like interlooping of the chains. Rubber-like response to rapid deformations is obtained because the strands between coupling points can adjust rapidly, while considerably more time is required for entire molecules to slip around one another s contours and allow flow or the completion of stress relaxation. 

It was also found that the diluents added after the end-linking were more easily removed, possibly because they were less entangled with the network structure, and this could correspond to differences in diluent chain conformations. Such comparisons can thus provide valuable information on the arrangements and transport of chains as constrained within complex network structures [20]. 

Another group of theories is based upon intermolecular strain dependent effects caused 1) by orientationally active short chains, 2) by excluded volume, and 3) by a structuring in the network, including entanglements. The first two do not yield a sufficiently large C2. For the third, several proposals have been made, but they are either qualitative or, as yet, incomplete. The structuring hypothesis needs special emphasis because we have seen that many networks may indeed exhibit much more structure than is implied by the normal picture of coiling-chain networks. 

A pseudo solid-like behavior of the T2 relaxation is also observed in i) high Mn fractionated linear polydimethylsiloxanes (PDMS), ii) crosslinked PDMS networks, with a single FID and the line shape follows the Weibull function (p = 1.5)88> and iii) in uncrosslinked c/.s-polyisoprenes with Mn > 30000, when the presence of entanglements produces a transient network structure. Irradiation crosslinking of polyisoprenes having smaller Mn leads to a similar effect91 . The non-Lorentzian free-induction decay can be a consequence of a) anisotropic molecular motion or b) residual dipolar interactions in the viscoelastic state. 

The mass fractions of these three phases are shown in Table 14. the crystalline fraction is relatively small as 0.43 or 0.44. This low level of crystallinity may arise from relatively strong molecular entanglement due to the network structure. 

Network Structure in Oil-Extended Rubbers - Effect of Chain Entanglements... 

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