Effects of Dangling Chains

Values of the maximum extensibility of the networks are shown as a function of the high deformation modulus 2 Ci in Fig. The networks containing the dangling ends 

As pointed out previously , even more thorough studies of the effects of dangling chains may be carried out by including some monosubstituted chains in the preparation of the model networks. In this way it would be possible to vary the lengths of the dangling ends, as well as their relative numbers. 

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Amouroux, N. Leger, L., Effect of Dangling Chains on Adhesion Hysteresis of Silicone Elastomers, Prohed hy JKR Test. Langmuir 2005,21,1396-1401. 

Mark and co-workers have studied the effect of dangling chains on the ultimate properties of model networks prepared by end-linking vinyl-terminated PDMS chains with a tetra functional crosslinker [34]. In this case the crosslinker was used in varying amounts smaller than those corresponding to a stoichiometric balance. The... 

It has been established by a number of studies l that in crosslinked rubbers the major contribution to hysteresis in the plateau region comes from the effect of dangling chain ends. Thus it is generally accepted that hysteresis, i.e. tan 6, should decrease with increasing crosslink density. 

Amouroux N, Leger L (2003) Effect of dangling chains on adhesion hysteresis of silicone elastomers, probed by JKRtest. Langmuir 19 1396... 

Effect of Dangling Chains. Since dangling chains represent imperfections in a network structure, their presence should have a detrimental effect on ultimate properties such as the tensile strength, as gauged by the stress at rupture. This expectation was confirmed by an extensive series of results obtained on PDMS networks which had been tetrafunctionally cross-linked using a variety of techniques. 

Despite its simplicity, the statistical method has been quite successful in predicting the effect of various chemical variables on network formation (cf. e.g. [29, 30, 34-37]). Since the internal structure of the gel can be characterized to a certain degree by the statistical method (e.g. average size of dangling chains and weight fraction of material in them), these methods offer a basis for correlations between structure and viscoelastic properties. 

Figure 2. Effect of crosslinking, chain termination, and plasticization on effective strain. Numbers on points refer to percent plasticizer, moles of chains X 10A/cc. or moles of chains X 10A /cc. dangling, respectively...

How do we take into account the contribution of dangling chains to Tg In linear polymers, we know that chain ends carry on a free volume excess and, thus, play a plasticizing effect that can expressed through a copolymer law ... 

From a fit of Equation (10) to spatially resolved relaxation curves, images of the parameters A, B, T2, q M2 have been obtained [3- - 32]. Here A/(A + B) can be interpreted as the concentration of cross-links and B/(A + B) as the concentration of dangling chains. In addition to A/(A + B) also q M2 is related to the cross-link density in this model. In practice also T2 has been found to depend on cross-link density and subsequently strain, an effect which has been exploited in calibration of the image in Figure 7.6. Interestingly, carbon-black as an active filler has little effect on the relaxation times, but silicate filler has. Consequently the chemical cross-link density of carbon-black filled elastomers can be determined by NMR. The apparent insensitivity of NMR to the interaction of the network chains with carbon black filler particles is explained with paramagnetic impurities of carbon black, which lead to rapid relaxation of the NMR signal in the vicinity of the filler particles. 

An interesting sub-dass of lonomers are telechelic lonomers, of which there are several noteworthy examples. The term "telechelic" indicates that the ions are attached exclusively at the chain termini and that every chain end contains an ionic moiety. Such placement provides a network free of dangling chain ends, and minimizes melt viscosity, since the telechelic polymer molecular weight is of the same order as the elastically effective molecular wei t between crosslinks. This is in direct contrast to "random"... 

The effect of pendent chains on the mechanical properties of model PDMS has been studied. It was foimd that the loss modulus of these networks was a function of the concentration and molecular weight of the dangling chains. The elastic properties of networks with pendent chains were found to be lower than those that were completely cross-linked (352). 

The elastic modulus is controlled by the polymer network structure and synthesis conditions. The number of cross-links added per mole of main monomer helps define the molecular mass between cross-links. The molecular mass of the backbone polymer chain (obtained if no cross-linker was used) affects the effectiveness of the cross-links in forming a fiilly connected network with no soluble fraction and minimum content of dangling chains. The polymer volume fraction during cross-linking t>2<> defines the extent of entanglement of the network chains, and therefore influences the elastic modulus. 

PDMS networks, these increases in modulus and ultimate strength are due to the low incidence of dangling-chain irregularities and to non-Gaussian effects arising from limited chain extemibility2 22). 

Networks obtained by anionic end-linking processes are not necessarily free of defects 106). There are always some dangling chains — which do not contribute to the elasticity of the network — and the formation of loops and of double connections cannot be excluded either. The probability of occurrence, of such defects decreases as the concentration of the reaction medium increases. Conversely, when the concentration is very high the network may contain entrapped entanglements which act as additional crosslinks. It remains that, upon reaction, the linear precursor chains (which are characterized independently) become elastically effective network chains, even though their number may be slightly lower than expected because of the defects. 

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