Chain dangling

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. 

As the reaction proceeds beyond the gel point, the molecular weight of EANCs decreases and the fraction of material in the EANCs increases. The fraction of material in dangling chains passes through a maximum but their molecular weight decreases. Figure 5.5 characterizes the behavior of simple polyurethane systems. 

Figure 5.4 Schematic representation of sol and a part of the gel DC dangling chains, EANC elastically active network chains, EAC elastically active crosslinks...

Figure 5.5 Calculated dependence of weight fractions of various substructures in the crosslinking system of H3 + I2 type on the initial molar ratio of H-groups to l-groups, ah (a polyurethane system) DC - dangling chains, BC - backbone chains, S - sol (backbone chains are elastically active network chains without dangling chains)...

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. 

TELECHELIC POL YMERS CONTROL OF PROPERTIES THROUGH DANGLING CHAINS... 

Figure 5.12 Structure of a polyurethane network with dangling chains prepared from F1 + F2 + F3 components...

Preparation of Dangling-Chain Networks (a) Excess difunctional chains... 

Figure 4.9 Two end-linking techniques for preparing networks having known numbers and lengths of dangling chains.237 Reproduced by permission of the American Chemical Society.

Macromolecular 10-100 nm Network chains/strands, crosslinks network defects (dangling chains) Macro- molecular science Rubber elasticity, solvent swelling... 

Homogeneous nonideal, e.g. open networks, obtained from the same chemistry as the previous ones. These networks contain dangling chains as a result of incomplete cure, nonstoichiometric composition, or presence of monofunctional monomers. 

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