Trifunctional junction points

C. C. Han, H. Yu and their colleagues (23) have presented some new SANS data on end-linked trifunctional isoprene networks. These are shown in Figure 10. Those materials of low molecular weight between crosslinks exhibit greater chain deformation consistent with the thesis that the junction points are fixed. This is the reverse of that found by Beltzung et al. for siloxane networks. 

Experimental results on reactions forming tri- and tetrafunctional polyurethane and trifunctional polyester networks are discussed with particular consideration of intramolecular reaction and its effect on shear modulus of the networks formed at complete reaction. The amount of pre-gel intramolecular reaction is shown to be significant for non-linear polymerisations, even for reactions in bulk. Gel-points are delayed by an amount which depends on the dilution of a reaction system and the functionalities and chain structures of the reactants. Shear moduli are generally markedly lower than those expected for the perfect networks corresponding to the various reaction systems, and are shown empirically to be closely related to amounts of pre-gel intramolecular reaction. Deviations from Gaussian stress-strain behaviour are reported which relate to the low molar-mass of chains between junction points. 

Some authors consider four chains (as opposed to three chains) to emanate from a crosslink, regarding a trifunctional crosslink as merely a junction point. 

On the other hand, the parts of each crosslinking molecule between two adjacent branch points can be taken as short network chains. In this case the junctions are trifunctional (f = 3) and the chains have a bimodal distribution. The total number of network chains,, is threefold the number of former a,u-divinyl chains, because two short chains and one long chain proceed from each crosslink. Vj is also tabulated in Table II. 

---