Tetrafunctional systems intramolecular reaction

In Figure 6, the larger values of A b for systems 4 and 5 compared with the other systems illustrate the increased opportunities for intramolecular reaction in tetrafunctional compared with trifunctional systems. Further, the smaller values of b for system 5 compared with those for system 1, with the same value of v, probably indicated that equation(3) relatively undercounts the opportunities for intramolecular reaction for tetrafunctional as compared with trifunctional reactants, so that smaller values of b are required in compensation. System 3, based on aromatic diisocyanate, gives the largest values of b, characteristic of its stiffer chain structure. 

The deviations from Gaussian stress-strain behaviour for the tetrafunctional polyurethane networks of Figure 9 are qualitatively similar to these found for the trifunctional polyester networks (Z5), and the error bars on the data points for systems 4 and 5 in Figure 9 indicate the resulting uncertainties in Mc/Mc. It is clear that such uncetainties do not mask the increases in Mc/Mc with amount of pre-gel intramolecular reaction. 

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. 

The relative posiMons of the lines for the various systems can be related to M(.o(or v), f, and the chain structures of the reactants(1.2.9-12). The slopes of the lines show that the reduction in modulus with pre-gel intramolecular reaction is larger for trifunctional compared with tetrafunctional networks (c.f. systems 1 and 2 with 4 and 5), although higher values of p- Q obtain for tetrafunctional reaction systems (c.f. Figure 2 . 

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