Surface Tension of Curing Oligomers

Completion of the reaction and the degree of conversion of the epoxy groups were determined by IR spectroscopy. To determine the equilibrium v llue of the surface tension, a quantity of PEPA was added such that the extent of epoxy group conversion would not exceed 30-35%. In this case the resin remained in the liquid state. 

At low degrees of epoxy conversion, CCMF of OP-10 does not change (see Fig. 2.7, curves 1-3). Only at 30% conversion does CCMF begin to decrease (curve 4). With increase of the degree of conversion, one observes the decrease of the system surface tension at the surfactant concentration that corresponds to CCMF. The kink on the surface tension isotherm at low surfactant concentrations can be related to the formation of the surfactant pre-associations in the system. 

The size of micelles is determined by energetic factors and concentration factors. The decrease of CCMF in a viscous system must result in decrease of the size of the micelles. For increase of the surfactant concentration, the micelle size increases, and spherical micelles transform into elhptic ones and then into plate-shaped ones as increase in the surfactant concentration in common solvents is accompanied by a free energy advantage. It can be assumed that such a process occurs with the increase of the degree of conversion up to 30%. In this case CCMF shifts into the area of lower surfactant concentrations without any change in the surface tension (see Fig. 2.7). 

Consider the change in the system surface tension with the extent of epoxy group conversion at different surfactant contents (Fig. 2.8). The transition of the smfactant molecules into the associated state with increase of the degree of epoxy groups conversion (with increase of the oligomer molecular weight) results in depletion of the surface layer of surfactant molecules by diffusion to lower-concentration regions, which causes the increase of the system surface tension (see Fig. 2.8, curves 2 and 3). Consequently, the formation of the 

This is related to the formation of a great number of large colloid particles at such a high surfactant concentration. Since at 10% conversion the system viscosity is still low, the colloid particles coalesce and the system separates into two phases (polymer and surfactant). The separation of surfactant from the oligomeric solvent results in the decrease of the system s optical density. 

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