Size and mass determination of clusters obtained by polycondensation near the gelation threshold

2014 The gelification process by polycondensation leading to self similar clusters of polyurethane is studied, near and below the gelation threshold. In dilute solutions, where light scattering experiments are performed, the interaction between clusters is a function of 03B5, the distance to the gelation threshold. This is a direct evidence of the swelling of the clusters. Thus, the experimental value of the exponent which links the size of the largest clusters to the distance from the gel point 03B5 is larger than the value predicted by the percolation theory. However, the exponent 03B3, which links the weight average molecular weight to 03B5, is determined and found to be equal to 1.71 ~ 0.06, in agreement with percolation theory. The gelification process by polycondensation and percolation belong to the same class of universality : they share two exponent values 03C4 and 03B3. J. Physique 48 (1987) 1809-1818 OCTOBRE 1987, Classification Physics Abstracts 61.40 82.35 05.20 Half a century ago a mean field theory of the gelation threshold [1, 2] was developed. This theory, well adapted to describe the chemical process (as for example the location of the gel point) was verified experimentally [3]. About ten years ago an analogy was done [4, 5] between gelation and percolation, the latter being a critical phenomenon of connectivity. This analogy was followed by Monte Carlo simulation [6] and by few experiments. Here we are interested in the formation of gel through chemical reaction. We show experimentally that the gelation process, near and below the gelation threshold, is well described by the critical theory of percolation. 1. Theoretical background. Percolation is a critical phenomenon of connectivity, all the characteristic quantities of this critical phenomenon are expressed as power laws of the distance to the critical point. Moreover one has to know the variation of only two of these quantities, and thus the corresponding exponent values, in order to know the behaviour of all the other characteristic quantities. First of all we will describe the percolation model Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198700480100180900