Semimicroscopic theory of elasticity near the vulcanization transition

Randomly cross-linked macromolecules undergo a liquid to amorphous-solid ph ase transition at a critical cross-link concentration. This transition has two main signatures: the random localization of a fraction of the monomers and the emergence of a nonzero static shear modulus. In this paper, a semim icroscopic statistical mechanical theory of the elastic properties of the a morphous solid state is developed. This theory takes into account both quen ched disorder and thermal fluctuations, and allows for the direct computati on of the free energy change of the sample due to a given macroscopic shear strain. This leads to an unambiguous determination of the static shear mod ulus. At the level of mean field theory, it is found (i) that the shear mod ulus grows continuously from zero at the transition, and does so with the c lassical exponent, i.e., with the third power of the excess cross-link dens ity and, quite surprisingly, (ii) that near the transition the external str esses do not spoil the spherical symmetry of the localization clouds of the particles.