Tailor-made networks: synthesis and properties
Methods were developed to synthesize homogeneous networks characterized by a well-known (and nearly constant) length of their linear elements, and in some cases by the constant functionality of the branch points. These methods are based on reacting the chain ends of 'precursor' polymer molecules to bind them with branch point sites. Crosslinking does not involve any major change in the segment concentration, nor does it Iead to solvent expulsion. Thus the gel point has a physical significance. These tailor-made networks were studied thoroughly using various techniques: equilibrium swelling and uniaxial compression measurements, as well as equilibrium partition coefficients, the latter being meant to characterize the average pore size of the gels by comparison with the molecular dimensions of linear polymers of the same nature. The experimental results were compared with the predictions of the existing theories, assuming that all 'precursor chains' have become elastic chains upon crosslinking (i.e. chains attached by their two ends to two different branch points), that they behave as Gaussian chains and that the fluctuations in the number of chains connected with one branch point are small within a sample. Results of equilibrium swelling measurements are in satisfactory agreement with theoretical predictions; and there is also a satisfactory fit for uniaxial deformation experiments. However, the relation between the concentration of segments upon crosslinking and the so-called 'memory term', which characterizes the relaxation state of the elastic chains in the network, has to be discussed thoroughly: results of the swelling and uniaxial compression measurements are used, as weil as some very recent results obtained by neutron smallangle scattering on partly deuterated polystyrene networks. The results so far obtained by this technique support expectations as to what concerns the homogeneity of the gels, and they also give indications on the average distance between branch points in the gel. Finally, the results obtained on measuring the equilibrium partition constants in a ternary system-solvent-linear polymer-gel-are also discussed, as they can be used as a measure of the pore size distribution within a gel. Account must be taken, however, of the partial deswelling of the tailor-made gels in the presence of a linear polymer (eyen one of the same chemical nature). Osmotic effects cannot account for the total effect observed. The existing theories of equilibrium swelling of polymer networks and of rubber elasticity are based upon some assumptions concerning the gel itself: