The authors of the present invention have devised a method for controlling the dimensions and the morphology of polymers, that is innovative, simple and low cost. The method provides polymers with different morphology starting from polymers synthesised with any procedure, without the use of emulsifiers. The method exploits a physical barrier, exemplified in dialysis membranes or more in general in semi-permeable membranes that allow the passive transport of solutes, to slow down the mixing of a polymer solution with a non solvent of the polymer itself (the miscibility of the solvent and of the non solvent must be high, preferably complete), in which the dialysis bag containing the solution of the polymer is immersed. As a variant, able further to slow the process, the bag can be immersed in the same solvent present inside, whereto is slowly added the non solvent by dripping (e.g. by means of a peristaltic pump). The gradual mixing of the solvent and of the non solvent inside the bag causes the mixture that is being formed to be progressively less able to solubilise the polymer. The consequent increase in the interface tension pushes the molecules of the polymer to aggregate in spheroidal particles, whose shape allows to minimise the energy of the system, which then form a precipitate. The speed whereat the process takes place is the factor that most influences the morphology of the final product and it is determined by various parameters, among them the solvent – non-solvent pair, the rate of addition of the non solvent to the system and the temperature. The morphology of the particles is then determined by kinetic and thermodynamic factors. From a thermodynamic viewpoint, the most important parameter is the free interface energy. When the precipitation process is slow enough, the morphology is not influenced by the kinetic parameters and the so-called equilibrium morphology is reached, i.e. the one that corresponds to the minimisation of the free energy of the system. With regard to the kinetic parameters, the ones with the greatest influence on particle morphology are: the diffusion and the phase rearrangement inside the particles themselves. The mobility of the polymer chains is restricted during their aggregation and hence phase separation and rearrangement can be slower than the aggregation rate.
In this case, kinetic factors prevail over thermodynamic ones and morphologies different from the spherical one are obtained, called non-equilibrium. The spherical morphology, reached when the thermodynamic factors prevail, is the equilibrium morphology].
Control over morphology is easily obtained acting on a few experimental parameters: choice of materials, concentrations, number and type of membranes, temperature. Polymer recovery is total and takes place by centrifuging and subsequent drying
