Phase transitions in polymer mixtures with application to compositional quenching

Novel microdispersions of one polymer in another were created by a new process called compositional quenching. A homogeneous solution of the polymers in a common solvent is plunged into the unstable (spinodal) region by flash devolatilization. Spinodal decomposition ensues, resulting in a wide variety of blends ranging from particulate dispersions to interconnected networks.; The theoretical understanding of this process is based on the Landau-Ginzburg functional, which is used to describe the free energy of inhomogeneous binary mixtures. An approach, based on the calculus of variation, was used to determine the equilibrium concentration profiles in these mixtures, which must necessarily be monotonic. New, more restrictive conditions were imposed, which eliminated non-physical and pseudoequilibrium solutions found in previous treatments. The non-linear Cahn-Hilliard equation required a subtle modification to be consistent with the present theory. However this modification did not affect the linearized theory or the long term predictions of the full equation.; The stability of periodic concentration profiles in unstable and metastable binary mixtures was then examined. It was shown that although these profiles are not equilibrium profiles (or global minimizers of the system free energy), they are local minimizers, and hence exhibit stability with respect to a certain range of perturbations.; Numerical solutions to the non-linear Cahn-Hilliard equation were obtained using finite difference techniques. Some preliminary observations regarding the non-linear nature of spinodal decomposition were made.; The phenomenological coefficient of the gradient energy in the Landau-Ginzburg functional for polymer-polymer-solvent systems was estimated based on enthalpic contributions only. This was used to analyse the spinodal decomposition of polymer-polymer-solvent systems during continuous solvent removal as is the case when a polymer blend is made by compositional quenching.; The entropic contribution towards the gradient energy in polymer-solvent systems was evaluated using a lattice model. Its effect on predictions regarding spinodal decomposition and interfacial tension was shown to be significant.