| The morphology and properties of any simultaneous interpenetrating network (SIN) depend on the time order of three key events during its synthesis. These are: gelation of polymer I, gelation of polymer II, and phase separation of polymer I from polymer II. Metastable phase diagrams for SINs are developed. Using these diagrams, the time order of these events can be visualized and thus, controlled more effectively. Reaction mixtures for a cross-polyurethane-inter-cross-poly(methyl methacrylate) (PU-PMMA) model SIN system were followed through phase separation and gelation in terms of conversions for each polymer. Fourier transform infrared spectroscopy (FTIR), dynamic mechanical spectroscopy (DMS) and differential scanning calorimetry (DSC) were utilized. The final isothermal diagram takes the shape of a tetrahedron with its four vertices representing the two monomers (MMA and "U") and the corresponding two polymers.; Within this three-dimensional metastable phase diagram for this model SIN system, phase separation and gelations of the two polymers are indicated by various surfaces. For example, gelation of PMMA at 8% conversion gives a planar surface, and that of PU at 65% conversion gives another. The third event, phase separation, yields a curvilinear surface within the tetrahedron. These surfaces intersect with each other at lines and curves, representing unique conditions of SIN synthesis, e.g., simultaneous gelation of the two polymers, or, simultaneous phase separation and gelation of polymer I, etc. These are critical conditions in terms of the development of SIN morphology. The positions of these critical conditions can be moved within the diagram in a controlled fashion via slight changes in the system. As an example, various amounts of N,N-dimethylacrylamide monomer (DMA) were incorporated into the PMMA network by means of copolymerization.; It was also found that during the late stages of MMA polymerization, the partition coefficient of MMA monomer across the PU- and PMMA-rich phases is nearly unity. Free energy calculations (statistical thermodynamics) confirm this experimental result, and indicate that the absorption (or retainment) of MMA in either phase and its partitioning across the phases is entropy-driven and not enthalpy-driven. |
