Phase separation reaction-induced

6 Modelling the Diffusion-Controlled Overall Kinetics and Cure Rate Law of Epoxy Systems 

The ejq)erimental MTDSC observations on anhydride-cured and amine-cured epoxies, described in the previous section, will now be modelled to illustrate the benefits of the technique to obtain a quantitative law of cure kinetics for such thermosetting systems. Because cure kinetics are often complicated by diffusion limitations and/or mobility restrictions, the effect of diffusion has to be incorporated into the overall reaction rate law. For this purpose, both heat capacity and non-reversing heat flow signals for quasi-isothermal and non-isothermal cure experiments are used. 

The thermodynamic condition for compatibility of a resin and a rubber is that the free energy change of mixing (AG ) at constant pressure (P) and temperature (T), should be negative [108]  

Combining the Flory-Huggins equation and the Hildebrand equation, the free energy of mixing can be expressed as  

Polarized Optical Microscopy Scanning Electron Microscopy Atomic Force Microscopy Transmission Electron Microscope 

The diffusivity of rubber in the resin medium (D ) is considered to be proportional to the temperature and viscosity ratio through the Stokes-Einstein equation [112]  

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Williams, R. /. Rozenberg, B. A., Pascault, J.-P.. Reaction Induced Phase Separation in Modi-... 

In comparison to the results obtained for the samples prepared with hexane, it can be concluded that the mean pore size and volume fraction do not depend on the initial concentration of the solvent, ( )o, but mainly on the difference between ( )o and (Fig. 25). Similar qualitative results are also reported for rubber-modified epoxies prepared via reaction induced phase separation [103]. 

Recently, a new concept in the preparation of TPVs has been introduced, based on the reaction-induced phase separation (RIPS) of miscible blends of a semicrystalline thermoplastic in combination with an elastomer, with the potential for obtaining submicrometer rubber dispersions. This RIPS can be applied to a variety of miscible blends, in which the elastomer precursor phase was selectively crosslinked to induce phase separation. Plausible schematic representation of the morphological evolution of dynamic vulcanization of immiscible and miscible blends is shown in Fig. 9. For immiscible blends, dynamic vulcanization leads to a decrease in the size... 

Reaction-induced phase separation is certainly also the reason for which an inhomogeneous structure is observed for photocured polyurethane acrylate networks based on polypropylene oxide (Barbeau et al., 1999). TEM analysis demonstrates the presence of inhomogeneities on the length scale of 10-200 nm, mostly constituted by clusters of small hard units (the diacrylated diisocyanate) connected by polyacrylate chains. In addition, a suborganization of the reacted diisocyanate hard segments inside the polyurethane acrylate matrix is revealed by SAXS measurements. Post-reaction increases the crosslink density inside the hard domains. The bimodal shape of the dynamic mechanical relaxation spectra corroborates the presence of a two-phase structure. 

The initial mixture is homogeneous, and phase separation takes place during the cure of the thermoset. This second technique is called reaction-induced phase separation (Williams et al., 1997) and may lead to several types of morphologies a dispersion of modifier-rich particles in a thermoset matrix a dispersion of thermoset-rich particles in a modifier matrix (phase-inverted morphology), or two bicontinuous phases. 

Description of the Reaction-Induced Phase Separation Process... 

Depending on applications, both procedures used to generate two-phase morphologies in modified-thermosets (a) reaction-induced phase separation (RIPS) and (b) the use of preformed particles - have their advantages... 

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