Epoxy phase separation

Mechanical properties of mbber-modifted epoxy resins depend on the extent of mbber-phase separation and on the morphological features of the mbber phase. Dissolved mbber causes plastic deformation and necking at low strains, but does not result in impact toughening. The presence of mbber particles is a necessary but not sufficient condition for achieving impact resistance. Optimum properties are obtained with materials comprising both dissolved and phase-separated mbber (305). 

Fig. 67 Schematic of phase behaviour for blend of novolac epoxy resin with nearly symmetric poly(methyl acrylate-co-glycidylmelhacrylate)-0-polyisoprene. Ordered L can be swollen with up to about 30% of resin before macroscopic phase separation occurs, producing heterogeneous morphologies containing various amounts of L, C, worm-like micelles and pristine epoxy. At lower concentrations, disordered worm-like micelles transform into vesicles in dilute limit. According to [201]. Copyright 2003 Wiley...

Keywords. Porous polymer, Solubility parameter, Phase separation. Epoxy, Cyanurate,... 

Phase Separation Mechanism in Hexane-Epoxy Systems. . Influence of Reaction Parameters on the Morphology of Cyclohexane-Modified Epoxy Networks Prepared via CIPS... 

Procedure for the Preparation of Solvent-Modified and Macroporous Epoxy Networks via Chemically Induced Phase Separation (CIPS)... 

Fig. 10. Phase separation lines for the synthesis of macroporous epoxies via CIPS...

Plotted in Fig. 10 are phase separation lines, which were obtained with the gradient oven for epoxies cured in the presence of the above solvents [88]. At the left side of these lines, no phase separation occurs and the materials stay transparent. The right side of these phase separation lines gives the temperature and composition ranges where phase separation occurs. The onset of the phase separation, which gives one single point on the phase separation line, can easily be detected for each concentration, as the samples become opaque as a consequence of the formation of liquid domains in the pm-range. 

One particularly interesting system is the epoxy 2,6-dimethyl-4-heptanone as up to 40 wt % of this solvent can be easily mixed together with the epoxy precursors to generate a phase separation process. This allows one to verify experimentally the possible morphologies which were predicted based on the schematic phase diagram at concentrations below the phase inversion (see Fig. 7). Shown... 

Fig. 17. Schematic phase diagram explaining the phase separation behavior observed during the curing of epoxies in the presence of hexane...

Fig.18a-b. Scanning electron micrographs on cryo fractured surfaces of a macroporous epoxy prepared with 6 wt % hexane via the Cl PS technique showing a narrow size distribution b macroporous epoxy prepared with 7.5 wt % hexane via the CIPS technique showing a narrow size distribution. Reprinted from Polymer, 37(25). J. Kiefer, J.G. Hilborn and J.L. Hedrick, Chemically induced phase separation a new technique for the synthesis of macroporous epoxy networks p 5719, Copyright (1996), with permission from Elsevier Science... 

A further increase in the amount of solvent leads to the development of a bi-modal pore size distribution, as observed with SEM on samples prepared with concentrations of 10-15 wt % hexane (Fig. 18c,d). Similar bimodal distributions have also been reported with the octane and decane based systems [88,89] as well as in in rubber-modified epoxies prepared via phase separation [67,95-98]. 

Phase Separation Mechanism in Hexane-Epoxy Systems... 

The previous discussion has shown that the CIPS technique allows one to produce macroporous epoxy networks with either a narrow or bimodal size distribution. However, no indication has been given on the type of phase separation mechanism to yield these morphologies. As discussed earlier, the formation of a closed cell morphology can result either from a nucleation and growth mechanism or from spinodal decomposition. 

Fig. 25. Mean pore diameter, d, of macroporous epoxies prepared via CIPS with different solvents. gives the difference between initial solvent concentration and critical solvent concentration for phase separation...

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]. 

Fig. 26. Influence of isothermal curing temperature on the phase separation behavior oi macroporous epoxies prepared via CIPS with cyclohexane...

In the next section we will examine the influence of phase separation on the toughness of solvent-modified and macroporous epoxies prepared via CIPS. 

Experimentally a cyanate ester precursor mixture consisting of BPEC, 1 wt % BPE, and 100 ppm cobaltacetylacetonate was prepared and subsequently mixed with the cyclohexane phase separating solvent [86]. Essentially the same procedure as for the epoxy is used for sample preparation with the difference that the curing was done at 80 °C and post drying at 240 °C. 

The melting enthalpy related to the sample mass, represented by the area of this melting peak, increases nearly Hnearly with increasing amount of solvent once phase separation has occurred (Fig. 54). The appearance of such a melting peak has also been observed for cyclohexane-modified epoxies prepared with more than 20 wt % cyclohexane, having domain sizes larger than around 5 pm and a volume fraction of higher than around 13%. 

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