The effect of chemical structure on the phase separation time/temperature dependencies in some thermoplastics modified thermoset systems

The analysis of the cure induced phase separation time/temperature dependence of thermoplastics (TP) modified thermosetting (TS) systems is necessary in the cure routine design process so as to optimize the final material mechanical properties. We found the cure induced phase separation time/temperature can well be described with the Arrhenius equation; the phase separation activation energy E_a(ps) generated from Arrhenius equation is not affected by the TP content, TP molecular weight, cure rate or phase separation detection means, and the chemical environments is presumed to play a vital role on E_a (ps). How the chemical environment will affect the miscibility between TP and TS species and further the phase separation activation energy E_a (ps), there is no theoretical or experimental work on such problems. In this paper, we make a detail analysis on the question how the chemical environments changes along the TP or TS chemical structure, and how the E_a (ps) is related to the chemical environments. As part of our serious work, our main focus here is on some TP/TS systems which show upper critical solution temperature (UCST) type phase behavior. The component chemical structures of thermoplastics, thermosets, crosslink agent and stoichiometric ratio etc . were changed in certain way, the corresponding cure induced phase separation activation energy were determined in a wide time-temperature window. The TP and TS species employed here are poly (ether imide ), epoxy, cyanate ester, diamine and anhydride. The study is carried out by means of transmission optical microscopy and differential scanning calorimetry. The results indicate that the phase separation activation energy E_a (ps) varied with the structures of thermoplastics, thermosetting monomers and crosslink agent, and also changed with the stoichiometric ratio. E_a (ps) decreases with the increase of EEW (epoxy equivalent weight) of DGEBA monomers. The structure of thermoplastic PEI also shows a remarkable effect on E_a (ps), the systems with PEI bearing large stereo obstacle groups favoring miscibility of the TP and TS present higher E_a (ps). At the same time E _a(ps) increases with the content of crosslink agent of DDM and MTHPA, while decrease with the content of DDS.The change of E_a(ps) with the chemical structure of the component can be explained in view of the interaction energy density theory which based on the Hildebrand-Scatchard regular solution theory. By calculation of the interaction activation energy density with the solubility parameter, it was found that the factors unfavoring miscibility between TP and TS species will decrease the value of E_a (ps), while those factors favoring miscibility of the systems will increase the value of E_a (ps), some systems with very small value of interaction energy density parameter will not show phase separation throughout the cure reaction. The phase separation diagram of thermoplastics with thermosetting oligomers can verify the interaction energy density approach in understanding the change of E_a (ps) with chemical structure of the components.