THERMODYNAMIC ANALYSIS OF PHASE-SEPARATION IN RUBBER-MODIFIED THERMOSETTING POLYMERS - INFLUENCE OF THE REACTIVE POLYMER POLYDISPERSITY

A thermodynamic simulation of the phase separation process in a modifi ed thermosetting polymer was carried out. The polydispersity of the ge nerated polymeric species was taken into account in the frame of a con ventional Flory-Huggins equation. The example considered in the simula tion was a diglycidyl ether of bisphenol-A (DGEBA)-ethylenediamine (ED A), epoxy-amine polymer, modified by the addition of 15 wt% castor oil (monodisperse modifier). The size increase of the oligomeric species and the corresponding decrease of the entropic contribution to the fre e energy of mixing made a modifier-rich phase (beta-phase) segregate f rom the matrix (alpha-phase) at a particular conversion level. The bet a-phase is enriched in monomers and low-molecular-weight species of th e polymer distribution. This produces a significant decrease of the be ta-phase conversion with respect to the overall conversion. The monome r with the smaller size and functionality is preferentially segregated into the beta-phase, leading to a stoichiometric imbalance. When a se mipermeable beta-phase is assumed, i.e. no oligomeric species are allo wed to transfer to the alpha-phase, a secondary phase separation insid e the beta-phase is generated. This leads to a sub-matrix (delta-phase ) which is rich in modifier, and a sub-segregated phase (gamma-phase) which is rich in thermosetting polymer. This process may continue well beyond the gelation of the alpha-phase, due to the low conversion lev el of the beta-phase at the time the alpha-phase gels. The thermodynam ic simulation explains some recent experimental observations in system s of commercial interest.