CONICET | Buscador de Institutos y Recursos Humanos

Polyhedral oligomeric silsesquioxanes (POSS), are nanosized cage structures that can be incorporated into linear or thermosetting polymers to improve their thermal and oxidation resistance, and reduced flammability. If the POSS has a functional group, it can be incorporated into the network structure as pendant groups. Complete incorporation of the POSS requires that the mixture of POSS and polymer remains homogeneous. It has been observed that a phase separation process may take place when employing a POSS bearing organic groups that are not compatible with the epoxy network (1,2). Phase separation leads to a heterogeneous material which may also lead to interesting properties. The objective of this work is to study phase separation in an epoxy matrix modified with a POSS in the course of polymerization. The modified epoxy network was obtained by polymerizing a stochiometric mixture of a diepoxy monomer (DGEBA, DER 332, Dow), with a weight per epoxy group equal to 174.25 g/mol, and 4,4-methylenebis(2,6-diethylaniline) (MDEA, Lonza), mixed with different amounts of Glycidyloxypropyl-heptaisobutyl POSS (Ibu-POSS). This POSS shows two different thermal transitions, possibly due to the presence of two different crystalline phases, at 138 and 118 ºC (1). The reaction was carried out at 135º and 115ºC. It has been shown that in these conditions the reaction between the POSS and the amine is negligible (2). Some samples were post-cured at 190ºC for 4 hours. Optical microscopy (TOM, Leica DM LB) with a heating stage (Linkam THMS600) was used to determine cloud-point diagrams and follow the evolution of morphologies. The final morphologies were also observed by scanning electron microscopy (SEM, Jeol JSM 6460LV). At 135ºC, the initial mixture of monomers and POSS is miscible for concentrations below 18 wt% POSS. A liquid-liquid (L-L) phase separation is observed in the course of the reaction for more than 3 wt% POSS, giving rise to a morphology consisting in dispersed POSS-rich-droplets in the micron range. When the mixture is polymerized at 115ºC (below the crystallization temperature of POSS), the situation is different. At this temperature mixtures with less than 10 wt% POSS are initially miscible, and phase-separate during the reaction if the concentration of POSS is higher than 2 wt%. The process of phase separation was followed by TOM for a mixture of 5% wt POSS, and it was observed that it took place in two steps. The first one is attributed to a solid-liquid (S-L) phase  separation, and gives rise to the formation of tetragonal POSS-rich-particles, which grow to a size of several micrometers. After some time, a second process attributed to a L-L phase separation takes place within the matrix. An optical microscopy of the material in this second stage showed large tetragonal particles coexist with smaller spherical particles. This coexistence of tetragons and spheres was confirmed by SEM images. This second phase separation can be explained considering that a metastable L-L equilibrium could exist, buried at temperatures below the stable S-L equilibrium. As conversion increases, the temperature of the L-L equilibrium shifts and eventually reaches the reaction temperature, and at this point L-L phase separation is kinetically favored over S-L phase separation.  This morphology consisting in dispersed tetragons and spheres was found in all phase-separated samples cured at 115ºC observed by SEM. The size of the tetragons increased with the concentration of POSS. After post-curing, the cubic morphology is conserved, despite the fact that POSS crystals are melted during post-curing for concentrations up to 5% wt POSS, while for 10% wt POSS only spheres are observed