PALS study of epoxy matrices: Self-assembly of block copolymers and its capability for nanostructuring thermosetting systems

Abstract

The free volume in epoxy systems based on diglycidyl ether of bisphenol-A (DGEBA) fully cured with aminic hardeners having different chemical structures has been studied by means of positron annihilation lifetimes spectroscopy. The results are compared with those obtained from the analysis of the macroscopic specific volume changes by using pressure-volume-temperature (PVT) experimental technique. An excellent correlation between the volumes measured at macro and nanoscales was found for the epoxy systems fully cured with aminic hardeners. On the other hand, a systematic study on the dependence of the volumes at nano-scale in epoxy systems cured with two selected aminic hardeners at different pre-cure temperatures revealed that the pre-cure temperature, as well as, the structure of the hardeners governs the packing of the molecular chains of the epoxy network and its influence in the volume and number density of the nanoholes. In the second part of this work, a systematic study of the effect of several epoxidation degrees of butadiene block in a commercial polystyrene-polybutadiene star block copolymer (SB) has been carried out to control nano-ordering in thermosetting epoxy systems. For this purpose, blends of DGEBA cured with 4,4′-methylenebis(3-chloro-2,6-diethylaniline) (MCDEA) containing 30 wt % neat SB and several epoxidized SB star block copolymers have been synthesized. At low epoxidation degrees, interactions with the epoxy matrix are not sufficiently favorable and macroscopic phase separation takes place, leading to a phase-inverted morphology where the block copolymer may become the matrix. For high epoxidation degrees, however, nanostructured thermosetting systems, being the epoxy-rich phase the matrix of the blends, have been obtained. It has been proved that, in blends containing SepB61 and SepB76 star block copolymers, the copolymer self-assembles into a well-defined hexagonally ordered structure, where cylinders are formed by PS arranged in an epoxy matrix containing both epoxidized and non-epoxidized butadiene units. SAXS experiments show that there are not strong differences in the long spacing between the different systems because the repeated distance between PS cylinders for all the blends is around 41-42 nm. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.