New thermosets obtained from bisphenol A diglycidyl ether and hydroxyl-ended hyperbranched polymers partially blocked with benzoyl and trimethylsilyl groups

New epoxy thermosets with improved flexibility were prepared by chemical modification of bisphenol A diglycidyl ether (DGEBA) with hyperbranched polymers (HBPs). Hydroxyl-ended hyperbranched polyesters were modified by blocking part of the hydroxyl groups with trimethylsilyl or benzoyl groups. The curing of mixtures of DGEBA with various proportions of two modified HBPs using ytterbium triflate as cationic initiator was investigated using differential scanning calorimetry and thermomechanical analysis. The characterization of these materials was performed using several thermal analysis techniques and their morphology was investigated using electron microscopy. High proportions of HBPs reduced the glass transition temperature and the relaxed storage modulus but barely affected gelation. The overall curing shrinkage was controlled by the content of hydroxyl groups and by the changes of HBP molecular interactions during curing. The results indicated that the relative proportion and type of terminal groups play a role in the evolution of the curing and the properties of the thermosets. Hydroxyl groups promoted the covalent incorporation of the HBP to the network via hydroxyl-induced chain-transfer reactions, whereas benzoyl groups promoted phase separation. Formulations containing HBP blocked with benzoyl groups showed two phases connected through covalent linkages between the HBP-rich phase and the epoxy matrix. (C) 2010 Society of Chemical Industry