Novel hyperbranched polyphenylsilsesquioxane‐modified cyanate ester resins with improved toughness and stiffness

High‐performance thermosetting resins should have good toughness and stiffness, so simultaneously toughening and stiffening is the main target in developing high‐performance resins. A novel modified cyanate ester resin with improved toughness and stiffness was developed by copolymerizing 2,2′‐bis(4‐cyanatophenyl)isopropylidene (CE) with hyperbranched polyphenylsilsesquioxane (HBPPSi). The mechanical properties and their nature were systematically investigated from the viewpoint of structure‐property relations using positron annihilation lifetime spectroscopy and spectral analyses. It is found that a suitable content of HBPPSi in CE resin can effectively improve toughness and stiffness. In the case of the CE resin modified with 10 wt% HBPPSi, its impact and flexural strengths are 21 kJ m −2 and 148 MPa, respectively, about 2.6 and 1.4 times of those of neat CE resin. The flexural modulus increases from 3.0 (for neat CE resin) to 3.4 GPa. The results of dynamic mechanical analyses also corroborate the static mechanical properties. The improved toughness and stiffness of CE resin can be attributed to the synergistic effect resulting from changes of both polymer chain structure and aggregation state structure. These attractive features of HBPPSi/CE resins suggest that the method proposed herein may be a new approach for the development of high‐performance resins for cutting‐edge industries. Copyright © 2011 Society of Chemical Industry