Synthesis of POSS-grafted epoxy-terminated hyperbranched polysiloxane for toughening epoxy resin at cryogenic temperature

A POSS-grafted epoxy-terminated hyperbranched polysiloxane was synthesized. • The reactive PEHP modifier formed a covalently bonded rigid-flexible epoxy network. • The modified epoxy achieved 112.26 MPa tensile strength at 77 K, a 22.4% increase. • The elongation at break at 77 K reached 2.19%, corresponding to a 37.7% increase. • Multiscale toughening arose from free-volume pore and Si-rich microphase domains. The brittleness of epoxy resins at cryogenic temperatures severely limits their reliable use in extreme engineering environments. Herein, a POSS-grafted epoxy-terminated hyperbranched polysiloxane (PEHP) was synthesized and employed to toughen epoxy resin, aiming to better satisfy the application requirements under extremely low-temperature conditions. Using POSS as a rigid multi-site core, a hybrid rigid-flexible dendritic branching structure of PEHP was constructed with precisely controlled molecular weight through alternating hydrosilylation and Piers-Rubinsztajn (PR) reactions. The modified epoxy resin exhibits an approximate 77.8% increase in room-temperature elongation at break and a 55.6% increase in fracture toughness. More importantly, at an extremely low temperature (77 K), its tensile strength, elongation at break, and fracture toughness increase by 22.4%, 37.7%, and 49.7%, respectively, compared to pure epoxy resin. This exceptional low temperature toughness stems from the synergistic interaction between rigid and flexible components at multiple scales. The bulky functionalized POSS disrupts local chain packing upon incorporation into the network, thereby enlarging the free-volume pore size and enhancing local chain mobility. The rigid core pins the crack front and promotes bowing to enhance fracture resistance, while flexible siloxane side chains increase chain mobility. Microphase separated domains further induce shear yielding under stress concentration and blunt the crack tip, impeding propagation. This synergistic multiscale dissipation mechanism gives the epoxy resin excellent cryogenic toughness, showing great potential for applications in extremely low temperature conditions. Download: Download high-res image (243KB) Download: Download full-size image