Toughening of a polysilsesquioxane network by homogeneous incorporation of polydimethylsiloxane segments

Methods were developed to characterize the fracture behavior of a condensation cure polysilsesquioxane network, and to toughen the network with homogeneously incorporated polydimethylsiloxane (PDMS) segments. Hydroxyl terminated PDMS short chains were end-capped with tetraethoxy silane and the end-capped segments were coupled with the oligomeric silsesquioxane bearing silanol ends. The endcapping and the coupling reactions were investigated by FT-IR, Si-29 and H-1 NMR, and GPC. The complete end-capping of the PDMS chains was achieved with minimum self-condensation and cyclization, and a complete coupling of the functionalized PDMS with the oligomeric silsesquioxane was achieved with no self-condensation of the PDMS chains. To toughen the network such a coupling reaction was necessary, otherwise PDMS chains formed a separate phase which was ineffective. Short PDMS chains and silsesquioxane oligomers were incompatible and a ternary phase diagram with toluene as the third component was constructed to define a concentration window for the coupling reaction. When homogeneously reacted into the resin network, all the PDMS chains of degrees of polymerization (DP) between 8 and 55 increased the fracture toughness, and within this range the longer chains were more effective. Ten parts of PDMS of DP 55 increased the K-1c from 0.253 to 0.456 MPa m(1/2), and G(Ic), from 34.1 to 151.11 J/m(2). TCA showed the thermal stability of the network was retained after PDMS toughening. Enhanced inelastic deformation was responsible for the increased fracture toughness. Upon re-initiation of a crack, the toughened network developed a plastic zone the size of which was consistent with the calculated zone from Irwin's model, while no evidence of such yielding was seen for the untoughened network. (C) 2000 Elsevier Science Ltd. All rights reserved.