Toughening of a polysilsesquioxane network by homogeneous incorporation of polydimethylsiloxane segments

Abstract

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 end-capping and the coupling reactions were investigated by FT-IR, 29Si and 1H 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 KIc from 0.253 to 0.456MPam1/2, and GIc from 34.1 to 151.11J/m2. TGA 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.