B. Zhu et al., Toughening of a polysilsesquioxane network by homogeneous incorporation ofpolydimethylsiloxane segments, POLYMER, 41(20), 2000, pp. 7559-7573
Methods were developed to characterize the fracture behavior of a condensat
ion cure polysilsesquioxane network, and to toughen the network with homoge
neously incorporated polydimethylsiloxane (PDMS) segments. Hydroxyl termina
ted PDMS short chains were end-capped with tetraethoxy silane and the end-c
apped segments were coupled with the oligomeric silsesquioxane bearing sila
nol 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 chain
s was achieved with minimum self-condensation and cyclization, and a comple
te coupling of the functionalized PDMS with the oligomeric silsesquioxane w
as achieved with no self-condensation of the PDMS chains. To toughen the ne
twork such a coupling reaction was necessary, otherwise PDMS chains formed
a separate phase which was ineffective. Short PDMS chains and silsesquioxan
e oligomers were incompatible and a ternary phase diagram with toluene as t
he 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 increase
d 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 0.4
56 MPa m(1/2), and G(Ic), from 34.1 to 151.11 J/m(2). TCA showed the therma
l stability of the network was retained after PDMS toughening. Enhanced ine
lastic deformation was responsible for the increased fracture toughness. Up
on 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 mod
el, while no evidence of such yielding was seen for the untoughened network
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