Toughening of brittle polymers.

The general principles of toughening brittle polymers are presented in this thesis. The effects of a process zone on R-curve behavior in a rubber-modified epoxy are first investigated to support the mechanistic principles of toughening brittle polymers. The existence of an R-curve behavior in the rubber-modified epoxy indicates that extrinsic toughening by the development of a process zone is significant. Furthermore, it appears that an increase in the intrinsic toughness is induced in this material. The effects of rate on crack growth in a rubber-modified epoxy are then examined to illustrate the nature of the crack instability in toughened brittle polymers. Dramatic rate effects on the fracture behavior are observed in a 10-phr rubber-modified epoxy. This behavior appears to arise from the velocity dependence of the intrinsic toughness for this material. Furthermore, the crack instability may result from a ductile (cavitation)-to-brittle (no cavitation) transition. The effects of the cross-link density on toughening efficiency are also studied for a rubber-modified epoxy to demonstrate the importance of the constitutive properties in polymer toughening. It is observed that the addition of a rubbery phase may effectively increase the ratio of cohesive strength to yield stress of the modified epoxies. In particular, the steady-state toughness can be substantially enhanced when this ratio is elevated to a sufficient level. The results described above suggest that two approaches can be employed to toughen a brittle polymer. The first is to manipulate the cohesive processes at a crack tip so as to increase the intrinsic toughness of the polymer. The second approach is to induce R-curve behavior by triggering non-linear deformations in a process zone. The essence of these two approaches is to modify the constitutive behavior of the system. This lies at the heart of the successful strategy to toughen brittle epoxies by the incorporation of second-phase rubber particles. Both the intrinsic cohesive processes and the non-linear processes in the process zone may be very rate-sensitive. These rate effects, and accompanying instabilities are of practical importance in interpreting the strength characteristics of toughened brittle polymers.Ph.D.Applied SciencesMaterials scienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132514/2/9964774.pd