How does a polymer glass resist fatigue crack growth?

We investigate fatigue crack growth in a polymer glass in which polymer chains are long and not crosslinked. Atoms bind by forces of two types: covalent bonds between repeat units along a chain, which resist chain scission, and noncovalent interactions between the chains, which resist chain slip. The covalent bonds are much stronger than the noncovalent interactions. When a crack impinges on a long chain, the chain slips and transmits tension over a segment of the chain. When the chain breaks at a single covalent bond, the energy stored in the segment dissipates. This molecular picture suggests a hypothesis: the fatigue threshold increases as the yield strength decreases. We analyze this hypothesis by developing a shear-lag model. We test the hypothesis by using high-molecular-weight poly(methyl methacrylate), and by modifying noncovalent interactions with plasticizers.