Following Kramer et al. who compared the maximum local draw ratio inside craze fibrils or shear deformation zones for different types of network structures, here the critical thickness (ID(c)), below which apparently brittle amorphous glassy polymers demonstrate a macroscopic draw ratio comparable with the high values determined on a microscopic scale, is studied for two - principally different - types of physical networks. The value of ID(c) is determined for network structures based on polystyrene-poly(2,6-dimethyl-1,4-phenylene ether) (PS-PPE) blends. Two types of networks are compared obtained via blending high molecular weight PPE with either (i) a (standard) high molecular weight PS (PS(h)) or (ii) an extremely low molecular weight PS (PS(l)). In contrast to PS(h) PS1 cannot contribute to the entanglement network structure of the blend since the molecular weight falls well below the entanglement molecular weight of PPE and, consequently, it can be considered as a (glassy) solvent for PPE. The critical thickness of two network structures with the same network density but with different values of the natural draw ratio (lambda(max)) is experimentally determined via addition of (non-adhering) core-shell rubbers. ID(c) proves to be inversely proportional to the natural draw ratio of the network structure. The simple energy-based model for the prediction of ID(c) quantitatively explains the differences found.