To improve the mechanical properties of glycidyl azide polymer (GAP)-based polyurethane network binders, new polyether-type block copolymer binder matrices were prepared using GAP and polyethylene glycol (PEG) at various wt. ratios, whilst varying the ratio of Desmodur N-100 pluriisocyante (N-100) and isophorone isocyanate (IPDI) as curatives. Swelling experiments were used to determine the solubility parameters using Gee’s plot and the cross-link densities, and the molecular weights of the polymer chains between cross-links (Mc) were determined using the Flory-Rehner equation. Larger quantities of IPDI and GAP resulted in looser network binders and, as expected, the elongation was improved by the increase in Mc resulting from increasing the IPDI content. Also, the introduction of PEG into binder recipes increased the elongation despite the decreased value of Mc. A high initial modulus was exhibited by copolyurethane networks containing more than 30 wt% PEG prepared using both N-100 and IPDI curatives, except for the N-100-based curative system; this finding is attributable to the stress-induced crystallization of PEG in the binder matrices. A greater content of PEG reduced the values of Tg of all of the copolyurethane networks. This property is a desirable one for solid-propellant applications, in which good low-temperature performance is required.