AbstractPolyisobutylene‐based model urethane networks have been prepared by crosslinking liquid α, ω‐di(hydroxyl)polyisobutylenes, i.e., PIB‐diols carrying exactly two CH2OH functions,Fn= 2.0 ± 0.1, and rather narrow molecular weight distributions,Mw/Mn= 1.5−1.6, with tritriphenylmethyl isocyanatedocumentclass{article}pagestyle{empty}begin{document}$$rm{HC} (hskip-6pthbox{---}prm{C}_6rm{H}_4hbox{---}rm{NCO})_3$$end{document}. Networks prepared withMn= 1400 and 7500 PIB‐diols, and with 90/10 and 80/20 mixtures of these PIB‐diols (bimodal networks), have been characterized by extraction, by the Flory–Rehner swelling method, and by the Mooney–Rivlin equilibrium modulus method, and tested by stress–strain measurements.Mcvalues of theMn= 1400 PIB‐diol network obtained by swelling (1550) and by equilibrium modulus studies (1500) were in excellent agreement with theMnof the prepolymer. Also theC2parameter was negligible in comparison toC1, suggesting the absence of interchain entanglements. This is the first hydrocarbon‐based polyurethane network that exhibits a negligibleC2value by stress–strain measurements of unswollen samples. TheMcvalues of theMn= 7500 PIB‐diol were also in good agreement with theMnof the prepolymer; however,C2was larger thanC1, indicating interchain entanglements. Evidence for strain‐induced toughening was observed with both networks prepared with theMn= 1400 and 7500 PIB‐diols. The ultimate properties of the two bimodal networks did not show improvement over those of the individual constituents; however, theMn's of the co
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