Entanglement network model relating tensile impact strength and the ductile‐brittle transition to molecular structure in amorphous polymers

A model is presented to account for the large variations in tensile and tensile impact strength of amorphous polymers from a consideration of an idealized entanglement network. The material strength under tensile impact conditions is shown to be predictable and to increase with the “fineness” of the entanglement network; a higher entanglement density leading to more molecular chains supporting the stress. The entanglement density is, in turn, shown to increase with number‐average molecular weight and the quotient of the length to the molecular weight of the chemical repeat unit (empirically found to be related to the critical enganglement molecular weight). Ductile behavior is demonstrated to occur under tensile impact conditions when the material strength σB exceeds the yield stress σy and brittle behavior when σy > σB. It is further demonstrated that the large variation in tensile impact strength among the amorphous polymers studied can be adequately accounted for in terms of the large and predictable variation in σB; the larger σB is relative to σy, the more the polymer can draw (absorbing energy in the process) until σB is reached. Surprisingly, the predictions of strength for high‐molecular‐weight polycrystalline materials also gave good agreement with experimental data.