Rubber of high molar mass

like cis-polybutadiene

shows a stress peak on theenineering stress-strain curve during stretching at room temperature. In this work cis-polybutadiene samples were swollen in a poor solvent

CHCl3/EtOH (1/1 v/v)

for differenttimes. It was found that both the initial modulus and the stress peak on stretching de-creased in magnitude with increasing swelling time and the peak disappeared entirely after1 hour of swelling. On further swelling the initial modulus increased somewhat and a small stress peak re-appeared after swelling for 2h.The disappearance of the stress peak after swelling is interpreted as the result of disruption of cohesional entanglements present in the initial rubber sample.The re-appearence of a small stress peak and the increase of modulus on further swelling are interpreted as being of the same nature as the phenomenon of anti-plasticization.It is the result of forming some new cohesional entanglements of larger binding energies through longer range chain segmental motions excited after the disruption of the previously existing cohesional entanglements in the rubber.Thus an understanding of the stress peak on stretching a high molar mass rubber and the phenomenon of anti-plasticization on molecular level has been put forward.