Recovery of shear modification of polypropylene melts

Long-chain branched polypropylenes (LCB-PP) of different degrees of branching (up to 1 branch/10⁴ carbon atoms) and a linear polypropylene (L-PP) are deformed at different shear conditions (rate, time and deformation) leading to reversible modifications of the entanglement structure. These modifications recover with time. Because of it the intensity of the modification and the rate of recovery are studied. At shear rates between 1 s⁻¹ and 10 s⁻¹ lower rates modify stronger. The intensity increases with shear time to a maximum at times of about 1 h where the final deformation does not control the intensity. Obviously, the disentanglement created by shearing competes with the Brownian motion coupling entanglements. Also, the intensity increases with the degree of LCB where the increase is stronger at low degrees. The rate of recovery not influenced noticeably by the initial modification strongly depends on the degree of LCB. The pertinent recovery functions grow exponentially to the limiting value of the unmodified state. Three different recovery processes are found. The fastest one with a recovery time shorter than 10³ s is assigned to linear chains. The process with a time of about 5·10³ s independent of the degree of LCB is assumed to describe the recovery of the backbones. The times for the very slow recovery of the side chains increase with the degree of LCB (between 10⁴ and 10⁵ s for the investigated samples). The recovery strength reflects the initial modification and depends on the degree of LCB. By that, the recovery behaviour provides information on the molecular structure.