COLD MILLING OF POLYISOBUTYLENE - MOLECULAR-WEIGHT DISTRIBUTION STUDY

High molecular weight polyisobutylene samples were degraded by milling at 320 K. The degradation process was followed by determining the course of the changes in molecular weight distributions (MWD) that were obtained by gel permeation chromatography. After long milling times degradation stops or at least the rate of rupture becomes extremely small, the molecular weight approaching an apparent minimum value (Mm) of 0.4 × 106. The rate of degradation decreases in the course of the first ½ to 1 hr from an initial value to one constant up to at least 3 hr if the maximum shear rate is higher than about 6 s−1. At lower shear rates the rate of scission is constant from the start. When milling is stopped for 24 hr, high initial rupture rate is observed on resumption of milling at high shear rate, again followed by a drop in rate to the same value as before the interruption. The initial rates are independent of shear rate, whereas the subsequent constant rates are proportional to the rate of shear. These observations are discussed in terms of an equilibrium between the formation of multimolecular “rheological units” and the tendency, due to thermal motion, to form a homogeneous entanglement network. The MWDs are compared with those calculated from a model based on a given relation between probability of scission (P) and molecular weight and an assumed probability distribution (Q) of rupture site along the length of the polymer moelecules. The observed MWDs are incompatible with those calculated from models in which Mm = 0 or in which breakage near the center of the molecule is favored. They agree rather well with computed MWDs based on the assumptions that P ∞ MW and Q is the symmetrical beta function between points along the molecule Mm removed from the ends, where Mm is (0.4–0.5) × 106. The mechanism of rupture appears to be the same for low and for high shear rates. Copyright © 1979 John Wiley & Sons, Inc.