Premium
Cold milling of polyisobutylene—A molecular weight distribution study
Author(s) -
Van Der Hoff B. M. E.,
Chopra V. K.
Publication year - 1979
Publication title -
journal of applied polymer science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.575
H-Index - 166
eISSN - 1097-4628
pISSN - 0021-8995
DOI - 10.1002/app.1979.070230815
Subject(s) - molar mass distribution , shear rate , reaction rate constant , polymer , rheology , materials science , gel permeation chromatography , composite material , shear (geology) , thermodynamics , constant (computer programming) , degradation (telecommunications) , chemistry , polymer chemistry , analytical chemistry (journal) , chromatography , kinetics , physics , classical mechanics , telecommunications , computer science , programming language
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 ( M m ) of 0.4 × 10 6 . 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 M m = 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 M m removed from the ends, where M m is (0.4–0.5) × 10 6 . The mechanism of rupture appears to be the same for low and for high shear rates.