Mechanischer Scherabbau von Polymeren in Lösung mittels turbulenter Strömung, 1. Kinetik,Molekulargewichtsabhängigkeit und Frage des Bruchmechanismus

The mechanical shear degradation of poly(decyl methacrylate) with weight average molecular weights 1,0·10⁶ ≤ M?_w ≤ 1,7·10⁶, and molecular polydispersity ratio M?_w/M?_n = 5 in dilute solutions is studied in turbulent flow as a function of molecular weight using a special apparatus consisting of two vessels connected by a capillary. Shear stress and shear rate remained constant during degradation. The rate of degradation was followed up by molecular weight distribution curves using gel permeation chromatography and described by ?dc_i/dt = k_i·cⁿ, i being a high molecular weight species of the distribution. The reaction was found to be of the first order (n = 1) independent of solvent and of capillary length. Rate constants k_i in the molecular weight range from 3,2·10⁶ to 13,5·10⁶ proved to be proportional to the hydrodynamic volume of the polymer molecules expressed in terms of the product of intrinsic viscosity and molecular weight [η]_i·M_i. This corresponds to a linear relationship between k_i and M_i^1,75. Additional experiments show that this type of dependence on molecular weight holds only for turbulent flow; in laminar flow the result of the literature could be confirmed that there is a linear relation between k_i and M_i¹. Both results are independent of capillary length. As to the mechanism of breakage in turbulent flow it seems that in one step each macromolecule is broken simultaneously into several smaller parts.