A Broad-Line NMR Investigation of Isoprene Rubber Filled with Carbon Black
Abstract The results of NMR measurements on IR filled with various carbon blacks lead to the following conclusions. The rubber matrix is a heterogeneous system characterized by two rubber phases of different chain mobilities. One phase is almost similar to unfilled IR at the temperature of measurement, while the properties of the other are nearly identical to those of pure IR at a lower temperature. The following observations apply to the phase with restricted mobility : 1. The quantities of rubber and carbon black in this phase are proportional below a critical loading, which supports the assumption that immobilization is limited to the immediate surroundings of the filler aggregates. The critical loading varies with structure and defines the packing density above which the formation of a coherent gel becomes important. 2. The rubber content decreases with rising temperature, showing that the activation energy for the onset of segmental motion varies within the phase. The mobility of the rubber chains directly bound to the surface is restricted even far above the glass transition temperature. 3. The degree of immobilization varies with temperature but is mostly independent of carbon black type and loading. 4. The actual distribution of immobilized rubber within an aggregate is not completely known, but some evidence exists for the immobilization of a thin surface layer as well as parts of the occluded rubber. A considerable immobilized rubber fraction is independent of surface area and structure.