ABSTRACT
The vulcanization of rubber is a complex process that produces a three-dimensional network structure. A network structure produced by any of the conventional vulcanization reactions may possess several types of flaws or imperfections. Among these are free chain ends, two of which exist for every primary polymer molecule. The two free chain ends per polymer molecule in the vulcanizates contribute little to the physical properties as predicted by Flory [1]. A schematic representation of a vulcanizate with the free ends emphasized is given in Figure 22. la. An estimate of the fraction of rubber existing as free chain ends is given by the simple relationship, 2Mc/M, where Me is the average molecular weight between crosslinkages and M is the average molecular weight of the primary molecules. In a normal styrene/butadiene rubber (SBR) tread vulcanizate this fraction probably amounts to over one-tenth of the polymer molecules. The incorporation of this fraction of the molecules into the network structure should contribute significantly to the properties of the vulcanizates [2,3]. This contribution is difficult to estimate because a terminal group chain linkage is trifunctional whereas an internal crosslink is tetrafunctional. The two types of linkages are illustrated in Figure 22. Ib.
