Structural Characterization of Sulfur-Vulcanized Rubber Networks

Abstract

Abstract Studies of the chemistry of vulcanization today occupy a central position in current efforts to achieve better product performance from available natural and synthetic rubbers. They provide technologists with increasingly realistic pictures of the molecular make-up of vulcanizates from which relations between physical properties and chemical structure may be deduced. In addition these studies are aimed at an understanding of vulcanization, sufficiently advanced in chemical mechanistic terms, to permit the effect of changes in curing system on vulcanizate structure to be rationally predicted. Perhaps the most important and fascinating aspect of current activities in vulcanization chemistry is the problem of determining the structure of rubber vulcanizates. Part of this problem forms the subject of this Review. In order to keep this article within reasonable proportions two limits have been observed. First, attention in Section IV has been restricted to “sulfur-vulcanized” rubbers, i.e., where vulcanization is effected using recipes containing elemental sulfur or sulfur donor (e.g., thiuram disulfides or dithiobis-amines). Secondly, we shall distinguish between the in toto vulcanizate and the corresponding network (as defined by Moore) restricting our attention to the latter. Thus, we are concerned with the crosslinked assembly of rubber chains (the network) apart from filler particles, residual curatives and simple molecular products of the vulcanization reactions. These latter constituents are extra to the network and may be examined as a separate analytical exercise. The physical and chemical analyses of rubber networks cannot be undertaken entirely by classical means, and some unique problems are encountered which have required, and still require, imaginative techniques for their solution. An attempt has been made to deal with the important progress which has been made to date, emphasis being given to the most practicable and reliable methods which have been adopted and practiced. It will become evident that there are wide gaps in our knowledge, some of which must be filled even before structural characterization of certain networks can be tackled. It is therefore part of the intention of this review to point out the important outstanding problems and to stimulate activity in these areas while defining as clearly as possible the numerous pitfalls which exist in this demanding type of investigation.