I. INTRODUCTION

Most rubber is produced from crosslinkable high molecular weight linear polymers with low glass temperatures [1 6]. The high molecular weight is necessary to obtain high extensibility in the ultimate elastomer, and the low glass temperature is required to obtain resilience. These precursors are collections of entangled linear molecules that ultimately are free to flow past one another and hence are viscoelastic liquids [1, 5, 7]. They are viscoelastic by virtue of their time-dependent mechanical response, which reflects the sluggish configurational changes of the molecules. Upon being crosslinked sufficiently, a chemical molecular network (rubber or elastomer) is formed that transforms the polymer into a viscoelastic solid, which does not flow. Like its precursor polymer, the viscoelastic properties are strongly dependent on time or frequency, temperature, pressure, and the presence of swelling solvent or filler. Among viscoelastic solids, rubber has the unique characteristic of preserving material integrity even when subjected to high stresses or strains, although the viscoelastic behavior is highly dependent on the large stresses or strains [3, 5, 8 10]. However, for sufficiently small stresses and strains, the viscoelastic behavior becomes invariant, and the linear viscoelastic regime prevails [1]. In this chapter, we describe the linear viscoelastic properties of rubber and its dependence on various parameters, including crosslink density, chemical structure, and molecular weight, with experimental data principally coming from measurements on a series of well-characterized and fully cured bisphenol-A-based epoxy...