The tensile properties of strain‐crystallising vulcanisates. I. A new theory to explain strengthening

Abstract

ABSTRACT:The tensile properties of conventional and peroxide vulcanisates were studied over a range of crosslink densities at room temperature and at 90°C. At 90°C the tensile strength and elongation at break of vulcanisates of lower crosslink density are superior to those at room temperature, while for vulcanisates of higher crosslink density the reverse applies. When strain-induced crystallites form, they act as crosslinks shortening chains within the network. Shortened chains have lower entropies and a larger force is required for their continued extension, i.e., for a further reduction in their entropy. It is proposed that because these stiffer chains resist deformation, other less stiff chains are preferentially extended. This alters the network deformation pattern, forcing more chains to become load bearing and delaying the development of taut chains or chain sequences. Thus the formation of strain-induced crystals leads to the slope of the stress-strain curve rising rapidly. At elevated temperatures the rate of nucleation of strain-induced crystallites is slower but data on stress-strain curves obtained with different temperature programs show that, while strain-induced crystallization is essential for the development of high tensile strength, delaying their formation to higher elongations is advantageous for high tensile properties. In vulcanisates of higher crosslink density the rate of crystallization at high temperatures becomes too slow. Rupture occurs before strain-induced crystallites can form and protect the network by altering the network deformation pattern.

The tensile properties of strain‐crystallizing vulcanizates. III. The superiority of conventional over peroxide vulcanizates in terms of network microstructure

“…Strain‐induced crystallization, which gives rise to the upturn in the stress–strain curve12 and to stress relaxation,14 is time dependent, i.e., nonequilibrium conditions are experienced during the extension of a sample. Ring samples of IR/DCP and IR/MBTS were extended at 500, 100, and 0.5 mm/min at room temperature and at 500 and 100 mm/min at 90°C.…”

“…In a previous paper12 it was proposed that the upturn in the stress–strain curve and the resulting high UTS of vulcanizates that undergo strain‐induced crystallization are due to chain shortening resulting from the formation of strain‐induced crystalline crosslinks in the network. Shorter chains have lower entropies, are stiffer, and require a larger force for their extension.…”

“…IR/DCP also contained 0.5 phr of the antioxidant N ‐isopropyl‐ N ′‐phenyl‐ p ‐phenylenediamine (IPPD) (Bayer, Leverkusen, Germany) and IR/MBTS 1 phr IPPD to prevent oxidation at elevated temperatures. The compounding, vulcanization, and crosslink density determination procedures have been detailed in the first paper of the series 12…”

“…Extension was either measured by an Instron long travel elastomeric extensometer and expressed as a strain measurement or was measured as the distance (in millimeter) traveled by the upper grip. Extensions at elevated temperatures were conducted in a thermostatted water tank, as described in Paper I of this series 12. Three different types of sample were used: (1) D size ASTM D 412 dumbbell, (2) custom dumbbell with a gauge length of 8 mm and neck width of 2 mm, and (3) ring‐shaped samples (inner diameter 16 mm, outer diameter 20 mm) cut with a nonrotating ring cutter.…”

“…Bimodal networks undergo nonaffine deformation, with the short chains deforming to a lesser degree than the more easily deformable long chains, redistributing strain. In the first paper of the series12 it was suggested that stiffer chains, ensuing from chain shortening by strain‐induced crystalline crosslinks, result in the preferred extension of other, less stiff chains. This alters the network deformation pattern, delays chains being drawn taut to the point of fracture, and leads to higher ultimate tensile stress (UTS) and strain values.…”