Ultimate tensile properties of elastomers. VII. Effect of crosslink density on time–temperature dependence

Data on tensile strength and elongation at break for a series of Viton A‐HV vulcanizates are discussed. The data were obtained at various extension rates at temperatures from −5 to 230°C (25 ≲ T — T g ≲ 260°C) on seven vulcanizates having crosslink densities v e (estimated from C 1 in the Mooney‐Rivlin equation) from 0.46 × 10 −5 to 24.4 × 10 −5 mole/cm 3 . At an extension rate of 1 min −1 , an increase in v e affects the tensile strength σ b (based on the undeformed cross‐sectional area) and the true tensile strength σ b σ b (based on the cross‐sectional area of a deformed specimen) as follows: σ b is essentially constant at a low temperature; it passes through a decided maximum at intermediate temperatures; and it increases to a plateau at elevated temperatures. In contrast, λ b σ b decreases markedly at all temperatures, an exception being the most lightly crosslinked vulcanizate(s). Application of time—temperature superposition to the ultimate‐property data gave log a T ; its temperature dependence is that typical of nonpolar rubbery polymers. Data on the vulcanizates were compared in corresponding temperature states by plotting log 273σ b / T , log 273λ b σ b / T , and (λ b — 1)/(λ b — 1) max against log t b /( t b ) max , where t b is the temperature‐reduced time to break and ( t b ) max is the value at which the ultimate extension ratio λ b attains its maximum, (λ b ) max . Except for the most lightly crosslink vulcanizate, the comparison shows that 273λ b σ b / T and (λ b — 1)/(λ b — 1) max are substantially independent of (or only weakly dependent on) crosslink density, that 273λ b / T increases with v e , and that 273λ b / T ∝︁ v e 0.6 and λ b ∝︁ v e −0.4 at a large value of t b /( t b ) max .