Evaluating the Linear Viscoelasticity of Vitrimers Using Start-up and Cessation Tests

Stress relaxation experiments are often performed on reprocessable polymer networks to evaluate their characteristic relaxation time, which represents the bond lifetime in the absence of structural contributions. While traditional cessation tests employ the application of an instantaneous small strain, the strain amplitude and the rate of strain applied during startup are known to influence the subsequent stress relaxation behavior. Here we illustrate this phenomenon using vitrimers with characteristic bond exchange times spanning seconds to hours. Our experiments show that controlling the dimensionless strain amplitude and shear rate applied during startup is critical to obtaining the true distribution of relaxation times for non-Maxwellian materials. In particular, consistent with rheology performed on thermoplastics, we find that experiments performed at small Deborah numbers, the dimensionless strain amplitude, and small Weissenburg numbers, the dimensionless shear rate, result in a stress relaxation time consistent with that observed in SAOS tests.