Abstract
This paper investigates the influence of molecular properties (molecular weight, polydispersity (PDI) and molecular weight distribution (MWD)) on the mechanical fatigue of model systems of anionic synthesized polystyrene (PS, low PDI), their blends (bimodal MWD) and commercial PS (broad PDI) under strain-controlled oscillatory shear (torsion). A power-law relation between lifetime Nf and strain amplitude γ0 (Wöhler curve) was found, with the exponent independent, but the pre-factor highly dependent on molecular characteristics. Additionally, nonlinear changes of the stress response, due to ongoing fatigue, were quantified via Fourier Transform (FT), to analyze besides the storage modulus G′, the intensities of the second (I2) and third (I3) harmonics over the fundamental one (I1). The behavior of G′ and I3/1 was found to depend on molecular properties, with I3/1 being more sensitive to failure onset than G′, while an increase of I2/1 above the noise level could be correlated with the occurrence of macroscopic cracks.
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