Linear and Nonlinear Uniaxial Extensional Behavior of A-B-A Thermoplastic Elastomers: Structure-Properties Relationship and Modeling

Luca Martinetti (University of Minnesota - Twin Cities)

At service temperatures, A-B-A thermoplastic elastomers (TPEs) behave similarly to filled (and often entangled) B-rich rubbers, and their viscoelastic behavior is largely dictated by chain mobility of the B block rather than by microstructural order. Relating the small- and large-strain response of undiluted A-B-A triblocks to molecular parameters is a prerequisite for designing associated TPE-based systems that can meet the desired linear and nonlinear rheological criteria. In this talk, the behavior is examined in uniaxial extension and two examples are discussed.
(1) The critical-gel-like linear viscoelasticity typical of rubbers and A-B-A TPEs is analyzed in terms of a simplified wedge distribution of relaxation times, and a relation between the observed power-law exponent and molecular structure is established. The measured low-frequency response, originating from the incipient glass transition of the A domains, is exploited and extrapolated to lower frequencies via a sequential application of the fractional Maxwell model and the fractional Zener model.
(2) The relationship between large-strain response and network structure is investigated, by (i) highlighting the effect of linear relaxation mechanisms on the tensile behavior, (ii) inferring the governing mechanism of rupture, and (iii) accounting for the strain rate dependence of the ultimate properties. It is shown that the Rubinstein-Panyukov non-affine slip-tube model is unable to capture the strain dependence of the stress, while the Edwards-Vilgis slip-link model provides a reasonable description of the data even at the higher rates of strain.

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