Paper Number
SM31 

Session
Polymer Solutions & Melts

Title
Continuous relaxation spectra for MAOS characterization

Presentation Date and Time
February 15, 2017 (Wednesday) 11:15

Track / Room
Track 5 / Snowy Egret

Authors (Click on name to view author profile)

1. Martinetti, Luca (University of Illinois at Urbana-Champaign, Department of Mechanical Science and Engineering)
2. Singh, Piyush K. (University of Illinois at Urbana-Champaign, Mechanical Science and Engineering)
3. Soulages, Johannes M. (ExxonMobil Research and Engineering, Corporate Strategic Research)
4. Ewoldt, Randy H. (University of Illinois at Urbana-Champaign, Mechanical Science and Engineering)

Author and Affiliation Lines (in printed abstract book)
Luca Martinetti¹, Piyush K. Singh¹, Johannes M. Soulages², and Randy H. Ewoldt^1
1Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801; ²Corporate Strategic Research, ExxonMobil Research and Engineering, Annandale, NJ 08801

Speaker / Presenter 
Martinetti, Luca

Text of Abstract
We show how any analytical prediction for (strain-controlled) medium-amplitude oscillatory shear (MAOS) can be coupled with a continuous distribution of relaxation times. This applies to models that are not inherently time-strain separable, and is important for improved certainty when inferring molecular information from these bulk rheological measurements.

Here, we demonstrate inference using continuous spectrum MAOS with aqueous poly(vinyl alcohol) (PVA) transiently-crosslinked by sodium tetraborate (Borax) [Bharadwaj et al.]*. To gain physical insight into network microstructure and modeling, the frequency-dependence and sign changes of all four MAOS measures ([e1](ω), [e3](ω), [v1](ω), [v3](ω)) are considered. The mesoscopic network model contains a single nonlinear parameter (χ) for elastic strain-stiffening, hypothesized to arise from a combination of both (i) finite chain extensibility, and (ii) stretch-induced crosslink formation. The use of a continuous relaxation spectrum derived from the log-normal distribution dramatically improves the SAOS fit, but surprisingly makes the MAOS fit worse if the nonlinear parameter is assumed constant for all relaxation times. Qualitative and quantitative agreement with the asymptotically-nonlinear signature is achieved with the simple use of a step-function-type distribution for the nonlinear parameter χ that excludes the longer relaxation modes. Our results suggest that MAOS is more sensitive than SAOS to the longer relaxation times.

While the work here is focused on a specific polymeric system, it represents the broad potential contribution of continuous relaxation spectra and asymptotic, leading-order nonlinearities to enable structure-rheology insight, and model selection for soft materials in general.

* Bharadwaj N. A., Schweizer K. S., Ewoldt R. H., “A strain stiffening theory for transient polymer networks under asymptotically-nonlinear oscillatory shear”, submitted (2016).