Paper Number
SF15 

Session
Self-assembly and Flow-induced Systems/Gels

Title
Microstructural origins of yield, strain hardening and hysteresis in thermoplastic elastomers under uniaxial deformation: An in-situ tensile-SANS study

Presentation Date and Time
October 6, 2014 (Monday) 5:40

Track / Room
Track 6 / Washington C

Authors (Click on name to view author profile)

1. López-Barrón, Carlos R. (ExxonMobil Chemical Company)
2. Eberle, Aaron P. (ExxonMobil)

Author and Affiliation Lines (in printed abstract book)
Carlos R. López-Barrón¹ and Aaron P. Eberle^2
1ExxonMobil Chemical Company, Baytown, TX; ²ExxonMobil, Annandale, NJ

Speaker / Presenter 
López-Barrón, Carlos R.

Text of Abstract
In-situ morphological characterization of elastomeric materials under periodic deformation is crucial to understand the structural origin of equilibrium mechanical properties and time dependent properties like hysteresis and fatigue. We present unique measurements that combine the power of small-angle neutron scattering (SANS) and precision mechanical measurements to measure the morphology on the nanoscale and the mechanical properties simultaneously. To this end a new instrument was developed at the NIST Center for Neutron Research (NCNR), consisting of a Sentmanat extensional rheometer (SER) to measure the uniaxial stress response during SANS measurements. The first in-situ tensile-SANS (tenSANS) measurements were performed on two commercial thermoplastic elastomers (TPEs) consisting on styrene-isoprene-styrene (SIS) triblock copolymers with two styrene contents: 14 wt% (SIS14) and 22 wt% (SIS22). Under quiescent conditions, SIS14 forms a bcc lattice with glassy (polystyrene) spheres, whereas SIS22 forms glassy rods arranged in hexagonal structures. The glassy domains are connected by (polyisoprene) amorphous tie chains. By using advanced SANS nano-metrology techniques, we were able to quantify the special arrangement of the glassy domains and thereby determine affine deformation at low strains (in the linear regime), and cluster formation at high strains where yield and strain hardening is measured. Remnant nano-deformation is observed after loading-unloading cycles, which is directly correlated to the macroscopic permanent set and deformation hysteresis. In addition, the orientation angle of the clusters associated to yield and the alignment of the rods in the SIS22 system were quantified. Complementary techniques (in-situ WAXS and SALS and birefringence measurments during extension) are being used to quantify cavitation, strain-induced crystallization, and molecular alignment during uniaxial extension, and ultimately correlate these microstructural features with the measured stress.