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Chain orientation in natural rubber, Part I: the inverse yielding effect
P-A Albouy1, J Marchal, J Rault
1Laboratoire de Physique des Solides, UMR 8502, Université de Paris-Sud, Orsay, France.
The European Physical Journal. E, Soft Matter
|June 23, 2005
Summary
Natural rubber (NR) exhibits inhomogeneous deformation and inverse yielding during unloading, a phenomenon linked to micronecking. This study investigates the mechanical and crystalline properties associated with this unique behavior in stretched rubber.
Area of Science:
- Materials Science
- Polymer Physics
- Rheology
Background:
- Inhomogeneous deformation, similar to necking in polymers, is observed in stretched natural rubber (NR).
- This phenomenon, termed inverse yielding, occurs during recovery after drawing in the hardening domain and is characterized by constant stress propagation.
- Understanding the underlying mechanisms of inverse yielding and micronecking in NR is crucial for predicting material behavior.
Purpose of the Study:
- To investigate the conditions for nucleation and propagation of inhomogeneous deformations in natural rubber.
- To analyze the mechanical and crystallinity properties of natural rubber samples with and without inverse yielding.
- To elucidate the deformation mechanisms within the necking zone using advanced characterization techniques.
Main Methods:
- Mechanical testing of natural rubber samples with varying crosslink densities under different stretching conditions.
- Analysis of crystallinity and orientation using Wide-Angle X-ray Scattering (WAXS) at a synchrotron source.
- Correlation of macroscopic mechanical behavior with microscopic structural changes.
Main Results:
- Inhomogeneous deformations and inverse yielding were observed in natural rubber, particularly at lower crosslink densities.
- The necking zone, characterized by constant stress, showed deformation following flow lines, as evidenced by crystallite orientation.
- Local crystallinity measurements within the neck allowed for the determination of the local draw ratio.
Conclusions:
- Inverse yielding in natural rubber is associated with micronecking, a phenomenon likely present in all rubbers exhibiting a recovery stress-strain plateau.
- Deformation within the necking zone aligns with flow lines, and local draw ratios can be determined from crystallite orientation.
- The findings contribute to a deeper understanding of rubber elasticity and deformation mechanisms within the framework of polymer theories like Flory's.