Uniaxial cold drawing of plastic - What happen to the material's modulus?

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Uniaxial cold drawing of polybutylene succinate results in increased strength and elongation at break along the machine direction (MD) with higher draw ratios, while modulus slightly decreases. In contrast, strength decreases in the transverse direction (TD), but elongation at break increases slightly, and modulus significantly rises from 500MPa to 750MPa at the highest draw ratio. This behavior contradicts existing studies that suggest modulus should decrease along the TD due to less rigid molecular bonds. The discussion seeks explanations for this unexpected increase in modulus along the TD. Insights into this phenomenon are requested from the community.
yijing84
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Hi all.
I am now working on the uniaxial cold drawing on the plastic sheet (polybutylene succinate, a semicrystalline polymer).
After uniaxial cold drawing, as expected, strength increased along the drawing direction/ machine direction (MD).
Strength and elongation at break drastically increased with increasing draw ratio along the MD.
However, modulus slightly reduced with the draw ratio.

Along the transverse direction (TD), strength reduced as expected with the draw ratio.
However, elongation at break slightly increased with draw ratio.
Furthermore, modulus significantly increased with draw ratio from 500MPa to 750MPa at the highest draw ratio along the TD.

According to most of the study, modulus should reduce along the TD with draw ratio as the less regid molecular bond along the TD as compared to along the MD (rigid covalent bond). However in my case, it is totally different.
I am here kindly ask for any possible explanation on this phenomena on the modulus along the TD?
Thank you so much.
 
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