Microstructure and Nonlinear Viscoelasticity in Oriented Polypropylene
MASSACHUSETTS INST OF TECH CAMBRIDGE DEPT OF MATERIALS SCIENCE AND ENGINEERING
Pagination or Media Count:
Oriented semi-crystalline polymers do not show linear viscoelastic mechanical behavior in useful ranges of strain. The relationship of the nonlinear viscoelasticity of the stress relaxation modulus to microstructural features, including crystallinity, crystalline orientation and amorphous orientation, was examined using oriented polypropylene film. The microstructure was altered by annealing in such a way that crystallinity and crystalline orientation were kept constant. Birefringence and sonic modulus were used to measure amorphous orientation. The strain dependence of the isochronal stress relaxation modulus and the amorphous orientation were then compared for unannealed and annealed films. By controlling crystallinity and crystalline orientation, structural changes required to cause changes in nonlinear behavior may have been prevented. The structural change caused by annealing was not a simple retraction of amorphous material, as evidenced by an increase in amorphous orientation. It is proposed that, upon annealing, micro-fibrillar motion and increased lamellar perfection cause a reduction in crystal-crystal connections and an increase in crystal-amorphous connections along the draw direction, thereby decreasing the modulus.