Origin of High Elastic Recovery of Hard-elastic Polypropylene Film at Room Temperature: the Mixed Contribution of Energy Elasticity and Entropy Elasticity

摘要

The crystalline and amorphous regions were alternately arranged in the hard elastic polypropylene(PP)films with row-nucleated lamellae.In this work,their structure evolution during stretching and recovery at room temperature was followed and the elastic recovery mechanism was discussed by twice cyclic tensile experiment.During the first stretching to 100%,the lamellae crystals are parallel separated and the intercrystallite crazing is formed at the first yield point.Many nano-cavities within the intercrystallite crazing appear when the strain reaches 20%.The strain-hardening process accompanies with the lamellae long period increasing and the intercrystallite crazing enlargement.After the secondary yield point,the lamellae cluster is further separated and more nano-cavities appear.The first and second recovery processes are complete overlap.During recovery,firstly,the energy elasticity provided by nano-cavities surface tension drives the shrinkage of material,and then the entropy elasticity related to amorphous chain relaxation plays a leading role when the strain is smaller than the secondary yield point.The elastic recovery process of hard elastic material is the co-contribution of energy elasticity and entropy elasticity.This work gives a clearer recognition about the source of hard elastic property and the role of amorphous region in material's deformation.