Study On Stress Oscillation Behavior Of Amorphous Polymers

Stress oscillation is a special mechanical phenomenon.Usually,when the tensile rate is constant,the stress of the polymer is basically unchanged after post yielding.However,under certain conditions,stress exhibit periodic fluctuation as the strain increases for some polymers.This phenomenon is always occurred in amorphous but crystallizable polymers(such as PET),and there are many reports on the stress oscillation of such polymers,but the mechanism of its fluctuation is till controversial.At the same time,there are few reports about the study on the mechanism of stress oscillation for amorphous non-crystallizable polymers(such as PC).Therefore,this present work will analyze the amorphous but crystalline polymer(PET)and amorphous non-crystalline polymer(PC)by stretching behavior,thermal behavior,crystallinity,macroscopic and microscopic morphology.It is expected that the mechanism of stress oscillation could be revealed initially from the molecular level in different polymers.1.Stress oscillation of amorphous but crystallizable polymer PET was studied.Stress oscillation behavior of PET has the dependence on molecular weight and tensile rates.The bright-dark alternation band structure occurred on the macroscopic morphology after the tensile stress.The microscopic structure of PET stress oscillation region was confirmed by SEM and DSC analysis,and which also confirmed the internal cavitation phenomenon was only the microscopic result of stress oscillation.The difference of the crystallinity of the “bright” and “dark” regions is the cause of the strain hardening and strain softening on the stress oscillation curve.2.Stress oscillation of amorphous non-crystallizable polymer PC was studied.Stress oscillation behavior of PC has the dependence on tensile rates and aging times.It was not the typical bright-dark alternation band structure,but the transparent prismatic structure which parallel to the fracture direction.The results of DSC,IT and SEM show that the mechanism of stress oscillation for PC was different from the previous mechanisms for amorphous but crystallizable polymers.It was considered that the stress oscillation of PC was consequent on the alternate effect of physical aging and stress rejuvenation.The physical aging increases stress,while stress rejuvenation reduces stress.In addition,a model was developed to describe the process of fluctuation period in stress oscillation.The model was similar to a two-element Maxwell unit,which can simulate the stage of stress increase and stress reduction for PC.