| Semicrystalline polymers are widely used as structural materials in various aspects of human production and life.As we all know,the microstructure of materials directly affects their mechanical properties.Therefore,it is necessary to deeply understand the stretching-induced structural transformation mechanism of samples,which is in favour of mastering the manufacture processing and improving the performance.Many studies on the deformation mechanism of semicrystalline polymers have been carried out in recent decades,but there are few studies on the evolution of free-volume holes of atomic and molecular dimensions in amorphous regions.The size,concentration,and distribution of free-volume holes are closely linked with the macroscopic properties of the polymers,such as mechanical,electrical,and thermal properties.Thus,the deformation mechanism on amorphous regions can be revealed by analyzing the evolution of the holes.Positron annihilation lifetime spectroscopy(PALS)is sensitive to nanoscale defects,holes and other microstructural information,and can directly figure out the size and concentration of free-volume holes.Hence,PALS plays a unique role in the field of materials science compared to other experimental techniques.However,limited by the count rate ranging from tens to hundreds cps(counts percent second),the traditional positron annihilation lifetime spectrometer usually takes several hours to record a reliable lifetime spectrum.For semicrystalline polymers that need to analyze the deformation process,it is difficult to record the process of rapid change.To reduce the interference of structural relaxation,the high counting rate positron annihilation lifetime(HCR-PAL)spectrometer rate is essential to in-situ measurements.The main research contents of this dissertation are as follows:(1)Using the latest developed PALS system(with a count rate up to 3000 cps),insitu PALS experiments were performed for the first time on the continuous stretching process of polymers to quantitatively analyze the minute-scale evolution of free-volume holes.According to the stress-strain relationship and PALS results of the four types of polyethylenes with different crystallinities,the deformed process could be divided into four stages:elastic,initial nonlinear(until yield point),post-yield,and strain hardening stages.The infinite spherical well model and ellipsoidal well model for free-volume hole were applied to analyze the deformed process of the polyethylenes,and the relationship between the microstructural evolution and mechanical properties was studied on the stretching polymers.The increase in the orthopositronium(o-Ps)lifetime in the first three stages demonstrated an enlargement of free-volume hole size with increasing strain.The decrease in the o-Ps lifetime in the last stage was due to the increase on the anisotropy of the holes.(2)The PALS results of four types of polyethylene could be compared to study the influence of amorphous region on the deformation process of the sample.For polyethylene with higher crystallinity(less amorphous region),the relative change in relative fractional free-volume was more significant throughout the entire tensile process,which illustrated that the structure relaxation tended to occur in amorphous regions.(3)The in-situ measurements were applied to study the continuous stretching process of isotropic polypropylene(iPP)films by HCR-PAL spectrometer.Combined the stress-strain relationship with PALS results,the deformation process was analyzed in different stages,and the evolution characteristics of free-volume holes were studied.The o-Ps intensity was basically unchanged in the first three stages,which demonstrated the concentration of free-volume holes.The decrease in the o-Ps intensity in the last stage was due to the aggregation of the holes.The relationship between the microstructure evolution and mechanical properties of iPP films was analyzed and compared to polyethylenes.The deformation mechanism of semicrystalline polymers was revealed more sufficiently. |
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