Study Of Deformation Induced Phase Transition Of Polyolefins With In-situ Synchrotron Radiation X-ray Scattering

All of the material properties are determined by the intrinsic structure, so the relationship between structure and properties is very important for optimizing and tuning material properties. In the field of polymer science, the relationship between structure and mechanical properties occupies very important status, which has been drawing attentions of scientists since half a century before. Because of the complexity of polymer structure and oneness of detecting method, the development in this issue is very slow. To solve this problem we can strive from two aspects, which are developing suitable detecting techniques and choosing appropriate research objects.In our study, the uniaxial tensile apparatus was adopted in combination with synchrotron radiation X-ray scattering, which can provide information both of structural evolution and mechanical response. Polybutene-1(PB-1) and isotactic polypropylene (iPP) was studied due to their multiphase structures. Under external simulation which is drawing deformation in this research, there are phase transitions such as the phase transition from forms Ⅱ to Ⅰ in PB-1and from a crystal to mesophase in iPP. By means of in-situ testing method, the structural evolution can be tracked easily, which affords the possibilities to establish the relationship between micro-structure and mechanical properties. The research contents of this work are as follows:i. The effect of temperature on the deformation induced phase transition of PB-1is studied with in situ synchrotron radiation wide angle X-ray scattering (WAXS), which verify that tensile deformation can accelerate the transition from forms Ⅱ to Ⅰ. The phase transition at different temperatures is interpreted based on either a direct crystal-crystal transition at lower temperature or an indirect approach via an intermediate stage of melt at higher temperature, namely a melting recrystallization process. A three-stage mechanical deformation including linear deformation, stress plateau and strain hardening is observed in the engineering stress strain curves, which corresponds to a process of incubation, nucleation and gelation of form Ⅰ crystals. It establishes a nice correlation between phase transition and mechanical behavior in this study.ii. Taking advantage of the time resolution of synchrotron radiation X-ray scattering, the effect of strain rates on deformation induced phase transition from forms Ⅱ to Ⅰ is studied. The phase transition is faster under larger strain rates. In this work, the beginning of phase transition is found to be strain-controlled, while the transformation degree is related to the mechanical work. After deformation under higher strain rates at80℃, the recrystallization of form Ⅱ occurs, which verifies the transition at higher temperature accords with the mechanics of deformation induced melting-recrystallization, while the phase transition at lower temperature follows Young’s dislocation model.iii. As we know, the phase transition from forms Ⅱ to Ⅰ will lead to shrinkage in different directions of lamellae, so in this work synchrotron radiation small angle X-ray is adopted to follow the change of long spacing. A micro-yield point on the engineering stress strain curve is found before macroscopic yielding, which may be the real beginning of form Ⅱ crystal’s destruction.iv. To study deformation induced transition from α-iPP crystal to mesophase, the cross-hatched structure that is the characteristic feature of iPP can orient along with two directions in flow field which is utilized in this work. A method to distinguish the parent-daughter lamellae with in situ environment is developed. Combining with synchrotron radiation WAXS, the stretching induced structural evolution of parent and daughter lamellae is studied from different directions. No matter what the tensile direction is, parent lamellae are destroyed before daughter ones. Mesophase is observed at very small strain, immediately after the damage of parent lamellae. Deformation induced mesophase is proved to be small crystal cluster which is transformed from parent-daughter lamellae.