The Effect Of Chain Relaxation On Flow Induced Crystallization Of Polymer

Polymers are widely used in the modern life as a developing organic material. Two-thirds of polymers being used are crystallizable. Naturally research about the crystallization of polymer is very important. While different from the small molecules, polymer owns rather high conformational entropy, which originates in the long chain character. The conformational entropy makes crystallization of polymer depend on not only the temperature but also the chain conformation. On the other hand, flow is inevitable during the processing of polymer, which will lead to great chain deformation. The deformation can accelerate the crystallization kinetics and modify the morphology of crystal. This is the commonly referred flow-induced crystallization of polymer (FIC). Since the potential of tuning the property of products, FIC of polymer has been a focus in research of polymer physics in the past decades. The central issue of FIC is to establish the relation between chain deformation and crystallization.In this thesis FIC researches based on narrow-dispersed polyethylene oxide (PEO) are introduced. The different effect of chain orientation and weak chain stretch on crystallization is demonstrated, where relaxation of chain deformation plays an important role. Technically the synchrotron radiation small angle X-ray scattering (SAXS) and extensional rheology are combined. Qualitative information about the chain deformation can be obtained by rheological measurement, and the structure evolution during crystallization is monitored by SAXS. The combination makes it possible to correlate the chain deformation and crystallization directly. The results are summarized as follows:(1) The effect of relaxation of chain orientation on the time evolution of long period is investigated. The strain is set constant1.2and the strain rate gradually increases. It is found that the evolution trend of long period in the early stage will change from decrease to increase when the strain rate passes10s-1. It is assumed that in the high orientation case, the linear nucleation density will decrease when chain relaxation happens, thus the distance between two nuclei will increase with time. Consequently, the time-averaged long period will increase. A theoretical description of this process is proposed based on the microrheological model. A good agreement with the experiment is achieved through fitting.(2) Experimentally the effect of chain stretch induced by crystallization during extension on FIC is shown. Crystallization can happen during stretching under a relatively low temperature. The formed crystals will physically crosslink the entanglement network of melt. A transient network composed by the crystals and free melt will form. In this network chain stretch can happen at a much lower strain rate, which corresponds to the significant strain hardening in stress-strain curve. It is found that the long period is sensitive to the occurrence of chain stretch. The changes include a jump in the initial value and the two-step decreasing in trend.(3) FIC of the heterogeneous melt is investigated, where a model system, PEO/NaBr blend, is used. The heterogeneous component, e.g. nanoparticles, crystals formed during flow, will affect the deformation of polymer chain and the subsequent crystallization. In the investigated system, PEO and NaBr will form a crystalline complex with high melting point. The structure maintains the folded chain character of lamellar crystal. Increasing the content of the crystalline complex, strain hardening will happen and the long period evolution changes correspondingly. No shish forms in the investigated concentration range.(4) The feedback effect of crystallization in the end-linked PEO network is investigated. Compared to the free melt, the relaxation of crosslinked network is negligible and a better correspondence between microscopic chain deformation and macroscopic strain can be obtained. Meanwhile, during the strain induced crystallization of crosslinked network, the increase in crystallinity will lead to a release of macroscopic stress, which corresponds to the relaxation of the microscopic chain deformation. It is found that the relaxation enhances the chain mobility of the amorphous and facilitates the increase of crystal dimension. The increase can be concluded by the evolution of scattering pattern, while a melting and fusing mechanism is further revealed through the oscillated stress. The major innovations are summarized as follows:(1) The time evolution of long period is used to differentiate the effect of chain orientation and weak chain stretch on flow-induced crystallization of polymer. This is totally different from the traditional way that correlating the chain deformation and crystallization through the morphological change of crystal. Commonly a strong chain stretch is assigned to the formation of shish-kebab structure. While for the chain orientation and weak chain stretch cases, the morphology keeps as oriented lamellar stack. It is impossible to distinguish the two cases from the morphology. Given the determinant effect of chain deformation on crystallization, the time-dependent chain deformation during the relaxation process may show some influences on the structure evolution. Experimentally it is found that the evolution of long period shows a close relation with the chain deformation. A more detailed chain deformation and crystallization correlation is given based on these findings.(2) The formation of crystalline complex in PEO/NaBr blend is made used to investigate the FIC of the heterogeneous melt. This model system has two advantages: First the content of the heterogeneous particle can be easily altered. Second, the surface interaction between heterogeneous particles and polymer chains is minimized, since the crystalline complex maintains the folded chain character. This is important in differentiating the effect of rheological property and surface interaction.(3) Technically the extensional rheology and in-situ synchrotron radiation SAXS are combined, different from the ex-situ structure analysis assistant rheological research and pure in-situ structure detection. This is crucial in establishing the relation between chain deformation and crystallization behavior.