Flow Induced Crystallization Of Polymers

Flow-induced crystallization of polymers is not only a fundamental Nonequilibrium thermodynamic problem but also of importance for polymer processing and the final properties of polymer products. During polymer processing, such as extrusion, injection molding, and fiber spinning, polymer materials are subjected to various flow fields. Flow can enhance the crystallization rate up to several orders of magnitude and induce the formation of the so-called shish-kebab structure, which significantly increases the stiffness and thermo deformation temperature. Although the benefit from flow field is widely recognized in academia and polymer industry, no satisfactory molecular theory has been achieved yet.In this thesis, two ideal systems were designed, one is a mixture containing short free chains and chemically end-linked network, and another is the pure end-linked network with different mesh size. Through studying the crystallization of different network system under stretching by the X-ray diffraction, the role of the long and short chains in the formation of shish and the necessarily of EDT in the formation of oriented nuclei were determined. Additional, the crystallization of natural rubber with high strain rates was investigated to answer the effects of strain rate on the crystallization kinetics and the crystal morphology. The main research work and conclusion are introduced as follows:(1) The nature of hierarchical crystallization of polymer under flow field was studied on two simplified mixtures of short free chains and end-linked network, which were designed to mimic the blend of short and long chains. When no disentanglement occurs, the acceleration of nucleation in the stretched mixtures stem from the synergetic effect between the orientation of the network and the fast diffusion of the free chains, which renders the oriented nuclei composing mainly of short free chains. With the occurrence of EDT, the long chains are the main contributor of the oriented nuclei, in agreement with the traditional view. The former may be the general case in polymer melt, while the latter occurs mainly in polymer solution.(2) The structural evolution of the network with different mesh size during stretching at high temperature was studied by in-situ small angle X-ray scattering. It is found that the chains in the network gradually change with the increase of draw ratio. The network chain would form shish when the draw ratio reaches a critical value. Additional, the results from crystallization of network under a strain after stretching with high strain rate indicated that the fringed micelle model is more suitable to describe the crystallization of deformed network.(3) Deformation-induced crystallization (DIC) of natural rubber (NR) samples with different strain rates at a fixed strain was investigated by synchrotron radiation X-ray diffraction measurements. It was found that the Avrami index was about 1 during the crystallization of NR after the cessation of deformation, which demonstrated that homogenous nucleation takes place during DIC process. The increase of the crystallinity was attributed to the increase of the number of new crystallites rather than the growth of the crystal size. An unexpected relationship between the final crystallinity and strain rates was observed. The increase of physical crosslink points was considered as the reason that leads to the non-monotonic variation of the final crystallinity with strain rates. Though the physical origins are different with high and low strain rates, the increase of nuclei stems from the same mechanism that is to increase the"crosslink density". At high strain rate, entanglement points act as"crosslink", while at low strain rate the crystallites are physical crosslink points.(4) The phase transition of quench-induced mesophase and deformed mesophase which was prepared by stretching the quench-induced mesophase during heating and isothermal annealing process was studied by in situ synchrotron radiation WAXD. The results show that the temperature at which the phase transition has a slope increase is higher with deformed mesophase than quench-induced mesophase; it indicated that the former one was much more stable. It was found that the regular conformation of helices along stretching direction has a larger long-range order in deformed mesophase. That reason was used to explain why the deformed mesophase has a higher melt point.Therefore, the main innovations of the present research work are listed as follows:(1) In order to clarify two specific fundamental issues: (i) what is the role of long and short chains in the formation of shish; and (ii) is EDT a necessary condition in the formation of the oriented nuclei? Two simplified mixtures containing short free chains and chemically end-linked network were designed which resemble the transient network of polymer blend with short and long chains. These systems minimize the interferences of relaxation and macroscopic inhomogeneous of flow field, which may complicate the interpretation on the observed experimental phenomenon. The nature of hierarchical crystallization of polymer under flow field was obtained.(2) In order to clarify another two specific fundamental issues: (i) could shish be formed by the stretched network; and (ii) is a critical strain required for shish formation? The network with different mesh size were designed which resemble the transient network of the entanglement polymer. These systems also minimize the interferences of relaxation and macroscopic inhomogeneous of flow field, which may complicate the interpretation on the observed experimental phenomenon. The result indicted that the deformed network could form shish above a critical draw ratio.(3) In order to obtain the structural variation of natural rubber in many extreme conditions. Deformation induced crystallization of vulcanized natural rubber with high strain rates is studied with a homemade two-drum extensional rheometer. The results show that the higher strain rate leads to a larger crystallinity. A new nucleation model during the crystallization of deformed network was also given based on the results.