| Polymers, as a newly developing organic material, are widely used in all the aspects of daily life in the society of humen being. Two-thirds of polymers being used are crystallizable. Thus, crystallization of polymer is of vital importance in both industry and academy. Polymer with a long chain character owns rather high conformational entropy, which makes crystallization of polymer depend on the chain conformation, not only the temperature. Flow, such as shear, compression, extension, or combination of them, is inevitable during the processing of polymer industry. Thus, flow-induced crystallization (FIC) of polymer has been a focus in research of polymer physics in the past decades.In this thesis, extension-induced crystallization of lightly cross-linked high density polyethylene (XL-HDPE) has been studied, with a combination of extensional rheology and in situ synchrotron radiation small-angle X-ray scattering (SR-SAXS) and wide-angle X-ray diffraction (WAXD) measurements. Following subjects have been invesitigated. The correlation between extension-induced conformations of chains and morphologies of nuclei; A modified SNM is developed and employed to analyze strain-temperature equivalence on nucleation in FIC as well as the natural transition from lamellar to shish nuclei; Through quantitative estimation of the thermodynamic parameters, the properties of variant flow-induced precursors are deduced.I:Extension-induced crystallization of lightly cross-linked high density polyethylene (XL-HDPE) has been studied with a combination of extensional rheology and in situ SAXS measurement, where XL-HDPE is a dynamic asymmetric system containing both cross-linked network and free chains (23wt%gel fraction). SR-SAXS results revealed that the nucleation morphologies can be divided into four regions in strain space, namely uncorrelated oriented point-nuclei, scaffold-network nuclei, microshish nuclei, and shish nuclei. The definition of these four regions coincides nicely with the transitions in stress-strain curves, which allows us to establish a correlation between extension-induced conformations of chains and morphologies of nuclei. With in situ WAXD measurement, we investigated the isothermal crystallization process in strained state based on varient nuclei. Accrodingly, we established a relationship between kinetics and strain, chain conformation, shish content, lateral size, and twisted extent of kebab.II:Extension-induced crystallization under nearequilibrium condition has been studied in a series of XL-HDPE with a combination of extensional rheology and in situ SR-SAXS and WAXD measurements. According to crystal morphology and structure, four regions were defined in straintemperature space, namely "orthorhombic lamellar crystal"(OLC),"orthorhombic shish crystal"(OSC),"hexagonal shish crystal"’(HSC) and "oriented shish precursor"(OSP), respectively. This indicates that flow not only induces entropic reduction of initial melt, but also modifies the free energies of the final states, which is overlooked in the classical stretched network model (SNM) for flow induced crystallization (FIC). Incorporating the free energies of various final states, a modified SNM is developed and employed to analyze strain-temperature equivalence on nucleation in FIC, which reveals that the critical nucleus thickness/at different regions leads to a natural transition from lamellar to shish nuclei.Ⅲ:Based on the incipient strains of SAXS and WAXD signals during extension in a large temperature range under near-equilibrium condition, strain-temperature diagrams for flow-induced precursor (FIP) and nucleation are constructed, respectively, which revealed that flow-induced crystallization (FIC) undergoes two stages, namely,"melt-precursor transition"(MPT) and "precursor-nuclei transition"(PNT). At different temperatures, FIP with different inner structures and morphologies can be induced by strain, whose embryo has a shape and structure related to the ones of the corresponding critical nuclei. With the strain-temperature diagrams, the thermodynamic properties of FIP are deduced, which shows that compared with the relative nuclei the FIP always has a lower bulk free energy and a much lower surface free energy. In extreme case (high temperature), surface free energy of FIP can be negligibly small. The quantitative estimation of the thermodynamic parameters suggest the existence of variant FIPs, which plays a vital important role for the subsequent progress PNT and the whole process of FIC.The major innovations are summarized as follows:(1) In this thesis, take advantage of60Co y-ray radiation source, a series of XL-HDPE samples were obtained. In this way cross-linkage with long relaxation time has been introduced. It could suppress chain relaxation after deformation, and could obtain a higher strain resolution when investigating the effect of strain.(2) In this thesis, with a combination of extensional rheology and in situ SR-SAXS and WAXD measurements, it allow us to obtain information of both chain conformation and structure evolution. Focusing on the subject of "point nuclei-shish nuclei" trainsiton of FIC, systematical investigations have been conducted in three aspects, namely:Correlation of "extension field-induced conformations of chains-morphologies of nuclei"; Nucleation theory for FIC under near-equilibrium; And thermodynamic properties of FIP, respectively.(3) In this thesis, an under near-equilibrium condition was designed. Incipient strains of SAXS and WAXD signals during extension in a large temperature range were obtained. Considering not only flow induces entropic reduction of initial melt, but also modifies the free energies of the final states, a modified SNM is developed. The modified SNM has been employed to analyze strain-temperature equivalence on nucleation in FIC, as well as the thermodynamic properties of FIP... |
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