‘Intermediate phase’ in poly(ethylene) as elucidated by the WAXS. Analysis of crystallization kinetics (original) (raw)
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The small strain mechanical behavior of bulk polyethylene was investigated at the local scale of the lamella stackings by means of combined in situ SAXS and WAXS at different testing temperatures. Three different thermal treatments on four materials afforded studying a wide range of crystallinities (X c) and microstructures. The local strain in tensile direction of the amorphous phase in equatorial region of the spherulites was determined via SAXS. The amorphous to macroscopic strain ratio proved to be fairly constant in the preyield strain domain for every materials. This ratio also proved to be strongly dependent on X c. The local tensile stress on the amorphous phase in equatorial region was assessed from the strain on the crystals as measured by WAXS, using theoretical values of the elastic constants. The apparent tensile modulus of the amorphous phase, M a , was shown to reach a maximum value of 300 MPa at RT for X c = 50% and exhibited a monotonic drop with increasing both X c and temperature. Evidence was given of the major role of the density of molecular stress transmitters on the amorphous phase stiffness over that of structural confinement. Comparison between M a and macroscopic modulus revealed a significant modification of the mechanical coupling of the crystalline lamellae in relation to X c that was assigned to an increasing lamella percolation throughout the spherulites with increasing X c .
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The small strain mechanical behavior of bulk polyethylene was investigated at the local scale of the lamella stackings by means of combined in situ SAXS and WAXS at different testing temperatures. Three different thermal treatments on four materials afforded studying a wide range of crystallinities (X c) and microstructures. The local strain in tensile direction of the amorphous phase in equatorial region of the spherulites was determined via SAXS. The amorphous to macroscopic strain ratio proved to be fairly constant in the preyield strain domain for every materials. This ratio also proved to be strongly dependent on X c. The local tensile stress on the amorphous phase in equatorial region was assessed from the strain on the crystals as measured by WAXS, using theoretical values of the elastic constants. The apparent tensile modulus of the amorphous phase, M a , was shown to reach a maximum value of 300 MPa at RT for X c = 50% and exhibited a monotonic drop with increasing both X c and temperature. Evidence was given of the major role of the density of molecular stress transmitters on the amorphous phase stiffness over that of structural confinement. Comparison between M a and macroscopic modulus revealed a significant modification of the mechanical coupling of the crystalline lamellae in relation to X c that was assigned to an increasing lamella percolation throughout the spherulites with increasing X c .
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Isothermal crystallization in bulk has been compared dilatometrically with linear polyethylene samples which have been molded in different ways, including melt orientation and sintering of precipitated powders from dilute solutions. These experiments show significant differences in the crystallization kinetics, which in some cases cannot be removed even by prolonged heating of the melt prior to crystallization. The data have been analyzed by an empirical expression in terms of nucleation and growth processes, according to the theoretical approach of Avrami. The results suggest that, in general, the melted polyethylene is not in a true thermodynamic equilibrium and includes some ``quasi-indestructible'' clusters, which act as heterogeneous nuclei in the crystallization process. The nature and the amount of these nuclei depend on the molding, melting, and mechanical history of the sample, so that the semicrystalline material may have a wide variety of structure.
Macromolecules, 2010
Dynamic processes of isothermally cold-crystallized poly(trimethylene terephthalate) (PTT) was investigated by fluorescence spectroscopy of its amorphous region. The key issue lay in the fact that the emission at 390 nm originating from the main chain phenylene ring in the amorphous phase was correlated to the coldcrystallization. A reduction in this emission peak intensity corresponded to a gradual transition of the phenylene ring in the amorphous to crystal region. Accordingly, molecular chain movement and structure evolution of PTT in the course of cold-crystallization were carefully revealed. The experimental results indicated that the kinetics parameters measured by the fluorescence spectroscopy method are in good agreement with those by differential scanning calorimetry (DSC). What is more, the former was capable of providing detailed information about structural variations during cold-crystallization, for example, molecule arrangement in induction phase.
Polymer, 2005
The combination of Fourier transform Raman spectroscopy and thermal analysis has been proved to be adequate for the study of the quantitative structural changes which take place in amorphous poly(ethylene 2,6-naphthalate) on annealing. Different conformer contents were found in the annealed samples depending on annealing conditions. In general, annealing of the amorphous poly(ethylene 2,6naphthalate) from the glassy state induces a conformational transition of gauche to trans. The structure obtained during crystallization is characterized by a three-phase conformational model, including an amorphous phase, a rigid amorphous phase and a crystalline phase. The crystallization is further characterized by a three-zone process, firstly a primary crystallization process, secondly a variation of the rigid amorphous phase with a constant value of the crystalline phase and thirdly a secondary crystallization process. The bandwidth at half intensity at 1721 cm K1 in the Raman spectrum varied between 32 cm K1 for the complete amorphous phase and 7 cm K1 for the total rigid phase, the sum of the rigid amorphous and crystalline phase. The bandwidth at half intensity at 1721 cm K1 was directly related to the amount of the total rigid phase and confirmed by the variation of the heat capacity increase at the glass transition temperature. Two complementary bands in the Raman spectrum, at 1107 and 1098 cm K1 , were found to be related to the trans and gauche isomers. A difference was measured between the total trans content and the amount of rigid phase due to the presence of some trans conformations in the amorphous phase. The extrapolation of the bandwidth at half intensity at 1721 cm K1 to the value of zero, corresponding to the complete crystalline phase, gave a melting enthalpy of 196 J/g and the corresponding density of the crystalline phase was 1.4390 g/cm 3 . A complete rigid phase structure was obtained by a melting enthalpy of 144 J/g and a density of 1.4070 g/cm 3 .