Abstract:With two‐dimensional time‐domain NMR analysis in the proton spin rotating frame, three phases are identifiable in the solid polyethylene. The major proton magnetization fraction is due to the polymer's crystalline region, where the motion is least isotropic and slowest. A magnetization fraction with intermediate relaxation rate is also intermediate in magnitude. This component is proposed to comprise chain loops on the surfaces of crystallites and effectively entangled chain segments. The most mobile fraction,… Show more
“…In polyethylene, a simple two-phase model cannot account for the observations from Raman measurements and other methods [40,41]. A third intermediate phase has been proposed that is preferentially located at the crystallite fold surface boundary.…”
No clear picture of the deformation and wear mechanisms of Ultra High Molecular Weight Polyethylene (UHMWPE) in artificial knee joints exists up to today. Tribological tests were conducted under relevant loads and the worn samples were extensively studied by XRD, DSC, Raman, and SEM. It was shown that stresses close to the surface in areas where high relative velocity in combination with high normal loads are applied are most effective in changing the microstructure and therefore most detrimental to produce wear particles, while in the depth the same deformed structure as under unidirectional cyclic loading is found. A model was proposed which reflects the deformation at different zones in the depth of a tribologically loaded UHMWPE sample. This model shows amazing analogy to the orientation of collagen fibrils in natural cartilage.
“…In polyethylene, a simple two-phase model cannot account for the observations from Raman measurements and other methods [40,41]. A third intermediate phase has been proposed that is preferentially located at the crystallite fold surface boundary.…”
No clear picture of the deformation and wear mechanisms of Ultra High Molecular Weight Polyethylene (UHMWPE) in artificial knee joints exists up to today. Tribological tests were conducted under relevant loads and the worn samples were extensively studied by XRD, DSC, Raman, and SEM. It was shown that stresses close to the surface in areas where high relative velocity in combination with high normal loads are applied are most effective in changing the microstructure and therefore most detrimental to produce wear particles, while in the depth the same deformed structure as under unidirectional cyclic loading is found. A model was proposed which reflects the deformation at different zones in the depth of a tribologically loaded UHMWPE sample. This model shows amazing analogy to the orientation of collagen fibrils in natural cartilage.
“…The relative fractions of the components for m -PE, % T 2 s and % T 2 l , correspond to the content of hydrogen in rigid and soft phases, respectively. It is noted that no statistically relevant analysis of the T 2 decay for m -PE can be obtained with a three-component model which is usually used for linear PE. − It might be suggested that (1) chain mobility at crystal-amorphous interface in m -PE is larger compared to that to that for linear PE because of small size of crystals and crystal imperfections and (2) broad distribution of the correlation time for chain motions exists in the amorphous phase which can cause “apparent” single-exponential relaxation. 7 Decay of the transverse magnetization (points) at different temperatures for undeformed m -PE.…”
Section: Resultsmentioning
confidence: 99%
“…The polymer most frequently studied using this method is poly(ethylene) (PE). [9][10][11][12][13][14][15][16][17] In general, the results of these experiments suggest a three-phase model. According to these experiments, PE samples are composed of a crystalline phase, a semirigid phase and a soft amorphous phase.…”
Section: Introductionmentioning
confidence: 99%
“…Phase composition in semicrystalline polymers and its changes upon deformation have been often studied by proton T 2 relaxation. The polymer most frequently studied using this method is poly(ethylene) (PE). − In general, the results of these experiments suggest a three-phase model. According to these experiments, PE samples are composed of a crystalline phase, a semirigid phase and a soft amorphous phase.…”
The strain-induced phenomena in cured amorphous EPDM and semicrystalline ethaneoctene copolymer (m-PE) were studied using 1 H T2 NMR relaxation. Samples studied differed with regard to network structure; i.e., EPDM had a larger density of chemical cross-links, whereas the total network density in m-PE was significantly larger because of chain anchoring to crystallites. The effect of these differences on elastic properties was compared. A compression unit was developed for these experiments. This device made it possible to perform NMR experiments under fixed uniaxial compression and to record applied force. Despite the larger total network density in m-PE, the orientation of elastic chains under compression in EPDM is significantly greater, suggesting that the density of chemical cross-links largely determines the elastic behavior. It appears that crystallites in m-PE rearrange upon compression without a change in the crystallinity. It can be concluded from this study that an increase in the density of chemical cross-links is required to improve the compression set of m-PE.
“…Although the origin and the underlying mechanisms of the mechanical α-process in polyethylene have been extensively studied, − there is still no general agreement about the microscopic origin. Two points are mainly discussed: first, whether there are two α-processes (α I and α II in the order of increasing temperature) as often seen in measurements using dynamic mechanical analysis (DMA) ,− or whether there is only one high-temperature process as observed in nuclear magnetic resonance (NMR) − and dielectric spectroscopy measurements; second, even if the existence of the two α-processes is accepted, there is no agreement as to whether they are both due to processes in the crystalline phase or whether one of them is linked to the amorphous phase. On one hand, many authors believe that the α I -process is due to the interlamellar amorphous phase because the β-process often is not seen in the DMA data of high-density polyethylene (HDPE); it is thought that the α I -process coincides with the β-process, which is shifted to high temperatures because of the constraints imposed by the high entanglement density in the amorphous phase after crystallization.…”
Introduction.Although the origin and the underlying mechanisms of the mechanical R-process in polyethylene have been extensively studied, [1][2][3][4][5][6][7][8] there is still no general agreement about the microscopic origin. Two points are mainly discussed: first, whether there are two R-processes (R I and R II in the order of increasing temperature) as often seen in measurements using dynamic mechanical analysis (DMA) 5,7-15 or whether there is only one high-temperature process as observed in nuclear magnetic resonance (NMR) [16][17][18][19][20][21][22][23][24][25][26][27] and dielectric spectroscopy measurements; 1 second, even if the existence of the two R-processes is accepted, there is no agreement as to whether they are both due to processes in the crystalline phase or whether one of them is linked to the amorphous phase. On one hand, many authors believe that the R I -process is due to the interlamellar amorphous phase because the β-process often is not seen in the DMA data of high-density polyethylene (HDPE); 28 it is thought that the R I -process coincides with the β-process, which is shifted to high temperatures because of the constraints imposed by the high entanglement density in the amorphous phase after crystallization. On the other hand, pioneering work of Takayanagi has led to an assignment of the R I -process to an intralamellar slip process of mosaic blocks. 8 This process can be identified with the block slip process postulated by Strobl et al. 29,30 Different from the R I -process, there is agreement about the assignment of the R II -process: it is believed that the R II -process is due to the diffusionlike motion of polymer segments in the crystals. 3,4,7,8 In this report, we present a comparative study on the dynamic mechanical properties of three selected polyethylene samples with similar crystallinity, melting behavior, and long period but with different molecular weight distributions, branch content, and branch length to clarify the existence of the two R-processes and to clarify the assignment of the R I -and R II -processes. Our results support the original assignment of Takayanagi that the R I -process is due to the intralamellar block grain boundary slip process and the R II -process is due to the diffusion-like motion of chain segments in the crystallites.Experimental Section. Polyethylene samples used in this work were provided by Basell Polyolefine GmbH. Samples were prepared by compression-molding the pellets into sheets with a thickness of about 2 mm at 190 °C for 5 min. Crystallization occurred during cooling in the press to room temperature at a rate of 15 K/min. Some key parameters for the three samples are summarized in Table 1. It can be concluded that the three samples used in this study show similar properties with
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