Mechanical α-Process in Polyethylene

Abstract

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

Rheo-optical Raman study of microscopic deformation in high-density polyethylene under hot drawing

“…The α 1 and α 2 relaxations were assigned as relaxation in the grain boundaries and the thermal activation of the intrachain motion, respectively. These assignments were confirmed by dynamic mechanical analysis (DMA) of PE single-crystal mats [5] and PE with different molecular weight distributions and branch structures [6]. Because molecular motion is activated in the α relaxation region, the drawability of polymeric materials is highly enhanced.…”

Cyclic thermo-viscoplasticity of high density polyethylene

“…and testing conditions (frequency of oscillations, heating rate, intensity of preloading, etc.). For recent DMTA (dynamic mechanical thermal analysis) observations on polyethylenes, see Matthews et al (1999), Kuwabara et al (2000), Mano (2001), Sirotkin and Brooks (2001), Matsuo et al (2003), Men et al (2003), Kolesov et al (2005), Stadler et al (2005), Na et al (2007), Guan and Phillips (2007), and the references therein.…”

“…Several hypotheses have been suggested regarding molecular mechanisms of a-relaxation: c-shear within crystalline lamellae (homogeneous shear of crystal blocks in the direction of their c-axis) (Sirotkin and Brooks, 2001;Guan and Phillips, 2007), inter-lamellar shear (slip of crystalline blocks past each other) (Matthews et al, 1999;Jiang et al, 2009), sliding of tie molecules and motion of chain folds and loops through crystals (Khanna et al, 1985), diffusion of chains (Men et al, 2003) and defects (Alberola et al, 1990) in crystallites, growth of mobility of chains in the interphase (transition regions located in the close vicinity of lamellar surfaces) (Rastogi et al, 2007), weakening of lamellar coupling and enhancement of segment exchange between crystalline and amorphous phases (Kolesov et al, 2005;Na et al, 2007;Zubova et al, 2007).…”

Molecular mechanisms of the chain diffusion between crystalline and amorphous fractions in polyethylene

“…[25] and references therein), and the a 2 -peak (at higher temperatures) e to crystalline deformation. Only after the direct NMR observation of the flips of chains in crystallites [6], several authors began to associate one of the mechanically observed a-peaks (a 1 [27] or a 2 [26]) with the chain diffusion through crystallites and so to identify one of the mechanical processes with the dielectrically and NMR observed H-process. Kolesov et al [28] uses this idea of the chain diffusion for the qualitative explanation of the influence of crystal morphology and chain branching on parameters of the unresolved mechanical process.…”