Deformation mechanism of high-density polyethylene probed by in situ Raman spectroscopy

Abstract

Semi-crystalline polymers such as polyethylene (PE) and polypropylene (PP) show complicated morphologies composed of a mixture of crystalline and amorphous phases. The diversity in the structure with a wide range of length scales is responsible for the durability or high toughness of semi-crystalline polymers [1,2].When a high-density polyethylene (HDPE) specimen is uniaxially drawn, the elastic deformation is followed by double yield points referred to as the first and the second yields. During such dual plastic deformations, large-scale transformation of the semi-crystalline morphology takes place, and collapse of the spherulite structures, necking of the sample specimens, and rearrangement of the lamellar crystals and the polymer chains have been observed [1][2][3][4][5][6]. Although various models, such as the lamellar local-melting and recrystallization model [4,7], and the lamellar cluster model [3,[8][9][10], have been proposed for the deformation mechanism, such complicated changes in the hierarchical structures of semicrystalline polymers are still controversial.Spectroscopic techniques have been applied for the orientational behavior of PE during elongation [11][12][13][14][15][16][17]. The infrared (IR) spectroscopy has been applied intensively, and it has been established that the molecular chains in the crystalline and amorphous phases orientate toward the draw direction [11][12][13][14]. It has also been demonstrated that the orientation proceeds rapidly after the first yield, and gradually in the neck-propagation and the strain-hardening regions.However, the IR spectroscopy gives one orientation parameter 〈P 2 〉 which corresponds to the averaged orientation. For PE, the vibrations of the side chains, such as CH 2 twisting and wagging, are IR-active, and the skeletal vibrations of the C-C bonds are IR-inactive, then the load sharing on the polymer chains could not be directly observed with the IR spectroscopy.Raman spectroscopy has several advantages to investigate the deformation behavior of polymeric materials [18][19][20][21][22][23][24]: it is applicable to opaque and thick samples, and molecular environments can be separately detected for both the crystalline and the amorphous phases. Raman scattering is an optical technique that provides a complementary approach to IR absorption when transparent and very thin (typically <100 µm thickness) films are required. In addition, it is advantageous to use Raman spectroscopy because the vibrations of the skeletal C-C bonds in main chains are directly observed [20,25]. Because the C-C stretching vibration is strongly Raman active, microenvironments of the polymer chains are probed by the spectral shifts [26,27]. Polarized Raman spectroscopy gives not only a measure of the average orientation, which is obtained by the usual spectroscopies such as birefringence and IR dichrom, but also the orientation distribution function (ODF, N(θ)) [28][29][30]. In the yielding region where the neck of the specimen is formed, complicated deformation mechanism is expect...

Effect of Strain Rate on Microscopic Deformation Behavior of High-density Polyethylene under Uniaxial Stretching

“…The details of in situ Raman spectroscopy have been described in the previous papers [22][23][24][25]. A DPSS laser (LASOS, Jena, Germany, 639.7 nm in the wavelength, 200 mW in laser power) was used as the incident laser light and a monochromator (PIXIS 100 and Spectra Pro 2300i, Princeton Instruments, Trenton, USA, NJ) was used as the detector.…”

“…For PE sample, the Raman band at 1130 cm -1 assigned to the C-C symmetric stretching mode of crystalline chains has been used to obtain the orientation parameters [22,35]. In this study, two orientation parameters were determined from the intensities of three polarized Raman spectra as follows: [36][37][38][39] (1) (2) Here,  is the angle between the stretching direction and the principal axis of the Raman tensor.…”

“…For in-situ study of the structural changes during deformation, one of the most powerful tools is the rheooptics, which is the simultaneous measurements of mechanical testing and optical measurements such as birefringence, infrared (IR) spectroscopy, X-ray scattering, or Raman spectroscopy[9, [15][16][17][18][19][20][21][22][23][24][25][26][27][28]. Such a technique enables us to detect bulk properties (sress, strain, and strain rate) and coresponding local structural parameters (molecular orientation, microscopic stress, crystallinity, chain conformation) at same time during deformation.…”

“…Such a technique enables us to detect bulk properties (sress, strain, and strain rate) and coresponding local structural parameters (molecular orientation, microscopic stress, crystallinity, chain conformation) at same time during deformation. Especially, in-situ Raman spectroscopy have been useful to evaluate the molecular orientation distribution [22,24] and the microsocpic stress applied to the skeletal chains for PE and PP under deformation [22,25,29].…”

“…Although the rheo-Raman spectroscopy was applied, the strain rate effect was not clearly observed [18], presumably due to the low strain rate of ~10 -3 s -1 . Recent development of optical apparatus enable us to measure Raman spectra at sufficiently short exposure time of ~1 s. We have developed custom-made in-situ Raman spectroscopy [22,24] with the minimum exposure time during deformation is about 0.1 s. In this study, we performed rheo-Raman spectroscopy of highdensity polyethylene (HDPE) to evaluate the effects of strain rate on the microscopic deformation behavior. The molecular orientation and the load sharing of molecular chains of HDPE were evaluated during uniaxial stretching process.…”