Rheo‐optical FT‐IR Spectroscopy of LLDPE: Effect of Comonomer and Composite Materials

Abstract

Summary: In order to further evaluate the potential of FT‐IR spectroscopic investigations on molecular processes during tensile testing experiments, the behavior of monolayer LLDPE films, made with ethylene‐butene and ethylene‐octene copolymers, was studied. Additionally, multilayer LLDPE films based on the same C4 and C8 copolymers were investigated. The stress‐strain data obtained from the monolayer films indicate differences in the strain hardening region. It seems that the film samples PE 469‐30‐2 (C4‐gas phase process) and PE 469‐30‐5 (C8 solution process) behave similarly whereas the strain hardening for the PE 469‐30‐3 (C4 solution process) requires lower stress values. The orientation function changes during the stretching of the films indicating that unfolding of the polymer chains occurs at lower strain for PE 469‐30‐5 (C8) than in the C4 materials. In the multilayer systems the Primplast 44 material (C8) shows a lower tendency for reorientation in the strain hardening region than the Coex 82 (C4) material. In this region of the stress‐strain curve the lamellar structure is already transformed into the fibrillar arrangement. Regarding the orientation behavior of the material above 200% strain, a small increase in fb was observed, which led to a decrease of fc. In the octene product possibly the bulky side chains influence the unfolding significantly, producing a higher resistance to unfolding and alignment along the stretching direction. In part, this is potentially caused by the more perfect lamellae in the octene copolymer, which do not include the side chains, while the butene copolymer may have weaker lamellae because they contain a fraction of the side chains which create defects. Consequently, the octene copolymer requires higher stress values to be stretched and finally results in a lower stretchability of this material, as observed on an industrial scale during pallet wrapping tests. Based on the ratio of the structural absorbance parameters of the signals at 729 and 719 cm−1 changes in the crystallinity were studied. For the continuous stretching experiment, no monoclinic phase was detected even after Fourier self‐deconvolution and peak fitting approaches. Literature data, however, describe that this crystalline transformation takes place as a result of mechanical deformation. Therefore, stepwise stretching experiments which allow an improvement of the spectral resolution to 1 cm−1 were carried out. In the deconvoluted spectra the monoclinic, orthorhombic and amorphous LLDPE modifications could be assigned. Ultimate stretchability and stretching force of the films, both monolayer and multilayer, was well correlated to the development of crystalline orientation in the films upon stretching. Other mechanical properties like Elmendorf tear and dart impact can also be better understood with these results.

In situ near-infrared spectroscopic studies of the structural changes of polyethylene during melting

“…In situ Fourier transform infrared spectroscopic studies of the conformational changes of semicrystalline polymers such as PE and polypropylene have been conducted. [1][2][3] The mid-infrared region from 2500-25 000 nm is diagnostic for these polyolefins. However, the very high absorption in the mid-infrared region makes it difficult to assign spectra for industrial polyolefin bulk specimens; very thin films are usually required.…”

Rheo‐Raman spectroscopic study of microscopic deformation behavior for ultra‐low‐density polyethylene

“…The rheo‐optical technique, which is a combined method of mechanical tests and spectroscopy such as IR spectroscopy and XRD, is a powerful tool to investigate the mechanical behavior from a structural point of view. Rheo‐optical IR spectroscopy has been widely used to evaluate the conformational changes and the molecular orientation during elongation for semicrystalline polymers . In situ XRD measurements are also useful to elucidate the microscopic deformation behavior of PE since XRD is sensitive to changes in crystallinity and the crystalline lattice .…”

In Situ Monitoring of Orientation Parameters and Orientation Distribution Functions of Polyethylenes during Tensile Tests

“…While the plastic products are commonly formed under deformation, it has been established that the mechanical properties of the polymeric solids are strongly affected by the molecular orientation. The orientation behaviors of polyethylenes including high‐ and low‐density polyethylenes (HDPE and LDPE) have long been investigated by a variety of methods . The wide‐angle X‐ray diffraction (WAXD) has been employed to reveal the orientation of the lamellar crystals .…”

“…The wide‐angle X‐ray diffraction (WAXD) has been employed to reveal the orientation of the lamellar crystals . The infrared (IR) spectroscopy has been utilized the orientation of the amorphous chains, as well as the crystalline chains …”