Elsevier

Polymer

Volume 52, Issue 20, 12 September 2011, Pages 4610-4618
Polymer

An in-situ X-ray scattering study during uniaxial stretching of ionic liquid/ultra-high molecular weight polyethylene blends

https://doi.org/10.1016/j.polymer.2011.07.034Get rights and content

Abstract

An ionic liquid (IL) 1-docosanyl-3-methylimidazolium bromide was incorporated into ultra-high molecular weight polyethylene (UHMWPE) and formed IL/UHMWPE blends by solution mixing. The structure evolution of these blends during uniaxial stretching was followed by in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. During deformation at room temperature, deformation-induced phase transformation from orthorhombic to monoclinic phase was observed in both IL/UHMWPE blends and neat UHMWPE. The elongation-to-break ratios of IL/UHMWPE blends were found to increase by 2–3 times compared with that of pure UHMWPE, while the tensile strength remained about the same. In contrast, during deformation at high temperature (120 °C), no phase transformation was observed. However, the blend samples showed much better toughness, higher crystal orientation and higher tilting extent of lamellar structure at high strains.

Introduction

Recently, the incorporation of ionic liquids into the polymer system has been an interesting topic because ionic liquids can be used as solvents, processing aides and plasticizers to facilitate the synthesis and processing of polymers, as well as to enhance the polymer properties [1], [2], [3], [4]. Ionic liquid is a salt with a low melting point, allowing it to stay in the liquid state at relatively low temperatures. Different from ordinary organic solvents, ionic liquids consist entirely of ions and thus have many unique properties, e.g. they are nonflammable, thermally stable, non-volatile, and have high ion conductivity [1], [5], [6]. The most notable application of ionic liquids in polymers is their usage as polymerization solvents. Because of the unique properties of ionic liquids, the course of polymerization is often changed from that of common solvents. This has been seen in radical polymerization, ionic polymerization, polycondensation and atom transfer radical polymerization (ATPR) involving ionic liquids [7], [8], [9], [10], [11], [12]. In addition, ionic liquids can also be used as solvents to dissolve polymers with poor solubility in common solvents. These polymers include biopolymers such as silk, wool [13], [14], [15] and cellulose [16], just to name a few.

In this study, we explore the subject related to another application, which is the formation of polymeric blends with ionic liquids. The involatility, thermal stability and high conductivity of ionic liquids and their interactions with the polymer matrix make them good candidates as solid electrolytes, suitable for battery and fuel cell applications [3]. The ionic liquids can also be added to solid polymers as plasticizers, which would increase the flexibility of the matrix as well as facilitate its processibility. For example, Scott et al. [17], [18] reported that imidazolium based ionic liquids are good plasticizers for processing of poly methyl methacrylate (PMMA). They found that both glass transition temperature and elastic modulus decreased with the increasing content of ionic liquid in this material. A similar plasticization effect was also observed in systems of different polymers and ionic liquids [19], [20]. It was found that many traditional plasticizers are not suitable for high temperature usage, but ionic liquids can sustain their high temperature applications.

The goal of this study is to investigate the role of ionic liquids in affecting the structure and morphology of semi-crystalline polymers under deformation, where ionic liquids are used as plasticizers. The chosen polymer matrix was ultra-high molecular weight polyethylene (UHMWPE) and the chosen ionic liquid (IL) was 1-docosanyl-3-methylimidazolium bromide. Since the molecular weight of UHMWPE is extremely high (the weight average molecular weight Mw is usually in the range of several millions), the polymer possesses a great deal of chain entanglements and is very difficult to melt process. The dense entanglement structure greatly affects the crystalline structure and morphology of UHMWPE, resulting in high modulus, high tensile strength but very low elongation-to-break ratio. Various kinds of low molar mass additives have been developed and tested to improve the processibility of UHMWPE. One example is the use of low molecular weight paraffin that can enhance the mobility of highly entangled UHMWPE chains during processing and be removed after the process. In this work, we demonstrate that the type of ionic liquid, having a long aliphatic tail that is compatible to the polyethylene backbone, can also be used for the same purpose and remained in the UHMWPE matrix to create a new material. The chosen IL/UHMWPE blends were made by the solution mixing method to ensure the homogenous distribution of IL in the UHMWPE matrix. Simultaneous synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) measurements, in combination with uniaxial tensile deformation, were performed on the blends and the control sample (i.e., pure UHMWPE) to understand the structure, processing and performance relationships.

Section snippets

Materials and preparation

The UHMWPE sample Hizex 340M was obtained from Mitsui Chemical Co. Ltd., Japan. It had a weight average molecular weight (Mw) of 1.5 × 106 g/mol and a polydispersity of about 10.5. The chosen ionic liquid was 1-docosanyl-3-methylimidazolium bromide ionic liquid (IL)

, synthesized in our laboratory using the procedures described elsewhere [21]. The nanocomposite was prepared based on the solution mixing method as follows. First, the desired amount of IL was dissolved in decalin to form a

Results and discussion

Simultaneous X-ray and tensile deformation measurements were first carried out at 25 °C. The stress–strain curves for all three samples are shown in Fig. 3. It was found that by adding ionic liquid, the yield strength decreased (the 3% IL/UHMWPE exhibited the lowest yield point value), but the elongation-to-break ratio increased (e.g., 104% for pure UHMWPE, 338% for 0.6% IL/UHMWPE and 252% for 3% IL/UHMWPE). The final values of tensile strength for the three samples, however, were quite

Conclusions

The addition of a small amount of ionic liquid (e.g. 0.6%) to UHMWPE can significantly increase the elongation-to-break ratio at both low and high temperatures (e.g. 25 °C and 120 °C), while maintaining comparable or better tensile strength. This can be attributed to the increase of chain mobility by the plasticization effect of ionic liquid. However, too much loading of ionic liquid (e.g. 3%) does not further improve the toughness of the UHMWPE matrix. In this study, the 3% IL/UHMWPE sample

Acknowledgements

This work is supported by the National Science Foundation (DMR-0906512). The authors also acknowledge the assistance of Drs. Lixia Rong and Jie Zhu for the synchrotron SAXS and WAXD experimental setup.

References (39)

  • Y.S. Vygodskii et al.

    Polymer

    (2004)
  • M.P. Scott et al.

    Eur Polym J

    (2003)
  • A. Lewandowski et al.

    Solid State Ionics

    (2003)
  • K.E. Russell et al.

    Polymer

    (1997)
  • M.F. Butler et al.

    Polymer

    (1998)
  • J. Jang et al.

    Polymer

    (2003)
  • T. Welton

    Chem Rev

    (1999)
  • P.J. Kubisa

    Polym Sci. Part A

    (2005)
  • N.J. Winterton

    Mater Chem

    (2006)
  • T. Ueki et al.

    Macromolecules

    (2008)
  • P. Wasserscheid et al.

    Angew Chem Int Ed

    (2000)
  • J.S. Wilkes

    Green Chem

    (2002)
  • S. Harrisson et al.

    Macromolecules

    (2003)
  • V. Strehmel et al.

    Macromolecules

    (2006)
  • T. Biedroń et al.

    J Polym Sci Part A Polym Chem

    (2004)
  • J.L. Kaar et al.

    J Am Chem Soc

    (2003)
  • A.J. Carmichael et al.

    Chem Commun

    (2000)
  • K. Fujita et al.

    Chem Commun

    (2005)
  • D.M. Phillips et al.

    Mater Chem

    (2005)
  • Cited by (0)

    View full text