Ion transport model in exfoliated and intercalated polymer–clay nanocomposites Namrata Shukla, Awalendra K. Thakur ⁎ Department of Physics and Meteorology Indian Institute of Technology, Kharagpur-721302, India abstract article info Article history: Received 15 April 2010 Received in revised form 14 May 2010 Accepted 20 May 2010 Keywords: X-ray diffraction Fourier transform infrared spectroscopy Intercalated nanocomposite Ion transport Transport properties of ion conducting polymer–clay nanocomposites (PNCs) are reported. Experimental results indicate the possibility of exfoliation at lower clay loading and intercalation with increase in clay concentration in the composite matrix. The intercalation of cation coordinated polymer into the nanometric clay galleries has been confirmed by XRD and TEM analyses. Convincing evidences of polymer–ion, polymer– clay, ion–ion and ion–clay interactions have been observed in FTIR results. The estimated fraction of free charge carriers has shown excellent correlation with measured conductivity. Substantial enhancement in d.c. conductivity in the PNC films relative to the polymer salt complex (PS) is attributed to clay induced interaction among the composite components. A model for conductivity enhancement is proposed to understand the ion transport process in exfoliated and intercalated PNCs. The conceptual basis of the model seems consistent with the experimental results. The ion transport number (t ion ) for all the PNC films has been estimated to be N99%. An improvement in voltage stability has been observed with addition of clay. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Solid polymer electrolytes (SPE), prepared by dissolving salts in a polymer matrix have received considerable attention as a separator component in energy storage/conversion devices [1,2]. They impart special features in devices such as low weight, flexibility, ability to form desired shape, good adherence to electrodes and low manufac- turing cost. Applicability of ion conducting polymer films in devices requires high conductivity (∼ 10 -3 S cm -1 ). This, in turn, depends critically on the polymer salt ratio (Ö/Li + ), where Ö represents electron donating group in the polymer backbone and Li + refers to the mobile charge carrier (cation). In this light, an optimized (Ö/Li + ) ratio plays significant role in governing the suitability of SPE film as the separator for energy storage device fabrication. A large number of polymers such as PEO [3], PMMA [4], PAN [5] and PVDF [6] etc. have been used as the host for ion conducting membrane preparation with this advantage in mind. However, the poor ionic conductivity of the SPE imposes serious limitations on their application. The main cause of poor ionic conductivity in SPEs is attributed to the ion pair formation, which reduces the number of mobile charge carriers. Many efforts have been made to eliminate ion-pairing effect. Formation of intercalated polymer nanocomposite (PNC) is considered an effective approach where ion pair formation, due to Coulombic interaction between cations and anions, can be minimized if cannot be eliminated. Excellent reviews and reports on intercalated PNCs describing such feasibility are also available in literature [7–13]. Intercalation of ion conducting polymer matrix into organically modified hydrophobic nanometric channels of the clay ensures an effective separation between the cations (Li + ) with its counter ion due to the presence of negative (-ve) charges on the surface of the organophilic clay. As a consequence, concentration of free mobile charges (Li + ) in the polymer–clay nanocomposites may be expected to be higher. This in turn, may be expected to cause enhancement in the conductivity in accordance with the relation, σ = ∑ i n i z i μ i where, n i , μ i , and z i refer to the number of mobile charge carriers, ionic mobility and the ionic charge respectively. In sharp contrast to this feasibility, the addition of clay also results in lowering of the polymer chain mobility due to a possible increase in viscosity of the composite phase. It results in lowering of ion dynamics. These two opposite features in a heterogeneous nanocomposite induce two contrasting effects, of which one is favorable and the other is detrimental for ionic transport. As a result, no unanimous view on ion transport mechanism in a polymer nanocomposite in general and intercalated PNC in particular has emerged yet. In this paper, we report the results of our investigation on polymer nanocomposite films based on (PMMA) 4 –LiClO 4 +x wt.% DMMT over a wide range of organo-modified clay (DMMT) concentration (0–20 wt.%). The role of clay concentration on ion–ion, ion–polymer and ion–clay interactions has been analyzed. A model for ion conduction has been proposed to explain clay concentration dependence of conductivity. 2. Experimental Free-standing polymer–clay nanocomposite (PNC) films were prepared using PMMA (M.W. ∼ 2×10 5 ) from Aldrich (USA), salt (LiClO 4 ) from (M/s Acros Organics) and Na-montmorillonite (MMT) Solid State Ionics 181 (2010) 921–932 ⁎ Corresponding author. Tel.:+91 3222 283834; fax: + 91 3222 255303. E-mail address: akt@phy.iitkgp.ernet.in (A.K. Thakur). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.05.023 Contents lists available at ScienceDirect Solid State Ionics journal homepage: www.elsevier.com/locate/ssi