Delivered by Ingenta to: Chonbuk National University - Central Library IP : 210.117.158.168 Wed, 08 Dec 2010 01:34:14 Copyright © 2011 American Scientific Publishers All rights reserved Printed in the United States of America RESEARCH ARTICLE Advanced Science Letters Vol. 4, 65–73, 2011 Structure–Property of Thermoplastic Polyurethane–Clay Nanocomposite Based on Covalent and Dual-Modified Laponite Ananta K. Mishra 1 , P. R. Rajamohanan 2 , Golok B. Nando 1 ∗ , and Santanu Chattopadhyay 1 ∗ 1 Rubber Technology Center, Indian Institute of Technology Kharagpur, Kharagpur 721302, India 2 Central NMR Facility, National Chemical Laboratory, Pune 411008, India Modification of surface of clay platelets by ionic and covalent modification techniques renders it to be easily dispersed in polymers like, Thermoplastic Polyurethane (TPU). Only ionic or covalent modification techniques in isolation are not sufficient to achieve uniform nanoscale dispersion of Laponite (synthetic hectorite nanoclay) in TPU. Hence, the dual modification of Laponite (both ionic and covalent) is performed and the effects of the modification on the morphology, thermal and rheological behaviors of the TPU-modified clay nanocomposites have been studied. The degree of exfoliation of clay platelet in TPU matrix is found to be higher for dual func- tionalized nanoclays compared to their singly modified counterparts. Interestingly, dual modified Laponite clays prepared by using two different techniques (ionic followed by covalent and covalent followed by ionic) exhibit different morphology and properties. The dual modified clays significantly alter the equilibrium morphology of TPU. The storage modulus of the dual modified Laponite-TPU nanocomposite in the glassy region (at -60  C) and in the rubbery region +98  C) is improved by 172.8% and 85%, respectively as compared to the neat TPU. Similarly, the onset of degradation is found to be enhanced by 28.7  C as compared to the neat TPU. Keywords: Thermoplastic Polyurethane, Laponite Nanocomposite, Dual Modification, Morphology, Structure-Property. 1. INTRODUCTION Polymer–clay nanocomposites with a high degree of exfoli- ated morphology of nanoclays have gained increasing interest in recent years. 1 These composites offer improved mechanical properties, 2 higher heat distortion temperature, 2 better barrier properties, 3 lower water absorption, 4 increased thermal stabil- ity and reduced flammability. 5 In this regard TPU is of popu- lar choice in contrast to other polymers because of its wider applicability many engineering works. Several grades of clays are available (natural and synthetic) which can be used as fillers in polymers (Table I). Sheng et al. 6 have observed that matrix stiffening effect (commonly encoun- tered in case of clay with greater size and aspect ratio) can be greatly reduced with nanoclay possessing lower aspect ratio. Among the nanoclays listed in Table I, Laponite RD possesses the advantage of lower size (25–30 nm), which nearly matches with the size scale of the phase separated hard domains present in TPU. It is also chemically pure as it is derived synthetically. It has an empirical molecular formula of Na 07 (Si 8 Mg 55 Li 03  O 20 (OH) 4 . It is insoluble in water but forms a clear and colorless ∗ Authors to whom correspondence should be addressed. colloidal dispersion. However, to make it compatible with poly- mer, the surface of the nanoclay should be made hydrophobic. This is possible by the organic modification of the surface and can be done in two major ways. The Na + ions present in the inter- gallery spacing of the nanoclay can be ion exchanged with long chain alkyl ammonium ions. Alternately, the –OH group present on the edge of the nanoclay can also be covalently bonded with alkoxy silane groups. It is well known that optimum property can be achieved by increasing the degree of exfoliation of the clay platelets. In an earlier communications 7 8 the authors have reported about the difficulties in dispersing Laponite RD in an organic solvent, even if it is modified with either dodecylammonium chloride or cetyl trimethyl ammonium bromide through simple ion exchange pro- cess. Based on the previous experience, the present work has been undertaken for dual modification of Laponite RD. Both ionic modification 9–14 and covalent modification (silylation) 15–18 techniques of clay is well established in the literature. However, the dual modification (simultaneous covalent and ionic modifica- tion) of clay has been scarcely reported. 19–21 This paper embod- ies a novel approach for improving the dispersion of Laponite clay in the TPU matrix to uplift the property spectrum of the nanocomposites. Adv. Sci. Lett. Vol. 4, No. 1, 2011 1936-6612/2011/4/065/009 doi:10.1166/asl.2011.1174 65