Effect of a Dodecylsulfate-Modified Magnesium– Aluminum Layered Double Hydroxide on the Morphology and Fracture of Polystyrene and Poly(styrene-co- acrylonitrile) Composites V. Realinho, 1 M. Antunes, 1 D. Arenco ´n, 1 A. I. Ferna ´ndez, 2 J. I. Velasco 1 1 Centre Catala ` del Pla `stic, Departament de Cie `ncia dels Materials i Enginyeria Metallu ´ rgica, Universitat Polite `cnica de Catalunya, Colom 114, E-08222 Terrassa (Barcelona), Spain 2 Departament de Cie `ncia dels Materials i Enginyeria Metallu ´ rgica, Universitat de Barcelona, Martı ´ i Franque ´s 1, E-08028 Barcelona, Spain Received 3 July 2008; accepted 26 August 2008 DOI 10.1002/app.29288 Published online 25 November 2008 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Composites of polystyrene (PS) and poly (styrene-co-acrylonitrile) (SAN) containing a fraction of a dodecylsulfate-modified Mg–Al layered double hydroxide (LDH) were prepared by means of a melt-extrusion pro- cess. The structure and morphology were analyzed with wide-angle X-ray scattering and transmission electron mi- croscopy, respectively. The X-ray spectra of the PS matrix composite displayed the diffraction peak characteristic of the hybrid LDH basal plane at 2y ¼ 3.1 deg. The SAN ma- trix composite did not exhibit such a diffraction peak. Both PS and SAN composites displayed an intercalated type of morphology with respect to the LDH platelets, as assessed by transmission electron microscopy. A plasticiz- ing effect due to the hybrid LDH particles was observed for all composites and was supported by a decrease in the glass-transition temperature values and by Fourier trans- form infrared spectra. Besides tensile properties, the frac- ture toughness of the composites was compared with that of the pure polymers through the linear elastic fracture mechanics parameters. They were determined from frac- ture tests under a three-point-bending configuration. The results indicated that the effect of adding a small fraction of modified LDH particles to SAN caused an improve- ment in fracture toughness of 50% with respect to that of the pure polymer. Moreover, the relative increase in the fracture energy was about 200%. For PS matrix compo- sites, both tensile properties and linear elastic fracture mechanics fracture parameters remained unaffected. These results were explained on the basis of the different plasticities developed by both polymers around the par- ticles. V V C 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2574–2583, 2009 Key words: fracture; nanocomposites; polystyrene INTRODUCTION Despite all the thorough research work on polymer– clay nanocomposites in the last decade, only scat- tered investigations have considered the synthesis and characterization of polymer nanocomposites based on layered double hydroxides (LDHs). 1–10 A couple of works have focused on how LDH particles may affect some thermal and mechanical characteris- tics of the base polymers, such as the thermal stabil- ity and barrier properties of polymers used for packaging and medical applications, 11 electrical cable insulation and water impermeability, 12 flame retardancy, 13 and thermal conductivity, 14 or on how their presence alters the mechanical properties. 15,16 LDHs, also known as anionic clays because of the presence of anions between the brucite-like layers, show the general formula M 2þ 1x M 3þ x (OH) 2 A x=n n  mH 2 O, where x has a value between 0.17 and 0.33. 17 Partial M 2þ /M 3þ substitution positively charges these brucite-like layers, which are charge-balanced by the presence of interlayer anions, which are nor- mally a carbonate-type (CO 2 3 ) in most of the sam- ples found in nature. This particular structure results in strong interlayer electrostatic interactions between the layers, which make it difficult to exfoli- ate natural LDH platelets via normal procedures such as melt mixing. To weaken these electrostatic interactions and facilitate LDH delamination and polymer dispersion by conventional methods, their easy anionic interchange can be used to replace the original inorganic anions (e.g., carbonate with or- ganic ones), creating hybrid LDH precursors with expanded layer distances, weakening layer-to-layer interactions, and ultimately promoting platelet delamination. 18 This increase also facilitates Journal of Applied Polymer Science, Vol. 111, 2574–2583 (2009) V V C 2008 Wiley Periodicals, Inc. Correspondence to: V. Realinho (vera.realinho@upc.edu). Contract grant sponsor: Spanish Ministry of Science and Education; contract grant number: MAT2007-62956.