Dielectric properties of polylactides and their nanocomposites with montmorillonite J.K. Jeszka a, * , L. Pietrzak a , M. Pluta a , G. Boiteux b a Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland b Universite de Lyon, Universite Lyon 1, Laboratoire des Materiaux Polymeres et des Biomateriaux, UMR CNRS 5223, F-69622 Villeurbanne, France article info Article history: Available online 28 January 2010 Keywords: Dielectric properties Relaxation Electric modulus Nanocomposites Polymers Organics abstract The influence of the preparation method on the dielectric properties of two polylactides and their nano- composites with an organoclay (Cloisite Ò 30B) is investigated. The samples obtained by melt-mixing fol- lowed by hot pressing and by solution casting are compared. Dielectric properties of the nanocomposites show a weak influence of the nanoclay on segmental (a S ) and local (b) relaxations in PLA. Above T g a strong increase of dc conductivity attributed to ionic species in the clay is observed. It gives rise also to the Maxwell–Wagner–Sillars interfacial polarization and both real and imaginary parts of e * (x) strongly increase. In the temperature dependence of low frequency dielectric constant an additional max- imum around 80–90 °C is observed due to cold crystallization of PLA which is significantly different for the two kinds of PLA. After cold crystallisation conductivity and a-relaxation decrease while MWS relax- ation only shifts to higher temperatures. Dielectric spectra of solvent-cast and melt-blended nanocom- posites suggest better dispersion in the latter case. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Polylactide (PLA) is one of the most important biodegradable and biocompatible polymers. It can be derived from renewable re- sources. It is environment-friendly and exhibits interesting physi- cal properties, which can be further tailored by filling with selected additives by blending in the molten state [1,2]. PLA is the subject of growing scientific and practical interest in the last years. There are many papers concerning polylactide filled with layered silicates of different nature and properties [2–7]. Such nanocomposites exhibit improved physical properties, comparing to those of the unfilled polymer matrix: mechanical strength, bar- rier properties, thermal resistance and dimensional stability, even at low filler concentration (1–5 wt%). Dielectric properties of PLA were studied by several groups [8– 12]. In our previous paper [13] we compared mechanical, rheolog- ical and dielectric properties of polylactide-based nanocomposites containing layered silicate prepared in a standard way – melt-mix- ing and hot pressing. In this work we show the influence of preparation method on dielectric properties of two polylactides (Cargill and Hycail) and their nanocomposites with an organoclay (Cloisite Ò 30B). Amor- phous and semicrystalline materials obtained by hot pressing or solution casting are compared. 2. Experimental We studied two kinds of poly(L-lactide): one was manufactured by Hycail BV (The Netherlands) (PLAH), with 6.2% of D-lactide according to the producer, and M w of 108 kg/mol. The second was produced by Cargill–Dow (PLAC) with 4.1% of D-lactide. Resid- ual lactide contents were 0.5% and 0.1% respectively. The two polymers were chosen because they differ in crystalli- zation properties. PLAC after cold crystallization during heating 0.5 °C/min to 120 °C was 39% crystalline while PLAH reached only ca. 15.9% crystallinity (as determined by DSC assuming heat of fu- sion 106 J/g [14]). Organically treated montmorillonite Cloisite Ò 30B from Southern Clay Products (Gonzales, TX) was used as a fil- ler. This organoclay contains methyl-bis(2-hydroxyethyl) tallo- walkyl ammonium cations (29.2 wt%). PLA was melt-blended with other components in a counterrotating internal mixer (Brab- ender OHG, Duisburg, Germany). Unfilled PLA was also melt pro- cessed to have a suitable reference material. Melt processing was carried out at the rotation speed of 50 rpm for 20 min, in a dry nitrogen. Solvent-cast films were obtained using ethylene chloride (EtCl) or chlorobenzene (ClB) (Fluka) as a solvent. Montmorillonite was first sonicated in EtCl for 30 min then a proper amount of the poly- mer was added. The solution was sonicated for another 60 min and then cast on a glass plate at room temperature (EtCl) or 90 °C (ClB). The samples were dried at 40 °C first in air and then in vacuum for at least 12 h. 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.06.057 * Corresponding author. Tel.: +48 42 6803 228; fax: +48 42 6803 261. E-mail address: jkjeszka@cbmm.lodz.pl (J.K. Jeszka). Journal of Non-Crystalline Solids 356 (2010) 818–821 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol