Polymer Communication Viscoelasticity of main chain liquid crystalline elastomers Marta Giamberini a, * , Veronica Ambrogi b , Pierfrancesco Cerruti b,c , Cosimo Carfagna b,c a Istituto per i Materiali Compositi e Biomedici, Consiglio Nazionale delle Ricerche, Piazzale Tecchio 80, 80125 Napoli, Italy b Dipartimento di Ingegneria dei Materiali e della Produzione, Universita ` di Napoli ‘Federico II’, Piazzale Tecchio 80, 80125 Napoli, Italy c Istituto di Chimica e Tecnologia dei Polimeri, ICTP-CNR, Via Campi Flegrei 34, 80078 Pozzuoli, Naples, Italy Received 7 December 2005; received in revised form 17 March 2006; accepted 18 April 2006 Available online 19 May 2006 Abstract Time–temperature superposition (TTS) principle was applied to dynamic mechanical analysis performed on two main-chain polydomain elastomers exhibiting a nematic and a smectic A phase. It was found that TTS did not hold neither across the nematic–isotropic nor the smectic– isotropic transitions. The nematic elastomer showed an increase in the storage modulus in the isotropic phase with respect to the nematic phase: this could be explained by means of dynamic soft elasticity, which has been claimed in some literature for side-chain liquid crystalline elastomers (SCLCEs), or in terms of the de Gennes model by a macroscopic/hydrodynamic description. The presence of the mesogen directly incorporated into the main chain increases the lifetimes of the elastic modes both in the isotropic and in the liquid crystalline (LC) phases, with respect to the SCLCEs. In the case of the smectic A elastomer, lifetimes on the order of 10 9 s could be estimated. q 2006 Elsevier Ltd. All rights reserved. Keywords: Liquid crystalline elastomers (LCE); Viscoelasticity; Time–temperature superposition 1. Introduction Liquid crystalline elastomers have been the object of growing interest in recent years, due to their peculiar features, which arise from the combination of elastic properties of conventional elastomers and orientational properties of liquid crystals [1,2]. Interest in such materials grows from their potential applications in the field of mechanical actuators (artificial muscles), optics and coatings of materials, which can efficiently dissipate mechanical energy for, e.g. automotive and aerospace industry. Many papers recently appeared, which deal with mechanical, dynamic-mechanical and rheological proper- ties of side-chain liquid crystalline elastomers (SCLCEs) both on polydomain samples (in which the mesogenic groups are macroscopically disordered) and on monodomain samples (in which the mesogenic groups are macroscopically ordered) [3–18]. On the contrary, few papers examined the mechanical and dynamic-mechanical properties of main-chain liquid crystalline elastomers [19–22] or SCLCEs containing a main- chain LC polymer [11]. To our knowledge, in the literature no papers exist which deal with the viscoelastic properties of main-chain LCEs. In recent papers, we reported the synthesis, physico-chemical and mechanical properties of main-chain LCEs obtained by reacting mesogenic epoxy terminated molecules with dicarboxylic aliphatic acids of different length [23,24]. In this paper, we performed dynamic mechanical thermal analysis (DMTA) frequency sweep analyses in the shear sandwich configuration on two main-chain polydomain elastomers, namely PHBHQ–AA and PHBHQ–SA, which exhibit nematic and smectic A phase, respectively. We subsequently applied the time–temperature superposition (TTS) principle to DMTA data and compared our results with the ones reported for SCLCEs in the literature. 2. Experimental part 4 0 -(2,3-Epoxypropoxy)phenyl-4-(2,3-epoxypropoxy)benzo- ate (PHBHQ) was synthesized as previously reported [23]. Adipic acid (AA) and decanedioic acid (SA) were purchased from Aldrich Chemical Co. and used without any further purification. The LCE PHBHQ–AA and PHBHQ–SA were prepared as described in Ref. [23]. Molecular formulas of the LCE under investigation are reported in Scheme 1. The range of linear viscoelasticity of all the samples was checked by means of a strain controlled rotational rheometer (mod. ARES L.S., Rheometric Scientific). Dynamic strain sweep tests were carried out by using parallel plates (diameter 8 mm) in inert atmosphere with a gap between the plates of Polymer 47 (2006) 4490–4496 www.elsevier.com/locate/polymer 0032-3861/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2006.04.021 * Corresponding author. Tel.: C34 09775558286. E-mail address: marta.giambeini@urv.net (M. Giamberini).