Shear modulus of polydomain, mono-domain and non-mesomorphic side-chain elastomers: Influence of the nematic order

Abstract

We investigate the behavior of the complex shear modulus of a series of elastomers including mono-domain and poly-domain liquid crystal samples, and a non-mesomorphic sample. We find that the dynamics of the glass transition are strongly modified by the nematic order. This result explains why the truly elastic response of liquid crystal elastomers can only be observed in the isotropic phase at very high temperatures and at very low frequencies. Between the elastic regime and the glassy state, the elastomers have a visco-elastic regime, which is characterized by a Rouse-like behavior for mono-domain and poly-domain samples, and by a Zimm-like behavior for the non-mesomorphic sample. We also show that the mono-domain sample exhibits marked anisotropy of the shear-modulus G . This anisotropy, which is observed for the first time, is a function of frequency and is inverted between low and high frequencies, due to relaxation effects of the orientational order.

Dynamic soft elasticity in monodomain nematic elastomers

“…The reason could be that polydomain nematic elastomers were used, although we have earlier seen the effect of dynamic soft elasticity in a variety of polydomain elastomers [26]. The drop in G ′ below T ni is smaller in polydomain samples, but it only disappears in the monodomain V -geometry, as in fact the more recent study [11] confirms. It is also interesting that many authors, e.g.…”

“…In the isotropic phase, the exponent was found to be 0.5, similar to the Rouse model prediction. A more detailed recent study of nematic LCE rheology by Stein et al [11] also reported G ′ ∝ ω 0.5 in a polydomain nematic elastomers, but could not obtain Master Curves for monodomain, aligned nematic materials. Working with such monodomain nematic rubbers, Clarke et al [12,13] reported a significant difference in response between polydomain samples (where the nematic director is disordered on the scale above few microns) and monodomain nematic rubbers (with the director is macroscopically aligned along the shear direction, or perpendicular to it).…”

“…In this paper we describe a simultaneous study of dynamic-mechanical and stress-relaxation responses for the two well-characterised nematic LCE materials. One sample is a "classical" siloxane side-chain nematic rubber first prepared and described in great detail by Finkelmann et al and others [19,20,11,12]. The other material had a high content of main-chain nematic polymer strands, which makes the effective chain anisotropy very high [21], but also leads to a very slow dynamics.…”

Low–frequency acoustic waves in nematic elastomers

“…A pioneering study of oscillating dynamic-mechanical properties (Gallani et al . 1996) has been followed by experiments on aligned monodomain nematic LCEs (Stein et al . 2001;Clarke et al 2001), and a theoretical work , which demonstrated a dramatic reduction of storage shear modulus G 0 and the associated increase in the loss factor tan¯in certain geometries of deformation, which involve the induced director rotation.…”

Liquid crystalline elastomers: dynamics and relaxation of microstructure

“…1998), indicating a complex role played by the underlying constraints imposed by the nematic order. A pioneering study of oscillating dynamic-mechanical properties (Gallani et al 1996) has been followed by experiments on aligned monodomain nematic LCEs (Stein et al . 2001;Clarke et al .…”