Criticality Controlled by Cross-Linking Density in Liquid Single-Crystal Elastomers

A high-resolution calorimetry and deuteron-nuclear magnetic resonance study of a paranematic-nematic phase transition was performed on liquid single-crystal elastomers. We show that density variations of both rodlike and pointlike cross-links strongly affect the mean value and the dispersion of local mechanical fields. The system exhibits an inherent weakly disordered orientational state composed of regions with the temperature profile of the nematic order parameter ranging from first order to supercritical. On increasing the cross-linking density the predominantly first order thermodynamic response transforms into a predominantly supercritical one.

Figure 1

(a) Temperature dependence of ac (solid symbols) and relaxation mode (open symbols) $C_p$ data obtained for V6 concentrations. Circles stand for $x=0.075$, rhombs for $x=0.105$, down triangles for $x=0.125$, squares for $x=0.15$, and up triangles for $x=0.16$ concentration. On the top the chemical formula of V6 cross-link is given. (b) Temperature dependence of ac (solid symbols) and relaxation mode (open symbols) $C_p$ data obtained for V3 concentrations. Rhombs stand for $x=0.066$, circles for $x=0.075$, squares for $x=0.105$, and up triangles for $x=0.125$. On the top the chemical formula of V3 cross-link is given.

Figure 2

Above: (a) Simultaneous fit of $M_1(T)$ and $M_2(T)$ values extracted from the measured spectra of the $x=0.105$ V6-cross-link sample. (b) High-temperature close up of the $M_2(T)$ data from (a) fitted either with a distribution of $G$ (solid line) or of $T^{*}$ (dashed line). Below: A selection of NMR spectra for $x=0.075$ (c), 0.105 (d) and 0.15 (e). Arrows denote spectral features attributed to paranematic (PN) and nematic (N) phases.

Figure 3

The latent heat of PN-N transition for all the concentrations of the two cross-links V6 (solid circles) and V3 (open circles). Half-solid circles represent calculated latent heat based on the $d$-NMR data (see related text below).