Figure 3(a) displays the evolution of the birefringence at several temperatures above T_NI and also one temperature below. At T > T_NI and low shear rates, the birefringence is close to zero and slightly shear rate-dependent. This regime corresponds to the so-called paranematic phase [10], or flow birefringence [34]. Above a critical shear rate , one observes a strong departure of the birefringence. This non-linear behavior is interpreted, in the frame of the pre-transitional fluctuation coupling model [10] (Figure 3(b)), as a non-equilibrium, first-order phase transition, characterized by an alignment of the liquid crystal moieties along the velocity direction. The texture free from defects indicates a homogeneously aligned nematic phase (Figure 2). Figure 3(c) illustrates the critical shear rate corresponding to the onset of the emergence of the shear-induced birefringence. The (non-equilibrium) characteristic time (Figure 3(c)) varies from 0.1 s up to 10⁻² s. τ * is about 10² to 10⁵ larger than the orientational order fluctuations’ lifetime [35], excluding a coupling with the shear flow, in agreement with the first observations carried out on other SCLCPs [14]. This shear-induced birefringence resembles the foreseen non-equilibrium N-I transition but it is not. Is there an equivalence between the birefringence timescales (τ *) and the characteristic timescales of the rheological behavior (stress-optical rule)?

The second significant divergence with the prediction is the shape of the flow curve (stationary shear stress versus shear rate curve—for a description of the transient behavior, see [36,37]). The flow-induced phase transition is supposed to exhibit a 3-zone stress-shear rate curve (Figure 4(a)): at low shear rates, the paranematic phase corresponds to a Newtonian behavior, at intermediate strain rates, a stress plateau appears when the phase is induced and “phase-separates” into homogeneous bands flowing at strain rates and , to maintain the average imposed strain rate. Finally, at high shear rates, a new Newtonian branch should emerge corresponding to the induced nematic phase [10]. Concentrated systems of wormlike micelles [16,17,18,19,20,21,22,38,39] near a nematic transition are known to yield a non-monotonic flow curve of this form [10,16,17,18,19,20]; while more dilute systems exhibit a non-monotonic flow curve interpreted as an effect of flow on the reptation-reaction dynamics [40]. In this description, the stress plateau displayed upon applying an increasing shear rate is supposed to depend on the nature of the interface between the coexisting phases [10,12,13] but not on the nature of the substrate.

Figure 4(b,c) displays the shear stress versus steady-state shear rate curve of PANO2 at +3 °C above the NI transition temperature measured at two different gap thicknesses (using the same alumina substrate) or using two different substrates (the total wetting alumina and a partial wetting substrate quartz) respectively. In the isotropic phase, in absence of shear-induced phase, low degree of polymerization SCLCPs are expected to behave as Newtonian fluids, i.e., the viscosity (defined by the ratio: η = σ/ ) is constant. Both Figure 4(b) and Figure 4(c) display a non-Newtonian behavior indicating a coupling with the flow, i.e., the isotropic phase contains relaxation times of the order of the inverse of the shear rate (). At low shear rates, the shear stress is nearly linear with the shear rate defining a quasi-Newtonian regime. At about 20–30 s⁻¹, a second regime is identifiable by a slight overshoot followed by a lowering of the stress slope and even a stress plateau at 0.100 mm gap. The stress overshoot at about 20–30 s⁻¹ seems to be coherent with the onset of the shear-induced birefringence (Figure 3(a)). The second regime seems to be coherent with the appearance of the shear-induced phase. A thickness dependence of the flow curve is clearly observable over 30 s⁻¹, whereas the influence of the nature of the substrate (Figure 4(c)) is noticeable from the lowest shear rate (10 s⁻¹). In the case of the quartz substrate (which provides a weak wetting), the shear stress is lower and the non-linear behavior appears earlier when a wetting substrate is used. The affinity of the liquid to the substrate is a necessary condition to transmit the shear motion (see [33] for a relationship between wetting and boundary conditions).

This dependence on the nature of the substrate indicates a slip mechanism which is delayed when the interaction to the substrate is improved by using wetting surfaces. The dependence on the thickness (Figure 4(b)) is also in agreement with a slip mechanism. In this case, the deviation from Newtonian behavior (in particular the “stress-plateau”) cannot fulfill a constitutive equation but results from a mechanical instability [41]. An estimation of slippage rate can be undertaken via the determination of a sliding velocity v_s assuming that the flow can be modeled by a velocity gradient and that the slippage is located at both walls. This is a simplified model, but a detailed gradient and the location of the sliding zones are beyond the present scope.

In case of wall slip boundary conditions, the shear rate felt by the liquid is diminished from the applied one by a quantity: The effective shear rate is: .

The apparent shear rate is: , then . In this scheme, the sliding process is outlined by a no-zero velocity ν_s with respect to the walls (Figure 5 (a)). The sliding velocity, ν_s can be extracted from the values at the same shear stress of the shear rates measured at two different thicknesses following the relationship:

Figure 5(b) displays the sliding velocity ν_s versus shear stress. It shows that from the lowest shear rates, ν_s is not zero. The extrapolation of ν_s to zero gives the stress threshold at which the sliding mechanism is supposed to occur: σ ≈ 2 kPa. The slip manifests itself very early in an apparent Newtonian regime ( < 5 s⁻¹; see Figure 4(b,c)). The three distinct zones in Figure 5(b) can be interpreted as different sliding regimes and thus an interpretation of the flow curve (σversus ) (Figure 4(b,c)) in terms of instabilities.

The evolution of the sliding velocity as a function of strain is very similar to what was predicted by F. Brochard and P.-G. de Gennes, in 1992 [42] who explored the flow of a polymer melt near a solid surface on which chains of identical chemical nature were grafted. The low stress zone was interpreted by a chain extension. Under the effect of friction, the chain deformation causes a restoring elastic force. There is a limit of deformation of the chain, beyond a critical stress for which the chain tangles and disentangles. When the sliding velocity increases, this regime cannot survive indefinitely, the chain stretches and loses its entanglements.

In the present case, the isotropic phase behaves similarly as entangled polymers but without an entanglement/disentanglement mechanism, since the PANO2 chain is too short to be entangled (see Table 1). In addition, the slippage phenomena occur at low shear rates (typically 10 s⁻¹) as if the material was strongly entangled. It indicates that the intermolecular interactions play the role of entanglements. This cohesive or elastic property is inherent in the interactions in the isotropic state of SCLCPs. Therefore, the isotropic phase is a long-range correlated cohesive material.

We quantitatively access the force of this cohesion by measuring a finite low frequency shear modulus (elastic modulus) in the isotropic phase of PANO2. This measurement is extended to an ordinary polymer melt (PBuA) and finally to a small molecule (alkyl chains) in the liquid state.

The linear dynamic relaxation experiment consists of applying a weak shear stress/strain to the fluid. The solicitation has to be sufficiently low to not modify the material under stress (causality-linearity principle). The liquid submitted to a small (linear) shear solicitation is scanned at a constant shear strain γ₀, as a function of the frequency (ω), typically from 10⁻¹ rad/s up to 10² rad/s. The liquid-like character is identifiable by a vanishing response on a long timescale, i.e., at low frequency (flow regime) whereas a solid-like behavior exhibits a no-vanishing response on a long timescale (solid-like response).

This method provides a dynamic profile in terms of elastic (or shear) modulus (G′) and viscous (or loss) modulus (G″). Experimental and theoretical literature illustrate the effect of the liquid crystal anchoring and its orientation on the dynamic response [43,44,45,46]. In the present study, the reinforcement of the boundary conditions is achieved by improving the wetting of the liquid to the substrate (alumina substrate) and by probing sub-millimetre gaps. By reinforcing the contacts, the total wetting guaranties a better dynamic transfer and small gaps reduce the strain-induced displacement [28,29].

We now examine the dynamic response of a thin but macroscopic (several tens of microns) thickness of the isotropic phase of PANO2. Figure 6(a) displays the frequency dependence of the elastic and of the viscous moduli of PANO2 at 3 °C above the NI transition temperature (70 °C above the glass transition temperature (Tg = 33 °C)). A non-measurable shear modulus (G′) and a ω-scaling of G″ are expected if PANO2 exhibits a liquid behavior in this frequency range. The experiment shows, by way of contrast, a non-negligible shear elastic response (G′(PANO2) 10,000 Pa) which is about one order of magnitude higher than the viscous response (G′ > G″). The viscous component being negligible, the primary response of the isotropic phase is elastic, i.e., solid-like. Moreover, the quasi-constant shear elastic and viscous moduli versus frequency attest to a solid-like response. Under weak solicitation, the liquid does not flow but resists with significant elasticity. These elastic interactions cannot be related to reminiscent liquid crystal properties (no birefringence is detectable in the isotropic phase under weak strain solicitation). The isotropic phase is thus a fragile but long-range, elastically-correlated “self-assembly”. The action of the flow is a shear-melting. Above a threshold (defined by the elastic modulus), the “self-assembly” collapses and flows. Rheological and birefringence curves displayed in Figure 3 and Figure 4 reflect therefore what is happening after the collapse. At this state, the system is more or less shear-molten, depending on the shear rate. The timescales in the non-linear regime can hardly be related to any characteristic time.

Figure 6(b) displays the dynamic behavior of a polybutylacrylate (PBuA) of molecular weight (Mw = 20,000 g∙mol⁻¹). The study of a simple alkyl polyacrylate confirms that the elastic property is not specific to liquid crystal materials. This chain has the same architecture as PANO2. Only the terminal part of the liquid crystal moiety (the hard core) has been removed. This amorphous melt (PBuA) has been chosen with the same degree of polymerization as PANO2. At room temperature, i.e., at 90 °C above Tg, PBuA flows easily (the melt is below its entanglement regime 2. Me = 44,000 g∙mol⁻¹ [47]). As already reported in [29], this unentangled melt displays a non-negligible elastic plateau with G′ and G″ constant and G′ larger than G″ within the frequency range (which is the conventional measurement range). The left inset in Figure 6(b) displays the two waves corresponding to the sinusoidal strain amplitude and the output torque respectively. The almost superimposed signals indicate that the melt response is quasi-instantaneous. This is confirmed by the right inset displaying the strain amplitude versus the torque (Lissajous curve). A perfectly solid-like response would be a narrow line oriented at 45° in the Lissajous representation. The experimental result is in agreement with a quasi-solid-like response (cycle tilted at 45°). The closure of the cycle guarantees the reliability of the measurement. The width of the cycle indicates a slight viscous contribution to the signal. The three representations (viscoelastic moduli, input and output waves, Lissajous curve) point to an elastic response, i.e., a solid-like behavior.

This elasticity cannot be related to some unknown property of liquid crystal moiety since the polymer is amorphous. Entanglements cannot be at the origin of the elasticity since the chain is unentangled. The second interesting point is that PBuA and PANO2 have the same polymerization degree; the influence of the liquid crystal moiety can be directly estimated from the strength of the elastic plateau. G′(PBuA) 10³ Pa whereas G′(PANO2) 10⁴ Pa. Clearly, the liquid crystal moieties have an important cohesive role interplaying as connectors. The reinforcement is about 10 times stronger when the polyacrylate chain is substituted by nitrobiphenyl groups. This observation confirms the assumption that the elasticity of the isotropic phase originates from intermolecular interactions. These intermolecular interactions are reinforced by polar and/or polar-induced interactions when the molecules possess liquid crystal moieties. The intermolecular interactions of PBuA are essentially governed by weak van der Waals interactions.

Figure 6(c) displays the dynamic response of the liquid heptadecane. The heptadecane presents an abrupt first transition from crystalline state to liquid state at Tf = 21 °C. With 17 atoms of carbon, the heptadecane (above Tf) is an ordinary molecular liquid. This alkyl liquid presents the practical advantage of having a low vapor tension, and therefore a very weak evaporation. This is useful when the measurement requires long integration times. The spectrum displayed in Figure 6(c) has been carried out at 3 °C above the crystallization temperature. Any effect of the crystallization is excluded, since a short temperature gap away from Tf makes it integrally lose its crystalline properties (first order transition) in contrast to glass formers and possible glass transition effects. Figure 6(c) and the two insets indicate a (linear) solid-like response; the left inset in Figure 6(c) shows that the wave corresponding to the sinusoidal strain amplitude is quasi-superposed to the output torque, in agreement with the right inset displaying the Lissajous curve. The shear modulus G′ dominates the viscous component and both are weakly dependent on the frequency. This delicate elastic response obtained at low strain amplitude using total wetting surfaces shows for the first time that the elastic property can be also found on simple molecules. It excludes any glass transition effect. With a shear modulus of about G′ 80–100 Pa, i.e., 10 times smaller than the one of PBuA and about 100 times weaker than PANO2, we demonstrate that the solid-like property is a generic character inherent to the liquid state.