Effects of the variation of weight concentrations of PPTA (PPTA wt%: 18.5, 19 and 19.5%) and temperatures (75, 80, 85, 90 °C) on rheological curves of the PPTA-H₂SO₄ LC solution are shown in Figure 1. The solution exhibits a typical shear-thinning behavior even at a low shearing frequency (ω), i.e., its η^* decreases as ω increases, which results in the alignment of PPTA chains in parallel. With increasing ω, the density of junction points decreases since the crosslinks break significantly faster than they form owing to their shear-thinning behavior [22]. Such changes of chain configuration and orientation under shearing force indicate that both G″ and tanδ of PPTA-H₂SO₄ LC solution would increase as ω rises. At a high shearing rate, nearly all macromolecular chains orient, implying that G′ hardly follows the variation of frequency, since in this regard the first normal stress difference hardly follows frequency variation [23].However, when weight concentration of the solution increases, density of entanglement crosslinks and intermolecular forces both increase, and then macromolecular chains become more difficult to orient and diffuse, leading to the increase in all the rheological characteristics including η^*, G′, G″, and tanδ. On the other hand, as the temperature increases, thermal motion of macromolecular chains increases, flow resistance decreases, and rheological curves of η^*, G′, G″, and tanδ move down.

Supramolecular structure is key to affecting the rheological characterization of the polymer system. Figure 2 displays the rheological behaviours of PPTA-H₂SO₄ LC solution at 19.5 wt% of PPTA as a function of M_w (i.e., 28169, 56170, 68407, and 80135 g/mol) and temperature (i.e., 80, 85, and 90 °C). All four rheological parameters (η^*, G′, G″, and tanδ) increase with increasing M_w of polymers. Also, the larger M_w is, the longer relaxation time (λ) appears, meaning that oriented molecular chains upon shearing cannot resile, thus showing more distinct shear-thinning behaviour. This is greatly advantageous to obtain high degree of orientation in spinning process and hence obtain fibers with higher tenacity and modulus. Moreover, the larger M_w is, the more entanglement crosslinks are generated, meaning that the dynamic process of disentanglement and entanglement would again take place at less shear rate [24,25]. Hence, M_w of PPTA would be ideally neither high (difficult for molding) nor low (accelerating the relaxation).

Variation of complex viscosity as a function of SWNT weight concentration (SWNT wt%) in PPTA/SWNT/H₂SO₄ dopes is plotted in Figure 3, showing the nonmonotonic characteristic as a function of frequency and temperature. The η^* first gradually increases with increasing SWNT wt%, and then decreases abruptly when beyond ∼0.1 SWNT wt%, and arises again when beyond 0.2 SWNT wt% to reach about the same viscosity for three temperature-dependent curves. In general, this concentration dependent viscosity behavior of PPTA/SWNT/H₂SO₄ solution resembles that of SWNT/H₂SO₄ solution. As the SWNT concentration was below 0.1 wt%, the solution viscosity increased accordingly with increasing SWNT wt%. This can be considered as normal behavior of an isotropic solution. When SWNT concentration was exceeded 0.1 wt%, the viscosity decrease at increasing SWNT concentration can be considered as a unique behavior of anisotropic solution. It is caused by the liquid crystal behavior where SWNT are aligned upon shear and form domains of highly ordered structures.

It has been revealed in rheology and microscopy studies that SWNTs in H₂SO₄ exhibit roughly similar phase behavior as rod-like polymer solutions [21,26]. We have shown in optical microscopy studies that SWNTs in sulfuric acid solution tend to self-assemble into extremely long strandlike structures, which is typically characteristic of the LC phase [21]. The LC transition from a biphasic phase to a single phase occurs in the SWNT wt% range from 2.7 wt% (maximum viscosity) to 4 wt% (minimum viscosity) [21]. For a blend of SWNT with such a rigid-rod polymer as PPTA, SWNT appears easier to adopt a higher LC order as the transition occurs within the lower SWNT wt% range [27,28]. Thus, 0.1 wt% was the critical concentration of SWNT for PPTA/SWNT/H₂SO₄ system. A biphasic region of mixed isotropic and anisotropic phase existed between 0.1 wt% and 0.2 wt%. Beyond 0.2 wt%, the solution became an increasingly anisotropic-rich phase, and the solution viscosity increased again with increasing SWNT wt%.

Temperature effect on complex viscosity (η^*) as opposed to shearing frequency (ω) of PPTA/SWNT/H₂SO₄ dope is shown in Figure 4.

All three samples composed of pure PPTA and PPTA coupled with different portion of SWNT exhibit an obvious shear-thinning behavior through the entire ω range. As ω increases, η^* decreases in direct proportion to ω. The slope of such decreasing line on the logarithmic plot remains similar, at about −0.91, for all three samples. This shear-thinning behavior is thought to arise from entanglements between macromolecular chains. For rigid-rod polymers such as PPTA, shearing decreases the number of entanglements and thus affords macromolecules to rearrange the configurations readily. By comparison, PPTA/SWNT/H₂SO₄ dopes are more highly shear-sensitive. (1) The η^* of PPTA/H₂SO₄ dope decreases as temperature increases from 75 to 85 °C. Such behavior is often observed among conventional polymers. (2) The η^* of PPTA/SWNT (98/2 wt%)/H₂SO₄ dope decreases as temperature increases from 80 to 85 °C, while it becomes nearly independent of temperature ranging between 75 °C and 80 °C. (3) The η^* of PPTA/SWNT (97/3 wt%)/H₂SO₄ dope decreases slightly as temperature increases from 80 to 85 °C, but increases slightly as temperature increases from 75 to 80 °C.

To determine possible phase changes from 75 to 85 °C, we carried out thermal analysis on PPTA/SWNT/H₂SO₄ dopes. The DSC thermogram (Figure 5) reveals a broad endothermic peak for all samples, implying that multiple transitions occur including monothetic to the liquid state and also isotropic to nematic phase [25,29]. We also noted that with increasing SWNT wt%, the endothermic peak temperature increases from 73.6 to 79.9 °C, possibly because the curve of η^* vs. SWNT wt% becomes nearly independent of temperature [30].

Figure 6 presents the temperature dependent plots of the dynamic modulus of PPTA/SWNT/H₂SO₄ dopes over ω. As SWNT wt% increases, G′ and G″ arise sharply regardless of temperature. For any temperature and SWNT wt%, elastic characteristic such as G′ of PPTA/SWNT/H₂SO₄ dope is a dominant factor within the frequency range from 0.1 to 10 rad/s. As ω increases, the G′ rises slightly in direct proportion to ω. The slope of G′ versus ω on the logarithmic plot is 0.098, 0.085 and 0.063 as PPTA/SWNT is 100/0, 98/2 and 97/3, respectively. In a low ω region (ω < 1 s⁻¹), the G″ does not rise, while in other regions (ω > 1 s⁻¹), G″ rises faster than G′, but not in proportion to frequency.

According to the relative magnitude of G′ and G″, a logarithmic plot of G′ and G″ as a function of frequency comprising three zones: terminal, plateau and transition zone, has been proposed [31]. Referred to the literature, PPTA/SWNT/H₂SO₄ dope is in its plateau zone within the frequency range from 0.1 to 10 rad/s [32]. Rod-like macromolecules like PPTA are thought to exhibit longer relaxation times than flexible polymers due to the difficulty of molecular reorientation arising from the stiff molecular structure and close-packing. The dynamic behavior of Vectra^®, for example, supports this argument [33]. As SWNT wt% increases, PPTA/SWNT/H₂SO₄ dope seems to exhibit more intermolecular interaction and chain rigidity, thus giving rise to the plateau-zone behavior [34].

Sulfuric acid with 0.2 wt% excess SO₃ was prepared by mixing the fuming sulfuric acid (25 wt%) with concentrated sulfuric acid (98 wt%). The concentration of sulfuric acid was measured by acid-base titration. The SWNTs used in this study was acquired from Shenzhen Nanotechnologies Co., Ltd. and an average diameter of <2 nm. Finally, SWNTs were dried under vacuum overnight at 80–100 °C. Dispersions were prepared by mixing with a magnetic stir bar for 3 days in an anhydrous environment in a glovebox which was nitrogen atmosphere at room temperature [35]. The PPTA polymer was synthesized following the literature methods [36,37]. All glasswares were dried before use.

The PPTA-H₂SO₄ LC solutions were prepared with four different molecular weight polymers (M_w = 28169, 56170, 68407, 80135 g/mol), at different temperatures (75, 80, 85, 90 °C) and with different weight concentrations (18.5, 19, 19.5 wt%). For example, into a 250 mL three-neck flask equipped with a mechanical stirrer and a nitrogen inlet/outlet, were placed 34.01 g of PPTA and 140.4 g of H₂SO₄ at 75 °C. The mixture was then stirred for 2.5 h and opalescence was observed during this step. Likewise, the LC solutions of PPTA/SWNT/H₂SO₄, in which PPTA was kept at 19.5 wt% and PPTA/SWNT is 99/1 wt%, were prepared in a 250 mL three-neck flask. SWNTs (0.3401 g) were dispersed in H₂SO₄ (140.4 g) under an inert and anhydrous atmosphere with mechanical stirring and at room temperature for at least 3 days [21]. Into this flask were then added 34.01 g of PPTA at 80 °C. The mixture was stirred for 2.5 h and stir opalescence was observed during this step.

Subsequently, the PPTA-H₂SO₄ LC and PPTA/SWNT/H₂SO₄ dope solutions were performed with dynamical scanning on an ARES rotational rheometer (TA Instruments, New Castle, DE, USA) at four temperatures (75, 80, 85, 90 °C) in the frequency range of 0.01–100 rad·s⁻¹ and 0.1–10 rad·s⁻¹, respectively. Testing fixtures parallel plates (50 mm) were made of stainless steel 316 to avoid corrosion. In order to prevent water exchange between the sample and environment, the sample surface between the plate and the cone was coated with a mineral oil [38,39]. The effects of dynamic frequency (ω) on rheological behaviors such as η^*, G′, G″ and tanδ have been examined.

Thermal analysis on the PPTA/SWNT/H₂SO₄ dope was carried out on a differential scanning calorimeter (DSC) (Mettler Toledo 822) under a N₂ atmosphere at a heating rate of 5 °C/min ranging from 25 to 90 °C.