The influence of ultrahigh molecular weight polyacrylonitrile on the copolymerization of acrylonitrile with itaconic acid and on the rheological properties of the resulting polyacrylonitrile solutions

Abstract

In this study, a series of ultrahigh molecular weight polyacrylonitrile (UHMWPAN)/dimethylsulfoxide dilute solutions were produced and they were used afterwards as reaction medium for the synthesis of copolymers of acrylonitrile with itaconic acid by solution polymerization. The influence of UHMWPAN was investigated on the copolymerization and on the rheological behavior of the copolymer solutions. It was found that the presence of UHMWPAN was greatly influential on the copolymerization and the rheological behavior of the solutions. Compared with the polyacrylonitrile solution without UHMWPAN, M_η and η increased significantly. The results of shear rheological measurements indicated that the existence of UHMWPAN increased the characteristic relaxation time of polyacrylonitrile solutions. What is more, the polyacrylonitrile solutions containing UHMWPAN exhibited much more evident strain-hardening behavior than the solution without UHMWPAN. It was thus determined that polyacrylonitrile solutions containing a small amount of UHMWPAN are suitable for dry-jet wet spinning and can provide enhanced properties in the preparation of high-performance polyacrylonitrile precursor fiber.

Keywords

ultrahigh molecular weight polyacrylonitrile copolymerization polyacrylonitrile solution rheological behavior

Polyacrylonitrile (PAN)-based fiber is widely acknowledged as an important and promising precursor,^1–5 which has been established as the strongest.^6,7 PAN-based carbon fibers are also known to possess enhanced properties for the preparation of high-performance carbon fibers (as compared to pitch, rayon, etc.), which can be attributed to its greater carbon yield and higher melting point.^8–10 As is well-known, the performance of PAN-based carbon fiber is greatly dependent on the properties of its precursor and spinning solution, which can have a direct effect on the structure and properties of the fibers.^11,12 Accordingly, there have been widespread attempts to examine the properties of PAN precursor solutions for the development of enhanced PAN precursor fibers.

The attainment of excellent PAN-based precursor is largely influenced by the spinnability of the solution. Spinnability refers to the process of fiber production from a given set of raw materials¹³ that is influenced by spinning variables, such as the spinning temperature, jet stretch, rheological properties of the solution, spinneret hole size and shape, etc.^14–16 Commercial PAN fibers are usually fabricated by wet spinning or dry-jet wet spinning, although dry-jet wet spun PAN fibers are usually more structurally compact with reduced core-shell difference and better mechanical properties.^14,17 However, a gel spinning method has also been employed in the production of PAN precursor fibers by using high or even ultrahigh molecular weight polyacrylonitrile (UHMWPAN) and semi-dilute spinning solutions.^18–20 Therefore, the choice of spinning method is highly influential for the performance of PAN precursor fibers.

In our previous work, we found that the addition of a small amount of UHMWPAN to concentrated medium molecular weight polyacrylonitrile (MMWPAN) solution could enhance the drawability of viscoelastic solution. This finding gives insights into the drawing in dry-jet wet spinning: a larger total draw ratio could be achieved through increases in the jet stretch ratio of the spinning filament.

In this study, a few dilute UHMWPAN/dimethylsulfoxide (DMSO) solutions were prepared, which were first used to synthesize acrylonitrile (AN)/itaconic acid (IA) copolymers in a solution copolymerization system as a reaction medium. The influence of UHMWPAN was discussed in the copolymerization of AN and IA, and the rheological behavior of the PAN solutions containing UHMWPAN were investigated using Haake RheoStress 150 L stress-controlled rheometer.

Experimental details

Materials

AN (chemically pure) was purchased from SECCO Petrochemical Co., Ltd. DMSO (analytically pure) was purchased from Bohr chemical reagent Co., Ltd. Azodiisobutyronitrile (AIBN, chemically pure) was purchased from Shanghai No.4 Reagent & H.V. Chemical Co., Ltd. UHMWPAN copolymers (M_η = 2.12 × 10⁶gmol⁻¹, AN:IA = 98:2, by molar) were synthesized in our own lab by aqueous polymerization. Sodium thiocyanate (NaSCN, analytically pure), IA (analytically pure) and dimethyl formamide (DMF, analytically pure) were all purchased from Sinopharm Chemical Reagent Co. Ltd.

Preparation of PAN solution

A small amount of UHMWPAN powder was evenly dispersed into DMSO in a three-neck bottle. After the powder had completely dissolved, the solutions were heated for 4 h at 15–18℃. The temperature was then raised gradually to 80℃, and the solutions were stirred by an electric paddle stirrer for 6 h to produce dilute solutions, which contain 0.0%–1.0% UHMWPAN.

AN was purified by distillation between 76 and 78℃. IA was recrystallized from water. The initiator-AIBN was recrystallized from ethyl alcohol. Copolymerization of AN and IA in the feed were carried out in a three-necked flask at 60℃ under nitrogen atmosphere using above-mentioned dilute solutions as a reaction medium. In the reaction system, the total monomer concentration was 22 wt.%, AN/IA was 98:2 and the initiator concentration was 0.4 wt.% (on the basis of total monomers); the reaction time was 20 h. The composition of PAN solution samples is listed in Table 1. Then the solutions were deaerated for 3 h in a vacuum drying oven at 70℃.

Table 1.

The composition of polyacrylonitrile solution samples

Sample no.	AN/IA	Content of UHMWPAN (wt.%)	Content of monomer (wt.%)
A1	98/2	0.0	22.0
A2	98/2	0.1	22.0
A3	98/2	0.3	22.0
A4	98/2	0.5	22.0
A5	98/2	0.7	22.0
A6	98/2	0.9	22.0
A7	98/2	1.0	22.0

AN: acrylonitrile; IA: itaconic acid; UHMWPAN: ultrahigh molecular weight polyacrylonitrile.

Characterization

Conversion measurements

A certain weight of polymer solution (m₁) was pressed into a film by sheet glasses. The film was washed by deionized water repeatedly until it was clean and then it was put into a constant temperature drying oven at 100℃ for 1 h. The weight of the dried film was recorded as m₂. The total concentration of the monomer was C₁. The conversion ratio can be calculated as follows:

Conversion (%) = \frac{m_{2}}{m_{1} \times C_{2}} \times 100

(1)

Viscosity-average molecular weight (M_η) measurements

The Ubbelohde viscometer was used to measure the intrinsic viscosity of the polymer, which was dissolved in 0.1 molL⁻¹ NaSCN/DMF solution at 30℃ ± 0.5℃. The viscosity-average molecular weight can be calculated as follows:

[η] = 3.35 \times 10^{- 4} {\bar{M}}_{η}^{0.72}

(2)

Viscosity measurements

The DV-II+ type viscometer (Brookfield, America) was used to obtain the viscosity of the polymer solution at 60℃ ± 0.5℃ with the NO.SC-34 rotor.

Rheological measurements

Rheological measurements for all of the samples were performed on a Haake RheoStress 150 L stress-controlled rheometer (Fisher Scientific, Newington, NH) with a cone-plate geometry (35 mm/1°) and 0.052 mm gap. Both the dynamic and steady shear tests were performed at temperatures of 40℃, 50℃, 60℃, 70℃, 80℃, while the temperature control was performed in a thermostatic bath within ±0.1℃ of the preset. In steady-state tests, the shear rate ranged from 0.01 to 1000 s⁻¹. In a dynamic frequency sweep, the shear oscillation frequencies at different temperatures ranged from 0.1 to 100 rads⁻¹. The plate of the cone-plate geometry on which the samples (the PAN solutions) were placed was 35 mm in diameter, the angle of the cone-plate geometry was 1° and the sample size was 2 mm. A thin layer of low-viscosity paraffin oil was applied to cover the samples, protecting them from dehydration or evaporation and thus minimizing testing errors.

Results and discussion

Characteristics of copolymer

The chemical reaction equation for free radical copolymerization of AN/IA initiated by AIBN is shown in Scheme 1. The results are shown in Table 2.

Scheme 1.

The chemical reaction equation for free radical copolymerization of acrylonitrile (AN)/itaconic acid (IA).

Table 2.

Results of polymerization of acrylonitrile (AN) with itaconic acid (IA) (AN:IA = 98:2, by weight)

	A1	A2	A3	A4	A5	A6	A7
Conversion (%)	93.23	92.34	92.06	91.87	91.09	90.94	90.63
M_η × 10⁴	7.98	16.36	19.66	24.91	30.68	34.78	40.03
η (Pa·s)	287.33	347.14	384.88	464.57	499.47	535.43	588.22

As we can see in Table 2, the conversion of polymerization decreases and M_η and η increase as the content of UHMWPAN increases. In the reaction system, UHMWPAN did not participate in the reaction, but played a very important role. The existential state of UHMWPAN in the solution can be described in Figure 1.

Figure 1.

The state of ultrahigh molecular weight polyacrylonitrile molecular chains in solvent dimethylsulfoxide: (a)–(c) the concentration of solution increased gradually.

We can see that the molecular chain of UHMWPAN was disordered in the solution and easy to entangle. As the content of UHMWPAN increased, a large carbon–carbon skeleton was formed. The new synthetic polymer was complexing with UHMWPAN so as to obtain a larger macromolecular chain net. Furthermore, with the increase of the content of carboxymethyl on the molecular chain, a stronger steric hindrance appeared, which made it more difficult for free radical attacking. Therefore, the reaction and conversion rate went down; in the meantime, the molecular weight and viscosity increased.

Steady shear rheology

Figure 2 shows changes in apparent shear viscosity for different samples across a range of shear rates. As shown in Figure 2, the shear viscosity (η_a) rose with increases in the content of UHMWPAN in the solution. The presence of UHMWPAN led to improvements in the shear viscosity of the solutions, which was attributed to more entanglements and thus smaller mobility in the molecular chains of the UHMWPAN.¹¹ Increases in UHMWPAN content showed a similar effect on the shear viscosity of the solutions, due to more entanglements in the solutions. As expected, all of the samples exhibited shear-thinning behavior. The effects of UHMWPAN on the steady shear viscosity at different temperatures are shown in Figure 3. The viscosity (η_a) decreased with an increase in the temperature, which could be attributed to the enhanced molecular mobility of the chains and the loosening of the macromolecular entanglements as a result of the temperature increase. Rheological behavior was thus comparable to that of the general spinning solution. Viscosity sharply decreased when the shear rate increased over 100 s⁻¹, which was ascribed to changes in the shear stress of the PAN macromolecules and the increased degree of molecular orientation due to increasing shear rate. As a result of the break in intermolecular van der Waals force, unentangled networks formed between the PAN molecules and the molecules aligned, which led to a decrease in viscosity.^21,22 More specifically, the PAN solution system with UHMWPAN provided a shear-thinning fluid or pseudo-plastic fluid.

Figure 2.

Shear viscosity as a function of shear rate at different ultrahigh molecular weight polyacrylonitrile concentrations.

Figure 3.

Shear viscosity as a function of shear rate at different temperatures (℃): the open symbols are for A3 and the solid symbols are for A6.

Dynamic rheological measurements

The influence of shear rate on the viscoelasticity of PAN solutions

Figure 4 shows the complex viscosity (η*) of six different solutions under various oscillation frequencies at 50℃. Complex viscosity (η*) was shown to decrease with increased oscillation frequency (ω) in all of the solutions. PAN solution is a typical non-Newtonian fluid and exhibits pronounced pseudo-plastic behavior under the constant high shear rate conditions of steady-state measurements.

Figure 4.

Relationship between complex viscosity and oscillation frequency of 21 wt.% polyacrylonitrile/dimethylsulfoxide solutions with different ultrahigh molecular weight polyacrylonitrile (UHMWPAN) concentration. Note: A1 without UHMWPAN, A2 with 0.1 wt% UHMWPAN, A3 with 0.3 wt% UHMWPAN, A4 with 0.5 wt% UHMWPAN, A6 with 0.9 wt% UHMWPAN, A7 with 1.0 wt% UHMWPAN.

According to Muthukumar and Winter,²³ the influence of viscosity on frequency can be estimated as follows:

η * (ω) ~ ω^{- 2 / (d_{f} + 2)}, ω > 0

(3)

where d_f represents fractal dimension, which provides a characterization of the gelation structure parameters and is always positive. According to Equation (3), η* is inversely proportional to ω. As shown in Figure 4, when ω is fixed, η* increased as the concentration rose, which was attributed to the easily aggregated PAN molecular chains in the solution and the increases in the density of the molecular chain entanglement. As a result of decreases in the average gap between the molecular chains, the solution achieved increased physical junctions and elasticity between the molecules in addition to larger flow units with increased resistance and less sliding. On the other hand, a small amount of UHMWPAN content provided decreasing η* with increased ω. Therefore, PAN solutions containing small amounts of UHMWPAN were shown to provide an effective shear-thinning fluid.

As shown in Figure 5, the dynamic storage modulus G′ and loss modulus G″ were also examined as functions of shear frequency at 50℃ in different PAN solutions. Increases in both G′ and G″ occurred with increased shear frequency in all of the solutions. In addition, a clear plateau modulus appeared at high frequencies, demonstrating typical solid-like behavior. As known from the literature,²⁴ the experimental probing time was too brief to allow relaxation in the chains due to the UHMWPAN in the solutions. At low frequencies, the solutions maintained liquid-like behavior and predominantly responded to the imposed deformation with viscous behavior. However, increases in the shear frequency beyond the frequency scale of the PAN molecular chain rearrangements resulted in somewhat stable entanglements with more solid-like behavior. Both G′ and G″ were sensitive to the content of UHMWPAN in the solutions. Greater UHMWPAN content was shown to result in higher G′ and G″ values, demonstrating high modulus sensitivity to the UHMWPAN content of the PAN solutions.

Figure 5.

Dynamic moduli as functions of shear frequency at 50℃: A1 without ultrahigh molecular weight polyacrylonitrile (UHMWPAN), A2 with 0.1 wt% UHMWPAN, A3 with 0.3 wt% UHMWPAN, A4 with 0.5 wt% UHMWPAN.

The influences of temperature and the content of UHMWPAN on the viscoelasticity of PAN solutions

Polymer fluid flow properties can be greatly influenced by temperature. From a molecular motion point of view, rises in temperature can lead to improved movement in copolymer segments; expanding volume; weakened intermolecular interaction force; and increases in liquidity. Therefore, an understanding of how temperature influences flow curve indifferent polymer solutions can lead to enhancements in polymer processing and spinning conditions.

Figure 6 shows the influence of temperature on the viscoelasticity of the spinning solutions with shear frequency at 6.3 rads^–1. As shown in Figure 6, storage modulus G′, loss modulus G″ and the complex viscosity η* of the spinning solutions were shown to decrease linearly with increasing temperature and increase with rises in UHMWPAN content. Moreover, the loss angle tangent (tanδ) was shown to increase with increasing temperature and decrease with increasing UHMWPAN content. This could be explained as follows: storage modulus G′ corresponded to the elastic part of the polymer, loss modulus G″ corresponded to the viscous part and the loss tangent tanδ was a measure of the ratio between lost and stored energy in the cyclic deformation as follows:

\tan δ = \frac{G ″}{G'}

(4)

Figure 6.

Variation of (a) storage modulus, (b) loss modulus, (c) loss angle tangent and (d) complex viscosity as a function of temperature for solutions at constant oscillation frequency 6.3 rad/s: A1 without ultrahigh molecular weight polyacrylonitrile (UHMWPAN), A2 with 0.1 wt% UHMWPAN, A3 with 0.3 wt% UHMWPAN, A5 with 0.7 wt% UHMWPAN, A6 with 0.9 wt% UHMWPAN.

Where ω is fixed, increases in temperature led to enhancements in the thermal motion of each motor unit, after sufficient energy was obtained in the polymer chain segments. In addition, the gap between the PAN molecules increased as a result of thermal expansion and the free volume of the polymer was thus augmented. Therefore, the molecules obtained moderate flexibility, which was advantageous in the formation of chains orientated towards shear stress and chain segments with jump diffusive motion towards the void.

Figure 7 shows the temperature dependence of G′ and G″ in the cooling process with shear frequency (ω) at 6.3 rads⁻¹. Both G′ and G″ were seen to increase with decreasing temperature, although G′ increased more rapidly than G″. The point where G′ and G″ met indicated the temperature at which the sol-gel transition occurred. As illustrated by the curves of G′ and G″ in Figures 6(a)–(d), increased UHMWPAN content led to a decrease in the gelation temperature of the solution. The spinning solution converted into the gel phase with UHWPAN content at 0.5% (as shown at A4 in Figure 6(d)) and temperature at 30℃. This can be explained as follows: increased UHWPAN content led to increased chain aggregation and entanglement, resulting in easier gelation. More specifically, the elevated temperature of the sol-gel transition in the UHWPAN-PAN/DMSO solution caused greater chain mobility. Increases in the content and molecular weight of UHWPAN also led to visible signs of hardening and elasticity in the solution.

Figure 7.

Relationship between the temperature and G′ and G″ in the cooling process with shear frequency (ω) at 6.3 rads⁻¹: (a) A1 without ultrahigh molecular weight polyacrylonitrile (UHMWPAN); (b) A2 with 0.1 wt% UHMWPAN; (c) A3 with 0.3 wt% UHMWPAN; (d) A4 with 0.5 wt% UHMWPAN.

Conclusion

In this study, dilute UHMWPAN/DMSO solutions were prepared and they were used as reaction media to synthesize AN/IA copolymers in a solution copolymerization system. The influence of UHMWPAN on the shear rheological behavior of PAN/DMSO solutions was subsequently studied. It was determined that the existence of UHMWPAN led to an increased number of entanglements in the molecular chains of the solutions, making some properties of the solutions different from those of the solution without UHMWPAN, such as M_η and η. From above, we could see all of the samples exhibited shear-thinning behavior. The rheological material parameters (G′, G″, tan δ, η*) of the PAN solutions were very sensitive to the existence of UHMWPAN. Visible increases in the hardening and elasticity of the PAN solutions were observed as a result of increased UHMWPAN content in the solutions. It was therefore concluded that PAN solutions containing a small amount of UHMWPAN can be utilized in dry-jet wet spinning for the preparation of high-performance PAN precursor fiber.

Footnotes

Funding

This work was supported by a project of the Hubei Province Department of Education (Q20121708).

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