Ultrahigh molecular weight, lignosulfonate-based polymers: preparation, self-assembly behaviours and dispersion property in coal–water slurry

Abstract

Using a novel and facile method, we synthesized a family of ultrahigh molecular weight, lignosulfonate-based polymers (ALSs) via alkyl chain coupling polymerization. Gel permeation chromatography (GPC) showed a significant increase in weight-average molecular weights (Mws), from 42 800 Da of ALS1 to 251 000 Da of ALS5—one of the highest Mws among reported lignosulfonates (LSs) to date. Functional group content measurements, FTIR and ¹H-NMR confirmed the efficient polymerization by nucleophilic substitution coupling mechanism and suggested a straightforward relationship between the polymerization of lignosulfonate (LS) and consumption of phenolic hydroxyl groups. Moreover, hollow nanospheres were obtained via self-assembly of water-soluble ALS and were investigated by DLS, SEM, TEM and AFM. The hollow sphere structure, with a hydrophilic core and a hydrophobic shell, was confirmed by XPS and elemental analysis. Stable, quasi-solid nanospheres were obtained from ALS by the addition of cetyl trimethyl ammonium bromide (CTAB). Furthermore, ALS2, with its relatively high molecular weight, showed unexpectedly better dispersion properties than the raw material LS and naphthalene sulfonate formaldehyde condensate (NSF) for coal–water slurry. The effective polymerization route to improving Mw and the self-assembly from polymer-only ALS provide novel avenues for high-value application of lignin, a sustainable and abundant bioresource.

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Introduction

Polymeric nano- and micro-materials have formed a growing research field over the past decades, as they have shown potential in applications as multi-functional materials in nanoscience and technology.^1–5 Generally, synthetic amphiphilic block or graft copolymers have been extensively studied in preparing nano-/micro-structures via several methods including template synthesis,^6,7 self-assembly,⁸ emulsion polymerization,^9,10 and core removal of dendrimers.¹¹ It is well known that biocompatibility and biodegradability are required for the design of polymers. A promising approach is to readily obtain water-soluble polymers from bioresources such as lignin or lignosulfonate (LS). In our group's recent studies on the self-assembly of lignin and LS to obtain colloidal spheres,^12,13 we found that the alkyl chain-coupled, lignosulfonate-based polymer (ALS), an amphiphilic biopolymer obtained from the alkyl chain coupling polymerization of lignosulfonate, can directly and readily produce polymer nanospheres in selected solvents via self-assembly without any templates. This result has been rarely reported.

With the increasing concern on the problems of fossil-based energy and environmental pollution, the efficient utilization of abundant and renewable bioresources, including cellulose^14–16 and lignin,^17–19 becomes extremely important. Lignin, the second most abundant renewable bioresource material, has stimulated great fundamental interest for widespread applications in industrial technology for a long time. Molecular weight is one of the critical factors that determine the properties of dispersants based on lignosulfonate. For example, LSs with relatively high Mws exhibit positive effects on the viscosity-reducing properties of coal–water slurry (CWS).^20,21 High Mw accompanied with large steric hindrance plays a key role in the dispersion properties of LS in TiO₂ suspension.²² Moreover, LSs with high Mws also exhibit good performance as dispersants for dimethomorph water-dispersible granules (DWG)²³ and as water-reducing agents for cement–water systems.²⁴ In addition, the exploitation of such a biocompatible and biodegradable material to prepare nanomaterials is of crucial importance for the development of novel and high value-added applications.^25–27

In order to fabricate nanomaterials using the LS polymer, LS should be modified to adjust its hydrophobic property and increase Mw. The alkyl chain coupling polymerization of LS could be treated as a hydrophobic modification to obtain suitable amphiphilic property for the self-assembly behaviour, which has rarely been reported to date. Numerous studies have been focused on the modification of LS by various technologies to increase Mw in order to facilitate its application in industrial fields. To our knowledge, the main technologies used to improve Mw include formaldehyde condensation polymerization,²⁸ graft copolymerization,²⁹ graft sulfonation,³⁰ and enzyme catalysis reaction.³¹ Compared with these modification routes, our method features easy processing and a highly efficient procedure. It is expected to improve the Mw of LS with low Mw from industrial processes such as SPORL (sulfite pretreatment to overcome recalcitrance of lignocelluloses), as reported by Zhou and co-workers.³²

In this paper, ultrahigh Mw, lignosulfonate-based polymer ALSs were prepared and used as an amphiphilic biopolymer to prepare hollow nanospheres via self-assembly. The formation mechanism of hollow spheres was investigated by dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Furthermore, stable quasi-solid nanospheres obtained from the complex between ALS and cetyl trimethyl ammonium bromide (CTAB) were also detected by TEM, AFM, elemental analysis and XPS. Thus, the method makes the ALS polymer potentially useful in new applications due to the presence of electrostatic attraction between sulfonic groups and guest molecules with cationic groups. Moreover, ALSs exhibited unexpectedly better dispersion properties than naphthalene sulfonate formaldehyde condensate (NSF) in coal–water slurry.

Experimental section

Materials

LS was supplied by the Quanlin paper mill (Shandong province, China). 1,6-Dibromohexane (C₆H₁₂Br₂) was supplied by Energy Chemical Co. Ltd. (Shanghai, China) with a purity of 97%. All other chemicals were of analytical grade, including sodium hydroxide (NaOH), potassium iodide (KI), hexane, ethanol (EtOH) and cetyl trimethyl ammonium bromide (CTAB). NSF, naphthalene sulfonate–formaldehyde condensate (commercially available high-performance dispersant from Zhanjiang additive company, Guangdong, China), was tested for comparison with ALSs.

Shenhua coal (Shenfu, Shanxi, China) was used to prepare coal–water slurry. Before the preparation of coal–water slurry, raw coal was comminuted with a ball mill to obtain coal powder. Then, the coal powder was screened through a 100-mesh screener (0.150 mm pore size) and 70-mesh screener (0.212 mm pore size) to obtain bimodal distribution products with an average particle size of 30 μm.

Preparation of ALSs

LS (10 g) was dissolved in 50 mL of H₂O, and NaOH was used to adjust the pH value of the aqueous solution to 11. Then, different proportions of C₆H₁₂Br₂ and a trace amount of KI (0.1 g) were added to the reaction liquid at 70 °C. The polymerization was stopped after reflux at 70 °C for 8 hours. The synthetic approach is shown in Scheme S1 in ESI.† The samples were then extracted with hexane to remove the excess C₆H₁₂Br₂, and the polymers in aqueous solution were filtered. The filtrates were diluted to 0.15 g mL⁻¹ and desalted by ion-exchange resin. The purified products were then freeze-dried to obtain yellow-brown powder samples. The ratios of m (LS) : m (C₆H₁₂Br₂) were 1 : 0.08, 1 : 0.12, 1 : 0.16, 1 : 0.24 and 1 : 0.30, which were named ALS1, ALS2, ALS3, ALS4 and ALS5, respectively. The proposed structure of the ALS polymer is shown in Fig. 1.

Fig. 1 The proposed structure of the ALS polymer.

Preparation of hollow nanospheres with ALS

ALS polymer was first dissolved in water to prepare 0.5 mg mL⁻¹ ALS aqueous solution, followed by the addition of ethanol to achieve a 0.05 mg mL⁻¹ solution. Then, the mixed solution was stirred for 3 h and left at room temperature for 1 h to obtain the ALS hollow nanospheres.

Preparation of quasi-solid nanospheres with ALS and CTAB

ALS polymer was first dissolved in water to prepare 0.5 mg mL⁻¹ ALS aqueous solution. CTAB, dissolved in ethanol, was added into the above ALS aqueous solution (CTAB/SO₃H = 1.5/1, n/n) to achieve a solution of 0.05 mg mL⁻¹. Then, the mixed solution was stirred for 3 h and left at room temperature for 1 h to obtain the quasi-solid nanospheres.

Characterization

Mw measurement was conducted by aqueous gel permeation chromatography, and the UV absorption at 280 nm was monitored with a Waters 2487 UV detector (Waters Co., Milford, MA, USA). Polystyrene sulfonate was used as calibration standard. A 0.10 M NaNO₃ solution (pH 8.0) was used as mobile phase (0.50 mL min⁻¹). All samples were dissolved and diluted into 0.3 wt% using double-distilled water and filtered through a 0.22 μm filter.

The phenolic hydroxyl group (–OH) content of samples was determined by the Folin–Ciocalteu colorimetric (FC) method.³¹ The sulfonic group (–SO₃H) content of LS and ALS was measured using an automatic potentiometric titrator (Type 809 Titrando, Metrohm Corp., Switzerland).³³

Fourier transform infrared (FTIR) spectrometry using Auto system XL/I-series/Spectrum 2000 (Thermo Nicolet Co., Madison, WI, USA) was used for infrared spectrum analysis, recording between 4000 and 400 cm⁻¹. The measurement method used was the potassium bromide pressed-disk technique. The ¹H-NMR spectra of samples were recorded using the DRX-400 spectrometer (Bruker Co., Ettlingen, Germany), with 30 mg of each sample dissolved in 0.5 mL of deuterium oxide (D₂O).

Scanning electron micrographs were recorded with a Nova Nano SEM instrument (Zeiss, Netherlands). Samples were mounted on aluminum stubs by means of double-sided conductive adhesive and sputtered with Au/Pd to reduce charging effects.

TEM images were obtained using a HITACHI H-7650 electron microscope with an accelerating voltage of 200 kV. The TEM samples were prepared by dropping diluted sample solutions onto the copper grids coated with a thin carbon film.

Dynamic light scattering (DLS) experiments were performed on a Zeta PALS instrument (Brookhaven, USA). The concentration of samples was 0.05 mg mL⁻¹ in ethanol/H₂O (v/v, 9 : 1) at around pH 7.0.

Atomic force microscopy (AFM) was employed to observe the morphology of hollow and quasi-solid spheres after carbonation. AFM images were recorded using Park XE-100 instrument in tapping mode. The hollow and quasi-solid spheres on the silicon wafer were charred in a tube furnace under nitrogen (N₂) atmosphere. The temperature increased from 25 °C to 800 °C with a heating rate of 5 °C min⁻¹. The carbonation process was held at 800 °C for 2.0 h. Then, the samples were cooled to room temperature naturally, and AFM was conducted to observe the morphology of their residues after pyrolysis.

Elemental analyses were performed using the Elementar Vario EL cube. Elemental distribution on the surface of ALS hollow nanospheres was also analyzed by X-ray photoelectron spectroscopy (XPS, Utra Axis DLD, Kratos, England). The elemental distribution of quasi-solid nanospheres obtained with the aggregates of the complex between ALS and CTAB produced via self-assembly was also detected by elemental analysis and XPS method.

Viscosity measurement of the coal–water slurry with LS, ALSs and NSF dispersants was performed using a rheometer (Haake MARS III). The shear rate range was up run with a range of 0–200 s⁻¹, and the temperature was kept at 25 °C, with a fluctuation of 1 °C. The dispersant dosage was 0.6 wt%, and the coal content of CWS was fixed at 60 wt%. The coal powder was slowly mixed in a pot, adding a fixed amount of dispersant and water. The slurry was stirred for 10 min at 1200 rpm to disperse the coal particles uniformly in CWS, and the apparent viscosity of the slurry was measured by rheometer. Table S1† shows the results of the proximate and ultimate analyses for Shenhua coal. The Shenhua coal powder has high inherent moisture (M_ad) and O/C ratio, which suggest that it is a type of low-rank coal, and it is difficult to prepare CWS with high coal content such as 60 wt%, even for a commercially used dispersant NSF with excellent industrial properties.

Results and discussions

Characterization of ALSs

As shown in Fig. 2a, the influence of C₆H₁₂Br₂ on the molecular weight of the ALSs is apparent. Mw gradually increased with the increasing proportion of C₆H₁₂Br₂. The results of Mw distribution are listed in Table 1. As shown in Table 1, when LSs were coupled with different amounts of C₆H₁₂Br₂, the LSs underwent more extensive polymerization.

Fig. 2 (a) Effect of 1,6-dibromohexane on Mw distributions of ALSs. (b) Effect of 1,6-dibromohexane on phenolic hydroxyl group contents of ALSs (photos inserted from left to right: LS, ALS1, ALS2, ALS3, ALS4, and ALS5 solution samples after the addition of FC reagent).

Table 1 Effect of 1,6-dibromohexane on the molecular weight distributions and functional group contents of ALSs

Sample	m (LS) : m (C6H12Br2)	Mw/Da	Mn/Da	PDI	–SO3H (mmol g^−1)	–OH (mmol g^−1)
LS	1 : 0.00	13 100	3350	3.91	1.82	0.81
ALS1	1 : 0.08	42 800	11 200	3.82	1.31	0.71
ALS2	1 : 0.12	95 400	21 300	4.48	1.21	0.54
ALS3	1 : 0.16	115 000	33 700	3.41	1.16	0.43
ALS4	1 : 0.24	135 000	38 100	3.54	1.03	0.32
ALS5	1 : 0.30	251 000	45 200	5.55	1.01	0.15

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With m (LS) : m (C₆H₁₂Br₂) of 1 : 0.30, the Mw of ALS achieved the maximum value by a factor of more than 20-fold that of LS. With different proportions of m (LS) : m (C₆H₁₂Br₂), ALS products with different Mws could be easily obtained, as shown in Table 1. The polydispersity indexes (PDIs) are also listed in Table 1; PDI of ALS5, which was generated by the higher molecular weight components, is the highest among the samples. However, from the viewpoint of environmental protection, 1,6-dibromohexane is toxic; a new and green reagent should be developed in future work.

In order to study the influence of C₆H₁₂Br₂ on the structure of LSs, functional groups, such as phenolic hydroxyl groups and sulfonic groups, were investigated. As shown in Fig. 2b, with different proportions of C₆H₁₂Br₂, Mws of ALSs obviously increased, accompanied by a remarkable decrease in phenolic hydroxyl group content. The results are listed in Table 1; the phenolic hydroxyl group content of ALS5, bearing the highest molecular weight, was just 0.15 mmol g⁻¹. This suggested a straightforward relationship between the polymerization of LS and consumption of phenolic hydroxyl groups.

The sulfonic group contents were also detected and analysed, as shown in Table 1. With the increase of proportion of m (LS) : m (C₆H₁₂Br₂), sulfonic group contents decreased slightly from 1.82 to 1.01 mmol g⁻¹. The results could be explained by the increase of Mw due to the increasing introduction amount of alkyl chains. The alkyl chain coupling polymerization using C₆H₁₂Br₂ to improve Mw of LS, could have changed the amphiphilic property by increasing hydrophobicity; however, the total amount of sulfonic groups may be unchanged by alkyl chain coupling polymerization, and thus the sulfonic group content (mmol g⁻¹) decreased only slightly with increased Mw.

The water solubility of ALS was tested carefully. The solubility of ALS with Mw less than 80 kDa is about 200 mg mL⁻¹, but that of ALS with Mw more than 100 kDa decreased to 60 mg mL⁻¹. As is known to us, low dosage of dispersant is usually required for many applications including coal–water slurry, pesticide suspension concentrate and cement–water suspensions, as mentioned in the previous introduction.^20–24 Therefore, ALS can meet this basic requirement in industrial applications.

To further investigate the influence of C₆H₁₂Br₂ on the chemical structure of raw material LS, the FTIR spectra of LS and the ALSs were measured, as shown in Fig. 3a. The areas of the bands at 2938 cm⁻¹ and 2864 cm⁻¹ correspond to methylene stretching vibrations from the alkyl chain.³⁴ The areas at 1141 cm⁻¹ correspond to a combination of C–O bond stretching vibrations.³⁵ As shown in Fig. 2a, the adsorption intensities of –CH₂–/CH₃– and C–O in the ALSs both increased compared to LS as a result of efficient substitutions of aliphatic chains on phenolic groups of LS. In addition, S–O bonds (bands at 1038 cm⁻¹ and 653 cm⁻¹)²¹ were all observed in LS and ALSs, but the adsorption intensities of the ALSs were decreased slightly compared to the raw feedstock LS, which was in good agreement with the results of sulfonic group content measurement. Moreover, the adsorption intensities of aromatic ring stretching vibrations (bands at 1512 cm⁻¹ and 1601 cm⁻¹)³⁴ were all detected in LS and the ALSs, which exhibited no observable change between raw material and ALSs, suggesting that the aromatic ring structure of LS molecules was not destroyed in the reaction process.

Fig. 3 Spectroscopic analyses of SL and ALSs: (a) IR spectra, (b) ¹H-NMR spectra.

As shown in Fig. 2b, the ¹H-NMR spectra of LS and ALSs using quantitative samples were also investigated to further study the change of chemical structure in LS. The ¹H-NMR spectra were obtained from 30 mg samples dissolved in 0.5 mL D₂O. The signals between 0.5 and 2.0 ppm were associated with the aliphatic chain.^36,37 The signal intensity of aliphatic protons exhibited a significant increase with the increase of ALS Mws, which was in good agreement with the FTIR results. The dramatic increase of aliphatic proton signals came from the efficient coupling of –C₆H₁₂– groups. In addition, it is worth mentioning that the signal intensity of methoxyl protons at 4.00–3.70 ppm exhibited no observable change between LS and ALSs, which means that the alkyl chain coupling polymerization did not destroy the methoxyl group.

The results of GPC, functional group content measurements, FTIR, and ¹H-NMR demonstrated that effective intermolecular nucleophilic substitution reactions between C₆H₁₂Br₂ and LS molecules simultaneously and efficiently occurred, which further confirmed the proposed structure shown in Fig. 1.

Self-assembly behaviours from ALS

ALS, obtained from the alkyl chain coupling polymerization of LS, is an amphiphilic biopolymer due to the presence of hydrophilic groups and its hydrophobic aromatic skeleton. In this work, ALS was used to prepare hollow nanospheres via self-assembly. The ALS was first dissolved in H₂O to prepare 0.50 mg mL⁻¹ ALS aqueous solution, followed by the addition of EtOH to achieve a 0.05 mg mL⁻¹ solution. The volume ratio of EtOH/H₂O was 9/1, and the pH of the solution was around 7.0. We studied the self-assembly carefully with a combination of SEM, TEM, DLS, AFM and XPS.

Fig. 4a and b shows SEM images of the nanospheres obtained from ALS solution. The spherical morphology is clearly demonstrated by scanning electron microscopy. The diameter of the nanospheres is about 100–500 nm. It is known that lignosulfonate exists as a nano-sized, oval-shaped aggregate between 10 and 20 nm in water solution.¹³ More importantly, the nanosphere contained a lot of ALS aggregates as observed by SEM (Fig. 4a, red arrows). This result is coincident with the result in a previous work¹³ and can be explained with the mechanism model in Fig. 6. ALS was firstly dissolved in water to achieve 0.50 mg mL⁻¹, which formed a lot of aggregates.¹³ EtOH was then added into the ALS aqueous solution, and the ALS aggregates began to self-assemble, finally forming smooth nanospheres (Fig. 4b). Compared to LS, the hydrophobic alkyl chain building block in ALS improved the C/S element ratio (see Table 2) and provided higher hydrophobicity, which may contribute to the self-assembly behaviours, thus forming nanospheres.^12,38

Fig. 4 (a and b) SEM image of nanospheres from ALS alone (ALS aggregates, see red arrow); (c) TEM image of ALS hollow nanospheres; (d) DLS of ALS solution of 0.05 mg mL⁻¹ in EtOH/H₂O (9/1, v/v); (e and f) TEM image of quasi-solid nanospheres from ALS and CTAB via self-assembly (aggregates, see blue box).

Table 2 Elemental distribution of LS and ALS bulk materials by elemental analysis method (left) and of the ALS nanosphere surface by XPS (right)^a

Samples	Elemental analysis			XPS method
	C%	S%	C/S	C%	S%	C/S	Br%
a Note: — means none detected.
LS	45.98	5.85	7.86	—	—	—	—
ALS1	53.84	4.18	12.88	71.19	1.84	38.69	—
ALS3	55.03	3.41	16.14	72.14	1.71	42.19	—
ALS5	55.72	3.29	16.94	73.78	1.69	43.66	—

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The hollow nature of these nanospheres was evidenced by transmission electron microscopy (TEM), as shown in Fig. 4c. The nanospheres exhibited an obvious contrast between the inner part and the periphery of the spheres, which further supported the hollow nature of the nanospheres detected by SEM. The diameter of the hollow nanospheres was 100–500 nm, which is very close to the diameter revealed by SEM.

Dynamic light scattering study of the ALS solution can give important information about the solution self-assembly process. As shown in Fig. 4c, the average diameter of the nanospheres was about 300 nm, which was in good agreement with the results measured by SEM and TEM. Additionally, particle size distribution of the ALS solution was about 100–700 nm, as shown in Fig. 4d. This means that nanospheres of different diameters existed in the solution, which may be mainly ascribed to the high polydispersity index (PDI) value of the ALS polymer.

ALS aggregates containing a lot of sulfonic groups tend to assemble with molecules containing cationic groups, such as CTAB, due to electrostatic interaction.³⁹ In this work, ALS polymers were employed for the assembly with CTAB. The preparation proceeded by adding CTAB dissolved in EtOH to the ALS aqueous solution, and the ratio of n(CTAB) : n(–SO₃H) was 1.5 : 1. As shown in Fig. S1,† nanospheres with different diameters, from 100 nm to 700 nm, were observed by SEM. The structure of the spheres was evidenced by TEM. As shown in Fig. 4f, homogeneous, quasi-solid nanospheres were obtained by the addition of CTAB molecules. More importantly, the transient moment that the 100 nm nanospheres contained many solid aggregates (∼15 nm) was also observed by TEM (blue box in Fig. 4e) at the same time. The result further confirmed the quasi-solid nature of the sphere. The formation mechanism we proposed in Fig. 6 clearly illustrates the process of self-assembly to obtain quasi-solid spheres. The complexes were formed from ALS and CTAB via electrostatic interaction, and then the quasi-solid spheres were fabricated from the complexes via self-assembly.

All the above evidence suggests that CTAB molecules, with the ammonium group, promoted the formation of quasi-solid nanospheres via the self-assembly of complexes. It is noteworthy to mention that quasi-solid spheres of different diameters were also observed from 100–500 nm (Fig. 4e and f), which is in agreement with the DLS result shown in Fig. 4d. This is mainly ascribed to the high PDI of the ultrahigh Mw ALS polymer.

The detailed structure of the hollow sphere and quasi-solid sphere was also investigated using AFM on the morphology of the biochar after pyrolysis.⁴⁰ The pyrolysis of both the hollow spheres and quasi-solid spheres were conducted comparatively in a tube furnace under N₂ atmosphere at 800 °C for 2.0 h. As shown in Fig. 5, AFM images of hollow spheres and quasi-solid spheres were significantly different. The middle of the hollow sphere was thinner than the periphery; a deep hole was detected after the hollow sphere collapsed in the carbonation process (Fig. 5a and c). In contrast, the middle of the quasi-solid sphere obtained from the ALS and CTAB complex was thicker than the periphery; the middle embossment was observed after the quasi-solid sphere collapsed in the carbonation process (Fig. 5b and d). The result is in good agreement with the TEM result above, which further confirms the structure of the hollow sphere and the quasi-solid sphere.

Fig. 5 AFM images of nanospheres after pyrolysis at 800 °C under N₂ atmosphere. (a and c) Hollow nanospheres from ALS alone, (b and d) quasi-solid nanospheres from ALS and CTAB via self-assembly.

That the ALS hollow nanosphere has a hydrophilic core and a hydrophobic shell can be proven by comparing the element ratio of C/S on the nanosphere surface and that of the ALS bulk materials after the freeze-drying process. The C/S ratio of ALS was measured by elemental analysis, while the C/S ratio on the hollow nanosphere surface was estimated by XPS. The results are shown in Table 2 and Fig. S2.† Hydrophobic association of ALS has a higher C/S element ratio compared with LS, which is a contributing factor to the self-assembly behaviour of ionic polymers.³⁸ Moreover, the C/S element ratio on the surface of ALS hollow nanospheres was obviously higher than those of ALS bulk materials. This gives direct evidence to support that the ALS hollow nanosphere has a hydrophilic core and a hydrophobic shell, which suggests a higher content of –SO₃H groups inside the hollow spheres and more hydrophobic groups on the surface of the hollow spheres.

In addition, X-ray photoelectron spectroscopy (XPS) of ALS spheres was conducted to test the content of bromine element (Fig. S2 in ESI†), and no Br was detected in all ALS samples shown in Table 2. If the LS molecules were connected linearly by –C₆H₁₂– groups, under excessive addition of C₆H₁₂Br₂, the –C₆H₁₂Br groups should be detected by XPS. However, no Br element was detected, which suggested that the mono-substitution reaction of C₆H₁₂Br₂ with the LS molecule was negligible, and the cross-linked network structure of ALSs is proposed to be produced during the polymerization reaction. Thus, the result further supports the structure of ALS we proposed, as shown in Fig. 1. Hence, the general formation mechanism of ALS hollow nanospheres is proposed in Fig. 6. ALS aggregates, in aqueous solution, self-assemble into hollow nanospheres by the addition of EtOH.

Fig. 6 The proposed formation mechanism of hollow and quasi-solid nanosphere from ALS aggregates.

The elemental distribution of the nanosphere obtained from ALS by the addition of CTAB was also detected by elemental analysis and XPS method, as shown in Table S2 and Fig. S3.† The C/N ratio detected by elemental analysis is very close to that detected by XPS method, which suggested that the homogeneous quasi-solid nanosphere was formed from ALS and CTAB via self-assembly due to the interaction between sulfonic groups of ALS aggregates and the amino group of CTAB. In other words, the C/N ratio on the surface of the quasi-solid nanosphere was the same as that of the entire nanosphere. This further supports the results of TEM above, that CTAB molecules promote the self-assembly of ALS aggregates to form quasi-solid nanospheres, as shown in Fig. 4e.

Furthermore, a 70.51% C content on the surface of quasi-solid nanospheres was detected by XPS, higher than that of the entire nanosphere (59.05%) detected by elemental analysis, as shown in Table S2.† This illustrates that the hydrophobic chain of some CTAB molecules tended to point away from the quasi-solid sphere. Therefore, the formation mechanism of ALS quasi-solid nanospheres is proposed in Fig. 6. The complexes were formed from ALS and CTAB via electrostatic interaction between the sulfonic group and amino group, and then the quasi-solid nanosphere was formed by the aggregation of complexes containing ALS and CTAB via self-assembly.

For coal–water slurry (CWS) used in industry, low viscosity value and high solid content are expected to ensure the ease of handling during transfer, storage and atomization.^21,41 In this study, LS, ALS1, ALS2, ALS3 and NSF were selected to prepare CWS using Shenhua coal, which is a type of low-rank coal (Table S1†) and is difficult to use in preparing CWS with a good dispersion property. The dispersant dosage and coal content of each CWS sample were the same, at 0.6 wt% and 60.0 wt%, respectively. The CWS flow curves are shown in Fig. 7a. Apparent viscosity of CWS using ALSs and NSF as dispersants decreased gradually with increasing shear rate. The apparent viscosity of CWS with LS was the highest, which meant that the dispersion property of LS was bad. The viscosity values of CWSs with ALSs were lower than NSF, which suggested that ALSs better reduced the viscosity of the highly concentrated, low-rank CWS compared to NSF. ALS2 and NSF were selected to study the effect of dispersant concentration on the apparent viscosity of the CWS, as shown in Fig. S4.† The dispersant concentrations varied from 0.3 to 1.4 wt% (on dry coal basis), and the solid concentration was kept at 60 wt%. Fig. S4† reveals that ALS2 had a better effect on reducing the CWS viscosity compared to NSF at different concentrations. The higher molecular weight the of lignosulfonate-based dispersant is an important factor influencing its dispersion property.²⁰ The effectiveness of ALS in reducing viscosity of the CWS may also be attributed to its cross-linked molecule configuration, which may have tighter adsorption affinity for the coal surface.²¹

Fig. 7 Apparent viscosity (a) and flow curve (b) of CWSs with Shenhua coal (60.0 wt% coal content). The dosages of LS, ALSs and NSF for all the CWSs were fixed at 0.6 wt%.

The applications of the coal–water slurry are influenced significantly by its rheological behavior. In this study, the shear stress/shear rate relationship (Fig. 7b) was fitted with the Herschel–Bulkley fluid obeying eqn (1), where τ₀, K and n denote yield stress, consistency coefficient and flow characteristic exponential, respectively.⁴² The rheological data of the calculated Herschel–Bulkley model parameters are listed in Table 3.

τ = τ₀ + Kγⁿ (1)

Table 3 Fitting rheological parameters calculated by Herschel–Bulkley model of 60 wt% CWS with different dispersants

Dispersants	τ0 (Pa)	K (Pa S^n)	n	R^2	Apparent viscosity (mPa s) (at 100 s^−1)
LS	32.03	0.93	0.98	0.9997	1145
NSF	24.07	0.88	1.02	0.9999	773
ALS1	10.51	0.83	0.93	0.9977	625
ALS2	5.79	0.73	0.88	0.9959	536
ALS3	15.45	0.87	0.91	0.9996	639

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Table 3 suggests that all rheological data represented good linear relationships, as coefficients of correlation (R²) were around 1. The values of flow characteristic exponential (n) for CWS with ALSs were slightly less than 1, which indicated that the slurries belonged to the pseudoplastic fluid class. The yield stress τ₀ and consistency coefficient K should be as low as possible for a good dispersant for CWS. As shown in Table 3, ALS2 displayed excellent rheological property because of lower yield stress (5.79 Pa) and lower consistency coefficient (0.73 Pa Sⁿ), which is in good agreement with the result of lower apparent viscosity (536 mPa s at shear rate of 100 s⁻¹). The results above show that ALSs have great potential for practical industry application.

Conclusions

A facile method for preparing ALSs with ultrahigh molecular weights by alkyl chain coupling polymerization using LS as raw materials has been developed for the first time. GPC, functional group content analyses, FTIR, ¹H-NMR, elemental analyses and XPS further confirmed the cross-linked network structure. The high hydrophobicity of ALS may induce the formation of hollow nanospheres using ALS alone, via self-assembly in selected solvents. In contrast, stable quasi-solid nanospheres were also obtained from ALS and CTAB via self-assembly. ALS2, with its moderate molecular weight, showed unexpectedly better dispersion properties than naphthalene sulfonate formaldehyde condensate (NSF) in coal–water slurry. Furthermore, we believe the design concept using LS as raw material can likely be applied to the synthesis of other lignin and lignosulfonate-based polymers for promising potential industrial applications. The successful fabrication of nanomaterials using the lignosulfonate-based polymer provide novel avenues for the high-value application of this abundant bioresource.

Acknowledgements

The authors would like to acknowledge the financial support of the National Basic Research Program of China 973 (2012CB215302), International S&T Cooperation Program of China (2013DFA41670), and the National Natural Science Foundation of China (21436004, 21402054).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic approach of ALS, the proximate and ultimate analyses for Shenhua coal sample, XPS spectrum of the surface of ALS hollow nanospheres and quasi-solid spheres, SEM and elemental distribution of the quasi-solid nanospheres with ALS and CTAB via self-assembly, Effect of dispersant dosage on the apparent viscosity of CWS with ALSs and NSF. See DOI: 10.1039/c5ra02157j

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