| As one of the most abundant biomass resources in nature, lignin is thought to be very promising petrochemical substitute resource. Lignosulfonate(LS) is the main derivative of lignin obtained through the traditional sulfite pulping process, which could be widely used in agriculture, energy, construction, and many other fields, owning the advantages of rich sources, low cost and non-toxic. However, only a small quantity of LS is utilized despite a wide range of applications, so to exploit new application fields of LS will be of great significance. The application performances of LS are closely related to its solution behaviors. Researches on aggregation behaviors and microstructures of LS are beneficial to guide its industrial applications. However, up to now, investigations on solution behaviors of LS are mainly limited to the dilute solution system. For the LS concentrated solution, investigations on its solution behaviors are rare due to the complex structure. However, in consideration of the application of concentrated LS solution in storage, transport, modified reactions and compound technique, studies on its solution behavior have both important theoretical and applied values. In this work, effects of external factors such as concentration, temperature, p H and cationic surfactant cetyltrimethylammonium bromide(CTAB) on aggregation behaviors and microstructures of sodium lignosulfonate(Na LS) in concentrated solutions were systematically investigated. On this basis, uniform nanoparticles were prepared from the Na LS/CTAB complex at the stoichiometric mass ratio in the concentrated system through electrostatic and hydrophobic self-assembly. Meanwhile, the applications of nanoparticles in polyvinyl plastics were explored.Firstly, four typical rheological models were used to fit the rheological curves of concentrated Na LS solutions, and the Herschel-Bulkley model was finally determined to be the most suitable due to the highest correlation coefficient. The effects of concentration and temperature on aggregation behaviors of Na LS in concentrated solutions were investigated by means of rheology and conductivity meter, respectively. The results showed that the aggregate effect strengthened with increasing concentrations, which resulted in the increased apparent viscosity(η), pseudoplasticity, stability, complex viscosity(η*), storage modulus(G’) and loss modulus(G″), and the decreased loss tangent(tanδ). When the temperature was in the range of 5-20 oC, η*, G’ and G″ decreased but the tanδ increased with increasing temperatures due to the faster Brownian motion. However, the result was just opposite when the temperature was higher 20 oC, which was caused by the increase of the aggregate degree and the strength of network structure.Secondly, the aggregation behaviors and the microstructure of Na LS in concentrated solutions at different p H and additions of straight-chain alcohols were studied by rheology, conductivity, acid-base titration and Zeta potential analyzer. Experimental results showed that the ionization degree of weakly ionized groups in Na LS increased with increasing p H(2.89-10.34), so the electrostatic repulsions increased, which led to the swelling of the Na LS hydrophobic cores and the increase of the network structure size, and thus the increased viscosity, pseudoplasticity, stability, thixotropy, η*, G’ and G″, and the decreased tanδ and crossover temperature of G′-T and G″-T curves. When p H exceeded 10.34, the experimental results were just opposite, which was resulted from the too-high electrostatic repulsions and the damage of hydrogen bondings. When small amounts of straight-chain alcohols were added in Na LS systems, their nonpolar carbon chains could insert into the hydrophobic cores of Na LS aggregates, which caused the decrease of aggregation degree, and thus the lower viscosity, weaker pseudoplastic and stability. As the dosage of straight-chain alcohols was bigger, they gradually began to exert viscosity enhancement effect on Na LS solutions. When the dosage reached 15 wt%, the viscosity increased, and the pseudoplastic and the stability improved due to the increase of hydrophobic interaction caused by the raised p H value. Besides, hydrophobic interactions played important roles in the aggregation behaviors of concentrated Na LS solutions.Thirdly, the influence of inorganic salts, urea and CTAB on aggregation behaviors of Na LS in the concentrated solutions was explored by means of rheology, conductivity meter, Zeta potential analyzer and surface tension meter. Results indicated that the increasing dosages of inorganic salts favored electrostatic repulsion shielding effect and solvent polarity enhancement, which both promoted the formation of larger aggregates, and resulted in the increased viscosity, pseudoplasticity, stability, thixotropy, η*, G’ and G″, and the decreased tanδ and crossover temperature of G′-T and G″-T curves. However, the result was just opposite after addition of urea due to the destruction of hydrogen bonds and the weakening of aggregation interaction. The above results also indicated that there were electrostatic and hydrogen bonding interactions in the aggregation of concentrated Na LS solutions. The increased additions of CTAB(m(CTAB)/m(Na LS)<1:7) would result in the increased η, η*, G’ and G″, and the decreased tanδ. This was caused by the electrostatic attractions between Na LS and CTAB and the hydrophobic interactions between CTAB molecules. The presence of CTAB, which actually acted as effective inter-polymer cross-linkers, led to the formation of mixed aggregates containing Na LS hydrophobes and CTAB molecules. This tends to strengthen the aggregation effect within the system. Upon the further addition of CTAB(m(CTAB)/m(Na LS)>1:7), the Na LS hydrophobes would be envelopped by excessive CTAB molecules. The electrostatic repulsions and the steric effects prevented the association in Na LS system, and thus the decreased η、η*、G’ and G″, and the increased tanδ. Moreover, the stoichiometric mass ratio(SMR) of Na LS/CTAB complex in the concentrated and relatively dilute systems separately was 1:7 and 1:2.82, where the hydrophobicity was strongest and to prepare colloidal spheres was feasible.Finally, uniform lignin nanoparticles were firstly prepared from the Na LS/CTAB complex at SMR in ethanol(Et OH) when water was continuously added into Na LS/CTAB/Et OH solutions. The structure and the formation mechanism of Na LS/CTAB nanoparticles were investigated by light scattering, TEM, contact angle measurement, XPS, elemental analysis and fourier transform infrared. Results showed that Na LS/CTAB molecules started to form nanoparticles through hydrophobic interactions at critical water content(CWC). When the initial concentration of Na LS/CTAB in Et OH solution was 1.0 mg/m L, CWC was estimated to be 58 vol%. The colloidal formation process was completed at a water content of 84 vol%, and the obtained Na LS/CTAB nanoparticles had <Dh> of about 539 nm, with a polydispersity index of 0.037. Nanoparticles with different average sizes and size distributions could be obtained by adjusting the initial concentration and the dropping rate of water. As Na LS/CTAB nanoparticles being blended into the high density polyethylene(HDPE), the mechanical property of the obtained Na LS/CTAB/HDPE composite materials were superior to that of Ca CO3/HDPE materials because they exhibited better interfacial compatibility with HDPE due to relatively stronger hydrophobicity, reaching the requirement of industrial use. Meanwhile, when Na LS/CTAB nanoparticles were added, they also played a role of “ball”, which caused the improved processing properties of composite materials.This work not only could provide theoretical foundation for the modification reactions and the improvement of application performance of lignin conducted in concentrated systems, but also expands the application of LS in material field, which gives a new way for the high-value applications of lignin. |
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