Preparation Of Lignin Self-assembled Multilayers And Its Application In Functionalization Of Material Surface

Technical lignin is a low-cost and abundant by-product of pulp and paper industry, amajor part of technical lignin is incinerated as a fuel for the production of energy, only1-2%of technical lignin is isolated from spent pulp liquors and employed in a wide range of finechemical products. In this paper, self-assembled multilayers were built up on proper solidsubstrate surface by using layer-by-layer (LBL) self-assembly technique based oncoordination interaction between technical lignin and metal ions or electrostatic interactionbetween technical lignin and positively charged substance. The growth modes ofself-assembled multilayers were monitored, the morphologies of self-assembled multilayerswere characterized, and the surface functionalization of solid substrates with self-assembledmultilayers was considered. This paper proposed a new way for the high-value use oftechnical lignin.Self-assembled multilayers of lignosulfonates (LS) and Cu²⁺were built up on both quartzslide and cellulose fiber surface by using LBL self-assembly technique based on coordinationinteraction between LS and Cu²⁺. The growth mode, surface chemical composition andmorphology of LS/Cu²⁺multilayers were analyzed, the effects of LS/Cu²⁺multilayers onfibers wetting properties and handsheet strength were investigated. The results showed thatLBL self-assembly of LS and Cu²⁺was feasible, stepwise LS/Cu²⁺multilayers growth wasoccurred and this deposition process was highly reproducible. The contents of characteristicelements (i.e. S and Cu) of LS/Cu²⁺multilayers increased with the number of LS layers,furthermore, the calculated surface LS content increased linearly as a function of LS layers,indicating that equal amount of LS was absorbed on each layer. The AFM phase imagesshowed that the cellulose microfibrils on fiber surface were gradually covered by LS particlesas self-assembly proceeded, resulting in the increase in average phase of AFM phase images.The dynamic contact angle results showed that the hydrophobicity of cellulose fiber waseffectively improved by constructing LS/Cu²⁺multilayers on fiber surface. The initial watercontact angle gradually increased and the attenuation rate of the water contact angle graduallydecreased with the number of LS layers. The initial water contact angle increased from0°to115.5°as the cellulose fiber was modified with a5-layer LS multilayer, and then decreased to97.9°after0.12s, suggesting that the controllable hydrophobicity of cellulose fiber can beachieved depending on the number of LS layers. However, it is unfortunate that LS/Cu²⁺multilayers have negative effect on strength property of handsheet.To improve the strength property of handsheet, multilayers composed of cationic polyacrylamide (CPAM) and LS were built up on both cellulose fiber surface and quartz slideusing electrostatic LBL self-assembly technique. The results showed that the electrostaticLBL self-assembly of CPAM and LS was feasible, the growth of CPAM/LS multilayers onboth quartz slide and cellulose fiber surface was uniform and successive. The Zeta potential offiber surface inversed after deposition of each layer, the contents of characteristic elements(i.e. S and N) of CPAM/LS multilayers increased with increasing bilayer number, and a goodlinear relationship was observed between the calculated surface LS content and the number ofbilayers. AFM phase images indicated that the cellulose microfibrils on fiber surface weregradually covered by LS granules as the multilayers grew, resulting in an increase in fibersurface roughness. The hydrophobicity of cellulose fiber was markedly increased throughfabricating CPAM/LS multilayers on fiber surface. More specifically, the initial contact angleincreased gradually with the number of bilayers. When a (CPAM/LS)5multilayer was formedon the fiber surface, the initial water contact angle reached to109.2°, suggesting that thisself-assembly strategy can lead to the formation of a hydrophobic surface on cellulose fiber.On the other hand, the attenuation rate of contact angle decreased with the number of bilayers,the initial contact angle changed from109.2°to90.7°after1.0s when the cellulose fibermodified with a (CPAM/LS)5multilayer. Moreover, the strength property and printability ofhandsheet were effectively improved by constructing CPAM/LS multilayers on cellulose fibersurface.Long aliphatic carbon chain was introduced to LS macromolecule through Mannichreaction. The modified LS (MLS) was applied to electrostatic LBL self-assembly system, andthe CPAM/MLS multilayers were built up for hydrophobicity modification of cellulose fibersurface. Moreover, LBL self-assembly system of CPAM and MLS was improved to obtain astable hydrophobic surface. The results indicated that the optimal Mannich reactionconditions were as follows: reaction temperature90oC, reaction time6h, n(LS):n(dodecylamine): n(formaldehyde)=1:1:1.3. The content of characteristic element (S) of theCPAM/MLS multilayers increased, and the Zeta potential of fiber surface inversed regularlyas the LBL self-assembly proceeded. The RMS roughness of fiber surface modified withCPAM/MLS multilayers was higher than that of fiber surface modified with CPAM/LSmultilayers when the number of bilayers was same. The hydrophobicity of fiber surfacemodified with CPAM/MLS multilayers was improved more significantly than that of fibersurface modified with CPAM/LS multilayers. Moreover, The CPAM/MLS multilayers haveno negative effect on the strength property of handsheet. The handsheet prepared from a(CPAM/MLS)5multilayer modified fibers was further coated with trimethoxytridecafluorooct -ylsilane, and a stable superhydrophobic surface having an initial contact angle of159.7°wasobtained.Photocatalytic multilayers composed of anatase TiO₂nanoparticles and LS werefabricated on quartz slides by the LBL self-assembly technique, and the photocatalyticproperties of TiO₂/LS multilayers were evaluated. The results showed that stepwise multilayergrowth occurred on the substrate and this deposition process is highly reproducible. AFMmorphological images showed that quartz slide was fully covered by particles and the averagethickness, the surface RMS roughness of the TiO₂/LS multilayers gradually increased as theLBL self-assembly proceeded. The increase in decomposition efficiency of methyl orangeunder the same UV irradiation time was linear with the number of TiO₂layers. The degree ofdecomposition of bacterial increased with the increasing number of bilayers when the samesubstance was in the outermost layer, and bacterial decomposition ability of the samples withTiO₂in the outermost layer was better than that of the samples with LS in the outermost layer.To prepare antioxidative paper, the multilayers consisted of cationic starch (CS) andalkali lignin (AL) were constructed on the pulp fiber surface using electrostatic LBLself-assembly technique, the antioxidant abilities of the pulp fibers modified with CS/ALmultilayers were investigated. The results indicated that the growth of CS/AL multilayers onpulp fibers was successive and uniform. The fiber surface was gradually covered by ALgranules with diameter of28nm30nm as the number of bilayer increased. For the pulpfibers modified with CS/AL multilayers, there was no obvious change in cellulose degree ofpolymerization and fiber strength after radical oxidation treatment. The modified pulp andbleached CTMP are equally matched in inhibition oxidative degradation of cellulose.However, the pulp modified with CS/AL multilayers has higher stability of brightness thanbleached CTMP pulp.