## 1 Introduction  
Cellulose and lignin are major components of lignocellulosic biomass [1; 2; 3; 4]. A variety of applications of cellulose and lignin have progressed in terms of resource recycling [1; 2].The molecular conversion of cellulose and lignin into value-added compounds is a promising methodology to effectively use biomass [2; 3; 4]. Several useful chemical platforms from cellulose are glucose [5; 6], fructose [5; 6], 5-hydroxymethylfurfurtal [7], glycerol [8; 9], lactic acid [8; 9], and others [10; 11], which are key compounds for Cn-chemistry. Furthermore, the use of cellulose has been advanced, i.e., cellulose-based materials including cellulose nanofiber [12; 13; 14], cellulose nanocrystalline [15], and cellulose derivatives [16]. Molecular conversion from lignin is also being vigorously studied. Representative examples of cellulose-derived materials are cellulose (nano)fiber [12; 13; 14], cellulosic membranes for separation operation [17], reinforcing materials [13], labelled cellulose [18], and interfacial materials based on cellulose and lignin [16; 19]. Block copolymers including cellulose have also been used to develop novel functional materials. For further improvement of the mechanical stability of cellulose- and polymer-based materials, lignin has been combined with them [12; 16; 19; 20]. Cellulose-based interfacial materials have been designed in terms of rheological and chemical mechanical properties, as well as functionality [21]. Two representative interfacial materials based on cellulose are (i) a wood-plastic composite (WPC), which is a composite of woody materials with plastic, and (ii) plate-like materials obtained from the molding of a mixture of cellulose and lignin, which is called a woody thin board (WTB) [16; 19]. WPC is the material obtained from woody biomass by a top-down approach, whereas WTB is the material obtained from pure cellulose and lignin by a bottom-up approach. The surface property of WPC depends on the content and chain length of plastics (polymers) due to the entanglement between cellulose and polymer chains [22]. In contrast, lignin can modulate the interfacial property of WTBs such as hydrophobicity and water repellency [19]. Lignin can also act as a glue between cellulose beads to reinforce the mechanical stability of WTBs [16]. However, a high lignin content in a mixture of cellulose and lignin makes it impossible to form WTBs [16; 19]. Therefore, the improved effect of water repellency in WTBs by lignin is limited to an increase from 45 deg to 60 deg [19]. As another approach, the chemical modification of cellulose by some chemical compounds has been examined: e.g., Ag nanoparticles [22] and diblock copolymers based on end-modified cellulose derivatives [16; 20; 23]. In our previous research, a dipping of poly-L-lactic acid (PLLA) could improve water repellency up to 80 deg [16]. For further improvement, PLLA-\(g\)-cellulose was synthesized by introducing PLLA into cellulose since PLLA is hydrophobic relative to cellulose. The use of PLLA-\(g\)-cellulose succeeded in the improvement of water repellency (water contact angle: 50 deg to 126 deg) by the orientation of PLLA-\(g\)-cellulose particles at the surface of WTB, because the chain of PLLA is an obstacle for burying in the grain boundary between cellulose particles [16]. On the other hand, the immersion of WTBs into the hydrophobic PLLA solution did not give higher hydrophobicity than the case using PLLA-\(g\)-cellulose [16]. However, the use of other hydrophobic polymers for the chemical modification of cellulose cannot always avoid the orientation into the grain boundary of cellulose particles, suggesting the need for alternative approaches. As one method, surface modification using adhesion phenomena between polymer particles having various surface properties and cellulose particles is considered. However, there are few studies related to the cellulose/lignin-based material interface.