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[Takehiro Fujita](https://orcid.org/0000-0001-9071-7133), Masami Shuta, Mika Mano, Shinnosuke Matsumoto, Atsushi Nagasawa, Akihiro Yamada, [Masanobu Naito](https://orcid.org/0000-0001-7198-819X)

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[Forced Gradient Copolymer for Rational Design of Mussel-Inspired Adhesives and Dispersants](https://mdr.nims.go.jp/datasets/f5b5bab9-a756-4b15-8cf3-197de94e1f28)

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Forced Gradient Copolymer for Rational Design of Mussel-Inspired Adhesives and DispersantsCitation: Fujita, T.; Shuta, M.; Mano,M.; Matsumoto, S.; Nagasawa, A.;Yamada, A.; Naito, M. ForcedGradient Copolymer for RationalDesign of Mussel-Inspired Adhesivesand Dispersants. Materials 2023, 16,266. https://doi.org/10.3390/ma16010266Academic Editor: Manuela ZubiturReceived: 18 November 2022Revised: 21 December 2022Accepted: 22 December 2022Published: 27 December 2022Copyright: © 2022 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).materialsArticleForced Gradient Copolymer for Rational Design ofMussel-Inspired Adhesives and DispersantsTakehiro Fujita 1,2 , Masami Shuta 1, Mika Mano 1, Shinnosuke Matsumoto 3, Atsushi Nagasawa 3,Akihiro Yamada 3 and Masanobu Naito 1,2,*1 Data-Driven Polymer Design Group, Research and Services Division of Materials Data and IntegratedSystem (MaDIS), National Institute for Materials Science (NIMS), Ibaraki 305-0047, Japan2 Program in Materials Science and Engineering, Graduate School of Pure and Applied Sciences,University of Tsukuba, Ibaraki 305-8577, Japan3 Oleo & Speciality Chemicals Research Lab., NOF Corporation, Hyogo 660-0095, Japan* Correspondence: naito.masanobu@nims.go.jpAbstract: In recent years, there has been considerable research into functional materials inspired byliving things. Much attention has been paid to the development of adhesive materials that mimic theadhesive proteins secreted by a mussel’s foot. These mussel-inspired materials have superior adhe-siveness to various adherents owing to the non-covalent interactions of their polyphenolic moieties,e.g., hydrogen bonding, electrostatic interactions, and even hydrophobic interactions. Various factorssignificantly affect the adhesiveness of mussel-inspired polymers, such as the molecular weight,cross-linking density, and composition ratio of the components, as well as the chemical structure ofthe polyphenolic adhesive moieties, such as L-3,4-dihydroxyphenylalanine (L-Dopa). However, thecontributions of the position and distribution of the adhesive moiety in mussel-inspired polymers areoften underestimated. In the present study, we prepared a series of mussel-inspired alkyl methacry-late copolymers by controlling the position and distribution of the adhesive moiety, which are knownas “forced gradient copolymers”. We used a newly designed gallic-acid-bearing methacrylate (GMA)as the polyphenolic adhesive moiety and copolymerized it with 2-ethylhexyl methacrylate (EHMA).The resulting forced gradient adhesive copolymer of GMA and EHMA (poly(GMA-co-EHMA), Poly1)was subjected to adhesion and dispersion tests with an aluminum substrate and a BaTiO3 nanopar-ticle in organic solvents, respectively. In particular, this study aims to clarify how the monomerposition and distribution of the adhesive moiety in the mussel-inspired polymer affect its adhesionand dispersion behavior on a flat metal oxide surface and spherical inorganic oxide surfaces of severaltens of nanometers in diameter, respectively. Here, forced gradient copolymer Poly1 consisted of ahomopolymer moiety of EHMA (Poly3) and a random copolymer moiety of EHMA and GMA (Poly4).The composition ratio of GMA and the molecular weight were kept constant among the Poly1 series.Simultaneous control of the molecular lengths of Poly3 and Poly4 allowed us to discuss the effects onthe distribution of GMA in Poly1. Poly1 exhibited apparent distribution dependency with regard tothe adhesiveness and the dispersibility of BaTiO3. Poly1 showed the highest adhesion strength whenthe composition ratio of GMA was approximately 9 mol% in the portion of the Poly4 segment. Incontrast, the block copolymer consisting of the Poly3 segment and Poly4 segment with only adhesivemoiety 1 showed the lowest viscosity for dispersion of BaTiO3 nanoparticles. These results indicatethat copolymers with mussel-inspired adhesive motifs require the proper design of the monomerposition and distribution in Poly1 according to the shape and characteristics of the adherend tomaximize their functionality. This research will facilitate the rational design of bio-inspired adhesivematerials derived from plants that outperform natural materials, and it will eventually contribute toa sustainable circular economy.Keywords: mussel-inspired polymer; adhesive; dispersant; forced gradient polymer; L-3,4-dihydroxyphenylalanine; dopamine; gallic acid; BaTiO3Materials 2023, 16, 266. https://doi.org/10.3390/ma16010266 https://www.mdpi.com/journal/materialshttps://doi.org/10.3390/ma16010266https://doi.org/10.3390/ma16010266https://creativecommons.org/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://www.mdpi.com/journal/materialshttps://www.mdpi.comhttps://orcid.org/0000-0001-9071-7133https://orcid.org/0000-0001-7198-819Xhttps://doi.org/10.3390/ma16010266https://www.mdpi.com/journal/materialshttps://www.mdpi.com/article/10.3390/ma16010266?type=check_update&version=2Materials 2023, 16, 266 2 of 121. IntroductionNon-human living things have a variety of capabilities that surpass those of humansand enable them to remain active even in the harshest of environments. Such superior capa-bilities have inspired the creation of biomimetic materials [1,2]. For example, the adhesivecapability of a mussel’s foot has been studied extensively. It is well known that catechol,which is a polyphenol, is expressed in the proteins secreted by a mussel’s foot. The byssus,which is a bundle of filaments secreted by many species of bivalve mollusk, plays animportant role in underwater adhesion to target surfaces. It achieves this via non-covalentinteractions, such as hydrogen bonding, electrostatic interactions, chelation, hydrophobicinteractions, and oxidative cross-linking [3–5]. A variety of mussel-inspired adhesives thatmimic a mussel’s adhesion mechanism have been developed by introducing a catecholmoiety into various types of polymers, such as polypeptides [6,7], polyethylene glycol [8],polyacrylates [9–14], and polystyrene [15–19]. These studies have revealed that the adhe-sive capabilities of mussel-inspired polymers are significantly affected by their primarystructures. For example, Wilker and co-workers [20] prepared poly((3,4-dihydroxystyrene)-co-styrene) copolymers with various molecular weights by living anionic polymerization.They found that the adhesive strength of the copolymer increases linearly as a function of itsmolecular weight in the range of 20,000–100,000 kDa. Similarly, Kohri and co-workers [21]reported that branched poly(N-(2-(2,2-dimethylbenzo-1,3-dioxol-5-yl)ethyl)-acrylamide)prepared by controlled polymerization has superior adhesion properties to similar poly-mers with linear structures. Ejima and co-workers [22–25] investigated the effect of thenumber of aromatic hydroxyl groups on the benzene rings of the adhesive moieties insuch copolymers. They found that the adhesion capability increases as the number ofOH groups on the polyphenol groups increases. However, it is still unclear how the posi-tion and distribution of the adhesive moieties in such polymers affect their adhesiveness,despite the fact that mussel-foot proteins have been shown to contribute profoundly tothe L-3,4-dihydroxyphenylalanine (L-Dopa) sequence, not only in regulating the bindingstrength of L-Dopa residues but also in achieving adaptability to various surfaces [26,27].Within this context, we have attempted to demonstrate the effects of the monomerposition and distribution of the adhesive moieties in mussel-inspired adhesive polymers ontheir adhesion and dispersion capabilities. To realize this, we designed a gallic-acid-bearingmethacrylate monomer (1) that has the potential as an adhesive moiety (Scheme 1A) [12,14].To demonstrate how the monomer position and distribution of 1 affect the adhesion anddispersion capabilities of a mussel-inspired adhesive polymer, we focused on a forcedgradient copolymer (FGCP) [28,29]. An FGCP is prepared by controlling the timing ofthe addition of the second monomer during the polymerization of the first monomer(Scheme S1). It allows the compatibility of immiscible polymer blends or the stability ofemulsions/dispersions to be systematically investigated.Scheme 1. (A) One-pot synthesis of methacrylate-bearing gallic acid monomer 1 and preparation ofpoly(1-co-(EHMA) (Poly1, EHMA = 2-ethylhexyl methacrylate, 1 = gallic-acid-bearing methacrylate).(B) Structure and chemical synthesis of Poly2.Materials 2023, 16, 266 3 of 12In this study, to clearly characterize the position and distribution of 1 in poly(1-co-EHMA) (Poly1, EHMA = 2-ethylhexyl methacrylate, 1 = gallic-acid-bearing methacrylate),a series of Poly1 copolymers with different molecular lengths of the homo- and randompolymer segments was prepared (Scheme S1). The resulting forced gradient Poly1 copoly-mers were subjected to adhesive tests on aluminum substrates and dispersion tests withnanoparticles of barium titanate (BaTiO3) perovskite oxides (50 nm in average diameter)in organic solvents (Scheme 1A). Here, the aluminum substrate and BaTiO3 nanoparticlewere used as a flat metal oxide surface and a spherical inorganic oxide surface, respectively.The adhesion and dispersion capabilities were significantly affected by the position anddistribution of 1 in Poly1 in different ways.2. Materials and Methods2.1. GeneralAll of the reactions involving the oxygen- and moisture-sensitive compounds werecarried out in a dry reaction vessel under nitrogen or argon. The reaction mixtures weredegassed using the freeze–pump–thaw method before the radical polymerization reactionscommenced. Flash column chromatography was performed with a silica gel (60N, sphericalneutral; Kanto Chemical Co., Inc., Tokyo, Japan). Analytical thin-layer chromatographywas performed with a silica gel coated with fluorescent indicator F254 (silica gel 60 F254,Art 5715, 0.25 mm, Merck, Darmstadt, Germany).2.2. MaterialsTriethyl silyl chloride, N,N-dimethylformamide (super-dehydrated), NaHCO3, tetra-butylammonium fluoride, tetrahydrofuran solution (~1 mol/L), acetic acid, and BaTiO3were purchased from FUJIFILM Wako Pure Chemical Co.(Osaka, Japan)., and they wereused as received. Imidazole was purchased from Tokyo Chemical Industry Co., Ltd (Tokyo,Japan)., and it was used as received. 1,2-Dichloroethane (super-dehydrated) was purchasedfrom Kanto Chemical Co., Inc., and it was used as received. 2-Ethylhexyl methacrylatewas purchased from FUJIFILM Wako Pure Chemical Co., and it was passed throughan A2O3 column before use to remove the radical inhibitor. 2-Cyano-2-propyl dodecyltrithiocarbonate was purchased from Sigma-Aldrich Co., and it was used as received.2,2′-Azobisisobutyronitrile was purchased from FUJIFILM Wako Pure Chemical Co., and itwas recrystallized from methanol before use. 1 was provided by NOF CORPORATION(Hyogo, Japan).2.3. Preparation of the Mussel-Inspired Forced Gradient CopolymersTo investigate the forced gradient adhesive moiety in adhesive polymers, we designedgallic-acid-bearing methacrylate monomer 1 that has the potential as an adhesive moiety(Scheme 1A). A galloyl group with three phenolic hydroxyl groups is expected to showgreater adhesiveness than a catechol group with two hydroxyl groups (such as in L-Dopa),owing to the greater number of vicinal hydroxyl groups in the benzene ring [22–25]. Fromthe viewpoint of practical application, gallic acid is more affordable than L-Dopa andits derivatives.In a previous study, we reported the rational design of a mussel-inspired adhesivecomprising a dopamine-functionalized copolymer (Poly2). We varied the alkyl-chainlengths/structural isomers of the alkyl methacrylate comonomers and their ratio withdopamine-functionalized methacrylamide 2 (Scheme 1B) [14]. The resulting EHMA-basedadhesive copolymer with 8 mol% dopamine exhibited tough, strong, and ductile adhesiveproperties. The relatively long and branched alkyl chain of EHMA is also expected toprevent oxidation of the catechol unit through hydrophobic interactions [30]. Here, wedesigned a gallic-acid-functionalized adhesive polymer (Poly1) based on Poly2 becausegallic acid, which has three hydroxyl groups, confers greater adhesiveness than L-Dopa,which has two hydroxyl groups [22–25]. Poly1 copolymers were synthesized from EHMAand 1 by varying the position and distribution of 1 (Figure 1).Materials 2023, 16, 266 4 of 12Materials 2023, 16, x FOR PEER REVIEW 4 of 12   In a previous study, we reported the rational design of a mussel-inspired adhesive comprising a dopamine-functionalized copolymer (Poly2). We varied the alkyl-chain lengths/structural isomers of the alkyl methacrylate comonomers and their ratio with do-pamine-functionalized methacrylamide 2 (Scheme 1B) [14]. The resulting EHMA-based adhesive copolymer with 8 mol% dopamine exhibited tough, strong, and ductile adhesive properties. The relatively long and branched alkyl chain of EHMA is also expected to pre-vent oxidation of the catechol unit through hydrophobic interactions [30]. Here, we de-signed a gallic-acid-functionalized adhesive polymer (Poly1) based on Poly2 because gal-lic acid, which has three hydroxyl groups, confers greater adhesiveness than L-Dopa, which has two hydroxyl groups [22–25]. Poly1 copolymers were synthesized from EHMA and 1 by varying the position and distribution of 1 (Figure 1).  Figure 1. (A) Summary of the chemical structures of the polymers in this work. Poly3 is a homopol-ymer domain of EHMA. Poly4 is a random copolymer domain of EHMA and 1 in Poly1a–1e. (B) Chemical and abbreviated structures of synthesized copolymers Poly1a–1e. We prepared a series of gallic-acid-bearing Poly1 copolymers by reversible addition–fragmentation chain-transfer polymerization. The resulting Poly1 copolymers were char-acterized by proton nuclear magnetic resonance (1H NMR) spectroscopy and gel permea-tion chromatography (GPC). The detailed procedures used in the present study are de-scribed in the Supporting Information, and the characteristics of the copolymers are sum-marized in Table 1. In the Poly1a–1e copolymers, the ratio of 1 to EHMA and the degree of polymerization (Dp) were standardized to 7:93 and 200, respectively. Here, Poly1a and Figure 1. (A) Summary of the chemical structures of the polymers in this work. Poly3 is a ho-mopolymer domain of EHMA. Poly4 is a random copolymer domain of EHMA and 1 in Poly1a–1e.(B) Chemical and abbreviated structures of synthesized copolymers Poly1a–1e.We prepared a series of gallic-acid-bearing Poly1 copolymers by reversible addition–fragmentation chain-transfer polymerization. The resulting Poly1 copolymers were charac-terized by proton nuclear magnetic resonance (1H NMR) spectroscopy and gel permeationchromatography (GPC). The detailed procedures used in the present study are describedin the Supporting Information, and the characteristics of the copolymers are summarizedin Table 1. In the Poly1a–1e copolymers, the ratio of 1 to EHMA and the degree of poly-merization (Dp) were standardized to 7:93 and 200, respectively. Here, Poly1a and Poly1ecorrespond to random and diblock copolymers of EHMA and 1, respectively. The Poly1b–1d FGCPs comprised two domains: a homopolymer domain of EHMA (Poly3) and arandom copolymer domain of EHMA and 1 (Poly4). Here, Dp of the Poly3 segment inPoly1b–1d was varied (49, 98, and 147). It is noteworthy that, in this system, the composi-tion ratio of 1 in the Poly4 domain can be manipulated by tuning Dp of the Poly3 domain.This allows a systematic evaluation of the adhesive capability of 1 regarding the monomerposition and distribution of adhesive moiety 1 in the Poly1 series.Materials 2023, 16, 266 5 of 12Table 1. Physical Properties of the Gallic-Acid-Bearing Polymers.Polymer EHMA:1 a Mn (×104) b, c Ð b, cPoly1a 93.5:6.5 5.1 1.1Poly1b 94.4:5.6 4.3 1.2Poly1c 93.8:6.2 4.3 1.1Poly1d 93.7:6.3 5 1.1Poly1e 95.6:4.4 4.1 1.1Poly3 100:0 3.1 1.2a Ratio of EHMA to 1 in the copolymer, as determined by 1H NMR. b Results for triethylsilyl-protected polymers.c Determined by gel permeation chromatography against poly(methyl methacrylate) standards at 40 ◦C.2.4. Adhesion TestWe evaluated the adhesive strengths of the Poly1 copolymers by butt tensile tests usinga centrifugal adhesion test analyzer (Supplementary Materials, Figure S2A) (LUMiFrac;LUM GmbH, Germany). Each Poly1 copolymer was coated on an aluminum butt at acoverage rate of 9.4 mg·mm−2, and the coated area was pre-cured at 60 ◦C to preventunexpected void formation. The resulting Poly1-coated aluminum butt was placed onan aluminum plate, and the specimen was then cured at 80 or 120 ◦C for 1 h (Figure S1).From five to eight samples were loaded simultaneously into the measuring chamber(Figure S2B). A centrifugal force was applied to each test specimen at 5 N·s−1 and ambienttemperature. The adhesive strength (MPa) was determined from the adhesive force (N)when bonding failure occurred divided by the area (mm2) of Poly1 applied to the aluminumbutt specimen (Table 1).2.5. Dispersion Test2.5.1. Dispersion Test Evaluated by the Naked EyeA mixture comprising BaTiO3 nanoparticles (50 mg, 0.21 mmol), Poly1 (25 mg,~5.0 × 10−4 mmol), and chloroform (5 mL) was sonicated for 60 min, and the resultingmixture was stirred at room temperature. The ability of the Poly1 copolymers to act asdispersants was evaluated by the naked eye.2.5.2. Dispersion Test Evaluated Using a RheometerA mixture comprising BaTiO3 nanoparticles (10 g, 43 mmol), Poly1 (10 mg,~2.0 × 10−4 mmol), and 1,1,2,2-tetrachloroethane (7.5 mL) was stirred at a shear rateof 1000 s−1, and the viscosity of the resulting mixture was monitored at a shear rate of10–100 s−1 at room temperature by a rheometer (MCR302; Anton-Paar, Austria).3. Results3.1. Effect of the Monomer Position on the AdhesivenessFirst, to confirm the ability of gallic-acid-functionalized methacrylate monomer 1 to actas an adhesive moiety, we carried out butt tensile tests on Poly1a, Poly2, and Poly3 using acentrifugal adhesion test analyzer. Poly2 was prepared by free-radical copolymerization of2 and EHMA according to our previous report so that molar ratio of 2 in the copolymerwas 8 mol% (see the Supporting Information) [15]. Poly3, which did not contain 1, wasused as a model polymer. Consequently, Poly1a exhibited similar adhesive strength toPoly2 (7.9 and 8.8 MPa, respectively). In contrast, Poly3 showed relatively low adhesivestrength (2.8 MPa), suggesting that 1 acted as an adhesive moiety in a similar manner to2. Furthermore, Poly1a and Poly2, which both contained polyphenolic adhesive moieties,exhibited cohesive failure resulting from strong interactions with the oxidized aluminumsurface (Figure S3). However, Poly3 exhibited surface failure, probably owing to the loweradhesiveness than the adherend. Next, we subjected Poly1 copolymers comprising 1 atvarious positions and with different distributions (Poly1b–1e) to butt adhesive tests. Notethat Poly1b–1d consisted of two domains: Poly3 comprising a homopolymer of EHMA andPoly4 comprising a random copolymer of EHMA and 1. The Dp values and compositionMaterials 2023, 16, 266 6 of 12ratios (mol%) of 1 were almost identical among the Poly1 copolymers, whereas the ratiosof Dp between Poly3 and Poly4 varied. The tensile strength values of the copolymers inmegapascals are listed in Table 2.Table 2. Adhesive Strength of the Gallic-Acid-Bearing Polymers.Polymer Dp of Poly3 1 in Poly4(mol%)Adhesive Strength(MPa)Poly1a 0 7 7.9Poly1b 49 9 9.8Poly1c 98 14 8.6Poly1d 147 26 4.1Poly1e 186 100 2.9Poly3 200 0 2.8To further clarify the effects of the position and distribution of 1 on the adhesivenessof Poly1, we plotted the adhesive strength versus Dp of the Poly3 domain (Figure 2A) andthe adhesive strength versus the ratio of 1 in the Poly4 domain (mol%, Figure 2B). Thecurve shown in Figure 2A is rather gradual and monomodal, whereas that in Figure 2B issteep, with a maximum of 9 mol%. These results provide important insights into the effectsof the monomer position and distribution on adhesion. The fact that the adhesive strengthchanged by only 8–10 MPa, even when the Dp value varied from 0 to 50, indicates thata non-adhesive Poly3 domain at one end of the Poly1 copolymer did not have a markedeffect on its adhesive performance, even if that domain occupied half of its total length. Incontrast, the adhesive strength clearly indicated the dependency on the composition ratioof 1 in the Poly4 domain of Poly1. The adhesive strength of Poly1 reached a maximumwhen it contained 9 mol% 1, but it decreased sharply when the ratio of 1 exceeded thatlevel. In a previous study, we found that the adhesive strength of Poly2 depends on thecomposition ratio of 2, and it reaches a maximum when the ratio is approximately 8 mol%.This arises from the balance between the polymer chain mobility and the adhesivenessconferred by the polyphenolic adhesive moiety dopamine. Considering that the samecomonomer, EHMA, was used in Poly1, we believe that a similar phenomenon occurred inPoly1. Overall, the results of the adhesion tests on the forced gradient Poly1 copolymersindicated that in Poly4, the maximum adhesiveness was more dependent on the ratio of 1than the domain length of non-adhesive Poly3.Materials 2023, 16, x FOR PEER REVIEW 7 of 12    Figure 2. Relationships between the adhesive strength (MPa) and the (A) degree of polymerization (Dp) of Poly3 (n) and (B) ratio of 1 in Poly4 (mol%). 3.2. Effect of the Monomer Position on the Dispersity With the miniaturization of commercial electronic products and devices, the size of electronic components has become smaller than ever. BaTiO3 is widely used in electronic components, such as multilayer ceramic capacitors (MLCCs), owing to its high dielectric constant [30,31]. MLCCs with even higher capacitance, smaller size, and higher reliability are required. Reducing the thickness and grain size of the dielectric layer leads to an in-crease in the capacitance-to-volume ratio, but such measures lead to an increase in the electric-field strength applied to the dielectric layer. Therefore, it is important to improve the dispersibility of the dielectric ceramic nanoparticles and make a low-viscosity slurry of the ceramic nanoparticles to fabricate defect-free ceramic dielectric layers. In general, MLCCs are manufactured as follows. Powdered dielectric ceramics, mainly BaTiO3, are dispersed in aqueous/organic solvents to obtain a slurry, which is coated on a carrier film to prepare a green sheet (raw sheet). Here, the choice of the dispersant is crucial to obtain a low-viscosity slurry of ceramic nanoparticles because a multilayer structure is formed by screen printing the inner electrode patterns on the green sheet. Eventually, the stack is sintered at 1000–1400 °C. Various types of polymer dispersants are used for the dispersion of functional inorganic/organic nanoparticles [32]. Polymeric dispersants consist of a func-tional group that serves as an anchor and a soluble polymer chain. For a polymeric dis-persant to adsorb on inorganic oxide (nano)particles, the anchor groups must be able to strongly adsorb to the (nano)particles. Much effort has been made to find suitable poly-mers for this purpose. Examples of typical functional groups for this purpose are amine, ammonium, and quaternary ammonium, carboxylic acid, sulfonic acid, and phosphoric acid groups [33–36]. L-Dopa has also been introduced at the terminus of polymer chains as the anchoring moiety, leading to a highly stable iron-oxide-nanoparticle colloidal sus-pension [37]. However, the position of the anchoring moiety in such polymer dispersants to create L-Dopa-functionalized polymers that are suitable as dispersants remains unclear. In the present study, we demonstrated the effects of the monomer position and distribu-tion of 1 in Poly1 on the dispersibility of BaTiO3 nanoparticles in organic solvents. In brief, inorganic BaTiO3 nanoparticles (averaged diameter 50 nm) were added to chloroform to produce a dispersion with a BaTiO3 concentration of 8.4 × 10−2 mol·L−1. This dispersion was then sonicated for 60 min. The resulting suspension of BaTiO3 particles immediately pro-duced a precipitate (Figure 3A). However, when 2.5 mL of a chloroform solution of Poly1a–1e (~2 × 10−4 mol·L−1) was added to the BaTiO3 suspension (2.5 mL) and the result-ing mixture was successively sonicated for 60 min, the sediment-prone suspension of Ba-TiO3 particles turned into a stable dispersion, which could be stored for more than 24 h. Unlike Poly1a–1e, Poly3 did not exhibit any dispensability with regard to BaTiO3 nano-particles using the same procedure. To quantitatively evaluate the dispersibility of Poly1 with regard to BaTiO3 nanoparticles, the viscosity of the dispersion of BaTiO3 was evalu-Figure 2. Relationships between the adhesive strength (MPa) and the (A) degree of polymerization(Dp) of Poly3 (n) and (B) ratio of 1 in Poly4 (mol%).3.2. Effect of the Monomer Position on the DispersityWith the miniaturization of commercial electronic products and devices, the size ofelectronic components has become smaller than ever. BaTiO3 is widely used in electroniccomponents, such as multilayer ceramic capacitors (MLCCs), owing to its high dielectricconstant [30,31]. MLCCs with even higher capacitance, smaller size, and higher reliabilityMaterials 2023, 16, 266 7 of 12are required. Reducing the thickness and grain size of the dielectric layer leads to anincrease in the capacitance-to-volume ratio, but such measures lead to an increase in theelectric-field strength applied to the dielectric layer. Therefore, it is important to improvethe dispersibility of the dielectric ceramic nanoparticles and make a low-viscosity slurryof the ceramic nanoparticles to fabricate defect-free ceramic dielectric layers. In general,MLCCs are manufactured as follows. Powdered dielectric ceramics, mainly BaTiO3, aredispersed in aqueous/organic solvents to obtain a slurry, which is coated on a carrierfilm to prepare a green sheet (raw sheet). Here, the choice of the dispersant is crucial toobtain a low-viscosity slurry of ceramic nanoparticles because a multilayer structure isformed by screen printing the inner electrode patterns on the green sheet. Eventually,the stack is sintered at 1000–1400 ◦C. Various types of polymer dispersants are used forthe dispersion of functional inorganic/organic nanoparticles [32]. Polymeric dispersantsconsist of a functional group that serves as an anchor and a soluble polymer chain. Fora polymeric dispersant to adsorb on inorganic oxide (nano)particles, the anchor groupsmust be able to strongly adsorb to the (nano)particles. Much effort has been made tofind suitable polymers for this purpose. Examples of typical functional groups for thispurpose are amine, ammonium, and quaternary ammonium, carboxylic acid, sulfonic acid,and phosphoric acid groups [33–36]. L-Dopa has also been introduced at the terminus ofpolymer chains as the anchoring moiety, leading to a highly stable iron-oxide-nanoparticlecolloidal suspension [37]. However, the position of the anchoring moiety in such polymerdispersants to create L-Dopa-functionalized polymers that are suitable as dispersantsremains unclear. In the present study, we demonstrated the effects of the monomer positionand distribution of 1 in Poly1 on the dispersibility of BaTiO3 nanoparticles in organicsolvents. In brief, inorganic BaTiO3 nanoparticles (averaged diameter 50 nm) were addedto chloroform to produce a dispersion with a BaTiO3 concentration of 8.4 × 10−2 mol·L−1.This dispersion was then sonicated for 60 min. The resulting suspension of BaTiO3 particlesimmediately produced a precipitate (Figure 3A). However, when 2.5 mL of a chloroformsolution of Poly1a–1e (~2 × 10−4 mol·L−1) was added to the BaTiO3 suspension (2.5 mL)and the resulting mixture was successively sonicated for 60 min, the sediment-pronesuspension of BaTiO3 particles turned into a stable dispersion, which could be storedfor more than 24 h. Unlike Poly1a–1e, Poly3 did not exhibit any dispensability withregard to BaTiO3 nanoparticles using the same procedure. To quantitatively evaluate thedispersibility of Poly1 with regard to BaTiO3 nanoparticles, the viscosity of the dispersionof BaTiO3 was evaluated using a rheometer. First, the BaTiO3 nanoparticles were dispersedin 1,1,2,2-tetrachloroethane (5.7 mol·L−1) at a shear rate of 1000 s−1, and the viscosityof the resulting dispersion was monitored at shear rates of 10–100 s−1 (Figure 3B). Theviscosity of the dispersion decreased exponentially from 589 to 165 Pa·s−1 as the shear rateincreased from 10 to 100 s−1. However, a stable dispersion of BaTiO3 nanoparticles with asignificantly lower viscosity was produced following the addition of ~2.0 × 10−4 mmolof Poly1a–1e. Moreover, it was obvious that the degree by which the viscosity decreaseddepended on the monomer sequence of the Poly1 copolymer used.To further clarify the effect of the monomer position of Poly1 on the dispersibilityof BaTiO3 at a shear rate of 100 s−1, we plotted the viscosity versus Dp of the domainof Poly3 (n) and the viscosity versus the composition ratio (mol%) of 1 in the domain ofPoly4 in Poly1 (Figure 4). To our surprise, the viscosity behavior was completely differentfrom that of the adhesive: the viscosity decreased linearly with Dp, whereas it decreasedexponentially with the composition ratio of 1 in the Poly4 domain of Poly1. These resultssuggest that the adhesive moiety 1 should preferably be concentrated at one end of Poly1,and a longer Poly3 domain contributes to better dispersion.Materials 2023, 16, 266 8 of 12Materials 2023, 16, x FOR PEER REVIEW 8 of 12   ated using a rheometer. First, the BaTiO3 nanoparticles were dispersed in 1,1,2,2-tetrachlo-roethane (5.7 mol·L−1) at a shear rate of 1000 s−1, and the viscosity of the resulting disper-sion was monitored at shear rates of 10–100 s−1 (Figure 3B). The viscosity of the dispersion decreased exponentially from 589 to 165 Pa·s−1 as the shear rate increased from 10 to 100 s−1. However, a stable dispersion of BaTiO3 nanoparticles with a significantly lower vis-cosity was produced following the addition of ~2.0 × 10−4 mmol of Poly1a–1e. Moreover, it was obvious that the degree by which the viscosity decreased depended on the mono-mer sequence of the Poly1 copolymer used.  Figure 3. (A) Photographs of BaTiO3 dispersed in a chloroform solution in the presence of Poly1a–1e. (B) Relationships between the shear rate and the viscosity in dispersions comprising the copol-ymers, BaTiO3, and 1,1,2,2-tetrachloroethane. To further clarify the effect of the monomer position of Poly1 on the dispersibility of BaTiO3 at a shear rate of 100 s−1, we plotted the viscosity versus Dp of the domain of Poly3 (n) and the viscosity versus the composition ratio (mol%) of 1 in the domain of Poly4 in Poly1 (Figure 4). To our surprise, the viscosity behavior was completely different from that of the adhesive: the viscosity decreased linearly with Dp, whereas it decreased expo-nentially with the composition ratio of 1 in the Poly4 domain of Poly1. These results sug-gest that the adhesive moiety 1 should preferably be concentrated at one end of Poly1, and a longer Poly3 domain contributes to better dispersion.   Figure 4. Relationships between the viscosity (mPa s−1) and (A) Dp of Poly3 (n) and (B) the compo-sition ratio of 1 in the Poly4 domain of Poly1 (mol%) at a shear rate of 100 s−1. 3.3. Discussion It is worth considering why the monomer position and distribution of Poly1 affected adhesion and dispersion in different ways. In the foot proteins secreted by mussels, the sequence of L-Dopa is known to be important for the adhesion ability [38]. However, de-spite the numerous reports of adhesive polymers inspired by mussel proteins, the rela-tionship between the positions of the adhesive moieties, such as L-Dopa, and the adhesive Figure 3. (A) Photographs of BaTiO3 dispersed in a chloroform solution in the presence of Poly1a–1e.(B) Relationships between the shear rate and the viscosity in dispersions comprising the copolymers,BaTiO3, and 1,1,2,2-tetrachloroethane.Materials 2023, 16, x FOR PEER REVIEW 8 of 12   ated using a rheometer. First, the BaTiO3 nanoparticles were dispersed in 1,1,2,2-tetrachlo-roethane (5.7 mol·L−1) at a shear rate of 1000 s−1, and the viscosity of the resulting disper-sion was monitored at shear rates of 10–100 s−1 (Figure 3B). The viscosity of the dispersion decreased exponentially from 589 to 165 Pa·s−1 as the shear rate increased from 10 to 100 s−1. However, a stable dispersion of BaTiO3 nanoparticles with a significantly lower vis-cosity was produced following the addition of ~2.0 × 10−4 mmol of Poly1a–1e. Moreover, it was obvious that the degree by which the viscosity decreased depended on the mono-mer sequence of the Poly1 copolymer used.  Figure 3. (A) Photographs of BaTiO3 dispersed in a chloroform solution in the presence of Poly1a–1e. (B) Relationships between the shear rate and the viscosity in dispersions comprising the copol-ymers, BaTiO3, and 1,1,2,2-tetrachloroethane. To further clarify the effect of the monomer position of Poly1 on the dispersibility of BaTiO3 at a shear rate of 100 s−1, we plotted the viscosity versus Dp of the domain of Poly3 (n) and the viscosity versus the composition ratio (mol%) of 1 in the domain of Poly4 in Poly1 (Figure 4). To our surprise, the viscosity behavior was completely different from that of the adhesive: the viscosity decreased linearly with Dp, whereas it decreased expo-nentially with the composition ratio of 1 in the Poly4 domain of Poly1. These results sug-gest that the adhesive moiety 1 should preferably be concentrated at one end of Poly1, and a longer Poly3 domain contributes to better dispersion.   Figure 4. Relationships between the viscosity (mPa s−1) and (A) Dp of Poly3 (n) and (B) the compo-sition ratio of 1 in the Poly4 domain of Poly1 (mol%) at a shear rate of 100 s−1. 3.3. Discussion It is worth considering why the monomer position and distribution of Poly1 affected adhesion and dispersion in different ways. In the foot proteins secreted by mussels, the sequence of L-Dopa is known to be important for the adhesion ability [38]. However, de-spite the numerous reports of adhesive polymers inspired by mussel proteins, the rela-tionship between the positions of the adhesive moieties, such as L-Dopa, and the adhesive Figure 4. Relationships between the viscosity (mPa s−1) and (A) Dp of Poly3 (n) and (B) the composi-tion ratio of 1 in the Poly4 domain of Poly1 (mol%) at a shear rate of 100 s−1.3.3. DiscussionIt is worth considering why the monomer position and distribution of Poly1 affectedadhesion and dispersion in different ways. In the foot proteins secreted by mussels, thesequence of L-Dopa is known to be important for the adhesion ability [38]. However, despitethe numerous reports of adhesive polymers inspired by mussel proteins, the relationshipbetween the positions of the adhesive moieties, such as L-Dopa, and the adhesive propertieshas rarely been explored. We used the Poly2 series to identify an appropriate polymerdesign that maximizes the adhesive strength by systematically varying the alkyl-chainlength of the methyl alkylate and the composition ratio of the L-Dopa monomer [14]. Themaximum adhesive strength of Poly2 was achieved when the composition ratio of theL-Dopa monomer was approximately 8 mol% in EHMA as methyl alkylate. In addition,we found that the ductility of Poly2 was maximized in this combination, resulting in highadhesive strength. Given the chemical structure of Poly1, the Poly4 domain can be regardedas an analog of Poly2. Therefore, it is reasonable that the adhesive strength was maximizedwhen the composition ratio of 1 in Poly4 was approximately 9 mol%, probably becauseof an appropriate combination of adhesive strength and ductility of the Poly4 domain(Figure 2B). In addition, when 1 is evenly located in Poly1, the adhesive monomer 1 notonly contributes to adhesion to the adherend, but it also acts as a non-covalent cross-linkageamong the gallic acid moieties (1), which may also contribute to the cohesion force. Indeed,the Poly3 domain without adhesive moiety 1 was found to have a negligible effect on theadhesive ability up to Dp of approximately 100 (Figure 2A). However, when the length ofthe Poly3 domain exceeds Dp = 100, adhesive monomer 1 is unevenly distributed at theend of poly1, making it difficult to cross-link the polymer chains by non-covalent bondsbetween adhesive monomers. Consequently, the cohesive force does not act on the entireadhesive layer, which is thought to lower the adhesive strength (Figure 5B).Materials 2023, 16, 266 9 of 12Materials 2023, 16, x FOR PEER REVIEW 10 of 12    Figure 5. (A) Abbreviated structures of the copolymers. (B) Model structure showing copolymers bonding to aluminum plates. (C) Model structure showing copolymers adsorbing on a BaTiO3 na-noparticle. 4. Conclusions In the present study, we investigated how the monomer position and distribution of the adhesive component of a polymer affect its adhesive strength and dispersion capabil-ity to develop an adhesive material that mimics the behavior of a mussel’s foot. We syn-thesized various FGCPs comprising EHMA by carefully controlling the position and dis-tribution of the gallic-acid-based adhesive component. A copolymer consisting of an EHMA homopolymer as the first domain and a random copolymer of EHMA/1 as the second domain showed the highest adhesion strength. A block copolymer consisting of Poly3 and Poly4 showed the greatest dispersion ability with regard to BaTiO3 nanoparti-cles. The results of the present research demonstrate the importance of the position and distribution of the monomers in the development of functional polymer materials. Be-cause the adhesion and dispersion properties of Poly1 are comparable with those obtained in previous studies, we are considering potential applications of Poly1 as an adhesive and a dispersant. Furthermore, because the search for optimal sequences proposed in this pa-per is not limited to biomimetic materials, such as L-Dopa, FGCPs could facilitate the de-velopment of various functional polymer materials. Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Synthesis procedures of Poly1–3, preparation procedure of a test specimen for the adhesion test, Scheme S1:General synthesis method of a forced gradient copolymer; Figure S1: Preparation of a test specimen; Figure S2: (A) Photograph of LUMiFrac® (LUM GmbH, Ger-many). (B) Photograph of the measuring chamber; Figure S3: Photograph of the fracture surfaces of adhesion tests of (a) Poly1a, (b)Poly1b, (c)Poly1c, (d)Poly1d, (e)Poly1e and (f)Poly3; Figure S4: GPC profiles of (a) poly1b, (b) poly1c, (c) poly1d, and (d) poly1e; Figure S5: 1H NMR spectrum of 1’ in Figure 5. (A) Abbreviated structures of the copolymers. (B) Model structure showing copolymers bond-ing to aluminum plates. (C) Model structure showing copolymers adsorbing on a BaTiO3 nanoparticle.When Poly1 was used as a dispersant for inorganic oxide nanoparticles, the adhesivePoly4 segment as an anchor moiety should strongly adsorb to the nanoparticle surface,while the soluble polymer moiety should be positioned to maximize the entropy. Con-sidering that the viscosity of the dispersion of BaTiO3 nanoparticles linearly decreasedas a function of Dp of Poly3, in the dispersion mechanism of the Poly1 series, adhesivemoiety 1 in the Poly4 segment adsorbed to BaTiO3, and the remaining Poly3 at one end isthought to determine the dispersibility (Figure 4A). Figure 5C suggests a more detaileddispersion mechanism. The conformation of the adsorbed polymer is a major controllingfactor in determining the stability and dispersibility of inorganic (nano)particles. In general,adsorbed polymers have three possible segments: (a) an interfacial segment (train), (b) asegment attached to the train at both ends (loop), and (c) a segment attached to the trainat one end (tail) [32]. Effective stabilization requires high coverage, effective anchoring,elongated tails (and possibly loops), and a favorable solvent environment for the segmentsto tail and/or loop. Our polymer design of FGCP Poly1 allows systematic variation ofthe lengths of the trains, loops, and tails, and the adsorption conformation and thicknessof the adsorption layer can be controlled. Thus, adhesive moiety 1 acts as the train parts,and the length of the loop parts can be manipulated by the composition ratio of adhesivemoiety 1 in Poly4. The length of the tail parts can be controlled by Dp of Poly3. That is,when the composition ratio of adhesive moiety 1 in the Poly4 segment is low, Poly1 has adominant loop conformation. Conversely, when the composition ratio of adhesive moiety1 in the Poly4 segment is high, one end adsorbs to adhesive moiety 1, and the remainingportion behaves as a tail in a diblock polymer conformation. As a result, the viscositydecreases exponentially as the composition ratio of adhesive moiety 1 increases, in otherwords, as the Poly1 copolymer becomes more diblock in nature, as shown in Figure 4B. Thisresult suggests that diblock polymers with 1 at one end are most suitable for the molecularMaterials 2023, 16, 266 10 of 12design of BaTiO3 dispersants. Furthermore, the present method is also an effective way toefficiently search for the most suitable molecular design of a dispersant consisting of ananchor moiety and a soluble polymer moiety.4. ConclusionsIn the present study, we investigated how the monomer position and distribution ofthe adhesive component of a polymer affect its adhesive strength and dispersion capa-bility to develop an adhesive material that mimics the behavior of a mussel’s foot. Wesynthesized various FGCPs comprising EHMA by carefully controlling the position anddistribution of the gallic-acid-based adhesive component. A copolymer consisting of anEHMA homopolymer as the first domain and a random copolymer of EHMA/1 as thesecond domain showed the highest adhesion strength. A block copolymer consisting ofPoly3 and Poly4 showed the greatest dispersion ability with regard to BaTiO3 nanoparti-cles. The results of the present research demonstrate the importance of the position anddistribution of the monomers in the development of functional polymer materials. Becausethe adhesion and dispersion properties of Poly1 are comparable with those obtained inprevious studies, we are considering potential applications of Poly1 as an adhesive anda dispersant. Furthermore, because the search for optimal sequences proposed in thispaper is not limited to biomimetic materials, such as L-Dopa, FGCPs could facilitate thedevelopment of various functional polymer materials.Supplementary Materials: The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ma16010266/s1, Synthesis procedures of Poly1–3, preparation procedureof a test specimen for the adhesion test, Scheme S1:General synthesis method of a forced gradientcopolymer; Figure S1: Preparation of a test specimen; Figure S2: (A) Photograph of LUMiFrac®(LUMGmbH, Germany). (B) Photograph of the measuring chamber; Figure S3: Photograph of the fracturesurfaces of adhesion tests of (a) Poly1a, (b)Poly1b, (c)Poly1c, (d)Poly1d, (e)Poly1e and (f)Poly3; Figure S4:GPC profiles of (a) poly1b, (b) poly1c, (c) poly1d, and (d) poly1e; Figure S5: 1H NMR spectrum of 1’ inCDCl3; Figure S6: 13C NMR spectrum of 1’ in CDCl3; Figure S7: 1H NMR spectrum of poly1a in acetone;Figure S8: 1H NMR spectrum of poly1b in acetone; Figure S9: 1H NMR spectrum of poly1c in acetone;Figure S10: 1H NMR spectrum of poly1d in acetone; Figure S11: 1H NMR spectrum of poly1e in acetone;Figure S12: 1H NMR spectrum of poly3 in CDCl3.Author Contributions: Conceptualization, T.F. and M.N.; data curation; T.F., M.S., M.M., S.M., A.N.,and A.Y.; investigation, T.F., M.S., M.M., S.M., A.N., and A.Y.; resources, S.M., A.N., and A.Y.; writing—original draft preparation, T.F.; writing—review and editing T.F. and M.N.; supervision, M.N.; projectadministration, M.N.; funding acquisition, M.N. All authors have read and agreed to the publishedversion of the manuscript.Funding: This work was supported by the Core Research for Evolutional Science and Technology(CREST) program “Revolution material development by fusion of strong experiments with theory/datascience” of the Japan Science and Technology Agency (JST), Japan (grant number JPMJCR19J3).Institutional Review Board Statement: Not applicable.Informed Consent Statement: Not applicable.Data Availability Statement: Not applicable.Acknowledgments: We thank Naoko Akimoto, Mariko Saito, and Kayoko Yamaguchi from theNational Institute for Materials Science (NIMS) for synthesizing the monomers and copolymers.Conflicts of Interest: The authors declare no conflict of interest.References1. Lazarus, B.S.; Velasco-Hogan, A.; Río, T.G.-D.; Meyers, M.A.; Jasiuk, I. A review of impact resistant biological and bioinspiredmaterials and structures. J. Mater. Res. Technol. 2020, 9, 15705–15738. [CrossRef]2. Wang, Y.; Naleway, S.E.; Wang, B. 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MDPI and/or the editor(s) disclaim responsibility for any injury topeople or property resulting from any ideas, methods, instructions or products referred to in the content.http://doi.org/10.1016/j.cis.2009.07.004http://www.ncbi.nlm.nih.gov/pubmed/19691945http://doi.org/10.1016/j.jcis.2004.02.026http://www.ncbi.nlm.nih.gov/pubmed/15120275http://doi.org/10.1007/BF02692706http://doi.org/10.1016/S0301-7516(03)00088-7http://doi.org/10.1007/BF02720518http://doi.org/10.1021/nl902212qhttp://doi.org/10.1126/sciadv.abb7620 Introduction  Materials and Methods  General  Materials  Preparation of the Mussel-Inspired Forced Gradient Copolymers  Adhesion Test  Dispersion Test  Dispersion Test Evaluated by the Naked Eye  Dispersion Test Evaluated Using a Rheometer  Results  Effect of the Monomer Position on the Adhesiveness  Effect of the Monomer Position on the Dispersity  Discussion  Conclusions  References