Dependency of Nanodiamond Particle Size and Outermost-Surface

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Dependency of Nanodiamond Particle Size and Outermost-Surface Composition on Organo-Modification – Evaluation by Formation of Organized Molecular Films and Nano-Hybridization with Organic Polymers – Taira Tasaki, Yifei Guo, Qi Meng, Muhammad Abdullah Al Mamun, Yusuke Kasahara, Shuichi Akasaka, and Atsuhiro Fujimori ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b02001 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 16, 2017

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ACS Applied Materials & Interfaces

Nanodiamond

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OH2 nanolayer 3~5 nm Organic compounds

Nanodispersion

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Single particle layer 60 50 40 30 20 10 0 100 80

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Dispersion 5 mm Neat polymer

Graphical abstract

Transparent!!

5 mm Hybrid film

T. Tasaki, et al.

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Dependency of Nanodiamond Particle Size and Outermost-Surface Composition on Organo-Modification – Evaluation by Formation of Organized Molecular Films and Nano-Hybridization with Organic Polymers –

Taira Tasaki,a Yifei Guo,a Qi Meng,a Muhammad Abdullah Al Mamun,a Yusuke Kasahara,a Shuichi Akasaka,b Atsuhiro Fujimoria,*

a

Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku,

Saitama 338-8570, Japan

b

Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama 2-12-1,

Meguro-ku, Tokyo 152-8550, Japan

*Corresponding Author Tel. and Fax: +81 48 858 3503. E-mail address: [email protected]

Keywords: Nanodiamond; organo-modification; surface treatment; single-particle layer; polymer/nanodiamond nanocomposite


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ABSTRACT The formation behavior of organized organo-modified-nanodiamond films and polymer nanocomposites has been investigated using nanodiamonds of several different particle sizes and outermost-surface compositions. The nanodiamond particle sizes used in this study were 3 and 5 nm and the outermost surface contained –OH and/or –COOH groups. The nanodiamond was organo-modified to prepare –OH2+ cation and –COO− anions on the outermost surface by carboxylic anion of fatty acid and long-chain phosphonium cation, respectively. The surface of nanodiamond is known to be covered with a nanolayer of adsorbed water, which was exploited here for the organo-modification of nanodiamond with long-chain fatty acids via adsorption, leading to nano-dispersions of nanodiamond in general organic solvents as a mimic of solvency. Particle multilayers were then formed via the Langmuir-Blodgett technique and subjected to fine structural analysis. The organo-modification enabled integration and multilayer formation of inorganic nanoparticles due to enhancement of the van der Waals interactions between the chains. Therefore, "encounters" between the organo-modifying chain and the inorganic particles led to solubilization of the inorganic particles and enhanced interactions between the particles; this can be regarded as imparting a new functionality to the organic molecules. Nanocomposites with a transparent crystalline polymer were fabricated by nano-dispersing the nanodiamond into the polymer matrix, which was achievable due to the organo-modification. The resulting transparent nanocomposites displayed enhanced degrees of crystallization and improved crystallization temperatures, compared with the neat polymer, due to a nucleation effect.


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INTRODUCTION Nanodiamond1 is a nanocarbon material along with fullerenes2, graphene3, and carbon nanotubes4; especially, nanodiamond has biocompatible, has antimicrobial properties, and a high refractive index.

In addition, nanodiamond is relatively inexpensive compared to other

nanocarbon materials and exhibits high Mohs hardness (10) and excellent thermal conductivity. It is also possible to impart electrical conductivity5 and fluorescence emission6 properties to nanodiamond using specific treatments. Noteworthy, electrochemistry of several diamond derivatives using planar macroscopic films has been widely investigated. 7 Since the heterogeneous doping in diamond materials, boundary effects, and the varied ratios of graphite to diamond in these case, such systems are able to provide electrochemical signals over the whole electrode. Furthermore, nanodiamond is already used commercially as an abrasive and dispersant. Although nanometer-scale diamond particles were first produced by Russian Researchers using the detonation method in the 1960s,8 they remained essentially unknown to the wider world until the end of the 1980s.9 A number of important developments in the late 1990s brought wider interest to these particles, which are now known as "nanodiamonds".10-13 Nanodiamond, of single digit nanometer size, manufactured by the detonation method is stabilized by covering the outermost surface with an adsorbed water layer. 14 Attempts have been made to utilize other nanocarbon materials as fillers in polymer composites/hybrids;15, 16 3 ACS Paragon Plus Environment

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however, aggregation forces arising from strong van der Waals interactions have hindered these attempts. Nanodiamond, which is a spherical nanocarbon material, was expected to achieve uniform dispersion in a polymer matrix. Polymer/nanocarbon hybrid materials17, 18 have now captured the attention of scientists and engineers due to their remarkably high dimensional stability and gas-barrier performance, in addition to their superior mechanical properties relative to conventional composite materials. 19, 20 The high thermal conductivity,21, 22

high refractive index,23,

24

antibacterial activity,25,

26

increased conductivity, 27,

28

and

non-photobleaching fluorescence29, 30 of nanodiamonds has spurred tireless research efforts on using these materials in products such as heat spreaders,31, 32 photonic crystals,33, 34 medical applications,35 electronics and sensors,36, 37 and biosensors.38, 39 However, the influence of the water-nanolayer adsorbed on the outermost surface prevents dispersion of nanodiamond fillers in hydrophobic polymers;40 aggregates tend to form due to affinity interactions between the

adsorbed

water

layers.

Researchers

have

previously attempted

to produce

single/multi-particle layers of inorganic materials at the air/water interface and nanocomposites with hydrophobic polymers by chemical surface modification of nanoclays,41 zirconia,42 zinc oxide nanodiscs,43 and magnetic nanoparticles44 using long-chain compounds (ammonium cations or carboxylates). Recent research has revealed that the aforementioned method is also applicable to the outermost adsorbed water layer on nanodiamond.45, 46 Oleophilication of the surface imparts 4 ACS Paragon Plus Environment


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amphiphilicity to the nanoparticles and improves their affinity with hydrophobic polymers. In addition, using explosives as the carbon source in nanodiamond preparation can introduce surface functional groups on the resultant nanodiamond that derive from species of the explosive materials. Surface termination with specific functional groups has also been performed on carbon nanotubes47 and graphene48 etc. Selective functionalization is achieved by introducing organic species through chemical reaction with the terminated functional group as a target. The adsorbed water on nanodiamond positively charges as oxonium cations and, thus, interaction with anions is expected. In addition, anions derived from N-containing species can be introduced to the nanodiamond surface and cationic modification has been achieved using trinitrotoluene (TNT) or 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) explosives. Furthermore, even for nanodiamond with a detonation primary-particle size of about 5 nm, the particle size can be controlled by scraping the surface on the gas phase treatment. It is interesting to elucidate the effects of increasing the nanodiamond-particle surface area and contact area with the polymer matrix in the composite. In this study, we investigated the effects of nanodiamond particle size and outermost-surface composition on organo-modification for the formation of organized molecular films and nano-hybridization with organic

polymers.

Organo-modified

nanodiamond was first synthesized by modifying 3 and 5 nm nanodiamond particles with long-chain cations and anion as shown in Fig. 1(a). We then prepared high-density low-defect 5 ACS Paragon Plus Environment

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single-particle layers and multi-particle layers, via organized layering, at the air/water interface, and elucidation of interaction supporting. Nanodiamond was also nano-dispersed in a transparent polymer matrix in an attempt to prepare a transparent polymer/filler nanocomposite. Furthermore, we estimated the enhancement of the resultant nanocomposite physical properties. We expect that the materials prepared in this study will be useful as ultra-thin water-repellent films imparted specific surface properties to nanodiamond, antimicrobial transparent film introduced the original characteristics of filler into polymer, and 3D wearable glass having both transparency and projection ability (Fig. 1 (b)).

EXPERIMENTAL SECTION Synthesis and characterization of organo-modified nanodiamond. Figure 2 presents a schematic illustration of the protocol for organo-nanodiamond synthesis. Three kinds of neat nanodiamond with cubic crystal systems were used in this study: 3 or 5 nm diameter particles (Fig. 2(a), ND3 and ND5, respectively, obtained from New Metals and Chemicals Co., Ltd. and Daicel Corporation, respectively) with only –OH groups present on the outermost surface, and 5 nm diameter particles with both –OH and –COOH groups present on the outermost surface (carboND5). As a result of following surface-modification, four types of organo-modified nanodiamonds is used in this study.

For the neat nanodiamond with a

diameter of 5 nm, the measured transmission electron microscope (TEM) is shown (Fig. 6 ACS Paragon Plus Environment


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S1(a)). It is an essential fact that the nanodiamond outermost layer covered with water nano-layers is terminated with hydroxyl groups. On the other hand, carboxyl-terminated nanodiamond was kindly achieved by a special treatment by a supplier (Daicel Corporation). In this case, the carboxyl surface groups were introduced by blowing carbon dioxide gas during post-processing in the detonation process and X-ray photoelectron spectroscopy was used to confirm the presence of carboxyl groups. Figures S1(b) and (c) show zeta potential measurements of nanodiamond having only –OH group on the surface, and it with both –OH and –COOH groups, respectively. It can be seen that nanodiamond having only –OH group on the surface is positively charged. On the other hand, it can be seen that the nanodiamond having both –OH and –COOH groups is negatively charged. We expect that the –OH and –COOH groups on the outermost surface will exist as charged –OH2+ and –COO− groups, respectively, in aqueous solutions. Dispersions were prepared by combining an aqueous solution of 3 or 5 nm-diameter nanodiamond with a methanolic solution of stearic acid or tributyl hexadecyl phosphonium bromide (Figs. 2(b) and (c), a detailed nanodiamond organo-modification procedure is present in refs. 45 and 46). Toluene was then poured into the nanodiamond dispersion with stirring (Fig. 2(d)) and the organo-nanodiamond particles migrated from the methanolic dispersion into the toluene phase. Water, methanol, and the remaining unreacted reagents were then removed by rotary evaporation under reduced pressure and decantation. 7 ACS Paragon Plus Environment

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This process modifies the surface-absorbed hydronium ions (R–H2O+) in the water nanolayer on the particle surface to long-chain alkyl carboxylates, as confirmed by infrared (IR) spectroscopy analysis (see Fig. S2), and also modifies the surface-absorbed carboxylate (R–COO−) groups on the outermost particle surface to phosphonium groups. Therefore, as a result of above process, the four resultant organo-modified nanodiamonds are denoted as St-ND3, St-ND5, St-carboND5 (stearate-modified ND3, ND5, and

carbo

ND5), and P-carboND5

(phosphonium-modified carboND5). Figure S2 shows IR spectra of the organo-nanodiamonds, organo-modified agents, and neat nanodiamonds in the bulk phase. The IR spectra were acquired using a 2000 spectrometer system (Perkin-Elmer Co., Ltd.). As described above, it is well known that the nanodiamond surface is coated with a water layer that stabilizes its structure.13 Here, we propose that carboxylic anions (–COO−) and phosphonium cations (–P+) are adsorbed by the oxonium cations and carboxylic anions, respectively, on the nanodiamond surface. The IR spectrum of neat nanodiamond displayed an O–H stretching vibration band at 3480 cm−1, which is assigned to the hydroxyl groups of the water-layer adsorbed on the particle surface. C–H symmetric and antisymmetric stretching vibrations, assigned to the alkyl-chain of the fatty acids, were also clearly observed around 2910–2840 cm–1. Both of the aforementioned bands were present in the organo-nanodiamond IR spectrum. Thus, we concluded that the organo-modification of the nanodiamond particle surface was successful. Schematic 8 ACS Paragon Plus Environment


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illustrations of the bonding state at the organo-nanodiamond surface are shown in Figs. 2(e) and (f). The matrix polymer used in the nanohybridization experiment was poly[vinylidene fluoride-co-(tetrafluoroethylene)] (P(VDF-TeFE)); its chemical and three-dimensional structures are shown in Fig. 2(g).

Estimation of structure in bulk. Figure 3(a) shows thermogravimetric (TG) curves of the bulk organo-nanodiamonds. TG measurements were performed using an Al container in a nitrogen atmosphere at a heating rate of 10 °C min –1 with an EXSTAR TG/DTA6200 (Seiko Instruments) instrument. The neat-nanodiamond TG measurement shows that a subtle reduction of up to about 10 % is a change derived from the raw material (Fig. 3 (b)). The TG analysis of the organo-nanodiamonds shows that a loss of about 15~60 % of the modified organo-chain occurred with increasing temperature. The organo-modification ratio was calculated for each nanodiamond, based on the nanodiamond surface areas (estimated from the particle sizes), and are summarized in Table 1 along with the thermal degradation temperatures. The thermal degradation (organo-chain desorption) temperature of the long-chain carboxylic-anion-modified nanodiamonds ranged from 140–155 °C. Compared with the 5 nm-nanodiamond, the surface-modification rate of the 3 nm-nanodiamond was relatively low. Scraping the surface by gas phase treatment appears to decrease the number of –OH terminal groups, which act as the adsorption sites. The modification rate for the carboND5 9 ACS Paragon Plus Environment

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with –COOH groups, introduced by CO2 blowing treatment, decreased to almost half the value. Conversely, the desorption temperature with temperature increasing of the nanodiamond modified with phosphonium cations was remarkably improved. The excellent heat resistance of phosphorus-based modifiers has also been reported in other papers.49 This is considered to be related to the difference in acid-dissociation constant between carboxylic acids and phosphonium. In addition, an innovative improvement in surface modification ratio was also confirmed. From these results, we conclude that the cation-modification is highly efficient compared with the anion modification. The packing mode of neat nanodiamond was examined using powder X-ray diffraction (XRD, Rigaku, Rint-Ultima III; Cu Kα radiation, 40 kV, 40 mA, equipped with a graphite monochromator). The powder diffraction data indicate that the nanodiamond particles crystallize in a cubic diamond lattice (ac = 3.571 Å, Fig. 2(a)).

Monolayer formation at the air/water interface and observation of molecular arrangement in films. It was possible to achieve uniform size dispersion of the prepared organo-nanodiamonds on a nanometer scale; this process can essentially be considered as dissolving the organo-nanodiamonds in an organic solvent. Therefore, it was also possible to prepare a spreading solvent for formation of inorganic-nanoparticles monolayers on the water surface. A toluene solution containing organo-nanodiamond (ca. 1.0  10−4 M) was spread on 10 ACS Paragon Plus Environment


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ultra-pure water (18.2 MΩ·cm) to form an organo-nanodiamond monolayer on the water surface. After evaporating the toluene for 30 min, surface pressure-area (π-A) isotherms were recorded at a compression speed of 0.08 mm·s−1. The temperature of the air/water interface was maintained at 15 °C by circulating thermostatically controlled water around the trough. A USI-3-22 Teflon-coated LB trough (USI Instruments) was used to measure the monolayer properties and perform Langmuir-Blodgett (LB) film transfer. The monolayers were transferred onto mica (atomic force microscopy (AFM) samples) or glass substrates (XRDs samples) using the LB method at 15 °C with a ferric stearate monolayer used as a hydrophobic underlayer.

Surface morphology and particle arrangement in organized films. The surface morphology of the transferred monomolecular films was observed using a scanning probe microscope (Dynamic Force Mode, Seiko Instruments, SPA300 with a SPI-3800 probe station) with microfabricated rectangular Si cantilevers and integrated rectangular tips; a spring constant of 1.4 N·m–1 was applied during this process. The long spacing between the layers of the films transferred onto the glass substrates was measured using an out-of-plane X-ray diffractometer (Rigaku, Rint-Ultima III; CuKα radiation, 40 kV, 40 mA) equipped with a graphite monochromator. The in-plane spacing of the two-dimensional lattice of the films was determined using an X-ray diffractometer setup with different geometrical 11 ACS Paragon Plus Environment

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arrangements50, 51 (Bruker AXS, MXP-BX; Cu Kα radiation, 40 kV, 40 mA, an instrument especially made to order) and equipped with a parabolic graded multilayer mirror. The X-rays were incident at an angle of 0.2° and the films were slow-scanned at a speed of 0.05°/80 s.

Nanohybrid

formation

with

organic

polymer

matrix.

A

P(VDF-TeFE)/organo-modified-nanodiamond mixture was extruded at 145 °C using a twin-screw

extruder

(Labo

kneader

mill,

Toshin

Co.,

Ltd.).

The

organo-modified-nanodiamond content in the nanohybrid material was 0.2 or 1.0 wt%. The P(VDF-TeFE) and its nanohybrid were molded into 500 µm films between two polyimide sheets (Kapton1 HN, Toray-DuPont Co., Ltd.) using a hot press at 145 °C and 20 MPa for 10 min, followed by quenching to room temperature. The melt-quenched film specimens (width: 20 mm, length: 30 mm) cut from these films were drawn using hand-drawing apparatus in an air oven at 110 °C in order to form the transparent films.46 The film-specimen surface was marked at intervals of 1 mm in order to measure the draw ratio. The drawing speed was fixed at 10 mm/min and the film was annealed at 110 °C for 5 min before drawing. The nanohybrid structure was estimated from high-performance wide-angle X-ray diffraction measurements (WAXD, Rigaku R-axis Rapid, 40 kV, 200 mA, Rotating anode CuKα radiation, imaging plate detector). The thermal properties were estimated from differential scanning calorimeter measurements (DSC, SII EXSTAR/DSC6200).

RESULTS AND DISCUSSION

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The influence of the various components in organo-modified inorganic fine particles on their interfacial monolayer behavior is extremely interesting. Fig. 4 shows the π-A isotherms of single layers of the different organo-nanodiamond particles on a water surface. The horizontal axis shows the "compression percentage", where indicates the "mean area per hydrophobic repeating unit" in the case of nanodiamond. In this case, the concept of molecular weight cannot be applied because the total number of atoms in a nanodiamond particle unit cannot be determined exactly. Therefore, we expect that "compression percentage", which is the compression-ratio mean versus the LB-trough surface area, will give the most accurate comparison of weight. The π-A curves of the St-ND5 and St-carboND5 monolayers display a two-dimensional phase transition from the expanded phase to the condensed phase. This phase transition is an effect derived from the organic component, while the clear crystalline phase in the high-pressure region is believed to be an effect of the inorganic component. The π-A curve of the St-ND3 monolayer on the water surface shows only a single phase, which may correspond to the solid-condensed phase due to domination of the inorganic content. Conversely, the P-carboND5-monolayer condensability is inferior and it tends to expand; the behavior of this monolayer appears to be similar to that of many organic amphiphiles. In addition, the collapsed-surface-pressure value is relatively low (Table 2). Contrary to St-ND3, P-carboND5 monomolecular film will be strongly affected by its organic components. Organo-modified 13 ACS Paragon Plus Environment

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nanodiamonds that are similar in this manner will behave significantly differently on the water surface depending on their surface-modified chains. The single-particle layers were transferred from the water surface to mica substrates at various temperatures using the upstroke-LB method. The AFM measurements (Fig. 5) indicate that the height direction is homogeneous and consistent with single particle size for the St-ND5 and St-ND3 single-particle layers, though the tendency towards particle integration of St-ND5 monolayer in the in-plane direction is not dense. The size of the visible particles in the St-ND3 monolayer is almost 30 nm; thus, it appears that these are first-order coagulates of the 3 nm particles. Also, we inferred from the cross-section data that these films formed a quite-highly-ordered layer arrangement along their height direction. However, because the height is about three times the particle diameter for the St-carboND5 and P-carboND5 monolayers, it appears that there are sites where the particles aggregate and pile up along the height direction. The out-of-plane X-ray diffraction measurements of the LB multilayers indicate that the films display a clear long-spacing value at around 5 nm. This value corresponds to the particle layering period as shown the profiles in Fig. 6. The (001) crystallite sizes (Table 3) were calculated from the Scherrer equation and the St-ND5 multilayers displayed the highest value. However, XRD reflection peaks based on the layering order is not confirmed from the P-carboND5 multilayers. 14 ACS Paragon Plus Environment


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At this stage, the trade-off relationship derived from the modified chain species of the organo-nanodiamond is clarified. P-carboND5 displays the highest organic-chain-desorption temperature and its surface-modification rate is also remarkably high. Consequently, the P-carboND5 monolayer on the water surface displayed the most organic behavior of the different organo-nanodiamonds and there is no layered regularity in its multilayers. St-ND3 has a low modification rate and the behavior of its monolayer on the water surface is less affected by the organic components and consists only of a solid condensed phase with no phase transition displayed; the surface morphology of its monolayer indicates that St-ND3 forms the most densely-integrated particle layer of the different organo-nanodiamond particles. St-carboND5 has a low anion-modification rate because carboxyl groups are present on its outermost surface. The St-carboND5 monolayer displays a two-dimensional phase transition on the water surface similar to the St-ND5 particles of the same diameter, though the proportion of the organic chain is half. The St-ND5 multilayers displayed the highest layered regularity of the different organo-nanodiamonds; the other characteristics of St-ND5 are also excellent on average. Figs. 7(a)-(d) shows the in-plane X-ray diffraction profiles of the organo-nanodiamond multi-particle layers that were transferred at different temperatures. The region where the diffraction peak was observed represents the in-plane spacing of the alkyl chain (sub-cell52). Clear signals are observed in the multilayers of all of the organo-nanodiamonds, which are 15 ACS Paragon Plus Environment

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assigned to two-dimensional hexagonal crystals or orthorhombic crystals. This data indicates that the modified chain forms a two-dimensional crystal between the integrated particle layers; that is to say, the organo-nanodiamond layer structure appears to be supported by enhanced van der Waals interactions that works on the formation of two-dimensional crystal between the organic chains. A single peak corresponding to the isotropic hexagonal (100) plane was apparent at 4.1 Å for the multi-particle layers fabricated at a subphase temperature of 5 °C. Additionally, the appearance of double peaks for the multi-particle layers fabricated at subphase temperatures of 15 and 25 °C indicates that the modified alkyl chains form a slightly anisotropic orthorhombic system. In view of the stearic-acid-surface-modification ratio of the particles, the possibility that the long alkyl chain is densely packed on the particle surface is extremely low. It is also possible that the long-period value remains unchanged from 5 nm because the long hydrocarbon chains between the particles form an interdigitated structure. Accordingly, the alkyl-chain packing is expected to generate a two-dimensional alkyl-chain crystal array between the individual particles. In other words, two-dimensional organo-nanodiamond integration and formation of a layered structure occurs, indicating that crystal formation arises from interactions between the modified alkyl chains. This inferred model is represented in Fig. 7(e), (f). We think that the nano-particle integrated organization in this system originates from sub-cell42 formation by the hydrocarbons between the particles. 16 ACS Paragon Plus Environment


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Figure 8 shows detailed 3D crystalline arrangement of NDs in LB multilayers estimated by para-crystal analysis53 and calculation of Scherrer equation. This figure clarified the difference in 3D regularity in the four systems.

In the para-crystal analysis, the regularity of

the multi-particle layered materials as a pseudo-crystalline property can be quantified by calculating the distortion of the crystalline order. As a result, it was found that the NDs multi-particle layers having a diameter of 3 nm exhibits an extremely high crystallinity order, and the 3D regularity of the two kind of

carbo

ND5 multilayers is extremely low.

Referring to

the previous literatures,43, 53, 54, the value of g = 0.51 indicated by St-ND5 is typically an average value. Many colloidal crystals have a crystalline order of 30 to 50 %, and it finds that the 3D regularity St-ND3 remarkable high. All of the particle layers in this study are supported by two-dimensional crystals in which modified-chains are formed, but the order of the St-carboND5 layered organization is 0.2 % crystalline, and P-carboND5 cannot calculate because of great disordering. On the other hand, the growth of crystallites having the largest diameter which can be calculated as the crystallite spread in the direction perpendicular to the (00l) plane, is indicated to the layered organization of St-ND5. This is followed by the values of St-ND3, and St-carboND5 in this order. Estimating the 3D order from a plurality of analyzing means in such a multi-viewing manner clearly shows the structure as a multi-particle layer. It is thought that the miniaturization of the particle diameter has an effect of reducing the distortion of the layered organization, but it can be concluded that the regularity of single 17 ACS Paragon Plus Environment

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layer formed in the same plane is sufficiently high even with nanodiamond of 5 nm particle diameter. By the way, in comparison between octadecyl and hexadecyl chains of modifier, the chain length dependency of fatty acid in formation of monolayer was not so noticeable. From these results, it was found that the change in fatty acid chain length has little effect on the film thickness within this length range. However, when trial of dioctadecyl cationic modifier (quaternary ammonium cation) was performed, the surface modification rate tended to increase, and contributing to improvement of stability and dispersibility in the following estimation. While this is a preliminary consideration, there is a possibility of causing a breakthrough for future research development. Figure

9(a)

shows

pictures

P(VDF-TeFE)/organo-modified

of

the

nanodiamond

surface (St-ND5)

of

a

hybrid

press-molded that

was

film

of

made

by

melt-compounding. The sample was cooled to 110 °C after being melted at 150 °C, and the free surface was then observed. This photo indicates that relatively-hard nanoparticles exist in the polymer matrix. Furthermore, the transparency of the ‘crystalline’ partially fluorinated polymer nanohybrid has been retained, which indicates that the organo-modified nanodiamonds are uniformly dispersed throughout the fluorinated polymer matrix formed by melt-compounding with P(VDF-TeFE).



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Figure 9(b) shows WAXD profiles of neat P(VDF-TeFE) and the P(VDF-TeFE) composite with organo-modified nanodiamond. The positions of the clear (110), (200) convoluted peak, (020) reflection, and (111), (201) convoluted reflection are invariant, which indicates that the crystal system is not changed by the nanocomposite. However, the ratio of the amorphous-curve intensity around 2θ = 15–20° to the crystalline peak intensity changes. Table 4 shows the crystalline sizes of nanocomposites with 0.2 and 1.0 wt% of filler added. Adding St-ND5 improves the crystallinity of the nanocomposite remarkably, with the improvement dependent on the amount of St-ND5 added. Conversely, P-carboND5 has minimal effect on the nanocomposite crystallinity. Adding small (0.2 wt%) amounts of St-ND3 and St-carboND5 increased the nanocomposite crystallinity; however, increasing the amount of St-ND3/St-carboND5 added causes the crystallinity to decrease. The D110and200 crystallite size calculated from the Scherrer equation corresponds to the diameter of the crystal along the ab-plane of this polymer forming switch-board type lamellae. Therefore, as shown in Fig. 8(c), introducing organo-modified nanodiamond increases the crystallite size in the ab-plane. Figure 10(a) shows the DSC crystallization peak of the nanocomposites. The crystallization temperature of these nanocomposites is of importance, here. Table 5 shows the dependence of the nanocomposite crystallization temperature on the amount of organo-nanodiamond added. Generally, an increase in crystallization temperature corresponds to an increase in the crystalline-polymer lamellae thickness. The nanocomposite containing 19 ACS Paragon Plus Environment

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0.2 wt% of St-carboND5 displayed the highest crystallization temperature. Nanocomposites containing 1.0 wt% of St-ND5 and StND3 also displayed increased crystallization temperatures, compared to the neat polymer, though adding 0.2 wt% P-carboND5 markedly lowered the nanocomposite crystallization temperature. As shown in Fig. 10(b), raising the crystallization temperature corresponds to an increase in the thickness of the lamellar crystal portion in the c-axis direction. The improvement in the lamellar thickness is expected to be due to the addition of a nucleation agent;45 that is to say, the organo-modified nanodiamond acts as crystal nucleation agent for P(VDF-TeFE). When the polymer chain solidifies from the molten state, its terminal-end adsorbs to the modified -chain end to form a thicker lamella and crystallize. Since the P(VDF-TeFE) fluorinated chain is not fluorine-terminated, it easily interacts with the hydrocarbon chain. In addition, since the stearic-acid a-axis length55 is close to the polymer b-axis length, we expect both to contribute to lamellae formation by epitaxial growth (Fig. 11). Conversely, long-chain phosphonium can inhibit the P(VDF-TeFE) crystallization, be formed thin lamellae, and reduce its crystallinity. Long-chain-phosphonium modification can be used to improve the elastic modulus without increasing the material hardness and, thus, prepare nanocomposites with a high amorphous tendency. The expedient phase diagram in Fig. 12 summarizes the findings of this study. The role of the surface-modifying organic chains has been analyzed by preparing two-dimensional 20 ACS Paragon Plus Environment


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integrated

organo-modified-nanodiamond

layers

and

Page 22 of 53

fabricating

a

transparent

crystalline-polymer/filler composite. When an amphipathic long-chain organic compound encounters inorganic particles at the contact interface, new functionality can be imparted to the inorganic particles. The presence of an organic chain was effective for promoting layered organization, though this modifier did not induce the formation of a layered structure in modified fine particles; however, it has a high modification rate, excellent heat resistance, and is able to form polymer nanocomposites that have a high melting point. Conversely, anion-modified nanodiamond with long-chain carboxylic acids formed a lamellar structure, which

improved

the

crystallinity

and

crystallization

temperature.

The

various

organo-modified nanodiamonds prepared with different modifier molecules were well dispersed in the polymer matrix and maintained the transparency of the crystalline polymer. As mentioned above, it believes that suppression of phase-separation with polymers of organo-ND is an affinity between the modified-chain and the terminal group of polymer chain, that is, miscibility. That is to say, it is not an exaggeration to say that this paper describes the effect of the modified-chain. At the time of forming a two-dimensional film, its affinity is added to the modified-chain. Also, at the time of nanocomposite formation with polymer, the modifying chain gives its miscibility and suppresses phase separation.

CONCLUSIONS 21 ACS Paragon Plus Environment

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The formation behavior of organized molecular films and

organic-polymer

nanocomposites was investigated using organo-modified nanodiamonds of different particle size and the outermost-surface composition. "Encounters" between the organo-modified chain and the inorganic particles led to solubilization of the nanodiamond and enhanced interactions between the particles; this can be regarded as imparting new functionality to the organic molecules of the modified agents. The nanodiamonds used in this study had diameters of 3 or 5 nm and contained –OH and/or –COOH groups on their outermost surface. Nanodiamond surface-modification was performed to produce –OH2+ cations or –COO– anions on the outermost surface by carboxylic anion of fatty acid and long-chain phosphonium cation, respectively. Multi-particle layers of four different organo-nanodiamonds were then formed via the Langmuir-Blodgett technique and subjected to fine structural analysis. The effects of organo-modification enabled nanodiamond integration and multilayer formation due to enhancement of the van der Waals interactions between the chains. Nanohybridyzation with a transparent crystalline polymer by nano-dispersing the nanodiamonds into the polymer matrix was also achieved due to the organo-modification. The resultant transparent nanocomposites generally displayed enhanced crystallinity and an increased crystallization temperature due to a nucleation effect.

ACKNOWLEDGMENTS 22 ACS Paragon Plus Environment


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The authors greatly appreciate the Ministry of Education, Culture, Sports, Science and Technology (MEXT) for providing a Grant-in-Aid for Scientific Research (C, 17K05986 (A.F.)). Further, authors would like to thank Mr. Koichi Umemoto, Dr. Daisuke Shiro, Mr. Atsushi Kume, and Mr. Hisayoshi Ito of DAICEL Corporation for providing nanodiamond samples. Finally, A.F. offer my heartfelt condolences to my mentor Professor Hiroo Nakahara, Saitama University, who died on December 12, 2016.


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Table and Figure Captions Table 1. Thermal-decomposition temperatures of organo-modified nanodiamonds and their organo-modification ratio calculated by TG analysis. Table 2. Collapsed surface pressures of organo-modified nanodiamond calculated from π−A isotherms. Table 3. Crystalline sizes of organo-modified nanodiamond calculated from the Scherrer equation based on out-of-plane XRD profiles. Table 4. Crystalline sizes of nanohybrids calculated from powder XRD profiles. Table 5. Crystallization temperatures of nanohybrids calculated from DSC thermograms.

Figure 1. (a) Summary of the research strategy used in this study. (b) Applications of particle layers and nanohybrid materials using organo-modified nanodiamond. Figure 2. (a) Schematic illustration of the structure of nanodiamond covered with a nanolayer of

adsorbed

water.

Chemical

structure

of

(b)

stearic

acid and

(c)

tributylhexadecylphosphonium cation used to modify the nanodiamond surface. (d) Schematic of the method used to form organo-modified nanodiamond. Schematic of the surface bonds of nanodiamond modified by (e) fatty-acid carboxylic anions and (f) long-chain phosphonium cations. (g) Chemical structure of the P(VDF-TeFE) polymer matrix used in this study. 32 ACS Paragon Plus Environment


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Figure 3. TG curves of (a) organo-modified nanodiamonds and (b) neat nanodiamond. Figure 4. π−A isotherms of organo-modified-nanodiamond monolayers. Figure 5. Atomic force micrographs of Z-type monolayers of (a) St-ND5, (b) St-ND3, (c) St-carboND5 and (d) P-carboND5 on a solid substrate. Figure 6. Out-of-plane XRD profiles of Langmuir-Blodgett multilayers of (a) St-ND5, (b) St-ND3, (c) St-carboND5 and (d) P-carboND5 particles. Figure 7. In-plane XRD profiles of Langmuir-Blodgett multilayers of (a) St-ND5, (b) St-ND3, (c) St-carboND5 and (d) P-carboND5 particles. Schematic models of (e) orthorhombic and (f) hexagonal packed two-dimensional lattices (sub-cells) formed by long hydrocarbons on the surface of organo-modified-nanodiamond LB films. Figure 8. Schematic models of multilayers of several nanodiamond estimated by Scherrer equation and paracrystal analysis: (a) St-ND5, (b) St-ND3, (c) St-carboND5 and (d) P-carboND5 particles. Figure 9. (a) Photographs of transparent polymer and its nanohybrid film (0.2 wt%). (b) WAXD profiles of neat polymer and the different nanodiamond-hybrid films. (c) Schematic illustration of changes in the crystalline size of neat P(VDF-TeFE) and its nanodiamond-hybrid film. Figure 10. (a) DSC thermograms of neat P(VDF-TeFE) and hybrid films (second cooling). (b) Schematic illustration of lamellar thickness model. 33 ACS Paragon Plus Environment

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Figure 11. Schematic illustration of nucleation-effect mechanism. Figure 12. Correlation diagram showing the effects of adding organo-modified nanodiamond to the matrix polymer.

ASSOCIATED CONTENT

Supporting Information. Brief statement in non-sentence format listing the contents of the material supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org/



Nanodiamond

Organic solvent

+

OH2 nanolayer 3~5 nm Organic compounds

Nanodispersion

Single particle layer


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 53

60 50 40 30 20 10 0 100 80

[μm]

0 60

40

20

1 0


Nanocomposite


Dispersion 5 mm Neat polymer

Graphical abstract

Transparent!!

5 mm Hybrid film

T. Tasaki, et al. 35 ACS Paragon Plus Environment

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Table 1

T. Tasaki, et al.

St-ND5 St-ND3 St-carboND5 P- carboND5

Thermal degradation

Organo-modification

temperature (˚C) 155 150 143 270

ratio (%) 48 37 24 76



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 2

Page 38 of 53

T. Tasaki, et al.

Collapsed surface pressure (mN / m) 54 49 50 39

St-ND5 St-ND3 St- carboND5 P- carboND5


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Table 3

T. Tasaki, et al.

D001 (Å) 236.14 182.41 166.79 -




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 4

Page 40 of 53

T. Tasaki, et al.


0 wt% 110 Å 110 Å 110 Å 110 Å

0.2 wt% 124 Å 119.8 Å 115.5 Å 112.9 Å


1.0 wt% 129 Å 109.5 Å 106.3 Å 113.1 Å

Page 41 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 5

T. Tasaki, et al.


0 wt% 112.10 ˚C 112.10 ˚C 112.10 ˚C 112.10 ˚C

0.2 wt% 112.48 ˚C 112.85 ˚C 113.70 ˚C 109.72 ˚C


1.0 wt% 113.04 ˚C 113.04 ˚C 111.95 ˚C 109.62 ˚C


(a)

Dispersion Transparent!!

Nanodiamond OH2+ nanolayer


Nano-dispersion Organic solvent

Endo. ←ΔH→ Exo.

Hybrid film 3~5 nm Organic compounds

5 mm 2nd cooling Neat Polymer

100

60 50 40 30 20 10 [μm] 0 0 100 80 60 40 20 0 Compression percentage / %

(b)

110

Temperature / ˚C

120

Multi-particle layered organization Interdigitated structure d001 Intensity / a. u.

Single particle layer Surface Pressure / mNm-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 53

1

d001 = 48.0 Å

1

2 3 2 / degree

4

Sub-cell structure

Organo-modification of surface of nanodiamond

Dispersion

Single particle layer

Nanohybrid film Antibacterial

High refractive index

Nanodiamond

Water repellent finishing

Medical packaging material

Figure 1

3D wearable device T. Tasaki, et al.


Page 43 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b) OH

OH2+ nanolayer

O

ND (c)

CH2CH2CH3 P+

3~5 nm

CH2CH2CH3 CH2CH2CH3

Cubic system

(d) Toluene Methanol Water

Toluene

Vacuum distillation

Stirring

Water Methanol Nanodiamond Organo-modified agent

O

(e) H ⊝ O O H

C

(f) R1

Br+

O

O

C

H ⊝ C R1 O O H

R2 ⊝ P O R1 R2 R2 O C

carbo

ND5

ND

R2

⊝ O P R1 R2 R2 OH2+ nanolayer

(g)

n

C F H

[ (CF2– CH2 )l – CF2 – CF2 ]m Figure 2



(a) 0

Weight loss / %

-10 -20 -30

St-ND5 St-ND3 St-carboND5 P-carboND5

-40 -50 -60

50 100

200

300

Temperature / ˚C

(b)

400

×5

0

0

-10

-2

-20

-4

Weight loss / %

Weight loss / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 53

-30 -40 -50 -60

20 100

200

300

400

Temperature / ˚C

-6 -8 -10 -12 -14

20 100

500

Figure 3

200

300

400

Temperature / ˚C

500


Page 45 of 53

60 St-ND5 St-ND3 St-carboND5 P-carboND5

50


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 30 20 10 0 100

80

60

40

20

0


Figure 4



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

Page 46 of 53

(b) Single-particle layer

0

[μm]

1

[μm]

14.67 [nm] 0 1 0

(c)

0

4.7 [nm] 0

0 (d)

[μm]

2.6 [nm] 0 1

[μm]

14.83 [nm] 0 1

Figure 5


Page 47 of 53

d001 ≈ 50 Å d001 d001

Multi-particle layered organization (a)

Intensity / a. u.

Intensity / a. u.

(b)

d001 = 46.0 Å

1

2

3

4

2 / degree

5

6

2

3 4 2 / degree

5

6

Intensity / a. u.

(d)

d001 = 55.9 Å

1

d001 = 48.0 Å

1

(c)

Intensity / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

3

4

2 / degree

5

6

1

2

Figure 6

3

4

2 / degree

5

6



(a)

Intensity / a. u.

d = 4.2 Å

15

Intensity / a. u.

(b)

15

(e)

d1

d = 3.7 Å unit cell

20

25

34.6 ˚ 55.4 ˚ 9.0 Å 5.1 Å

30

2 / degree

d = 4.2 Å d3

d2

Orthorhombic a ≠ b ≠ c, α = β = γ = 90 ˚ d1 = 3.7 Å, d2 = d3 = 4.2 Å

d = 3.7 Å

20

25

30

2 / degree

Intensity / a. u.

(c)

15

(d)

(f)

d = 4.1 Å

20 25 2 / degree

30

4.7 Å 60 ˚

d = 4.1 Å

15

d1

unit cell

9.5 Å 120 ˚

d2

Intensity / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 53

d3 Hexagonal

a = b ≠ c, α = β = 90 ˚, γ = 120 ˚ d1 = d2 = d3 = 4.1 Å

20

25

30

2 / degree

Figure 7


Page 49 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3D Para-crystal Analysis g ≡ Δd / d g = 0 (No distortion) g = 1 (Distorted crystal)

g: Crystal distortion d: Distance between the lattice Δd: Quantity of change of the lattice

(a) St-ND5

(b) St-ND3 Crystallite size D001 = 236 Å

Crystal distortion g = 0.51 (49% crystallinity)

g = 0.27 (73% crystallinity)

d = 46 [Å] c

D001 = 182 Å

d = 48 [Å]

d b

d a

(c) St-carboND5

(d) P-carboND5

D001 = 167 Å

Extremely distortion

g = 0.98 (2% crystallinity)

d = 55.9 [Å]

Interdigitated structure

Sub-cell structure of hydrocarbon-chain

Relative strong van der Waals interaction between long-alkyl chains along c-axis Figure 8



(a)

5 mm Nanocomposite film

5 mm Neat polymer (b)

(110 , 200) P-carboND composite 5 (111 , 201) (020) St-carboND5 composite

Intensity / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 53

St-ND3 composite

St-ND5 composite

Neat polymer 10

20

30

2 / degree

40

50

(c) D110 and 200

D110 and 200

c-axis

c-axis

b-axis

b-axis a-axis

a-axis Organo-modified ND

Switch-board type lamellae Figure 9


Page 51 of 53

(a) Cooling process

Endo.←ΔH→ Exo.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

P-

St-ND5 (1 wt%) Neat polymer

St-carboND5 (0.2 wt%)

carbo

ND5

(0.2 wt%) St-ND3 (1 wt%) 110 Temperature / ˚C

120

(b) Addition and Nanohybridization

Stearic acid modified ND

Formation of thicker lamellae along the c-axis

Figure 10



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 52 of 53

P(VDF-TeFE) polymer chain

Organo-ND monoclinic (Stearic acid) [a ＝ 5.59 Å, b = 7.40 Å, c = 49.38 Å, β = 117°]

a ＝ 9.13 Å c = 2.51 Å

b = 5.27 Å

orthorhombic [a ＝ 9.13 Å, b = 5.27 Å, c = 2.51 Å]

Figure 11


Page 53 of 53

O

Crystallization temperature of nanohybrid (˚C)

C17H35

C

115

−

H O+ O

・・

H

3 nm

Crystallite size of multiparticle layer (Å)

C4H9 C4H9 C4H9 + C H P − 16 33 O C O 5 nm

110

105

230

250

210

190

170 150 100 20 100 0

40

60

80

100

110

Organo-modification ratio (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


・ O

C17H35

C

120

O −

−

O+ O

H

5 nm

C17H35

C H

H

O+ O

H

Crystallite size of nanohybrid (Å)

Figure 12

−

COO

130

5 nm


Dependency of Nanodiamond Particle Size and Outermost-Surface

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