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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 44079−44087

Strategy for the Improvement of the Mechanical Properties of Cellulose Nanofiber-Reinforced High-Density Polyethylene Nanocomposites Using Diblock Copolymer Dispersants Keita Sakakibara,*,† Yoshihito Moriki,† Hiroyuki Yano,‡ and Yoshinobu Tsujii*,† †

Institute for Chemical Research, Kyoto University, Gokasho, Uji Kyoto 611-0011, Japan Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji Kyoto 611-0011, Japan

ACS Appl. Mater. Interfaces 2017.9:44079-44087. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 08/13/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Cellulose nanofibers (CNFs) hold great potential as sustainable reinforcement fillers with excellent mechanical, thermal, and chemical properties. However, in polyolefin nanocomposite materials, the rational control of dispersion and the improvement of interfacial strength remain challenging. Herein we propose the tuning of the interface between CNF and high-density polyethylene by the design of polymer dispersants on the basis of surface free energy and the glass transition temperature. The former is related to the wettability against the polymer matrix and is therefore critical to the dispersion of CNF whereas the latter is related to the interfacial strength between CNF and HDPE. As a result of this investigation, we discovered a suitable dispersant for CNFs, poly(dicyclopentenyloxyethyl methacrylate)-block-poly(2-hydroxyethyl methacrylate), which played a pivotal role in achieving both a uniform dispersion of CNF and greatly improved mechanical properties, including a 4-fold increase of the Young’s modulus over that of neat HDPE with 10 wt % CNF loading. KEYWORDS: polymer dispersant, cellulose nanofiber, nanocomposite, diblock copolymer, interfacial strength, dispersion

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INTRODUCTION Wood-derived cellulose nanofibers (CNFs) have been of great interest since their discovery,1−3 because of their unique physical properties: a high Young’s modulus of around 140 GPa,4−7 a high mechanical strength of around 2 GPa,8 optical transparency,9 and low thermal expansion.10 Much research has been focused on CNF as a reinforcement filler in polymer nanocomposite materials,11−17 which can be a promising alternative to conventional filler-reinforced composites using glass fiber, carbon black, and talc from the viewpoints of low cost, low density, biodegradability, high stiffness, and safety. Maximum reinforcement requires uniform dispersion of CNF in the polymer matrix; however, intrinsically hydrophilic CNF is incompatible with hydrophobic polymers. To resolve this type of incompatibility, surface modification of CNF is required to reduce fiber−fiber van der Waals and hydrogen bonding attraction while optimizing wettability against the polymer matrix.18−20 One promising strategy is surface modification by polymeric surfactants which adsorb onto CNF. For example, Volk and co-workers have utilized poly(ethylene oxide)-blockpolyethylene (PEO-b-PE) as a compatibilizer for the preparation of microfibrillated cellulose (MFC)-reinforced linear low-density polyethylene (LLDPE) through meltextrusion.21 Their nanocomposites showed almost no visible aggregates, and the tensile properties were improved. © 2017 American Chemical Society

Nagalakshmaiah and co-workers have demonstrated a triblock copolymer-type dispersant (PEO-b-PPO-b-PEO) for cellulose nanocrystal (CNC) toward melt extrusion with LLDPE.22 Their nanocomposites exhibited improved dispersibility as well as suppression of thermal degradation. In our previous work (Figure 1a), dispersion of CNF in hydrophobic high-density polyethylene (HDPE) was achieved using the adsorption of a diblock copolymer-type dispersant, poly(lauryl methacrylate)block-poly(2-hydroxyethyl methacrylate) (PLMA-bPHEMA).23 PLMA and PHEMA were selected as resincompatible and cellulose-interactive blocks, termed the dispersion and immobilization blocks, respectively. PLMA has its long-chain alkyl group, analogous to the chemical structure of PE. In fact, the surface free energy (γ) of the PLMA model layer was comparable to that of HDPE. Few CNF aggregates were visible in X-ray CT images of the nanocomposites, indicating that CNF was well-dispersed at the subhundred nanometer level. While this reduction of the interfacial free energy between CNF surface and HDPE is crucial to preparing defect-free CNF dispersions, the improvement in the Young’s modulus of the CNF nanocomposite was limited (Eactual = 1.5 Received: September 14, 2017 Accepted: November 29, 2017 Published: November 29, 2017 44079

DOI: 10.1021/acsami.7b13963 ACS Appl. Mater. Interfaces 2017, 9, 44079−44087

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic illustration of CNF-reinforced HDPE nanocomposite materials using a diblock copolymer dispersant. (b) Interplay between the CNF and HDPE interface on the wetting (related to γ) and a local rheological property (related to Tg) controlled by designed polymer dispersant layers. (c) Chemical structures of the polymer dispersants used in this study.

Table 1. Molecular Property of Polymer Dispersants first block a

3

total −1

dispersant

monomer

Mn (×10 g mol )

Mw/ Mn

PMMA-b-PHEMA PBMA-b-PHEMA PLMA-b-PHEMA PCHMA-b-PHEMA PBCHMA-b-PHEMA PDCPMA-b-PHEMA

MMA BMA LMA CHMA BCHMA DCPMA

2.6 5.4 7.0 5.3 4.4 5.0

1.2 1.4 1.2 1.4 1.2 1.4

a

Mna

3

−1

(×10 g mol ) 5.4 7.9 8.4 7.9 7.7 7.3

Mw/ Mna

DPn, first: DPn, secondb

Tgc (°C)

1.6 1.5 1.3 1.4 1.5 1.4

26:10 38:19 28:13 26:14 24:18 19:19

105 20 −65 83 76 28

a Number-average molecular weight (Mn) and polydispersity index (Mw/Mn) obtained from GPC (THF as an eluent). bComposition ratio determined by 1H NMR. cGlass transition temperature of the dispersion blocks shown in refs 39 and 40.

GPa). Such less reinforcement effect has been frequently observed in the previously reported nanocomposites.21−23 We attribute this limited reinforcement to the weakness of the CNF/HDPE interfacial strength because pure PLMA, the resincompatible block, is a viscous and sticky liquid at ambient temperature, which may contribute to a highly mobile adsorbed polymer layer. Accordingly, the central goal of this study is to examine the chemical structures of polymer dispersants in detail for their role in achieving effective interfacial reinforcement between CNF and HDPE. It is well-recognized that mechanical behavior of polymer nanocomposites is controlled by matrix−filler interphases between the adsorbed or grafted polymer layers on the fillers or nanofillers and matrix chains.24−30 Thus, the design of the adsorbed/grafted polymer is critical for dispersion and interfacial strength in nanocomposites. Some researchers have taken this approach, especially in the case of the polymer grafted-nanofillers mixed with free chains of the same polymer.31−34 However, polyolefins including PE and polypropylene suffer from a lack of suitable graft polymers due to synthetic difficulties in the preparation of polyolefin blocks or graft copolymers.35−37 Our research group is interested in polymethacrylate-type dispersants which can be synthesized via living radical polymerization at industrial scales. Hence, we design suitable polymethacrylates for dispersion blocks. As described above, low interfacial free energy between CNF and HDPE is needed for the proper dispersion of CNF. In addition,

we focus on the control of the viscoelastic properties at the interfaces, which are strongly affected by the molecular motion of the graft layer and therefore strongly related to the glass transition temperature (Tg) of the dispersion blocks. Consequently, our primary design principle for the realization of high interfacial reinforcement between CNF and HDPE is that dispersion blocks should have a Tg above ambient temperature (Figure 1b). Herein we report a systematic study using designer diblock copolymer-type dispersants with different hydrophobic (dispersion) blocks to reinforce the CNF-HDPE interface. Six different dispersants were used to tune the interface (Figure 1c), categorized by γ and Tg. We postulate that the former is related to the wettability against the polymer matrix and thus critical to the dispersion of CNF in HDPE whereas the latter must be related to the interfacial strength between CNF and HDPE. Each of these dispersants contained PHEMA as the immobilization block because of the previous findings which verified its efficacy as an anchor for the CNF surface.23 The relationship between the chemical structure of the dispersants and the mechanical properties of the CNF/HDPE nanocomposites will be discussed in detail in this work. Among these results, a suitable dispersant was discovered which played a significant role in both the proper dispersion of CNF and the significant improvement of the mechanical properties, with a 4fold increase of the Young’s modulus with 10 wt % CNF loading over that of neat HDPE. 44080


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filtered to yield the polymer adsorbed CNF slurry. Then, the CNF slurry was mixed with HDPE. Finally, some amount of the dispersant was supplied and dried under vacuum to yield a premix of HDPE/ CNF/dispersant (80/10/10, w/w/w) as a white powder. The premix obtained was kneaded by a twin-screw extruder (KZW 15TW-30/ 45MG-NH(−2200), Technovel Corp., Osaka, Japan). The temperature of the barrels was set between 120 °C and 140 °C, the screw speed at 200 rpm, and the compound output at approximately 50 g h−1. The detailed extruding conditions were described previously.20 Standard test pieces were prepared from the obtained extruded compounds using a manual injection molding machine (IMC-18D1, Imoto Machinery Co., Ltd., Kyoto, Japan) operating at an injection temperature of 165 °C. A mold die was used at rt. The specimens were dumbbell-shaped with dimensions of 74 mm (length) × 5 mm (width, narrow) × 2 mm (thickness). In some cases, an injection molding machine (NPX7-1F, Nissei-Plastic Industrial Co., Ltd., Nagano, Japan) was used at an injection temperature of 160 °C, a pressure of 100 MPa, a speed of 80 mm s−1, and a mold die temperature of 40 °C, which prepared test pieces with a thickness of 1 mm and a length of 80 mm.20 Characterization of Nanocomposites. The tensile strength and modulus of the samples were measured using universal mechanical testing equipment (model 3365; Instron Corp., MA, U.S.A.). The load cell and crosshead speed were 1 kN and 10 mm min−1, respectively. The tensile deformation was monitored by a CCD camera. The average values of the Young’s modulus, the tensile strength, and the elongation at break were calculated with the standard deviation taken as the error. Before mechanical testing, the samples were dried under vacuum at rt overnight. X-ray microcomputed tomography scanning was performed using skyscan 1172 (Bruker-microct, Belgium) in order to observe the three-dimensional morphology of the samples with the following scanning conditions: a current of 100 μA, voltage of 59 kV, pixel size of 0.79 μm, 360° of rotation, and a 0.100° step. The dumbbell-shaped sample was cut into a piece with dimension of 10 × 10 × 2 mm3. Polarized optical microscopy (POM) images were recorded on an Olympus BX60 polarized optical microscope. For POM, the dumbbell-shaped specimens were cut using a microtome (REM-710, Yamato Koki Co., Saitama, Japan) with a thickness of approximately 20 μm along the machine direction. The birefringence (Δn) of the samples was determined with a Berek compensator. The value Δn is defined as the difference, Δn = n∥ − n⊥, between the refractive index parallel (n∥) to the injection direction and that perpendicular (n⊥) to it. TEM images were acquired using a JEM-2100 (JEOL, Japan) microscopy (accelerating voltage, 200 kV) equipped with a CCD camera. Ultrathin sections were prepared using a Leica Ultracut R ultramicrotome with a diamond knife at rt. The block samples were stained with ruthenium tetraoxide (RuO4) prior to ultramicrotomy. Sections of the block samples of 100 nm in thickness were collected on a Cu grid and observed with TEM.

EXPERIMENTAL SECTION

Materials. CNF was disintegrated from needle-leaf bleached kraft pulp (NBKP; supplied by Oji Holdings Corp., Tokyo, Japan) by a bead mill apparatus, as described in detail in the previous work.23 The content of CNF was ca. 23.5%. Figure S1 of the Supporting Information (SI) shows scanning electron microscopic images of CNF used in this study. The width of CNFs varied from 20 to 200 nm. HDPE (HE-3040) was supplied by Sumitomo Seika Chemicals Co., Ltd. (Osaka, Japan), in the form of fine particles (diameter ≈ 11 μm) with a density of 0.961 kg cm−3. The melting temperature is approximately 130 °C. The polymer dispersants, poly(methyl methacrylate)-block-PHEMA (PMMA-b-PHEMA), poly(butyl methacrylate)-block-PHEMA (PBMA-b-PHEMA), PLMA-b-PHEMA,23 poly(cyclohexyl methacrylate)-block-PHEMA (PCHMA-b-PHEMA), poly(4-tert-butylcyclohexyl methacrylate)-block-PHEMA (PBCHMAb-PHEMA), and poly(dicyclopentenyloxyethyl methacrylate)-blockPHEMA (PDCPMA-b-PHEMA), were synthesized by the successive addition of hydrophobic monomers and then HEMA in reversible chain transfer catalyzed polymerization,38 supplied by Dainichiseika Color and Chemicals Mfg. Co., Ltd. (Tokyo, Japan). The molecular characteristics, including number-average molecular weight and polydispersity index, of the dispersants used in this study are presented in Table 1. The reported Tg values of the dispersion blocks are used.39,40 All other chemicals were obtained from commercial sources and used without further purification. Characterization of Polymer Dispersants. GPC analysis was carried out at 40 °C on a Shodex GPC-104-series high-speed liquid chromatography instrument (Shoko Co., Ltd., Kanagawa, Japan), equipped with a differential refractometer (Shodex RI-74S), a guard column (Shodex GPC KF-G), and two serial columns (Shodex GPC KF-404HQ). Calibration curves were obtained with PMMA standards. THF was used as the eluent at a flow rate of 0.3 mL min−1. 1H NMR spectra were recorded with a JEOL JNM-AL300 spectrometer in THFd8 with tetramethylsilane (Me4Si) as an internal standard. Preparation and Characterization of a Model Layer of Polymer Dispersants on a Silicon Wafer. A silicon substrate (Ferrotec Corp., Japan, one side chemically and mechanically polished, 525 μm in thickness) was cleaned by ultrasonic treatment in acetone, 2-propanol, and Milli-Q water for 15 min each and treated with UVozone cleaner for 10 min before use. The solution of polymer dispersants in THF (10 wt %) was casted on a silicon substrate and dried at 160 °C for 6 h under vacuum, and the solvent was leached twice in baths of fresh THF at rt via ultrasonic treatment; finally, the residual layer was dried under vacuum at rt. The adsorbed layer was characterized by ellipsometry (for thickness) and contact angle measurement (for surface free energy). Ellipsometric measurements were made on a compensator-rotating spectroscopic ellipsometer (M2000U, J. A. Woollam, Lincoln, NE) equipped with D2 and QTH lamps. The thickness of the adsorbed layer of polymer dispersants was calculated using the refractive index of PMMA in bulk. The contact angle was measured using a DropMaster 700 (Kyowa Interface Sci., Co., Ltd., Saitama, Japan). Distilled water (2 μL) or diiodomethane (CH2I2) (2 μL) was dropped onto the model surface, and a static contact angle was measured 1 s after dropping. Compounding and Injection Molding. CNF-reinforced HDPE nanocomposites were prepared by following a previously reported procedure (“method 3” in the reference).23 Here only main points are repeated. A polymer dispersant was first dissolved in N-methyl-2pyrrolidone (NMP) at a concentration of 20 wt %. Then, water was mixed up to the final weight ratio for NMP and water to be 1:1 with preparing emulsion by a magnetic stirrer and a homogenizer. Then, NMP (81.4 g: the same amount of water in wet CNF) was added into wet CNF (106.4 g; 25 g absolute dry mass), followed by the addition of the NMP-in-water emulsion stabilized with a polymer dispersant (12.5 g in 125 mL of NMP-water (1:1, w/w)). The slurry was mixed thoroughly and dehydrated under vacuum at 60 °C until reaching the constant weight, leading a selective solvent condition in which the percentage of the adsorption against CNF was evaluated by GPC.23 The NMP-wet CNF slurry obtained was washed with ethanol, and

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RESULTS AND DISCUSSION Interfacial Property of Polymer Dispersants. Two classes of polymethacrylates (Figure 1c) with linear and cyclic aliphatic side chains were selected as hydrophobic (dispersion) blocks. The polymethacrylates with linear aliphatic side chains are PMMA (Tg = 105 °C),39 PBMA (Tg = 20 °C),39 and PLMA (Tg = −65 °C),39 and those with cyclic aliphatic side chains are PCHMA (Tg = 83 °C),39 PBCHMA (Tg = 76 °C),39 and PDCPMA (Tg = 28 °C).40 Table 1 shows the characteristics of these copolymers. To understand the wetting properties of the interfacial layer, model films of the six polymer dispersants were prepared as follows: first, cast films of dispersants that had been prepared on silicone substrates were annealed at 160 °C for 6 h under vacuum. Then, solvent was leached twice in baths of fresh THF (a good solvent for each of the dispersants) at rt via ultrasonic treatment to remove nonadsorbed polymers from the substrates. Finally, the residual layer was dried under vacuum. 44081


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ACS Applied Materials & Interfaces Table 2. Thickness, Static Contact Angles, and Surface Free Energy of the Model Surface and HDPE

surface free energyc (mN m−1) −2

dispersant

d (nm)

σ (chains nm )

θwater (deg)

θCH2I2 (deg)

γd

γp

γ

PMMA-b-PHEMA PBMA-b-PHEMA PLMA-b-PHEMAd PCHMA-b-PHEMA PBCHMA-b-PHEMA PDCPMA-b-PHEMA HDPEd

6.2 6.0 5.3 6.3 4.2 7.6

0.6 0.4 0.3 0.5 0.3e 0.5e

73.6 88.9 96.5 89.6 96.7 91.0 89.9

37.4 41.7 45.6 36.8 39.9 52.7 49.4

36.7 37.3 36.5 40.2 39.9 31.1 33.0

6.6 1.4 0.3 1.0 0.1 1.9 1.8

43.3 38.7 36.8 41.2 40.0 33.0 34.8

a

b

a

Thickness of the adsorbed layer on the silicon substrate determined by ellipsometry. bGraft density. cDetermined with contact angles of water and CH2I2 using the following parameters: γwaterd = 21.8 mN m−1; γwaterp = 51.0 mN m−1; γwater = 72.8 mN m−1; γCH2I2d = 49.5 mN m−1; γCH2I2p = 1.3 mN m−1; γCH2I2 = 50.8 mN m−1. dData shown in ref 23. eDensity of PCHMA (1.07 g cm−3) was used for the calculation.

dispersant against CNF and the estimated graft density, evaluated through step I. Although these graft densities correlate to the dispersant structure to some extent, they are sufficiently high enough to make CNF hydrophobic. Note that each of the dispersants formed a stable physically adsorbed layer on the CNF surface, as demonstrated previously by the stability test.23 Thus, it was again confirmed that the PHEMA immobilization block acted as an anchor for CNF due to the strong attraction to the polar CNF surface. During step II, the excess dispersants were supplied to the premix due to partial desorption under shear flow during the kneading process (step III).23

The thickness and graft density of the obtained layer was around 5 nm and 0.3−0.6 chains nm−2, respectively (Table 2). These values are reasonable for the chosen model of the interfacial layer structure, as shown in Figure 1b. The contact angles and surface free energies (γ) for two different probe liquids (water and CH2I2) were measured on the dispersant-adsorbed silicon substrates (Table 2). Literature data for the γ-values of bulk polymers are also summarized in Table S1. As expected, the water contact angles of the model layers were relatively high (θwater = 73.6°−96.7°), indicating that the hydrophilic silicon substrate (θwater = 5.9°) became hydrophobic by adsorbing the dispersants. The γ-value was calculated on the basis of the theory developed by Owen and Wendt,41 where γ consisted of dispersion energy (γd) and polar energy (γp). Except for the PMMA layer, the values of γp for the dispersants were almost unity, indicating that the nonpolar hydrocarbon groups had accumulated on the surface. The γvalues decreased in the order of alkyl chain length (PMMA > PBMA > PLMA) and number of carbons (PCHMA > PBCHMA > PDCPMA). These results indicate that PLMA and PDCPMA-type dispersants are promising for the dispersion of CNF in nonpolar HDPE matrices. It should be noted that polyDCPMA with pendant bicyclo-alkenyl functionality displays unique characteristics: it has the same low surface energy as PLMA but with a comparatively higher Tg. Preparation of Nanocomposite Materials. We have previously described an optimized preparation process for CNF-reinforced HDPE nanocomposite materials using a PLMA-b-PHEMA dispersant.23 This process consists of three steps: (I) adsorption of a dispersant onto the CNF surface, (II) preparation of a premix of HDPE, CNF, and the dispersant in a weight ratio of 80:10:10, and (III) extruding and injection molding. This process was applied to each of the polymer dispersants. Table 3 shows the weight fraction (x) of adsorbed

Figure 2. Representative stress−strain curves of (a) neat HDPE, (b) HDPE/CNF (90/10, w/w), (c) HDPE/CNF/PMMA-b-PHEMA, (d) HDPE/CNF/PBMA-b-PHEMA, (e) HDPE/CNF/PLMA-b-PHEMA, (f) HDPE/CNF/PCHMA-b-PHEMA, (g) HDPE/CNF/PBCHMA-bPHEMA, and (h) HDPE/CNF/PDCPMA-b-PHEMA nanocomposites (80/10/10, w/w/w) prepared with a manual injection molding machine.

Table 3. Results of the Adsorption of Polymer Dispersants on the CNF Surface dispersant

xa

σb (chains nm−2)

PMMA-b-PHEMA PBMA-b-PHEMA PLMA-b-PHEMAc PCHMA-b-PHEMA PBCHMA-b-PHEMA PDCPMA-b-PHEMA

0.12 0.09 0.27 0.08 0.06 0.28

0.14 0.09 0.14 0.06 0.04 0.17

Mechanical Performance. Figure 2 and Table 4 show the characteristic tensile stress−strain curves and the resulting average tensile modulus, tensile strength, and elongation at break, respectively. The Young’s modulus and the tensile strength of the neat HDPE were 0.63 GPa and 19 MPa, respectively. The addition of CNF without any dispersants slightly increased these values to 1.14 GPa and 26 MPa, respectively. However, the dispersants enabled a higher Young’s modulus and tensile strength in all cases. Importantly, the mechanical properties were strongly dependent on the polymer

a

Weight fraction of the adsorbed polymer against CNF. bGraft density estimated by the equation in ref 23. 44082


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ACS Applied Materials & Interfaces Table 4. Tensile Properties of CNF-Reinforced HDPE Nanocompositesa dispersant PMMA-b-PHEMA PBMA-b-PHEMA PLMA-b-PHEMA PCHMA-b-PHEMA PBCHMA-b-PHEMA PDCPMA-b-PHEMA none a

Young’s modulus (GPa) 2.01 2.05 1.54 1.92 1.80 2.70 1.14

± ± ± ± ± ± ±

tensile strength (MPa)

0.07 0.07 0.04 0.11 0.05 0.12 0.03

30 37 35 31 32 39 26

± ± ± ± ± ± ±

0.26 0.39 0.88 0.95 0.28 2.2 0.18

elongation at break (%) 4.0 3.8 4.8 4.0 4.4 3.0 9.8

± ± ± ± ± ± ±

0.37 0.48 0.11 0.15 0.24 0.48 0.45

Weight percentage of CNF: 10%.

Figure 3. Reconstructed 3D X-ray CT images of injection-molded specimens of (a) neat HDPE, (b) HDPE/CNF, and HDPE/CNF with (c) PMMA-b-PHEMA, (d) PBMA-b-PHEMA, (e) PLMA-b-PHEMA, (f) PCHMA-b-PHEMA, (g) PBCHMA-b-PHEMA, and (h) PDCPMA-bPHEMA.

cross-section of approximately 600 × 600 × 150 μm3 (length × width × height). The blue areas represent the resin matrix while white ones indicate cellulose because of its higher electron density. At the 0.7 μm resolution used, this observation shows only CNF aggregation, and the dispersed CNFs are invisible. Fewer CNF aggregates appeared in the nanocomposites prepared using PLMA- and PDCPMA-type dispersants. This can be explained by their high compatibility with the HDPE matrix. On the other hand, the X-ray CT images for other nanocomposites consisted entirely of white portions, which were attributable to the aggregation of CNF due to the lesser wetting of these materials against HDPE. These aggregations created zones with accentuated fragility, resulting in lower mechanical strengths. To obtain further insights about the dispersion of CNF in an HDPE matrix, a TEM observation was conducted (Figure 4). The sample was stained using RuO4 to enhance the contrast; RuO4 stains mainly the amorphous region of PE by oxidation to yield a carboxylic acid group and a carboxylate (COORu) group.53 The PE crystals appear light whereas the amorphous region remains dark. As shown in Figure 4a, c, the discrete CNF diameter was at the submicron level, on the order of less than 0.6 μm. It was difficult to observe CNFs thinner than the sample sections (100 nm). The dark lines between CNF and HDPE was the well-stained boundary line, indicating that the dispersants existed between HDPE and CNF, as expected. Overall, from the TEM observation, the nanocomposites using PLMA- and PDCPMA-type dispersants make little difference regarding CNF dispersion. The anisotropy of the nanocomposites was evaluated using a polarized optical microscope (Figure S3). The birefringence values (Δn) of the specimens were low, on the order of 10−3, and almost equal for all dispersant systems, indicating that the present CNF-reinforced

dispersant used for the preparation of CNF nanocomposites. As expected, the PDCPMA-type dispersant, possessing both low γ and high Tg, yielded the highest Young’s modulus and tensile strength. This is related to the dispersion and interfacial strength as discussed below. Figure S2 compares relative Young’s modulus (EC/EM) and tensile strength (σC/σM), where the subscriptions, C and M, represent the values for the composite and matrix, respectively, for the CNF/HDPE/polymer dispersants nanocomposites and other types of cellulosic filler-reinforced polyethylenes reported in literatures.20,21,23,42−52 Some of the reported nanocomposites contain CNC,42−47 MFC,49−51 or CNF20,21,23,52 as a filler, being chemically modified on surface20,45,46,49,50 or surface activated by additives/compatibilizers,21,23,47,51,52 and prepared through melt-processing like screw extrusion20,23,46,47,51 or templating sol−gel approach.42 It is shown that the HDPE/ CNF/PDCPMA-b-PHEMA nanocomposite exhibited the superior Young’s modulus among others at 10% cellulosic filler content. This value was close to that of Sapkota and coworkers, where optimal CNC network in LDPE has been achieved through the templating sol−gel approach.42 In addition, the tensile strength value for the HDPE/CNF/ PDCPMA-b-PHEMA nanocomposite was also higher than others except that of Sapkota and co-workers.42 Because the templating approach is not industrially scalable at present, our nanocomposite materials, prepared through conventional extrusion and injection molding, are promising to be scalable to industrial production. Dispersion, Orientation, and Nucleation-Induced Crystallization. Figure 3 shows the reconstructed 3D image of the X-ray micro-CT observation of neat HDPE, nonmodified CNF/HDPE, and CNF-reinforced HDPE nanocomposites with each of six different dispersants over a sample volume 44083


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Figure 4. TEM images of a transverse section of the CNF-reinforced HDPE nanocomposites with (a, b) PLMA-b-PHEMA and (c, d) PDCPMA-b-PHEMA.

HDPE nanocomposites were relatively isotropic. This is because the pressure during manual injection molding was relatively low and significant shear orientation did not occur. Cellulose nanomaterials can serve as a nucleating agent for semicrystalline polymeric matrix. For example, de Menezes and co-workers have reported that ramie CNC, the surface being chemically modified or not, acted as a nucleating agent for LDPE in the nanocomposites, where the %crystallinity increased from 38 to around 50% at the filler content of 10%.46 In order to confirm such nucleation-induced change for the HDPE matrix in the present nanocomposites, differential scanning calorimetry (DSC) experiments were conducted (Figure S4 and Table S2). The DSC thermograms revealed roughly constant melting point between 129 °C and 130 °C upon the addition of dispersants-adsorbed CNFs, suggesting that the size of the crystallites was not affected by the filler.46 On the contrary, the %crystallinity slightly increased from 59% to around 60−66% for the nanocomposites. Polymer dispersants with cyclic aliphatic side chains had a tendency toward the nucleation effect, but the increment was limited. Thus, we consider that the primary cause of the reinforcement is not a change in the HDPE crystal morphology, but an interfacial strengthening by the polymer dispersants. Overall, the difference between PLMA- and PDCPMA-dispersants in mechanical properties could be mainly derived from the Tg effect, not from anisotropy or PE superstructures. Effect of the Polymer Adsorbed Layer. Figure 5 summarizes the effects of surface free energy (γ) (related to the dispersity) and Tg (related to the interfacial strength) on the Young’s modulus and tensile strength of the CNF/HDPE nanocomposite materials. We categorized the six dispersants into three groups: (I) high dispersity and high interfacial strength (PDCPMA-b-PHEMA); (II) high dispersity and low interfacial strength (PLMA-b-PHEMA); and (III) low dispersity and moderate interfacial strength (PMMA-, PBMA-, PCHMA-, and PBCHMA-b-PHEMA). Group I exhibits the most favorable Young’s modulus and tensile strength. Because the Young’s modulus in group III (less dispersion) is larger than that of group II (high dispersion), it is considered to be strongly affected by Tg. Since the Young’s modulus is related to the stiffness of the nanocomposites, rubbery-state dispersants such as PLMA-b-PHEMA are heavily inferior to the glassy-state

Figure 5. Summary of the three-dimensional library of polymer dispersants for (a) Young’s modulus and (b) tensile strength.

dispersants in group III. However, the tensile strength increases as the surface free energy (γ) decreases irrespective of the grouping. This is because the strength is related to the defects, so that the improved dispersion resulting from the low surface free energy yield the higher strength. This finding reemphasized that the PDCPMA-based dispersant led to a high Young’s modulus and tensile strength with both high dispersity and interfacial strength, successfully providing superior reinforcement efficiency. Comparison of Young’s Modulus with Predicted Model. The maximum Young’s modulus for the nanocomposites, 2.70 GPa for the PDCPMA-based nanocomposite, is compared to that of a predicted modulus (E) on the basis of the Halpin−Tsai composite theory for fiber reinforced polymer composites54 (eqs 1−3). E 3 ⎛ 1 + 2αηLVf ⎞ 5 ⎛ 1 + 2ηTVf ⎞ ⎟⎟ + ⎜⎜ ⎟⎟ = ⎜⎜ Em 8 ⎝ 1 − ηLVf ⎠ 8 ⎝ 1 − ηTVf ⎠

(1)

ηL =

E f / Em − 1 Ef /Em + 2α

(2)

ηT =

E f / Em − 1 E f / Em + 2

(3)

where Vf is the volume fraction of CNF (f, filler) (Vf = 0.063 assuming a CNF density of 1.6 g cm−3), Ef and Em are the 44084



Research Article

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CONCLUSIONS From our rational polymer dispersant designs, we have successfully achieved the improvement of the mechanical properties of CNF-reinforced HDPE nanocomposite materials using a novel dispersant, PDCPMA-b-PHEMA, possessing both low surface free energy and moderate Tg. The best Young’s modulus and tensile strength of the nanocomposites prepared via manual injection were as high as 2.7 GPa and 39 MPa with 10 wt % CNF loading, corresponding to 420 and 200% increases over those of neat HDPE, respectively. The highly dispersed CNF within the HDPE matrix using PDCPMA-type dispersants was revealed by X-ray CT. TEM observations revealed that a shish-kebab crystal morphology was formed under high shear conditions, which contributed to a slight increase in tensile strength of 44 MPa. Thus, PDCPMA was found to be an effective PE-compatible block which can be extended for use in other HDPE nanocomposite materials using different nanofillers. This approach can be easily adapted to large-scale industrial production.

elastic modulus of CELL-I crystal for the crystalline regions parallel to the chain axis (Ef = 138 GPa),6 and the Young’s modulus of HDPE obtained experimentally (Em = 0.63 GPa), respectively, and α is the aspect ratio of filler. Because α was too large to be evaluated, we assumed α was 1000. The predicted modulus was calculated as 3.61 GPa. The ratio of the actual Young’s modulus to the predicted moduli was about 0.75, higher than that of previously reported materials (ca. 0.43 for a CNF/HDPE nanocomposite using a PLMA-b-PHEMA dispersant).23 Hence, this study demonstrates that the optimized dispersant can maximize the mechanical properties of CNF as a reinforcement filler. Influence on Injection Molding. High-pressure injection molding afforded higher orientation of the nanocomposites using the PDCPMA-b-PHEMA dispersant. Figure 6a shows

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13963. Experimental procedure for FE-SEM and DSC; FE-SEM images of CNF; comparison of relative Young’s modulus and tensile strength of cellulosic (nano)filler reinforced polyethylene composites; polarized optical microscopic images of nanocomposites; DSC thermograms of nanocomposites; representative stress−strain curve of CNF-reinforced HDPE nanocomposite with PDCPMAb-PHEMA prepared with a high pressure injection molding machine; the data of surface free energies of polymers; and melting characteristics of nanocomposites (PDF)

Figure 6. CNF-reinforced HDPE nanocomposites with PDCPMA-bPHEMA prepared with a high pressure injection molding machine. (a) Polarized optical microscopic image, showing skin, subskin, and core regions. The birefringence values (Δn) of skin, subskin, and core regions were 1.7 × 10−2, 2.6 × 10−2, and 2.1 × 10−2, respectively (thickness: 20 μm). (b−d) TEM image of transverse section at skin (b), subskin (c), and core (d) regions. A schematic illustration of shish-kebabs is shown in c.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.S.). *E-mail: [email protected] (Y.T.). ORCID

Keita Sakakibara: 0000-0002-6013-695X

that the birefringence values (Δn), giving an indication of anisotropy, increased to be 1.7 × 10−2, 2.6 × 10−2, and 2.1 × 10−2 at the skin, subskin, and core regions, respectively. It is known that the subskin region is formed by elongation during the fountain flow of the polymer melt, leading to greater orientation in the direction of flow, whereas the core region shows reduced orientation because of the reduced shearing stress.55,56 The detailed crystal morphologies were observed by TEM (Figure 6b−d). The HDPE region reveals fibrillar crystals, which are known to consist of a central extended polyethylene chain crystal (shish) and folded chain lamellar crystals (kababs) (see particularly Figure 6c). As expected, the shish centers in the subskin region experienced more growth than those in the skin and core regions, which is related to birefringence. Such higher order structure has also been observed in the surface-modified CNF-reinforced HDPE nanocomposites.57 The shish-kebab crystal morphologies afforded an increased tensile strength, 44 MPa, for the nanocomposites created using PDCPMA-b-PHEMA as a dispersant (Figure S5).

Author Contributions

The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript. Funding

This work was partially supported by the New Energy and Industrial Technology Development Organization (NEDO)GSC program, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. T. Aoyagi and Mr. T. Imai (Dainichiseika Color & Chemicals Mfg. Co., Ltd., Japan) for supplying polymer dispersants, Dr. H. Sano (Mitsubishi Chemical Co., Japan) and Ms. Y. Kawano for TEM observation, Dr. H. Okumura for high pressure injection molding, and Ms. K. Konishi and Dr. H. Kagata for their contributions in the initial stage of this project. 44085


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