Learning from Natural Nacre: Constructing Layered Polymer

Subscriber access provided by GRIFFITH UNIVERSITY

Article

Learning from Natural Nacre: Constructing Layered Polymer Composites with High Thermal Conductivity Guiran Pan, Yimin Yao, Xiaoliang Zeng, Jiajia Sun, Jiantao Hu, Rong Sun, Jian-Bin Xu, and Ching-Ping Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10115 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 8, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

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

ACS Applied Materials & Interfaces

Learning from Natural Nacre: Constructing Layered Polymer Composites with High Thermal Conductivity Guiran Pan,

†, §

Yimin Yao,

Rong Sun, *, † Jian-Bin Xu, †

⊥

†, ‡

Xiaoliang Zeng,

Ching-Ping Wong †,

*, †

⊥,

Jiajia Sun,

†, ║

Jiantao Hu,

†, ║

#

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences,

Shenzhen 518055, China §

Department of Chemical Engineering, China University of Petroleum, Beijing

102249, China ‡

Shenzhen College of Advanced Technology, University of Chinese Academy of

Sciences, Shenzhen 518055, China ║

Department of Nano Science and Technology Institute, University of Science and

Technology of China, Suzhou 215123, China

⊥

Department of Electronics Engineering, The Chinese University of Hong Kong,

Hong Kong 999077, China #

School of Materials Science and Engineering, Georgia Institute of Technology,

Atlanta, Georgia 30332, United States 1 ACS Paragon Plus Environment


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 2 of 31

KEYWORDS: layered polymer composites; silver nanoparticles; Al2O3 platelets; thermal conductivity; natural nacre

ABSTRACT: Inspired by the microstructures of naturally layered and highly oriented materials, such as natural nacre, we report a thermally conductive polymer composite that consists of epoxy resin and Al2O3 platelets deposited with silver nanoparticles (AgNPs). Owing to their unique two-dimensional structure, Al2O3 platelets are stacked together via a hot-pressing technique, resulting in a brick-and mortar structure which is similar to the one of natural nacre. Moreover, the AgNPs deposited on the surfaces of the Al2O3 platelets act as bridges that link the adjacent Al2O3 platelets due to the reduced melting point of the AgNPs. As a result, the polymer composite with 50 wt% filler achieves a maximum thermal conductivity of 6.71 Wm−1K−1. In addition, the small addition of AgNPs (0.6 wt%) minimally affects the electrical insulation of the composites. Our bioinspired approach will find uses in the design and fabrication of thermally conductive materials for thermal management in modern electronics.

1. INTRODUCTION

The continuous miniaturization and increasing power density of modern devices make thermal management a crucial issue.1-3 Developing polymer composites with high thermal conductivity is believed to be an efficient way to address this issue. Among polymer composites, ceramic/polymer composite materials have attracted 2 ACS Paragon Plus Environment

Page 3 of 31

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


special attention in terms of their light weight, electrical insulation, excellent mechanical strength, and low cost. Unfortunately, most of the obtained polymer composites usually exhibit a poor thermal conductivity, lower than 5.0 Wm−1K−1.4,5 As demonstrated previously,6-8 the interfacial interaction and filler orientation are believed to be the two main factors that determine the thermal-conduction properties of polymer composites. Improving the interfacial interaction will reduce the interfacial thermal resistance and thus enhance the thermal conductivity of the corresponding polymer composites. Some studies have focused on filler functionalization to reduce the interfacial thermal resistance.9 In addition, other studies have demonstrated that when inorganic fillers form a highly ordered orientation in polymer composites, the composites can reach a higher thermal conductivity than polymer composites with randomly dispersed fillers.10-14 However, the ability to control the filler orientation and interfacial interactions simultaneously to achieve a high thermal conductivity in polymer composites remains a challenge. After billions of years of evolution, some naturally layered materials have successfully

overcome

these

two

difficulties,

achieving

exceptional

thermal-conduction or mechanical properties.15,16 For example, Wang and co-workers found that the thermal conductivity of spider dragline silk was as high as 340 Wm−1K−1 at a strain of 0%, and further increased to 416 Wm−1K−1 with a strain of 19.7%.17 This enhancement was attributed to the good orientation of the crystalline regions and strong intrachain and interchain H-bonds. In addition, natural nacre has a 3 ACS Paragon Plus Environment


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 4 of 31

precise inorganic-organic interface and an aligned brick-and-mortar structure that enable remarkable mechanical properties, including a high mechanical strength and Young’s modulus. Over the past decade, nacre-inspired materials have attracted significant attention, and composites have been constructed with various inorganic building blocks.18 We hypothesized that mimicking the microstructure of natural nacre by replacing thermally insulating CaCO3 platelets (2.613 Wm−1K−1)19 with thermally conductive platelets would result in polymer composites with a high thermal conductivity. We expected that the higher Young’s modulus would lead to a higher thermal conductivity in the bioinspired polymer composites compared with conventional polymer composites according to the following kinetic model: ݇ = ‫ܥ‬௣ ݈ඥ‫ߩܧ‬

(1)

where k is the thermal conductivity, Cp is the specific heat, E is the Young’s modulus, and l is the phonon mean free path.

Inspired by the hierarchical microstructure of natural nacre, we fabricated thermally conductive polymer composites consisting of Al2O3 platelets deposited with silver nanoparticles (AgNPs) and epoxy resins using a hot-pressing technique. The underlying rationale for using Al2O3 platelets is that Al2O3 platelets have a relatively high thermal conductivity (38 Wm−1K−1),20 and a much lower cost than AlN or BN.21 Furthermore, Al2O3 platelets possess two-dimensional structure which is easy to achieve a highly ordered arrangement within the polymer composites. To improve the interactions between the Al2O3 platelets, we deposited AgNPs with a diameter of 10– 4 ACS Paragon Plus Environment

Page 5 of 31

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


20 nm on the surface of the Al2O3 platelets using a liquid-phase chemical reduction method. The AgNPs could act as bridges that linked adjacent Al2O3 platelets to reduce the interfacial thermal resistance; this behavior is similar to our previous report on BN nanosheets deposited within AgNP/epoxy composites.22 As a result, the polymer composites achieved a thermal conductivity as high as 6.71 Wm−1K−1 at 50 wt% Al2O3. Our bioinspired approach will find uses for designing and fabricating thermally conductive materials for thermal management in modern electronics.

2. EXPERIMENTAL SECTION

2.1 Materials

Al2O3 platelets with an average thickness of 372 nm and a diameter of 8.2 µm were purchased from Merck, Germany. Liquid epoxy resin, E-54, was supplied by The Dow Chemical Company. N, N-Benzyldimethylamide (NNBDA, Sinopharm Chemical Reagent Co., Ltd.) and 4-methylhexahydrophtahlic anhydride (MHHPA, Sigma-Aldrich Co., Ltd.) were used as the catalyst and curing agent, respectively. Silver nitrate (AgNO3, 99.8%), ammonium per sulfate ((NH4)2S2O8, 98.0%), formic acid (88.0%) and absolute ethanol (99.7%) were supplied by Shanghai Ling Feng Chemical Reagent Co., Ltd. Sodium borohydride (NaBH4, 98%), and sodium citrate dihydrate were supplied by Aladdin Chemistry Co., Ltd. 3–Aminopropyl– triethoxysilane (APS, 98%) was obtained from J&K Scientific (GmbH, Germany). All the reagents were of analytical grade and used as received. 5 ACS Paragon Plus Environment


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 6 of 31

2.2 Surface modification of the Al2O3 platelets

The surface modification of the Al2O3 platelets with APS was performed using a reflux condensation method. In detail, 5 g of the Al2O3 platelets, 380 ml of absolute ethanol, and 20 ml of deionized water were mixed together in a 1000 ml three-mouth flask. After the mixture was uniformly dispersed, 1 g of APS and 0.5 g of formic acid were then added into the flask. The suspension was stirred for 24 h at 90 °C. After the reaction, the APS-Al2O3 platelets were collected using vacuum-assisted filtration, washed with deionized water and dried for 8 h at 50 °C.

2.3 Preparation of silver nanoparticle-decorated Al2O3 platelets (Al2O3-AgNPs)

To stabilize the AgNPs and control their sizes, 5 ml of sodium citrate dihydrate was added to a suspension containing 3 g of the APS-Al2O3 complex. NaBH4 acted as a reducing agent for the silver particles. The concentrations of the AgNO3 and NaBH4 were both 0.01 mol/L. The volume ratio of AgNO3 to NaBH4 was maintained at 1: 1.5 in order to ensure that all the AgNO3 was reduced. The volumes of the AgNO3 water solution ranging from 3 to 7 ml were dropped into the suspension to obtain different APS-Al2O3-AgNP hybrids. Finally, the suspension was subjected to vacuum-assisted filtration and dried for 12 h at 50 °C. The Al2O3 platelets used to fabricate the Al2O3-AgNP hybrids were all modified with APS.

2.4 Preparation of the Al2O3-AgNP/epoxy resins composites

6 ACS Paragon Plus Environment

Page 7 of 31

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


The epoxy resin (E-54) and curing agent (MHHPA) were first mixed with a weight ratio of epoxy to MHHPA of 1:1.03. The content of the N,N-benzyldimethylamide was 1 wt% of that of the MHHPA. A certain amount of the Al2O3-AgNPs and epoxy mixture was stirred in a blender mixer for 2 min at a rotation speed of 1500 r/min under vacuum. The epoxy resin with homogeneously dispersed Al2O3-AgNPs was easily coated on a copper film and then subjected to precuring at 100 °C for 1 h. Finally, the mixture was hot-pressed at 120 °C and 10 MPa for 15 min. The Al2O3-AgNP/epoxy composites were produced after curing at 120 °C for 1 h and 160 and 200 °C for 2 h. To obtain varying degrees of thermal conductivity in the Al2O3-AgNP/epoxy composites, the content of Al2O3 varied from 0 to 50 wt%. For comparison, APS-A2O3/epoxy composites both with and without hot-pressing were prepared using the same curing progress.

2.5 Characterization Scanning

electron

microscopy

(SEM)

images

of

the

A2O3/epoxy

and

Al2O3-AgNP/epoxy composites at 50 wt% Al2O3 were obtained using a field SEM (Nova NanoSEM 450, FEI) with an accelerating voltage of 10 kV. Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2001F transmission electron microscope. The TEM samples were prepared by dropping the Al2O3-AgNPs solution onto a copper grid and placing the samples in a drying oven. Fourier transform infrared spectroscopy (FTIR, Bruker Vertex 70) was employed to probe the functional groups on the Al2O3 surface. An X-ray photoelectron 7 ACS Paragon Plus Environment


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 8 of 31

spectrometry (XPS) analysis was carried out using a Kratos Axis Ultra DLD with Al Ka radiation (1486.6 eV). X-ray diffraction (XRD) measurements of the Al2O3-AgNP hybrids were recorded at a scan rate of 0.02° s–1 in the 2θ range of 5–90° using an X-ray powder diffractometer with Cu-K radiation. The in-plane thermal conductivity (K) value was measured using the laser flash technique (NETZSCH, LFA 467 Nano ash). The in-plane thermal conductivity is generally measured using the laser flash technique and was calculated using Equation (2): ‫ܥߙ = ܭ‬௣ ߩ

(2)

where ρ is the density of the composites, Cp is the specific heat capacity obtained using differential scanning calorimetry (DSC, TA Q2000) with the sapphire method (Figure S1, Supporting Information), and α is the in-plane thermal diffusivity.

3. RESULTS AND DISCUSSION

Our designed fabrication procedure involves the surface functionalization of Al2O3, the synthesis of Al2O3-AgNP hybrids and the preparation of Al2O3-AgNP/epoxy composites. The whole fabrication procedure of the Al2O3-AgNP/epoxy composites is depicted in Figure 1. First, Al2O3 platelets were functionalized with APS, which exposes hydroxyl and amino groups on the Al2O3 surface to improve the interactions between the Al2O3 platelets and polymer matrix. Second, a hybrid filler composed of Al2O3 platelets decorated with silver nanoparticles (AgNPs) was prepared using a liquid-phase chemical reduction method. Finally, using hot-pressing technology, 8 ACS Paragon Plus Environment

Page 9 of 31

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


Al2O3-AgNP/epoxy composites with highly ordered lamellar microstructure were fabricated. Two main aspects contribute to the microstructure of the composites. First, the Al2O3 platelets possess high aspect ratio and relatively large flat surface which can spread out in a plane and stack together orderly.23-25 Second, hot-pressing provides an external force along the vertical direction. During the hot pressing, the compressive stress enable the 2D Al2O3 platelets to gradually incline towards the horizontal direction, forming a stable in-plane stacking eventually, as reported previously.26,27

Figure

1.

Schematic

illustration

of

the

fabrication

procedure

of

the

Al2O3-AgNP/epoxy composites. Figure 2a shows the SEM micrograph of the Al2O3 platelets with sizes of 4–10 µm. The size histograms of the Al2O3 platelets determined from the SEM images indicate a mean size of 8.20 µm and a thickness of 372 nm (Figure S2, Supporting Information). FTIR was used to characterize the modified Al2O3 platelets. The 9 ACS Paragon Plus Environment


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 10 of 31

reactive mechanism of the Al2O3 modification is shown in Figure S3 (Supporting Information). The spectrum of the pristine Al2O3 platelets exhibits only the characteristic stretching vibration and bending vibration of –OH at 3435 and 1631 cm−1, respectively, as shown in Figure 2b.28 For APS-Al2O3, the asymmetric and symmetric stretching and bending vibrations of the –CH2 groups were observed at 2974 and 2884 cm−1, respectively. The –CH2 groups were present because it was very difficult to overcome the steric hindrance and fully hydrolyse the APS. The in-plane vibrational and bending vibration peaks of the –NH2 group at 1642 and 1090 cm−1 were also found. In addition, C–N stretching vibration peaks were present at 1394 and 1090 cm−1. On the basis of these observations, we confirm that the APS was covalently bonded to the Al2O3 surface. Due to the heterogeneous nucleation and consequent growth, the reduction of the Ag precursor (AgNO3) in the presence of the Al2O3 platelets resulted in the deposition of AgNPs with a size of 10–20 nm on the surface of the Al2O3 platelet, as shown in the TEM image (Figure 2c). The colour of the Al2O3 solution turned from white into yellow after the deposition of the AgNPs due to their small diameter (shown in the insert of Figure 2c). In addition, none of AgNPs were present in the area around the Al2O3-AgNP hybrids (Figure S4, Supporting Information), which indicates that all the AgNPs were attached on the Al2O3. Compared with the pristine Al2O3, the modified Al2O3 was decorated with AgNPs with the same size, but the number of AgNPs clearly increased both the edge length and surface area (Figure 2d). The XPS measurements of the Al2O3-AgNPs 10 ACS Paragon Plus Environment

Page 11 of 31

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


indicate the presence of not only Al2O3 peaks (Figure 2e) but also Ag3d5/2 and Ag3d3/2 core level peaks centred at 368.2 eV and 374.2 eV, respectively (Figure 2f). Compared with the previously reported values for AgNPs (367.9 eV and 373.9 eV),29 both the peaks were shifted to higher binding energies by 0.3 eV, validating the interaction between the AgNPs and Al2O3. Regarding the O1s peaks before and after the deposition of the AgNPs (Figure S5a and S5b, Supporting Information), the core level peaks exhibited a shift from 531.29 eV to 531.19 eV. Furthermore, the peaks of N1s (Figure S5c and S5d, Supporting Information) shifted from 401.29 eV to 401.98 eV. These results show that the deposited AgNPs interacted with the –OH and –NH2 groups, which agrees with the results from previous reports.30-32 The content of the AgNPs was approximately 2.25 wt% (Table S1, Supporting Information). Furthermore, the XRD patterns (Figure S6, Supporting Information) show that the characteristic peaks of Ag were not present in the Al2O3-AgNPs, indicating that the content was too low to be detected using XRD. By varying the AgNO3 content, we obtained Al2O3-AgNP hybrids with Ag content ranging from 0.96-2.25 wt%. The free Ag cations remained around the Al2O3 because of the coordination interactions between the Ag cations and hydroxyl groups or amino groups. After the reduction of the Ag cations, the interactions contributed to the attachment of AgNPs on the surface of the Al2O3, as shown in Figure 2g.



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 12 of 31

Figure 2. Characterization of the Al2O3-AgNP hybrids. (a) SEM micrograph of the pristine Al2O3. (b) FTIR spectra of the modified Al2O3. (c) TEM image of the Al2O3-AgNPs before the Al2O3 modification. The insets are digital photographs of the Al2O3 and Al2O3-AgNP dispersion. (d) TEM image of the Al2O3-AgNPs with modified the Al2O3. (e) XPS spectra of the Al2O3-AgNPs. (f) Ag3d spectrum of the Al2O3-AgNPs. (g) Schematic of the preparation of the Al2O3-AgNP hybrids. The prepared Al2O3-AgNP/epoxy composite exhibited good flexibility (Figure 3a). The microstructures of the Al2O3-AgNP/epoxy composites are revealed in the SEM images. The microstructure shows that the Al2O3-AgNP hybrids were stacked into a layer-by-layer structure throughout the transverse section, extending continuously 12 ACS Paragon Plus Environment

Page 13 of 31

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


along the longitudinal direction (Figure 3b), which is similar to the microstructure of natural nacre (Figure S7, Supporting Information). In addition, Figure 3c shows that the hot-pressing method imparted an advantageous orientation in the composites (Figure 3d), and the hot-pressed composites possessed good mechanical flexibility over the other composite without hot-pressing. However, the interfacial forces between the filler particles were lower in the hot-pressed Al2O3/epoxy than that in the hot-pressed Al2O3-AgNP/epoxy composites. This result was attributed to the fact that the AgNPs were sintered together during the curing process (Figure S8a and 8b, Supporting Information), which contributed to linking the adjacent Al2O3 platelets and forming a strong filler framework. In contrast, the composites made without the hot-pressing process had poor mechanical properties, the composite was too thick to bend (Figure 3e) and the microstructure was chaotic, as shown in Figure 3f. The high-resolution SEM image (Figure 3g) further confirms that the Al2O3 platelets were stacked into an oriented structure with the AgNPs. Therefore, the filler framework formed during the curing process contained sintered AgNPs between the adjacent Al2O3 platelets. Elemental area scanning further confirmed the existence of the elements of the AgNPs and qualitatively revealed the presence of 0.6 wt% AgNPs (Figure 3h).



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 14 of 31

Figure 3. Microstructure of 50 wt% Al2O3-based epoxy composite microstructures. (a) Optical image and (b) microstructure of the hot-pressed APS-Al2O3/epoxy composites with deposited AgNPs. (c) Optical image and (d) microstructure of the hot-pressed APS-Al2O3/epoxy composites without deposited AgNPs. (e) Optical image and (f) microstructure of the Al2O3/epoxy composites without deposited AgNPs and hot-pressing. (g) A high-resolution SEM micrograph of the Al2O3-AgNP/epoxy

composites.

(h)

Element

detection

of

the

50

wt%

Al2O3-AgNP/epoxy composites. The mechanical properties of materials greatly influence their practical applications. Figure 4 shows the stress−strain curves of the following composites: pure epoxy resins, Al2O3/epoxy composites, Al2O3/epoxy composites with hot-pressing and 14 ACS Paragon Plus Environment

Page 15 of 31

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


Al2O3−AgNP/epoxy composites with hot-pressing. Compared with the other composites, the Al2O3−AgNP/epoxy composite exhibited the highest tensile strength (66.17 MPa), tensile toughness (1.22 MJ m−3) and Young’s modulus (1.6 GPa). The tensile strength of the composites was close to that of the natural nacre.7 Four main aspects enhanced the mechanical performance. First, the modified Al2O3 platelets changed from hydrophilic to hydrophobic, which improved their compatibility with the epoxy resin.33 Second, the amino groups attached on the Al2O3 reacted with the epoxy, reinforcing the interfacial connections between the matrix and filler, which reduced the interfacial cavities and defects. Third, the nanoasperities consisting of the AgNPs made it difficult for adjoining interfaces to slip and pass each other under loading in tension parallel to the Al2O3 platelets, which resulted in the formation of dilatation bands instead of dominant brittle cracking.34,35 The rough surfaces of the Al2O3 platelets increased the frictional sliding during the application of a shear force. At last, the bridging Al2O3−AgNP hybrids generated sufficient resistance to restrain the sliding of the adjacent Al2O3−AgNP hybrids. The intensive stress was transferred to the next layer of Al2O3−AgNPs and induced potential sliding sites in the multiple adjacent Al2O3−AgNP hybrids. Under loading in tension along the platelet plane, the effective bridging and induction of multiple potential sliding sites resulted in mechanical interlocking between the adjacent platelets until the composites fractured, improving the gross energy dissipation.36 As a result, the energy dissipation pervaded



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 16 of 31

the Al2O3 platelets and AgNPs, resulting in cooperative sliding instead of sliding only at the fracture surface, which improved the mechanical properties.

Figure 4. Mechanical properties of the Al2O3-AgNP/epoxy composites at the Al2O3 loading of 50 wt%. (a) Stress-strain, (b) strength, (c) modulus, and (d) toughness curves

of

the

epoxy,

Al2O3/epoxy,

Al2O3/epoxy

with

hot-pressing

and

Al2O3−AgNP/epoxy with hot-pressing composites. To determine the effect of the AgNPs on the thermal conductivity of the polymer composites, Al2O3-AgNP hybrids with different AgNP contents were prepared. Figure 5a shows that the thermal conductivity increased continuously until the AgNP loading reached 0.6 wt% in the Al2O3-AgNP hybrids. The measured thermal conductivity 16 ACS Paragon Plus Environment

Page 17 of 31

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


reached 6.71 Wm−1K−1, representing an improvement of 2817% compared with that of the pure epoxy resin.37 When the fraction of AgNPs exceeded 0.6 wt%, the thermal conductivity reached saturation because the AgNPs were filtered out during the hybrid filler preparation progress. Therefore, we chose the AgNP content of 0.6 wt% as the optimal proportion for the Al2O3-AgNP/epoxy composites. Figure 5b shows the effect of the Al2O3 content on the thermal conductivity of the composites. The content of Al2O3 in the composites was accurately determined using TGA (Figure S9, Supporting Information). The Al2O3/epoxy composites without hot-pressing possessed the lowest thermal conductivity, due to the random stacking of the Al2O3 platelets, which decreased their contact area with the other Al2O3 platelets and epoxy, thus increasing the thermal resistance. However, the hot-pressed Al2O3/epoxy composites with highly ordered structures exhibited a higher thermal conductivity than that of the Al2O3/epoxy composites with randomly stacked Al2O3 platelets. This result proves the significance of the filler orientation. The thermal conductivities of the Al2O3/epoxy and Al2O3-AgNP/epoxy composites that were both hot-pressed, shared a similar variation tendency with the change in the Al2O3 loading. At a low weight percentage of Al2O3 (

Learning from Natural Nacre: Constructing Layered Polymer

Recommend Documents