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RETRACTED: A transparent pressure-sensitive adhesive with high electrical conductivity based on water-soluble nano core-shell hollow composite
Accepted Manuscript A transparent pressure-sensitive adhesive with high electrical conductivity based on water-soluble nano core-shell hollow composite Lipei Yue, Xiaoyong Zhang, Weidong Li, Yongping Bai, Yudong Huang PII:
S0266-3538(17)32827-0
DOI:
10.1016/j.compscitech.2018.03.012
Reference:
CSTE 7130
To appear in:
Composites Science and Technology
Received Date: 9 November 2017 Revised Date:
29 January 2018
Accepted Date: 10 March 2018
Please cite this article as: Yue L, Zhang X, Li W, Bai Y, Huang Y, A transparent pressure-sensitive adhesive with high electrical conductivity based on water-soluble nano core-shell hollow composite, Composites Science and Technology (2018), doi: 10.1016/j.compscitech.2018.03.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT A transparent pressure-sensitive adhesive with high electrical conductivity based on water-soluble nano core-shell hollow composite Lipei Yue a, Xiaoyong Zhang a, Weidong Li b, Yongping Bai a, b, *, Yudong Huang a, b a
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School of Chemical Engineering and Chemistry, Harbin Institute of Technology, Harbin, 150001, PR China b Wuxi HIT Limited Corporation & Research Institute of New Materials, Wuxi, 214183, China * Correspondence to: Yongping Bai (E-mail: [email protected])
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Abstract
The transparent conductive pressure sensitive adhesives (PSAs) are increasingly
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used for demanding applications, such as interconnects in electronic assemblies and transparent conductive films. Graphene is an efficient modifier for highly conductive PSAs, but it is hard to prepare homogenous composite and the transparency of PSA is bad, moreover, there is increased contact resistance during elevated temperature and
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humidity. In this work, we report a water-soluble core-shell hollow composite using graphene oxide (GO) modified by acrylamide as core through in situ polymerization.
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The composite is homogenous without any aggregation after testing in normal temperature for 100 days. The light transmittance of the PSA based on polyethylene
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terephthalate film is above 90% if the content of GO in the composite is under 0.2 wt%. The electrical conductivity of the PSA increases from 0.29 Sm-1 to 0.62 Sm-1 while the related humidity ranges from 0% to 90%. PSA film formed by core-shell composite can hold hydrone and exhibit advanced electrical conductivity in high humidity atmosphere. The conductivity of the composite is stable and unchanged in high temperature. Keywords: core-shell composite; graphene oxide; conductive; transparent
ACCEPTED MANUSCRIPT 1. Introduction Conductive pressure-sensitive adhesives (PSAs) are an indispensable component of optoelectronic devices, such as interconnects in electronic assemblies and
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transparent conductive films [1,2]. The transparent conductive PSAs, which provide both transmittance and electrical conductivity, are increasingly used for demanding applications, including touch screens, photovoltaic cells, liquid crystal displays, and
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light emitting diodes [3-5]. However, the potentially wide applications of transparent
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conductive PSAs have been slowed by some critical limitations, including electrical conductivity, increased contact resistance during elevated temperature and humidity aging [6]. In the past few years, there have been tremendous efforts addressing these issues; among these the effect of conductive filler on the properties of transparent
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conductive PSAs has been intensively investigated. Many materials, including carbon nanotubes [7,8], metallic nanowires [9-11], graphene [12-14], conductive polymer [15], metal grids and powders [16], have been investigated. However, conductive
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filler has some inherent shortcomings, including brittleness and expensiveness.
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Moreover, the conductive filler in the transparent conductive PSAs have large contact points, which make the network unstable [17]. Therefore, a new kind of homogeneous transparent conductive PSA without inorganic conductive filler and there is no increased contact resistance during elevated temperature and humidity is significative both to theory research and industrial manufacture. Graphene has attracted significant interests in the last few years owing to its outstanding electrical conductivities [18,19]. However, it is hard for graphene to be
ACCEPTED MANUSCRIPT distributed and incorporated into polymer homogeneously, and the tending of graphene to aggregate is not easy to be avoided [20]. With a strongly oxygenated and hydrophilic functionalized structure obtained by thermal, electrochemical, or chemical
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reduction processes from graphite, graphene oxide (GO) is considered to be a perfect candidate for the application of conductive polymer composites [21]. There are epoxy and hydroxyl groups on the surface of GO and carboxyl groups on the edge, which
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make the modifying of GO with other functional monomers possible [22]. Recent
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years have witnessed a great deal of researches on using GO to increase the conductivities of PSAs [23-25], but it is hard to prepare transparent PSAs and the research to avoid increased contact resistance of PSAs during elevated temperature and humidity is limited.
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Theoretically, in situ polymerization of monomers on the surface or edge of GO makes the composite a homogenous and stable phase which prevents aggregation in the process of film forming. The in situ polymerization technic enables strong
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interaction between GO and polymer through covalent bonds [26], electrostatic
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interaction [27], polar interaction [28] or hydrophilic interaction [29]. Composite with 3 dimensional core-shell structure synthesized through the polymerization of aliphatic monomers on the edge of GO can match configuration induces both electron and hole transfer channels on the single core-shell structured particle [30-32]. This kind of core-shell composite synthesized through in situ polymerization centered by GO can provide steady high electrical conductivity, but regretfully, there are little reports about this kind of composites with 3 dimensional core-shell structure, and the
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of
electrically
conductive
polymer
synthesized
through
in
situ
polymerization on the edge of GO has not been fully explored. Herein, we report a water-soluble hollow core-shell composite synthesized
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through in situ polymerization centered by the modified GO with acrylamide (AM) and surrounded by flexible aliphatic polyacrylate chains containing a large number of hydrophilic groups with an average of about 205 nm of diameter and is homogenous
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without any aggregation after testing in normal temperature for 100 days. The
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composite can be used as transparent electrically conductive PSA. The light transmittance of the PSA based on Polyethylene terephthalate (PET) film is above 90% if the content of GO in the composite is under 0.2 wt%. The electrical conductivity of the PSA film increases from 0.29 Sm-1 to 0.45 Sm-1 while the related humidity ranges
temperature.
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from 0% to 60%, with an improvement of 55%, and it is stable and unchanged in high
2.1. Materials
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2. Material and methods
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Expandable graphite (Grade 1721) was kindly provided By Xiamen Knano Graphene Technology Co., LTD. Concentrated sulfuric acid (H2SO4), potassium permanganate (KMnO4), sodium nitrate (NaNO3), hydrochloric acid (HCl), hydrogen peroxide (H2O2), and 2-allyl ether,3-hydroxypropane,1-sodium sulfonate (AEHPSS) were purchased from Shanghai Honesty Fine Chemical Co., LTD. Acrylamide (AM), absolute ethyl alcohol，azodiisobutyronitrile (AIBN), acrylic acid (AA), butyl acrylate (BA), N-methylolacrylamide (N-MAM) were all purchased from Aladdin.
ACCEPTED MANUSCRIPT 2.2. Synthesis of GO and preparation of GO-AM GO was prepared by the modified Hummers method [33]. Expandable graphite was oxidized and then be dried to obtain GO powder [34]. The modified GO-AM
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powder was obtained by 10 g of GO and 1.2 g of AM, heating at 160 oC, purging nitrogen for 3 hours.
2.3. Synthesis of core-shell composite through in situ polymerization
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20 g absolute ethyl alcohol was poured into a 100 mL of round three-necked
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flask, then 0.0224 g GO-AM, 10g BA, 6g AA, 6g N-MAM, 8g AEHPSS were added stirring for 1 hour until the temperature was stable at 78 oC. 0.1g AIBN was dissolved in 10 g absolute ethyl alcohol and then the solution was added into the flask in 2 hours, then the core-shell composite was obtained after 2 hours of reaction.
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2.4. PSA Film Forming
The composite was casted on a 50 µm thickness of Poly (ethylene terephthalate) (PET) and dried for 2 minutes at 100 oC, the thickness of the PSA film was 20 µm.
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2.5. Characterization
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FTIR measurements were carried out on a Perkin-Elmer Paragon 1000 PC Fourier transform infrared spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a VG ESCALAB MK II X-ray photoelectron spectroscopy, the energy is 1.254keV and the beam was 50 mA. The morphology of the composite was studied by SEM (Hitachi S-4800) at an accelerating voltage of 5 kV. TEM analysis was performed on a JEM-2100/INCA OXFORD instrument operating at an accelerating voltage of 200 KV. The samples were sprayed onto the
ACCEPTED MANUSCRIPT carbon-coated copper grids and air-dried at room temperature before measurement. Particle size was measured by dynamic light scattering (DLS) measurement in aqueous solution at 25 oC at a scattering angle of 90o, using a Malvern Zetasizer Nano
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S apparatus equipped with a 4.0 mW laser operating at λ = 633 nm. The electrical conductivity was measured by a four point probe (CMT-10 MP, Advanced Instrument Technology). Light transmittance of the PSA was carried out by WGT-2S
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transmittance tester. The transmission of the PSAs was recorded with PET substrate
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as blank sample. DSC test was carried out with a NETZSCH 209 DSC apparatus (Perkin Elmer, USA). The samples were heated from -60 oC to 150 oC at the rate of 5 o
C /min. A 25 mm-wide tape was attached to different substrates with a roller moving
on the samples back and forth 3 times to make a firm bond. After a dwelling time for
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1 h, the samples were tested for 180o peel strength on a tensile machine at the speed of 300 mm/min. The test temperature was 23±1 oC.
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3. Results and discussion
ACCEPTED MANUSCRIPT Fig. 1. Modifying of GO by AM and synthesis of hollow core-shell composite through in situ free radical polymerization.
3.1 Structure of GO and GO-AM The homemade GO was modified by AM through forming hydrogen bonds, and
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then the functional molecule GO-AM was established with vinyl groups around, which made the free radical in situ polymerization of acrylic monomers possible (Fig. 1). Fig. 2a shows the polar groups on GO surface by Fourier transform infrared
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spectroscopy (FTIR). The absorption peak at 3424 cm-1 (stretching vibration of OH)
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is presented owing to the absorbed water on the GO surface. The peaks at 1724 cm-1 (stretching vibration of C=O) and 1223 cm-1 (stretching vibration of OH) are ascribed the carboxyl groups on the edge of GO, and the stretching vibrations of OH from alcohol groups both on the surface and edge of GO are presented at 1050 cm-1, while
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the peak at 1625 cm-1 is ascribed the presence of un-oxidized graphitic domain of GO [35]. The morphology of the pristine components had been analyzed with transmission electron microscopy (TEM). Fig. 2b represents a typical TEM image of
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GO. The image shows hyaline GO sheet with a slightly wrinkled shape on the edge
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due to the polar oxygen-containing functional groups on the periphery, indicating that the prepared GO is single sheet structure without agglomerations. Surface analysis of GO was carried out using X-ray photoelectron spectroscopy
(XPS) and XPS analysis also confirmed the oxidation of GO (Fig. 2c). The C1s XPS spectrum of GO is split into four peaks which confirmed the SP2C-SP2C (284.5 eV), C-OH (286.5 eV), C-O (289.2 eV) and C=O (287.8 eV) bonds (Fig. 2d) [36]. The above results confirm the successful oxidization of graphite. The C=O percentage in
ACCEPTED MANUSCRIPT GO sample is estimated to be 4.8% by calculating the ratio of peak areas of C=O and the total peak areas, which indicates that 0.12g of AM is sufficient to react with 1g of
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GO by forming hydrogen bonds.
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Fig. 2. (a) FTIR spectrums of homemade GO. (b) TEM image of homemade GO. (c) Wide-scan XPS spectrum and (d) narrow-scan C1s XPS spectrum of the homemade GO.
The stretching vibrating of two peaks from AM at 3300-3500 cm-1 (characteristic
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peaks of primary amine) translates to one peak at 3300 cm-1 (characteristic peak of secondary amine) after 3 hours of high temperature processing with nitrogen (Fig. 3a),
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indicating thoroughly reaction of amidogen groups from AM and carboxyl groups on the edge of GO sheet. Modified by AM, the GO-AM shows more disordered structure than the original GO sheet. Hollow agglomerations can be seen from the scanning electron microscopy (SEM) image of GO-AM sample (Fig. 3b), which is due to the interfacial interactions of GO sheets containing hydroxyl and ether linkages on the surfaces. Furthermore, the vinyl groups around the edge of GO-AM make the functional molecule smoothness, and the surface of GO-AM inevitably to some extent
ACCEPTED MANUSCRIPT curls into hook face. The diameters of GO-AM hollow agglomerations with 2~3
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layers are about 100nm while each layer exhibits nanoscale thickness.
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Fig. 3. (a) FTIR spectrums of GO-AM. (b) SEM image of GO -AM.
3.2 Structure and mechanical property of core-shell composite The water-soluble core-shell composite was synthesized through in situ polymerization by using GO-AM as core, the vinyl groups around the edge of
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GO-AM and carried by the subsequent monomers like acrylic acid (AA), N-methylolacrylamide (N-MAM), 2-allyl ether,3-hydroxypropane,1-sodium sulfonate (AEHPSS) and butyl acrylate (BA) was reacted by free radical polymerization. The
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inorganic carbon layer of GO was then surrounded by flexible aliphatic chains of
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copolymers (Fig. 1). Because the morphology of the core-shell composite network centered by GO was preserved well, the water-soluble core-shell composite was homogenous without any aggregation and phase separation after testing in normal temperature for 100 days (Fig. 4e). There are two kinds of core-shell composite structures (Fig. 4a) coexisting in the sample. One is half core-shell composite (Fig. 4b) with about 182nm of diameter in which part of the GO sheet is exposed, the other is core-shell composite (Fig. 4c) with
ACCEPTED MANUSCRIPT about 210nm of diameter in which the GO sheet is fully surrounded by copolymer chains. The average diameter of the composite sample is 205nm (Fig. 4d), which is between that of the half core-shell composite (about 182 nm) with part of GO sheet
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exposed and that of the composite (about 210 nm) with core-shell structure. The coexisting of half core-shell structure and core-shell structure in the composite can be explained by the character of slow initiation, fast growth and rapid termination in free
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radical polymerization. The process of polymerization may be terminated before the
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aliphatic chains of acrylate copolymers grow long enough to cover the surface of GO. Thus, some carbon structures of GOs are exposed in the composite and make further
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contribution to the electrical conductivity of formed pressure-sensitive adhesives.
Fig. 4. Morphologies and size distribution by volume of water-soluble core-shell hollow composite synthesized by GO-AM, AA, N-MAM, AEHPSS and BA: (a) SEM image of core-shell composite. (b) TEM image of core-shell composite in which part of GO sheet is exposed with about 182 nm of diameter. (c) TEM image of core-shell composite nanoparticle with about 210 nm of diameter. (d) Size distribution by volume of core-shell hollow composite. (e) Photograph of the water-soluble core-shell composite samples after testing in normal temperature for 100 days, .indicating that the stability of the water-soluble core-shell composite is good.
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Fig. 5. (a) Differential Scanning calorimeter （DSC）curves of core-shell composite modified by 0.1 wt%
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of GO. (b) 180o peel strength of PSAs formed by composites containing different amount of GO on different substrates (PET, PE, PP).
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The composite with 0.1 wt% of GO exhibits two glass transition temperature (Tg) while the copolymer without GO shows a single Tg (Fig. 5a), indicating that the composite is core-shell structure. The Tg of shell structure of the composite is -42.2 o
C, 5.4 oC lower than that of copolymer without GO (-36.8 oC), which means the shell
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structure of the composite is flexible and can offer enhanced adhesion strength. Peel adhesion is the strength required to remove the adhesive from a surface at specific angle (90o-180o). The mechanical properties of peel adhesion were determined by peel
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strength at 180o. The results are shown in Fig. 5b. In both PSAs modified by GO on
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different substrates with different interface energies, the peel strength is increased with the increasing of GO concentration. It is related to the resistance of the internal structure of the material. The intermolecular interaction of the core-shell composite through the interaction of hydrogen bonds is increased when the GO amount is increased. In this case, the cohesion of the composite with increasing concentration of GO is enhanced. Meanwhile, with the same concentration of GO in the composite, the peel strength of PSA is increased with the increasing of interface energy of the
ACCEPTED MANUSCRIPT substrates (PP: 29 mN/m; PE: 33 mN/m; PET: 42 mN/m). This behavior is cause by polar groups in the shell structure of the composite for hydrogen bonding with the surface of the substrates; these increase the cohesive strength of the material and the
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interaction with the probe surface. In this case, the mechanical property is increased when the interface energy of the substrate is increased. That means the concentration of polar groups is higher on the substrate surface, which increases the interaction
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between the substrate and the PSA due to the formation of hydrogen bonds between
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the polymer chains, with the effect being stronger cohesion. Thus, this kind of PSA formed by core-shell composite can be used as protective films applying for touch screens, photovoltaic cells, liquid crystal displays, and light emitting diodes. 3.3 Transparency of PSAs formed by the core-shell composite
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Fig. 6a exhibits the photograph of the as-prepared PSA on the PET substrate. If the percentage of GO in the core-shell composite is 0.1 wt% (top) or 0.15 wt% (bottom), the PSA film shows high transparency, approximating to that of base
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material of PET. Fig. 6b exhibits the light transmittance of PSA with different
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percentage of GO in detail. PSA films modified by GO shows stably high transparency (the light transmittances of PSAs are 90.3%, 90%, 90.1% and 90% when the percentages of GO in the composite are 0 wt%, 0.05 wt %, 0.1 wt % and 0.15 wt %, while the light transmittance of PET is 90.3%). GO shows little influence on the transmittance of the core-shell composite network, and this is attributed to the high transparence of the shell structure of polyacrylate chains. There are two kinds of core-shell composite structures coexisting in the PSA film
ACCEPTED MANUSCRIPT (Fig. 4a). One is half core-shell composite in which part of the GO sheet is exposed; the other is core-shell composite in which the GO sheet is fully surrounded by polyacrylate chains. As shown in Fig. 6c，if the percentage of GO in the composite is
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under 0.2 wt %, the percentage of half core-shell composite is low and the area of exposed GO sheet is small, GO sheets in the composite do not influence the transparency of the PSA film. But if the percentage of GO in the composite increases
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to more than 0.2 wt %, the transparency of PSA film decreases obviously (Fig. 6b).
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Because the percentage of half core-shell composite is high and the area of exposed GO sheet is bigger relatively, some of the GO sheet may be exposed entirely. As can be seen from Fig. 6d, some of the GO sheets with bigger exposed areas or entirely exposed makes the PSA film with 0.25 wt % percentage of GO opaque and gray. So
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the percentage of GO in the core-shell composite should be lower than 0.2 wt % to
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guarantee the high transparency of the PSA film.
Fig. 6. (a) Photograph of the PSA film on PET substrate indicating that the film is transparent and flexible. (b) Light transmittance of the PSA with different percentage of GO based on the core-shell composite. (c) PSA film with 0.1 wt% percentage of GO on PET. (d) PSA film with 0.25 wt% percentage of GO on PET.
3.4 Electrical conductivity of PSAs formed by the core-shell composite
ACCEPTED MANUSCRIPT The
water-soluble
core-shell
composite
synthesized
through
in
situ
polymerization has high conductivity due to two structural features. One is that a large number of ion groups (sodium sulfonate groups from AEHPSS monomers) existing in
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the shell chains; the other is that the core-shell composite with part of inorganic carbon structure of GO sheet exposed making contribution to the conductivity of the homogenous composite. If the percentage of the GO in the composite increases, the
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content of core-shell composite with part of GO sheet exposed and the area of
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exposed GO sheet is bigger, resulting in the improved conductivity of the PSA films (Fig. 7a). The experiment of the influence of content of GO in the composite to the conductivity of PSA film is carried out in the atmosphere of 30 oC with 0% of relative
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humidity.
Fig. 7. (a) Conductivity of the PSAs with different content of GOs in the atmosphere of 30 oC with 0% of relative humidity. (b) Conductivity of the PSAs with 0.1 wt% content of GO in different temperatures with 0% of relative humidity. (c) Conductivity of the PSAs with 0.1 wt% content of GO
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When the related humidity is increasing and the film is surrounded by hydrone, it exhibits advanced electrical conductivity. As is shown in Fig. 7c (the content of GO in
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the composite is 0.1 wt % and the temperature is 30 oC), the electrical conductivity of the film increases from 0.29 Sm-1 to 0.62 Sm-1 while the related humidity ranging from 0% to 90%, with an improvement of 114%. The possible exploration of the
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advanced electrically conductive efficiency of the film in high humidity is that the
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composite with core-shell structure is hollow sphere centered by GO and covered by flexible aliphatic polyacrylate chains with nano level of interspaces, which is easy for hydrone to move in, and once the hydrone enters the interspaces of polyacrylate chains abounding sufficient oxygen groups (hydroxyl, ether bond, carbonyl and ester
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groups) and a large number of ion groups (sodium sulfonate groups from AEHPSS monomers), the interaction between hydrone and hydrophilic groups make it hard for the water monomer to move out (Fig. 8). On account of which the PSA films formed
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by composite with core-shell structure can hold hydrone and exhibits advanced
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electrical conductivities in high humidity atmosphere.
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Fig. 8. Hydrones surround hollow core-shell composite and enter the interspaces of polyacrylate chains and interact with the hydrophilic groups.
This kind of PSA based on composite with core-shell structure can be used in
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atmosphere of normal temperature and high humidity, but what if high temperature and high humidity? Is water can be locked in the composite in atmosphere of high
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temperature and high humidity? Fig. 7b exhibits the conductivity of PSA based on core-shell composite under different temperatures, it can be seen that the conductivity
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of PSA is stable and do not increase if we only increase temperature in relative humidity of 0%. Fig. 7d explores the conductivity of PSA based on core-shell composite under different temperatures in relative humidity of 60% and 80%, temperature does not influence the conductivity of PSA in the fixed humidity, indicating that water can be locked in the interspaces of shell structure in the composite with flexible aliphatic hydrophilic chains and this kind of PSA based on composite with core-shell structure can be used in atmosphere of high temperature
ACCEPTED MANUSCRIPT and high humidity. 4. Conclusions The water-soluble hollow core-shell composite is synthesized through in situ
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polymerization centered by the modified GO-AM and surrounded by flexible aliphatic polyacrylate chains containing a large number of hydrophilic groups with an average of about 205 nm of diameter and is homogenous without any aggregation after testing
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in normal temperature for 100 days.
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The composite can be used as transparent electrically conductive pressure sensitive adhesive. The light transmittance of the PSA based on PET film is above 90% if the content of GO in the composite is under 0.2 wt %. The electrical conductivity of the PSA film increases from 0.29 Sm-1 to 0.62 Sm-1 while the related humidity ranges
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from 0% to 90%, with an improvement of 114%, and it is stable and unchanged in high temperature. Because the flexible aliphatic polyacrylate chains with nano level of interspaces in the shell structure of composite makes it easy for hydrone to move in
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and interact with the oxygen and sodium sulfonate groups in the chains and then hard
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to move out even in high temperature, on account of which PSA formed by composite with core-shell structure can hold hydrone and exhibits advanced electrical conductivity in high humidity atmosphere. Acknowledgements
The authors gratefully acknowledge the financial support by the 2017 Innovation Ability Construction Project of Department of Science and Technology of Jiangsu Province (BM2017006), and 2017 Fundamental research Project of Department of
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ACCEPTED MANUSCRIPT [22] G. Eda, M. Chhowalla, Chemically Derived Graphene Oxide: Towards Large-Area Thin-Film Electronics and Optoelectronics, Adv. Mater. 22 (2010) 2392-2415.
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[23] S.T. Sun, P.Y. Wu, A one-step strategy for thermal- and pH-responsive graphene oxide interpenetrating polymer hydrogel networks, J. Mater. Chem. 21 (2011) 4095-4097.
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[26] J.F. Shen, M. Shi, B. Yan, H.W. Ma, N. Li, M.X. Ye, Ionic liquid-assisted one-step hydrothermal synthesis of TiO2-reduced graphene oxide composites, Phys. Lett. 484 (2010) 247-253.
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[27] F. Liu, S. Chung, G. Oh, T.S. Seo, Three-Dimensional Graphene Oxide
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S0266-3538(17)32827-0
DOI:
10.1016/j.compscitech.2018.03.012
Reference:
CSTE 7130
To appear in:
Composites Science and Technology
Received Date: 9 November 2017 Revised Date:
29 January 2018
Accepted Date: 10 March 2018
Please cite this article as: Yue L, Zhang X, Li W, Bai Y, Huang Y, A transparent pressure-sensitive adhesive with high electrical conductivity based on water-soluble nano core-shell hollow composite, Composites Science and Technology (2018), doi: 10.1016/j.compscitech.2018.03.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT A transparent pressure-sensitive adhesive with high electrical conductivity based on water-soluble nano core-shell hollow composite Lipei Yue a, Xiaoyong Zhang a, Weidong Li b, Yongping Bai a, b, *, Yudong Huang a, b a
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School of Chemical Engineering and Chemistry, Harbin Institute of Technology, Harbin, 150001, PR China b Wuxi HIT Limited Corporation & Research Institute of New Materials, Wuxi, 214183, China * Correspondence to: Yongping Bai (E-mail: [email protected])
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Abstract
The transparent conductive pressure sensitive adhesives (PSAs) are increasingly
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used for demanding applications, such as interconnects in electronic assemblies and transparent conductive films. Graphene is an efficient modifier for highly conductive PSAs, but it is hard to prepare homogenous composite and the transparency of PSA is bad, moreover, there is increased contact resistance during elevated temperature and
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humidity. In this work, we report a water-soluble core-shell hollow composite using graphene oxide (GO) modified by acrylamide as core through in situ polymerization.
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The composite is homogenous without any aggregation after testing in normal temperature for 100 days. The light transmittance of the PSA based on polyethylene
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terephthalate film is above 90% if the content of GO in the composite is under 0.2 wt%. The electrical conductivity of the PSA increases from 0.29 Sm-1 to 0.62 Sm-1 while the related humidity ranges from 0% to 90%. PSA film formed by core-shell composite can hold hydrone and exhibit advanced electrical conductivity in high humidity atmosphere. The conductivity of the composite is stable and unchanged in high temperature. Keywords: core-shell composite; graphene oxide; conductive; transparent
ACCEPTED MANUSCRIPT 1. Introduction Conductive pressure-sensitive adhesives (PSAs) are an indispensable component of optoelectronic devices, such as interconnects in electronic assemblies and
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transparent conductive films [1,2]. The transparent conductive PSAs, which provide both transmittance and electrical conductivity, are increasingly used for demanding applications, including touch screens, photovoltaic cells, liquid crystal displays, and
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light emitting diodes [3-5]. However, the potentially wide applications of transparent
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conductive PSAs have been slowed by some critical limitations, including electrical conductivity, increased contact resistance during elevated temperature and humidity aging [6]. In the past few years, there have been tremendous efforts addressing these issues; among these the effect of conductive filler on the properties of transparent
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conductive PSAs has been intensively investigated. Many materials, including carbon nanotubes [7,8], metallic nanowires [9-11], graphene [12-14], conductive polymer [15], metal grids and powders [16], have been investigated. However, conductive
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filler has some inherent shortcomings, including brittleness and expensiveness.
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Moreover, the conductive filler in the transparent conductive PSAs have large contact points, which make the network unstable [17]. Therefore, a new kind of homogeneous transparent conductive PSA without inorganic conductive filler and there is no increased contact resistance during elevated temperature and humidity is significative both to theory research and industrial manufacture. Graphene has attracted significant interests in the last few years owing to its outstanding electrical conductivities [18,19]. However, it is hard for graphene to be
ACCEPTED MANUSCRIPT distributed and incorporated into polymer homogeneously, and the tending of graphene to aggregate is not easy to be avoided [20]. With a strongly oxygenated and hydrophilic functionalized structure obtained by thermal, electrochemical, or chemical
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reduction processes from graphite, graphene oxide (GO) is considered to be a perfect candidate for the application of conductive polymer composites [21]. There are epoxy and hydroxyl groups on the surface of GO and carboxyl groups on the edge, which
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make the modifying of GO with other functional monomers possible [22]. Recent
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years have witnessed a great deal of researches on using GO to increase the conductivities of PSAs [23-25], but it is hard to prepare transparent PSAs and the research to avoid increased contact resistance of PSAs during elevated temperature and humidity is limited.
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Theoretically, in situ polymerization of monomers on the surface or edge of GO makes the composite a homogenous and stable phase which prevents aggregation in the process of film forming. The in situ polymerization technic enables strong
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interaction between GO and polymer through covalent bonds [26], electrostatic
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interaction [27], polar interaction [28] or hydrophilic interaction [29]. Composite with 3 dimensional core-shell structure synthesized through the polymerization of aliphatic monomers on the edge of GO can match configuration induces both electron and hole transfer channels on the single core-shell structured particle [30-32]. This kind of core-shell composite synthesized through in situ polymerization centered by GO can provide steady high electrical conductivity, but regretfully, there are little reports about this kind of composites with 3 dimensional core-shell structure, and the
ACCEPTED MANUSCRIPT potential
of
electrically
conductive
polymer
synthesized
through
in
situ
polymerization on the edge of GO has not been fully explored. Herein, we report a water-soluble hollow core-shell composite synthesized
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through in situ polymerization centered by the modified GO with acrylamide (AM) and surrounded by flexible aliphatic polyacrylate chains containing a large number of hydrophilic groups with an average of about 205 nm of diameter and is homogenous
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without any aggregation after testing in normal temperature for 100 days. The
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composite can be used as transparent electrically conductive PSA. The light transmittance of the PSA based on Polyethylene terephthalate (PET) film is above 90% if the content of GO in the composite is under 0.2 wt%. The electrical conductivity of the PSA film increases from 0.29 Sm-1 to 0.45 Sm-1 while the related humidity ranges
temperature.
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from 0% to 60%, with an improvement of 55%, and it is stable and unchanged in high
2.1. Materials
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2. Material and methods
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Expandable graphite (Grade 1721) was kindly provided By Xiamen Knano Graphene Technology Co., LTD. Concentrated sulfuric acid (H2SO4), potassium permanganate (KMnO4), sodium nitrate (NaNO3), hydrochloric acid (HCl), hydrogen peroxide (H2O2), and 2-allyl ether,3-hydroxypropane,1-sodium sulfonate (AEHPSS) were purchased from Shanghai Honesty Fine Chemical Co., LTD. Acrylamide (AM), absolute ethyl alcohol，azodiisobutyronitrile (AIBN), acrylic acid (AA), butyl acrylate (BA), N-methylolacrylamide (N-MAM) were all purchased from Aladdin.
ACCEPTED MANUSCRIPT 2.2. Synthesis of GO and preparation of GO-AM GO was prepared by the modified Hummers method [33]. Expandable graphite was oxidized and then be dried to obtain GO powder [34]. The modified GO-AM
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powder was obtained by 10 g of GO and 1.2 g of AM, heating at 160 oC, purging nitrogen for 3 hours.
2.3. Synthesis of core-shell composite through in situ polymerization
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20 g absolute ethyl alcohol was poured into a 100 mL of round three-necked
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flask, then 0.0224 g GO-AM, 10g BA, 6g AA, 6g N-MAM, 8g AEHPSS were added stirring for 1 hour until the temperature was stable at 78 oC. 0.1g AIBN was dissolved in 10 g absolute ethyl alcohol and then the solution was added into the flask in 2 hours, then the core-shell composite was obtained after 2 hours of reaction.
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2.4. PSA Film Forming
The composite was casted on a 50 µm thickness of Poly (ethylene terephthalate) (PET) and dried for 2 minutes at 100 oC, the thickness of the PSA film was 20 µm.
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2.5. Characterization
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FTIR measurements were carried out on a Perkin-Elmer Paragon 1000 PC Fourier transform infrared spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a VG ESCALAB MK II X-ray photoelectron spectroscopy, the energy is 1.254keV and the beam was 50 mA. The morphology of the composite was studied by SEM (Hitachi S-4800) at an accelerating voltage of 5 kV. TEM analysis was performed on a JEM-2100/INCA OXFORD instrument operating at an accelerating voltage of 200 KV. The samples were sprayed onto the
ACCEPTED MANUSCRIPT carbon-coated copper grids and air-dried at room temperature before measurement. Particle size was measured by dynamic light scattering (DLS) measurement in aqueous solution at 25 oC at a scattering angle of 90o, using a Malvern Zetasizer Nano
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S apparatus equipped with a 4.0 mW laser operating at λ = 633 nm. The electrical conductivity was measured by a four point probe (CMT-10 MP, Advanced Instrument Technology). Light transmittance of the PSA was carried out by WGT-2S
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transmittance tester. The transmission of the PSAs was recorded with PET substrate
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as blank sample. DSC test was carried out with a NETZSCH 209 DSC apparatus (Perkin Elmer, USA). The samples were heated from -60 oC to 150 oC at the rate of 5 o
C /min. A 25 mm-wide tape was attached to different substrates with a roller moving
on the samples back and forth 3 times to make a firm bond. After a dwelling time for
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1 h, the samples were tested for 180o peel strength on a tensile machine at the speed of 300 mm/min. The test temperature was 23±1 oC.
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3. Results and discussion
ACCEPTED MANUSCRIPT Fig. 1. Modifying of GO by AM and synthesis of hollow core-shell composite through in situ free radical polymerization.
3.1 Structure of GO and GO-AM The homemade GO was modified by AM through forming hydrogen bonds, and
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then the functional molecule GO-AM was established with vinyl groups around, which made the free radical in situ polymerization of acrylic monomers possible (Fig. 1). Fig. 2a shows the polar groups on GO surface by Fourier transform infrared
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spectroscopy (FTIR). The absorption peak at 3424 cm-1 (stretching vibration of OH)
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is presented owing to the absorbed water on the GO surface. The peaks at 1724 cm-1 (stretching vibration of C=O) and 1223 cm-1 (stretching vibration of OH) are ascribed the carboxyl groups on the edge of GO, and the stretching vibrations of OH from alcohol groups both on the surface and edge of GO are presented at 1050 cm-1, while
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the peak at 1625 cm-1 is ascribed the presence of un-oxidized graphitic domain of GO [35]. The morphology of the pristine components had been analyzed with transmission electron microscopy (TEM). Fig. 2b represents a typical TEM image of
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GO. The image shows hyaline GO sheet with a slightly wrinkled shape on the edge
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due to the polar oxygen-containing functional groups on the periphery, indicating that the prepared GO is single sheet structure without agglomerations. Surface analysis of GO was carried out using X-ray photoelectron spectroscopy
(XPS) and XPS analysis also confirmed the oxidation of GO (Fig. 2c). The C1s XPS spectrum of GO is split into four peaks which confirmed the SP2C-SP2C (284.5 eV), C-OH (286.5 eV), C-O (289.2 eV) and C=O (287.8 eV) bonds (Fig. 2d) [36]. The above results confirm the successful oxidization of graphite. The C=O percentage in
ACCEPTED MANUSCRIPT GO sample is estimated to be 4.8% by calculating the ratio of peak areas of C=O and the total peak areas, which indicates that 0.12g of AM is sufficient to react with 1g of
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GO by forming hydrogen bonds.
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Fig. 2. (a) FTIR spectrums of homemade GO. (b) TEM image of homemade GO. (c) Wide-scan XPS spectrum and (d) narrow-scan C1s XPS spectrum of the homemade GO.
The stretching vibrating of two peaks from AM at 3300-3500 cm-1 (characteristic
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peaks of primary amine) translates to one peak at 3300 cm-1 (characteristic peak of secondary amine) after 3 hours of high temperature processing with nitrogen (Fig. 3a),
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indicating thoroughly reaction of amidogen groups from AM and carboxyl groups on the edge of GO sheet. Modified by AM, the GO-AM shows more disordered structure than the original GO sheet. Hollow agglomerations can be seen from the scanning electron microscopy (SEM) image of GO-AM sample (Fig. 3b), which is due to the interfacial interactions of GO sheets containing hydroxyl and ether linkages on the surfaces. Furthermore, the vinyl groups around the edge of GO-AM make the functional molecule smoothness, and the surface of GO-AM inevitably to some extent
ACCEPTED MANUSCRIPT curls into hook face. The diameters of GO-AM hollow agglomerations with 2~3
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layers are about 100nm while each layer exhibits nanoscale thickness.
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Fig. 3. (a) FTIR spectrums of GO-AM. (b) SEM image of GO -AM.
3.2 Structure and mechanical property of core-shell composite The water-soluble core-shell composite was synthesized through in situ polymerization by using GO-AM as core, the vinyl groups around the edge of
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GO-AM and carried by the subsequent monomers like acrylic acid (AA), N-methylolacrylamide (N-MAM), 2-allyl ether,3-hydroxypropane,1-sodium sulfonate (AEHPSS) and butyl acrylate (BA) was reacted by free radical polymerization. The
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inorganic carbon layer of GO was then surrounded by flexible aliphatic chains of
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copolymers (Fig. 1). Because the morphology of the core-shell composite network centered by GO was preserved well, the water-soluble core-shell composite was homogenous without any aggregation and phase separation after testing in normal temperature for 100 days (Fig. 4e). There are two kinds of core-shell composite structures (Fig. 4a) coexisting in the sample. One is half core-shell composite (Fig. 4b) with about 182nm of diameter in which part of the GO sheet is exposed, the other is core-shell composite (Fig. 4c) with
ACCEPTED MANUSCRIPT about 210nm of diameter in which the GO sheet is fully surrounded by copolymer chains. The average diameter of the composite sample is 205nm (Fig. 4d), which is between that of the half core-shell composite (about 182 nm) with part of GO sheet
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exposed and that of the composite (about 210 nm) with core-shell structure. The coexisting of half core-shell structure and core-shell structure in the composite can be explained by the character of slow initiation, fast growth and rapid termination in free
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radical polymerization. The process of polymerization may be terminated before the
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aliphatic chains of acrylate copolymers grow long enough to cover the surface of GO. Thus, some carbon structures of GOs are exposed in the composite and make further
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contribution to the electrical conductivity of formed pressure-sensitive adhesives.
Fig. 4. Morphologies and size distribution by volume of water-soluble core-shell hollow composite synthesized by GO-AM, AA, N-MAM, AEHPSS and BA: (a) SEM image of core-shell composite. (b) TEM image of core-shell composite in which part of GO sheet is exposed with about 182 nm of diameter. (c) TEM image of core-shell composite nanoparticle with about 210 nm of diameter. (d) Size distribution by volume of core-shell hollow composite. (e) Photograph of the water-soluble core-shell composite samples after testing in normal temperature for 100 days, .indicating that the stability of the water-soluble core-shell composite is good.
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ACCEPTED MANUSCRIPT
Fig. 5. (a) Differential Scanning calorimeter （DSC）curves of core-shell composite modified by 0.1 wt%
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of GO. (b) 180o peel strength of PSAs formed by composites containing different amount of GO on different substrates (PET, PE, PP).
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The composite with 0.1 wt% of GO exhibits two glass transition temperature (Tg) while the copolymer without GO shows a single Tg (Fig. 5a), indicating that the composite is core-shell structure. The Tg of shell structure of the composite is -42.2 o
C, 5.4 oC lower than that of copolymer without GO (-36.8 oC), which means the shell
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structure of the composite is flexible and can offer enhanced adhesion strength. Peel adhesion is the strength required to remove the adhesive from a surface at specific angle (90o-180o). The mechanical properties of peel adhesion were determined by peel
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strength at 180o. The results are shown in Fig. 5b. In both PSAs modified by GO on
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different substrates with different interface energies, the peel strength is increased with the increasing of GO concentration. It is related to the resistance of the internal structure of the material. The intermolecular interaction of the core-shell composite through the interaction of hydrogen bonds is increased when the GO amount is increased. In this case, the cohesion of the composite with increasing concentration of GO is enhanced. Meanwhile, with the same concentration of GO in the composite, the peel strength of PSA is increased with the increasing of interface energy of the
ACCEPTED MANUSCRIPT substrates (PP: 29 mN/m; PE: 33 mN/m; PET: 42 mN/m). This behavior is cause by polar groups in the shell structure of the composite for hydrogen bonding with the surface of the substrates; these increase the cohesive strength of the material and the
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interaction with the probe surface. In this case, the mechanical property is increased when the interface energy of the substrate is increased. That means the concentration of polar groups is higher on the substrate surface, which increases the interaction
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between the substrate and the PSA due to the formation of hydrogen bonds between
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the polymer chains, with the effect being stronger cohesion. Thus, this kind of PSA formed by core-shell composite can be used as protective films applying for touch screens, photovoltaic cells, liquid crystal displays, and light emitting diodes. 3.3 Transparency of PSAs formed by the core-shell composite
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Fig. 6a exhibits the photograph of the as-prepared PSA on the PET substrate. If the percentage of GO in the core-shell composite is 0.1 wt% (top) or 0.15 wt% (bottom), the PSA film shows high transparency, approximating to that of base
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material of PET. Fig. 6b exhibits the light transmittance of PSA with different
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percentage of GO in detail. PSA films modified by GO shows stably high transparency (the light transmittances of PSAs are 90.3%, 90%, 90.1% and 90% when the percentages of GO in the composite are 0 wt%, 0.05 wt %, 0.1 wt % and 0.15 wt %, while the light transmittance of PET is 90.3%). GO shows little influence on the transmittance of the core-shell composite network, and this is attributed to the high transparence of the shell structure of polyacrylate chains. There are two kinds of core-shell composite structures coexisting in the PSA film
ACCEPTED MANUSCRIPT (Fig. 4a). One is half core-shell composite in which part of the GO sheet is exposed; the other is core-shell composite in which the GO sheet is fully surrounded by polyacrylate chains. As shown in Fig. 6c，if the percentage of GO in the composite is
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under 0.2 wt %, the percentage of half core-shell composite is low and the area of exposed GO sheet is small, GO sheets in the composite do not influence the transparency of the PSA film. But if the percentage of GO in the composite increases
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to more than 0.2 wt %, the transparency of PSA film decreases obviously (Fig. 6b).
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Because the percentage of half core-shell composite is high and the area of exposed GO sheet is bigger relatively, some of the GO sheet may be exposed entirely. As can be seen from Fig. 6d, some of the GO sheets with bigger exposed areas or entirely exposed makes the PSA film with 0.25 wt % percentage of GO opaque and gray. So
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the percentage of GO in the core-shell composite should be lower than 0.2 wt % to
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guarantee the high transparency of the PSA film.
Fig. 6. (a) Photograph of the PSA film on PET substrate indicating that the film is transparent and flexible. (b) Light transmittance of the PSA with different percentage of GO based on the core-shell composite. (c) PSA film with 0.1 wt% percentage of GO on PET. (d) PSA film with 0.25 wt% percentage of GO on PET.
3.4 Electrical conductivity of PSAs formed by the core-shell composite
ACCEPTED MANUSCRIPT The
water-soluble
core-shell
composite
synthesized
through
in
situ
polymerization has high conductivity due to two structural features. One is that a large number of ion groups (sodium sulfonate groups from AEHPSS monomers) existing in
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the shell chains; the other is that the core-shell composite with part of inorganic carbon structure of GO sheet exposed making contribution to the conductivity of the homogenous composite. If the percentage of the GO in the composite increases, the
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content of core-shell composite with part of GO sheet exposed and the area of
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exposed GO sheet is bigger, resulting in the improved conductivity of the PSA films (Fig. 7a). The experiment of the influence of content of GO in the composite to the conductivity of PSA film is carried out in the atmosphere of 30 oC with 0% of relative
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humidity.
Fig. 7. (a) Conductivity of the PSAs with different content of GOs in the atmosphere of 30 oC with 0% of relative humidity. (b) Conductivity of the PSAs with 0.1 wt% content of GO in different temperatures with 0% of relative humidity. (c) Conductivity of the PSAs with 0.1 wt% content of GO
ACCEPTED MANUSCRIPT in different humidity under30 oC. (d) Conductivity of the PSAs with 0.1 wt% content of GO in different temperatures with 60% and 80% of humidity.
When the related humidity is increasing and the film is surrounded by hydrone, it exhibits advanced electrical conductivity. As is shown in Fig. 7c (the content of GO in
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the composite is 0.1 wt % and the temperature is 30 oC), the electrical conductivity of the film increases from 0.29 Sm-1 to 0.62 Sm-1 while the related humidity ranging from 0% to 90%, with an improvement of 114%. The possible exploration of the
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advanced electrically conductive efficiency of the film in high humidity is that the
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composite with core-shell structure is hollow sphere centered by GO and covered by flexible aliphatic polyacrylate chains with nano level of interspaces, which is easy for hydrone to move in, and once the hydrone enters the interspaces of polyacrylate chains abounding sufficient oxygen groups (hydroxyl, ether bond, carbonyl and ester
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groups) and a large number of ion groups (sodium sulfonate groups from AEHPSS monomers), the interaction between hydrone and hydrophilic groups make it hard for the water monomer to move out (Fig. 8). On account of which the PSA films formed
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by composite with core-shell structure can hold hydrone and exhibits advanced
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electrical conductivities in high humidity atmosphere.
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Fig. 8. Hydrones surround hollow core-shell composite and enter the interspaces of polyacrylate chains and interact with the hydrophilic groups.
This kind of PSA based on composite with core-shell structure can be used in
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atmosphere of normal temperature and high humidity, but what if high temperature and high humidity? Is water can be locked in the composite in atmosphere of high
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temperature and high humidity? Fig. 7b exhibits the conductivity of PSA based on core-shell composite under different temperatures, it can be seen that the conductivity
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of PSA is stable and do not increase if we only increase temperature in relative humidity of 0%. Fig. 7d explores the conductivity of PSA based on core-shell composite under different temperatures in relative humidity of 60% and 80%, temperature does not influence the conductivity of PSA in the fixed humidity, indicating that water can be locked in the interspaces of shell structure in the composite with flexible aliphatic hydrophilic chains and this kind of PSA based on composite with core-shell structure can be used in atmosphere of high temperature
ACCEPTED MANUSCRIPT and high humidity. 4. Conclusions The water-soluble hollow core-shell composite is synthesized through in situ
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polymerization centered by the modified GO-AM and surrounded by flexible aliphatic polyacrylate chains containing a large number of hydrophilic groups with an average of about 205 nm of diameter and is homogenous without any aggregation after testing
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in normal temperature for 100 days.
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The composite can be used as transparent electrically conductive pressure sensitive adhesive. The light transmittance of the PSA based on PET film is above 90% if the content of GO in the composite is under 0.2 wt %. The electrical conductivity of the PSA film increases from 0.29 Sm-1 to 0.62 Sm-1 while the related humidity ranges
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from 0% to 90%, with an improvement of 114%, and it is stable and unchanged in high temperature. Because the flexible aliphatic polyacrylate chains with nano level of interspaces in the shell structure of composite makes it easy for hydrone to move in
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and interact with the oxygen and sodium sulfonate groups in the chains and then hard
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to move out even in high temperature, on account of which PSA formed by composite with core-shell structure can hold hydrone and exhibits advanced electrical conductivity in high humidity atmosphere. Acknowledgements
The authors gratefully acknowledge the financial support by the 2017 Innovation Ability Construction Project of Department of Science and Technology of Jiangsu Province (BM2017006), and 2017 Fundamental research Project of Department of
ACCEPTED MANUSCRIPT Science and Technology of Jiangsu Province (BK20171146). References [1] S.S. Baek, S.H. Jang, S.H. Hwang, Construction and adhesion performance of
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