A Sub-minute Curable Nanoadhesive with High Transparency, Strong

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A Sub-minute Curable Nanoadhesive with High Transparency, Strong Adhesion, and Excellent Flexibility Moo Jin Kwak, Do Heung Kim, Jae Bem You, Heeyeon Moon, Munkyu Joo, Eunjung Lee, and Sung Gap Im* Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea S Supporting Information *

ABSTRACT: To achieve adhesion between two arbitrary substrates, a subminute curable dry nanoadhesive was devised in a one-step manner. The dry adhesive is composed of a copolymer film containing poly(glycidyl methacrylate) (pGMA) and poly(2-(dimethylamino)ethyl methacrylate) (pDMAEMA) segments, where the tertiary amine moiety in pDMAEMA acts as an initiator that triggers the ring-opening reaction of the epoxy ring in pGMA, leading to a self-cross-linking of the epoxide groups in pGMA. Optimization of curing condition resulted in dramatic enhancement of the adhesion strength to values exceeding 250 N/cm2 of shear strength and 32.5 N/25 mm of peel strength. Also, strong bonds are observed in various types of substrate materials including glass, latex rubber, Si wafer, and many polymeric films to each other. Moreover, it maintained excellent adhesion against harsh mechanical, thermal, and chemical stresses. The copolymerbased nanoadhesive developed in this study will be highly advantageous for emerging flexible and foldable device applications.

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INTRODUCTION An adhesive is a material that can bind two surfaces together. Adhesives are used widely in our daily lives due to their advantages such as high bonding strength, easiness of application to various surfaces, and low manufacturing cost.1−7 Adhesives also allow for increased mechanical flexibility of binded substrates and the capability of releasing interfacial stress across the joint area; these advantages have in turn attracted huge interest in many developing fields including wearable electronics,8−10 thin film microfluidics, and flexible encapsulation layers.11,12 In such devices, it becomes critical to achieve reliable binding between two different substrates. Moreover, maintaining the flexibility of the bonded substrates while preserving the intact adhesion between the substrates is another essential factor to be achieved. Therefore, in addition to strong bonding, it is also highly desirable to secure an adhesive system that is mechanically flexible, optically transparent, and environmentally stable against various kinds of harsh chemical and mechanical stresses. One of the classic adhesive materials is liquid-phase epoxybased adhesive, whick is curable by UV light or heat.13−16 However, UV-curable adhesive is not applicable to opaque substrates, and thermally curable adhesive generally requires high process temperature (>100 °C) and long curing time (more than 2 h in most cases).14−16 Most importantly, the thickness of conventional liquid-phase adhesive is generally on the order of a few tens of micrometers or higher, which makes the adhesive extremely stiff and barely flexible after curing. Therefore, developing a versatile adhesive system that can fulfill © XXXX American Chemical Society

the requirements of short process time at low curing temperature and that can maintain high flexibility is critically important. In particular, to retain the flexibility without sacrificing the high adhesion strength, the thickness of the adhesive must be scaled down substantially. Several studies on thin adhesive layers have been performed using solvent-based processes such as spin coating or dip coating.17−19 However, the use of solvents may damage or deform the substrates, which limits the types of applicable substrate.20,21 The solvent in the adhesive can also degrade the performance of sensitive devices such as organic electronic devices.22,23 Dry adhesive can overcome such limits of liquidphase adhesives in that the solvent-free adhesive can coat substrate surfaces without damaging them. Indeed, there are actually various types of commercially available dry adhesives such as 3M tape, gecko-like adhesive, and dry adhesive that works by surface wrinkling.24−26 However, achieving submicrometer, ultrathin dry adhesives to secure both excellent bonding strength and mechanical flexibility has remained elusive. Recently, new types of dry nanoadhesive layers have been demonstrated using thin polymer films deposited in solvent-free manner via initiated chemical vapor deposition (iCVD). Dry nanoadhesives allow the attachment of various kinds of substrates and have been applied to the sealing of microfluidic devices.27−29 An epoxy-containing iCVD polymer, Received: September 28, 2017 Revised: January 9, 2018

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DOI: 10.1021/acs.macromol.7b02102 Macromolecules XXXX, XXX, XXX−XXX

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various substrates including slide glass, Si wafer, PET, latex rubber, PEN, PDMS, and PI. The monomers, glycidyl methacrylate (GMA, 97%, Aldrich, USA) and 2-(dimethylamino)ethyl methacrylate (DMAEMA, 95%, TCI, Japan), and initiator, tert-butyl peroxide (TBPO, 98%, Aldrich, USA), were vaporized and introduced into an iCVD reactor (Daeki Hi-Tech Co., Ltd.). All the chemicals in this work were used as purchased without any further purification. In order to obtain the required flow rates, GMA and DMAEMA were heated to 35 and 40 °C, but TBPO was kept at room temperature due to its high volatility. For the deposition of p(GMA-co-DMAEMA) with various chemical compositions, the flow rate of TBPO was fixed to 0.60 sccm, and the flow rates of GMA and DMAEMA were varied from 0.57 to 1.36 sccm and from 0.35 to 1.15, respectively. In all cases, the filament temperature was kept at 180 °C. The reactor pressure and substrate temperature were kept at 160 mTorr and 30 °C. To deposit pGMA and pDMAEMA, the reactor pressure and substrate temperature were kept at 160 and 100 mTorr and 25 and 30 °C. The thickness of the copolymer layer was 300 nm. All the process parameters of copolymers synthesized in this work are summarized in Table S1 of the Supporting Information. The thickness of the deposited film was monitored in situ by interferometry.34 Preparation of Adhesion Samples. Following the deposition of copolymer and polymer nanoadhesive films on target substrates, the adhesive-coated substrates were stacked together. The glass−glass pair pressurized using a hot press at 1 MPa at 80 °C for a designated time, 3−120 min, and PET−PET pair were adhered with the thickness ranging from 50 to 600 nm with 1 MPa at preset temperature and time ranging from 80 to 120 °C and 1 to 120 min, respectively, in total thickness of adhesive, 600 nm. The other pairs, PET−latex rubber, PET−PDMS, PEN−PDMS, PEN−latex rubber, and PI−PDMS, were pressurized with 0.005 MPa at 80 °C for 10 min or 120 °C for 1 min. Analysis of the Synthesized Copolymer Films. Cross-sectional images were obtained using a scanning electron microscope (SEM) (Nova 230, FEI). Fourier transform-infrared (FT-IR) spectra were obtained using ALPHA FT-IR in absorbance mode (Bruker Optics). A total of 64 scans were collected and averaged for each spectrum. The X-ray photoelectron spectroscopy (XPS) and high-resolution spectra results were obtained using Sigma Probe Multipurpose XPS (Thermo VG Scientic) with a monochoromatized Al Kα source. The UV absorption spectra were analyzed using a UV-3600 UV−vis−NIR spectrophotometer (Shimadzu, Japan) using a bare glass slide as a reference within the wavelengths of the visible spectrum (400−800 nm). The hardness of the copolymer layer was measured by a nanoindentation system (Nano Indenter XP, MTS). Thermogravimetric analysis (TGA) was performed in the range from 25 to 500 °C (heating rate was maintained with 10 °C/min) using a Mettler-Toledo TGA instrument. Differential scanning calorimetry was performed in the range from 0 to 200 °C (heating rate was maintained with 20 °C/ min) using a Netzsch DSC 214 Polyma. The shear and peel strengths of the G−D copolymer nanoadhesive were estimated by using an Instron 5583 (Instron Inc., USA). All samples were pulled from both sides to measure the lap shear and peel strength. The test was performed in tensile mode at a pulling speed of 20 mm min−1. Atomic force microscope (AFM) images of G−D copolymer coated PET and wafer were taken by a scanning probe microscope (SPM) (XE-100, Park System) at a scan size of 5 by 5 μm2. Stability Test. Chemical stability of the G−D copolymer nanoadhesive was tested by measuring the lap shear strength of bonded glass slides exposed to each test chemical. Pairs of glass slides were bonded (bonding area = 625 mm2) and were soaked in various chemicals such as THF (99%, Junsei), DMF (99.8%, Aldrich), acetone (99.8%, Aldrich), and aqueous solutions at pH 2 and 13 for 20 h. Acidic and basic solutions were prepared using HCl (99.8%, Aldrich) and KOH (99.8%, Aldrich) in deionized (DI) water, respectively. After 20 h of soaking, the samples were rinsed thoroughly with DI water and dried with N2 gas for measurement of lap shear strength. Similarly, thermal stability was tested by measuring the lap shear strengths of samples after the exposure to 200 °C for 20 h. Flexing test was also performed by repeatedly flexing PET−PET samples bonded with the G−D copolymer-based nanoadhesives at the bending radius of 5 and

poly(glycidyl methacrylate) (pGMA), was employed as the major adhesive component. Most importantly, a coating of just 200 nm was thick enough to achieve an adhesion strength of 60 N/cm2. Moreover, the nanoadhesive was highly flexible so that the laminated substrates could be bent and folded without bonding failure. However, additional post-treatment of the cross-linking agent was unavoidable,29 or a different type of adhesive layer had to be engaged on each substrate surface for the binding of two substrates.27,28 Most of all, the curing times of the nanoadhesives were considerably longmostly longer than 8 h. This long curing time as well as the complicated deposition procedure critically limits the expanded application of this adhesive to various fields. Here, we develop a copolymer-based ultrathin but subminute curable dry adhesive layer with enhanced adhesion strength synthesized in a one-step manner. The copolymer film consists of epoxy-containing pGMA and tertiary aminecontaining polymer, poly(2-(dimethylamino)ethyl methacrylate) (pDMAEMA) (termed as “G−D copolymer” hereafter). The tertiary amine group in pDMAEMA greatly prompts the curing reaction of the adhesive layer by triggering a self-crosslinking reaction among the epoxide groups in pGMA. Especially, the tertiary amine moiety, distributed homogeneously in the copolymer film, dramatically facilitates a ringopening reaction with the epoxide functionalities nearby, which leads to a reduction of curing time to less than 1 min at 120 °C. The formation of a homogeneous copolymer with epoxide and amine functionalities can play a pivotal role in the development of a fast-curing adhesive. The composition of the adhesive copolymer film was elaborately controlled by tuning the input flow rate of GMA and DMAEMA vapors in iCVD in order to optimize the adhesion performance of the copolymer-based nanoadhesive. As a result, a substantial enhancement of the adhesion strength was achieved compared to values repoted in previous works on iCVD-based nanoadhesives and microscale polymer adhesives.27−33 Various kinds of flexible substrates including latex rubber, poly(ethylene naphthalate) (PEN), polydimethylsiloxane (PDMS), poly(ethylene terephthalate) (PET), and polyimide (PI) can be bonded tightly to each other by G−D copolymer adhesive. All the bonded substrates were fully flexible, without bonding failure, because the total adhesive layer is extremely thin (typically less than 600 nm). Also, the adhesive strength was also fully retained even after 500 repeated flexing cycles with bending radius of 5 and 10 mm. The folded samples with the bending radius of 5−12 mm were exposed to a harsh environment (85 °C and 85% relative humidity) for 20 h, but no apparent degradation in the adhesion strength was observed. Moreover, the G−D copolymer-based nanoadhesive maintained its adhesion strength after exposure to high temperature (200 °C) and various chemicals including acetone, toluene, dimethylformamide (DMF), tetrahydrofuran (THF), strong acid (HCl solution, pH = 2), and strong base (KOH solution, pH = 13). The ultrathin, fast-curing, environmentally stable G−D copolymer-based nanoadhesive developed in this study will play a critical role in emerging flexible and foldable device applications.

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EXPERIMENTAL SECTION

Synthesis of G−D Copolymers, PDMAEMA, and PGMA via ICVD Process. Poly(glycidyl methacrylate-co-2-(dimethylamino)ethyl methacrylate) (G−D copolymer nanoadhesive)), poly(2-(dimethylamino)ethyl methacrylate) (pDMAEMA), and poly(glycidyl methacrylate) (pGMA) films were prepared using iCVD process onto B

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Figure 1. (A) Schematic illustration of overall adhesion process with G−D copolymer-based nanoadhesive. (B) Cascade adhesive reaction mechanism of G−D copolymer-based nanoadhesive with epoxy functionalities. (C) Schematic illustration and cross-sectional SEM image of PUA nanopattern bonded to PDMS using G−D copolymer-based nanoadhesive. 10 mm for 500 times using a bending machine (HanTech Co., Ltd.) and measuring the peel strength before and after flexing. Finally, the thermal stability test in highly humid conditions was executed by inserting the PET−PET samples bonded with the nanoadhesive and folded at the bending radius of 5−12 mm in a humid oven (85 °C and 85% of humidity) for 20 h. The peel strength of the samples was measured after the thermal treatment.

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reaction with tertiary amine in the G−D copolymer adhesive is mainly responsible for the adhesion: the tertiary amine moiety triggers the cascade epoxy ring-opening reaction to form a heavily cross-linked polymer network.35−37 Increasing the curing temperature to 80 °C was sufficient to activate the tertiary amine to react with the epoxy ring to generate a quaternary amine cross-linking site and alkoxide ion. Then, the formed alkoxide ion reacts with the neighboring epoxy group, and a chain reaction propagates rapidly to the entire G−D copolymer layer, leading to the self-cross-linking of the epoxide group (Figure 1B). It is worthwhile to note that the homogeneous distribution of amine moiety in the G−D copolymer is essential to induce a cascade cross-linking reaction evenly throughout the adhesive layer in simultaneous manner. As a result, fast curing of the adhesive layer was achieved in a very short curing time if less than 10 min with just mild heating at 80 °C or in 1 min at 120 °C. Indeed, the homogeneous copolymerization is a unique characteristic of vapor-phase polymer synthesis, in which more than two different kinds of monomers favor to form a homogeneous mixture thermodynamically in vapor phase due to the huge entropy gain of mixing.34,38 Therefore, a homogeneous, random copolymer from any arbitrary monomer mixture can be synthesized in vapor phase. During the deposition of the adhesive layer in this study, the GMA and DMAEMA monomers were founded to be favorably miscible in vapor phase without any phase segregation

RESULTS AND DISCUSSION

Figure 1A shows the overall procedure of laminating two substrates using G−D copolymer-based nanoadhesive. For the synthesis of an adhesive copolymer layer, a vaporized initiator, GMA, and DMAEMA monomers were introduced simultaneously into the iCVD chamber. The G−D copolymer adhesive can be deposited limitlessly on various arbitrary substrates such as Si wafer, glass, PET, PEN, PDMS, PI, and latex rubber. After the deposition of the adhesive layer, substrates were stacked together with the adhesive-coated surfaces facing each other. Unlike the previously reported iCVD-based nanoadhesives,27−29 the G−D copolymer layer is the one necessary adhesive component for the binding of two arbitrary substrates. The adhesive was deposited on both substrates in a one-step process, and no further post-treatment was required. Then, the substrate pairs were stacked together and pressured at 1 MPa at a temperature of 80 °C for 3−10 min (Figure 1A) to bond the two substrates. The thermally activated epoxy ring-opening C

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Figure 2. Chemical composition analysis of G−D copolymer-based nanoadhesives, G−D1, G−D2, G−D3, and G−D4, with the pGMA and pDMAEMA homopolymer controls: (A) FT-IR spectra, (B) XPS survey scan spectra, (C) XPS N 1s high-resolution scans, and (D) the calculated surface composition of pDMAEMA (black) and quaternary N+ (red).

while the peaks at 2767 and 2819 cm−1 (red area in Figure 2A) are associated with the tertiary amine moiety in pDMAEMA. In the FT-IR spectrum of pGMA, only epoxy ring peaks were detected. Increasing the flow rate ratio of DMAEMA to GMA in the iCVD process resulted in a gradual decrease in the intensity of peaks related to epoxide functionality and an increase in the peak intensity representing the tertiary amine component in the FT-IR spectra. XPS survey scan and highresolution scan of O 1s and N 1s analysis were also performed on the series of G−D copolymer films with different compositions. From the XPS survey scan, the atomic ratio of O to N was estimated quantitatively. Accordingly, it was possible to calculate the surface composition of the copolymer of each G−D copolymer using the atomic fraction obtained from the XPS results. The calculated DMAEMA/GMA ratio in copolymers, G−D1−4, was ranged from 8.5:1 to 0.2:1 (Table S2). In the XPS survey scan, as the input flow rate of the DMAEMA monomer increased, the XPS N 1s peak intensity increased, while that of O 1s gradually decreased (Figure 2B). The N 1s high-resolution scan analysis also indicates that the composition of G−D copolymer adhesive can be systematically tuned, which is fully consistent with the FT-IR results. It is worthwhile to note that both an [N] peak at 397 eV and an [N+] peak at 401 eV were detected in the N 1s high-resolution spectra of the G−D copolymer (Figure 2C), which strongly implies that in the course of the iCVD copolymerization process part of the epoxy ring in the GMA moiety underwent a ring-opening reaction with the tertiary amine groups in the DMAEMA moiety to form quaternary ammonium. In the polymerization process, the two monomers with reactive functionalities are introduced into the vacuum chamber simultaneously and then adsorbed on the surface of the cooled target substrate, which are polymerized to form a copolymer film. Before the adsorption, the two input monomers in vapor phase can collide with each other to go through an epoxy ringopening reaction, but the collision frequency is quite low (about 104 times less than that in liquid phase), and the reaction between the two functionalities can be suppressed

and to form a homogeneous copolymer film via the iCVD process. As a result, the epoxy ring-opening reaction occurred uniformly throughout the substrate by thermal curing; this can substantially reduce the curing time. Figure 1C shows a representative cross-sectional scanning electron microscope (SEM) image of the two substrates bonded to each other by the G−D copolymer nanoadhesive: a flat PDMS substrate was attached to a poly(urethane acrylate) (PUA) substrate with 1600 nm grating pattern. For this lamination, a 300 nm thick adhesive layer was applied to each substrate and cured for 10 min at 80 °C. It can be clearly seen in the cross-sectional SEM image that the glued interface did not show any apparent delamination. In particular, a series of micrometer-scale channels were successfully formed by laminating the grating pattern with flat substrates, thanks to the excellent conformal coverage characteristics of the submicrometer thick G−D copolymer adhesive on the grating pattern and the intact bonding of the two substrates. To optimize the performance of the adhesive layer, a series of G−D copolymer films were synthesized via the iCVD process. The detailed iCVD process conditions for G−D copolymer adhesives including the chamber pressure, substrate temperature, and the flow rate of each species of GMA, DMAEMA, and initiator are summarized in Table S1 of the Supporting Information. Notably, it was possible to elaborately control the chemical composition of each G−D copolymer adhesive by tuning the flow rate ratio of GMA and DMAEMA monomers during the deposition process. In this study, four kinds of G−D copolymer adhesive layers were synthesized with input flow rate ratios of GMA to DMAEMA ranging from 4:1 (G−D1) to 1:2 (G−D4); the chemical composition of each G−D copolymer film was characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses (Figure 2). FT-IR spectra of the G−D copolymers with various compositions are shown in Figure 2A and compared with those of corresponding homopolymers, pGMA and pDMAEMA. The peaks at 759, 847, and 907 cm−1 (gray area in Figure 2A) indicate the presence of the epoxide ring moiety in pGMA27,39 D

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Macromolecules efficiently. The mobility of the adsorbed monomer is significantly low compared to that in vapor phase, which can minimize the unwanted reaction between the two functionalities in the adsorbed monomers. Cooling the substrate to keep the substrate temperature sufficiently low is another way to limit the epoxy ring-opening reaction during the polymer synthesis. By harnessing this unique advantageous characteristics of the vapor phase synthesis method, we can successfully demonstrate a one-step fabrication of a series of copolymer films containing the two highly reactive species but maintaining their reactive species less reacted. As a result, the undesirable epoxy ring-opening reaction could further be reduced by controlling the input monomer flow ratio of GMA and DMAEMA and maintaining the substrate temperature low (30 °C). The fraction of pDMAEMA and its quaternarized fraction in the homopolymers and the G−D copolymer were quantitatively estimated by measuring the atomic composition of [O] and [N] in the XPS spectra (Figure 2D and Table S2). The contents of pDMAEMA gradually increased and, accordingly, the fraction of [N+] contents was also measured with the increase of input flow rate of DMAEMA. The intensity of the [N] peak increased in accordance with the increase of the DMAEMA flow ratio, while the [N+] peak intensity mostly remained constant, which supports the hypothesis of partial conversion of the epoxy ring during the iCVD process, as noted above. With the optimization of the polymerization reaction, the amount of the undesirable cross-linking reaction in copolymer could be kept less than 5%, which was confirmed by high-resolution XPS analysis as shown in Figure 2D. In addition, no [N+] peak was detected in the XPS spectra of the homopolymers of pGMA and pDMAEMA, which means that the iCVD process did not damage the epoxy or the tertiary amine moiety, as was observed previously (Figure 2C,D).27 Analogously, an O 1s high-resolution scan also confirmed the compositional change in the copolymer film with changing of the flow rate of the DMAEMA monomer (Figure S1). By increasing the input ratio of the DMAEMA/GMA monomer, the O 1s intensity in the XPS survey scan spectra decreases. Although both DMAEMA and GMA contain O elements in the −CO− and −C−O− in the ester group, the epoxy peak at 533.1 eV is only present in GMA. Therefore, with increased contents of DMAEMA in the copolymer, the epoxy peak fraction gradually decreases, which leads to a decrease in the total O 1s peak intensity in the XPS survey scan spectra (Figure S1). These results clearly demonstrate that increasing the flow of DMAEMA monomer definitely led to a increase of DMAEMA contents in the G−D copolymer adhesive. These FT-IR and XPS analysis results show that a series of G−D copolymers can be synthesized whose composition can also be precisely tuned; this is fully consistent with previous observations.34,40 In thermally curable adhesives, optimizing the curing temperature is essential to achieve high adhesion strength. In this regard, a differential scanning calorimetry (DSC) analysis was performed to determine the optimal curing temperature of the copolymer adhesive layer (Figure 3A). In the DSC scan curve of the G−D1 copolymer adhesive, a prominent, wide exothermic peak was detected; this peak ranges from 80 to 170 °C. Interestingly, the exothermic peak only appeared in the first DSC cycle and totally disappeared in the second cycle. This observation suggests that an exothermic, irreversible crosslinking reaction in the copolymer becomes dominant in this temperature range. We also used XPS and FT-IR analysis to

Figure 3. (A) DSC spectra of G−D1 copolymer adhesive with first and second cycle from 0 to 200 °C. (B) XPS N 1s high-resolution scan of G−D1 copolymer adhesive before (black) and after thermal annealing (red). (C) Zoomed-in FT-IR spectra of epoxy peak changes in G−D1 copolymer adhesive with before and after thermal annealing.

check for evidence of an amine−epoxy reaction (Figure 3B,C). During the adhesion process with the G−D copolymer-based nanoadhesive, a reaction between epoxide and tertiary amine occurred, as illustrated by the peak intensity change of [N+] in the XPS N 1s high-resolution scan and in the FT-IR peaks associated with the epoxide functionality. In the N 1s highresolution XPS spectra of G−D1, the intensity of [N] decreased and that of [N+] increased after thermal curing (80 °C for 10 min). Likewise, the epoxide-related peak intensity in the FT-IR spectra was found to decrease continuously in the thermally cured G−D1 copolymer adhesive due to the epoxy− amine reaction. Interestingly, a large amount of unreacted [N] still remained after thermal curing, which means that only a small amount of [N] is sufficient to trigger the epoxy−amine reaction. Figure 4A shows the peel strength of samples bonded with G−D1 copolymer adhesive at different curing temperatures in a range from 80 to 120 °C for the designated time of 1−120 min. The peel strength of the samples gradually increased with the increase of the curing temperature from 80 to 120 °C over the E

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Figure 4. (A) Schematic of peel strength test method (left) and peel strength variation of G−D1 copolymer adhesive with respect to the curing temperature and time (right). (B) Schematic of lap shear strength test method (left) and the obtained lap shear strength of glass−glass substrates bonded with pGMA and the series of G−D copolymer-based nanoadhesives (right). (C) Peel strength results of PET−PET substrates bonded with pGMA and the series of G−D copolymer-based nanoadhesive. (D) Image plot of shear adhesion strength of the series of G−D copolymer-based nanoadhesives for various curing time at 80 °C. (E) Peel strength results of PET−PET substrates bonded with pG−D1 with variation of total adhesive thickness, 50−600 nm.

fixed curing time of 1 min. This observation is fully consistent with the DSC results in that an exothermic reaction occurs in this temperature range. Especially, at the curing temperature of 120 °C, a value of more than 30 N/25 mm of peel adhesion was achieved with a curing time of less than 1 min. The peel strength values for all the samples were found to have increased with the increased curing time to 10 min, reaching a plateau value of 32.5 N/25 mm; however, no further significant increase was observed for any curing time longer than 10 min. Via the optimization procedure described above, a sufficiently high adhesion strength was achieved with just the low curing temperature of 80 °C, which is quite low compared to the temperatures required for other kinds of epoxy-based thermally curable adhesives.16,41,42 In the thermally curable adhesive, curing at low temperature is of paramount importance for the lamination of various thermally vulnerable substrates such as organic electronic devices; the low temperature is necessary to avoid damaging these fragile materials. Especially, the short curing time of only 10 min is highly desirable for the scale-up of the lamination process. Further increase of the curing temperature to 120 °C induced even faster curing of the adhesive, within 1 min. Hereafter, for further study, we fixed the curing temperature and time at 80 °C and 10 min, respectively. The adhesion strength of each copolymer (G−D1−4) was tested via lap shear and peel tests as shown in Figure 4B,C. Two glass substrates and two PET substrates were bonded to each other using the G−D copolymer-based nanoadhesive for lap shear strength and peel strength measurements. A pGMA layer was also used as a control adhesive for comparison. To bond the substrates, pGMA and G−D adhesive-coated substrates were cured at 80 °C for 10 min. The lap shear adhesion strength of the control pGMA adhesive without the amine component (DMAEMA) was measured and found to be 89 N/cm2, which value is similar to the value reported previously.27 The quite low peel strength of 7 N/25 mm was only obtained from the control pGMA adhesive due to the fact

that the ring-opening reaction of epoxides could not proceed sufficiently at such a low curing temperature without the help of tertiary amine functionality. Indeed, the amine-free, pristine pGMA adhesive requires a curing temperature higher than 130 °C in order for it to undergo the cross-linking reaction; a value of 30 N/25 mm of peel strength was obtained for the pGMA adhesive cured at 130 °C for 10 min (Figure S2).43 In the tertiary amine moiety-containing G−D copolymer-based nanoadhesive, shear and peel adhesion strengths were both found to increase prominently at a far lower curing temperature (80 °C) than that used for the pGMA adhesive. This observation clearly indicates that the addition of a tertiary amine substantially facilitates the epoxy−amine ring-opening reaction, which is followed by propagation of epoxy rings, leading to high adhesion strength. Interestingly, the lap shear strength and the peel strength were found to decrease with increase of the pDMAEMA ratio from G−D1 to G−D4. As a result, the G−D1 copolymer adhesive with the smallest amount of pDMAEMA (pGMA:pDMAEMA = 4:1) showed the maximum adhesion strength in both the lap shear and peel tests. This observation strongly implies that the minimum amount of tertiary amine moiety necessary to trigger the epoxy ring-opening reaction is quite low, which is completely consistent with the proposed crosslinking reaction mechanism, in which the amine activates the epoxide ring in pGMA. The activated epoxide group, in turn, reacts with the neighboring epoxides, leading to the selfpropagation of epoxide cross-linking. It is also interesting to mention that too high an amount of tertiary amine can even disturb the propagation of epoxy ring-opening reaction and consequently decrease the adhesion strength substantially. The G−D4 copolymer adhesive with the highest amine contents of 83%, showed practically no adhesion, as can be observed in Figure 4B,C. This clearly shows that the cross-linkable epoxide group is dominantly responsible for adhesion and that the F

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Figure 5. (A) Transmittance and (B) the hardness of G−D copolymer-based nanoadhesive before and after thermal curing. (C) Peel strength results with PET−PET sample laminated with G−D1 copolymer adhesive flexed repeatedly for 500 times with the bending radius of 5 and 10 mm. (D) Peel strength results of PET−PET film bonded with 600 nm thick G−D1 copolymer adhesive and folded with different bending radii, 5, 7, and 12 mm, and exposed to a high temperature and humidity (85 °C, 85% RH for 20 h) condition in folded states. (E) Image of thick PDMS bonded with thin PEN, PET, and PI using G−D1. (F) Image of PEN-laminated latex rubber put on human hand, stretchable Latex rubber bonded with various substrates, PET, and PEN (12 μm) with G−D1 copolymer adhesive.

is still outstanding despite its extremely low, submicrometer thickness.30−33 Using PET−PET laminated samples, the thickness dependency of the peel strength of the G−D1 was also investigated in a range of adhesive thickness from 50 to 600 nm (Figure 4E). The adhesion strength was found to be practically independent of the thickness variation in this range, implying that the adhesive thickness can be scaled down to 50 nm without sacrificing the adhesion strength, which is highly advantageous for future applications in which extremely thin adhesive is required. Figure 5A shows the light transmittance of the G−D copolymer before and after curing. The average transmittances of the G−D copolymer film before and after curing in the visible light region (300−800 nm) are both nearly 100% of the bare glass transmittance because the adhesive layer is extremely thin (300 nm). The excellent optical transparency of the G−D copolymer is a great advantage for its application to nextgeneration electronic and optical devices. In addition, the curing of 300 nm thick G−D copolymer adhesive caused an the increase of its hardness from 0.241 to 0.323 GPa due to the increased cross-linking of the adhesive layer (Figure 5B). However, the hardness of the fully cured adhesive layer is still quite low, which is also desirable to maintain the mechanical flexibility of the adhesive layer even after the thermal curing.

amine plays the role of an initiator of the cross-linking reaction in the G−D copolymer adhesive system. Changing the pDMAEMA fraction in the copolymer adhesive was also found to affect the curing time required to achieve a sufficient amount of adhesion strength. Figure 4D shows the dependency of the shear adhesion strength of each copolymer adhesive layer on the curing time at the fixed curing temperature of 80 °C. The G−D1 copolymer adhesive with the smallest amine moiety in the G−D copolymer film series showed the best adhesion performance. No shear adhesion was observed for the G−D4 copolymer adhesive layer even after 120 min of curing time, which is consistent with the observation above. As a result, with the best-performing G− D1 copolymer adhesive, the minimum time required to achieve shear bonding strength higher than 150 N/cm2 was just 3 min; this value increases to 30 min for G−D2 and G−D3 (Figure 4D). From these results, we can confirmed that the curing temperature of the G−D copolymer-based nanoadhesive of 80 °C in this study is reasonable, and that among the G−D copolymer adhesives developed in this study, the G−D1 copolymer adheisve with the lowest amine contents was shown to be the optimum composition to maximize the adhesion. Furthermore, compared to those of previously developed microscale thickness polymer adhesives, the adhesion performance of the G−D1 copolymer adhesive developed in this study G

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well, which indicates that the homogeneous copolymer system with two functionalities have good affinities with both metal and polymer surfaces, amine functionality which can possibly form a chelation bond with metal layer and epoxy functionality which has good cohesion after curing process (Figure S4). These results clearly demonstrate that the G−D copolymerbased nanoadhesive can quickly bond various substrate pairs with extreme simplicity. It is also worthwhile to note that this thermally curable adhesive can be applied to various types of nontransparent substrates such as latex rubber, Si wafer, and PI, which is extremely challenging to achieve with photocurable resin applicable only to the UV-transparent substrates. These special capabilities can offer many advantages for applications in area such as flexible microfluidics and wearable electronics, in which devices require high flexibility and stability. Not only the adhesion strength but also the stability of the adhesive layer against thermal and chemical stresses must be secured for long-term use of an adhesive layer. The thermal stability of the G−D copolymer-based nanoadhesive was assessed by estimating the change of lap shear strength of the copolymer adhesive layer before and after thermal stress applied at 200 °C for 20 h. As can be seen in Figure 6A, no

In Figure 5C, the peel strength of a G−D1 copolymer adhesive used to laminate two PET substrates was measured before and after repeated flexing cycles. The estimated peel strength of the laminated PET substrate after bending 500 times at a bending radius of 5 mm was 32 N/25 mm, which value is similar to that of the initial adhesion strength, 33.5 N/ 25 mm. Generally, the adhesion performance of conventional epoxy adhesives with thicknesses of a few tens of micrometers is prone to damage upon mechanical strain because the cured epoxy adhesive becomes stiff and brittle.44−46 Therefore, instead of being flexed, cracks or fractures are likely to form upon bending or folding.47,48 On the other hand, the thickness of G−D copolymer-based nanoadhesive is extremely low, which makes the adhesive highly flexible down to a bending radius of 5 mm without failure. At the same time, the highly cross-linked nature of the copolymer adhesive can still preserve strong bonding between substrates even in folded state. Long-term storage of the laminated PET substrate in the folded state also had little effect on the adhesion performance. The PET substrate was laminated with G−D1 copolymer adhesive and folded at bending radius of ranging from 5 to 12 mm for 20 h at a temperature 85 °C and an 85% of relative humidity (RH) condition. The adhesion strength of the folded substrate, incubated in humid conditions, at bending radius of 5 mm was still 33.5 N/25 mm, which is identical to that of the unfolded sample (Figure 5D). Even after long-term folding, no defects or buckling was observed, confirming the outstanding flexibility. The mechanical flexibility analysis verified that the G−D copolymer adhesive retains sufficient flexibility for its application to flexible and foldable applications. To demonstrate the versatility of the G−D copolymer-based nanoadhesive, various substrate pairs were bonded, including PDMS−PEN, PDMS−PET, PDMS−PI, latex rubber−PET, and latex rubber−PEN. All the tested substrates were tightly laminated using the G−D1 copolymer adhesive at a pressure of 0.005 MPa and temperature of 80 °C for 10 min or at 120 °C for 1 min. In Figure 5E, it can be seen that the thick PDMS substrate was attached to thin PEN, PET, and PI substrates using G−D1 copolymer adhesive. Harsh bending was not able to delaminate the attached thick and thin substrates. The G−D copolymer-based nanoadhesive was applied to laminates stretchable substrate on nonstretchable substrates such as latex rubber−PET and latex rubber−PEN. As can be seen in Figure 5F, the bonding remained intact even when the PENbonded latex rubber was put on a human hand and the fingers were flexed multiple times. All samples showed excellent adhesion; substrate failure often occurred before delamination of the substrates during peel strength measurement (Figure S3). Furthermore, the strong adhesion was preserved even when the latex rubber was pulled harshly in a continuous manner (Movie S1 and Movie S2). The versatility of the G−D copolymer-based nanoadhesive on various substrates, as well as its excellent mechanical properties and outstanding adhesion, can be ascribed to the conformal deposition of the iCVD copolymer, which can lead to good contact between G−D copolymer adhesive and the substrates to be laminated, which in turn enables the formation of strong adhesion between the target substrates as far as a good contact was achieved between the two coated substrates. For example, the PET−latex rubber pair was laminated within a minute by G−D1 using just hand pressure to ensure contact between the PET and the latex rubber surfaces at 120 °C (Movie S3). In addition to G−D copolymer adhesive can laminated Al foil and latex rubber quite

Figure 6. (A) Lap shear strength data of glass−glass substrates bonded with 600 nm thick G−D1 copolymer adhesive before and after the application of the thermal stress at 200 °C for 20 h. (B) Lap shear strength results of glass−glass substrates bonded with the 600 nm thick G−D1 copolymer adhesive and exposed to various types of solvents for 20 h.

significant change in the lap shear strength was observed, verifying the excellent thermal stability of the developed adhesive layer. Thermogravimetric analysis (TGA) results also support the result observed above; the TGA curve of the crosslinked G−D1 did not show any apparent mass loss up to 200 °C (Figure S5). The chemical stability of the adhesive film is also one of the most important aspects of adhesives. The lap shear strength of the G−D1 copolymer adhesive was measured to check the chemical stability. Prior to this lap shear strength H

DOI: 10.1021/acs.macromol.7b02102 Macromolecules XXXX, XXX, XXX−XXX

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measurement, the bonded substrates were immersed in various organic solvents such as THF, DMF, acetone, base (NaOH(aq), pH 13), and acid (HCl(aq), pH 2) for 20 h. As can be seen in Figure 6B, the samples bonded with G−D1 copolymer adhesive did not show any sign of degradation in the adhesive strength. The high degree of cross-linking as well as the extremely low thickness of the G−D copolymer adhesive layer effectively prevents the penetration of solvents into the adhesive layer, enhancing the stability against various strong chemicals.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.G.I.). ORCID

Sung Gap Im: 0000-0001-7562-2929 Author Contributions

M.J.K. and D.H.K. contributed equally. Notes

The authors declare no competing financial interest.

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CONCLUSION In this work, a novel ultrathin dry adhesive was synthesized by simple, one-step iCVD process. The G−D copolymer-based nanoadhesive employs the epoxy ring-opening reaction with a tertiary amine initiator and, compared to previously reported polymer adhesives, possesses several advantages over conventional dry adhesives including high shear and peel adhesion strength, with values greater than 250 N/cm2 and 32.5 N/25 mm. While maaintaining the ultralow thickness, the nanoadhesive developed in this study showed outstanding mechanical flexibility, high optical transprency, and excellent environmental stability against heat and various chemicals. Most of all, due to the fast propagation reaction of epoxides with the presence of tertiary amine moiety, the submicrometer thick adhesive can be cured within 3−10 min and at a curing temperature as low as 80 °C. The required curing time can be lower than 1 min at an increased curing temperature of 120 °C. The highly cross-linked G−D copolymer-based nanoadhesive also maintained excellent adhesion strength after harsh mechanical, thermal, and chemical stresses. The G−D copolymer-based nanoadhesive system was found to be applicable to various kinds of substrates including Si wafer, glass, Al foil, polymeric substrates, latex rubber, and PDMS. Unlike photocurable resin, this adhesive can be used for the lamination of nontransparent substrates. The adhesion strength is preserved even after bending 500 times at a 5 mm bending radius. These outstanding characteristics of the G−D copolymer adhesive are highly advantageous for the application of this material to various fields, such as flexible electronic devices or microfluidic systems.

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ACKNOWLEDGMENTS This work was supported by Wearable Platform Materials Technology Center (WMC) funded by the National Research Foundation of Korea (NRF) Grant of the Korean Government (MSIP) (No. 2016R1A5A1009926) and by the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (CASE2011-0031638).

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REFERENCES

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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/acs.macromol.7b02102. Summary of the iCVD process conditions, calculated surface compositions, XPS O 1s high-resolution scan of pGMA, G−D1 to G−D4, and pDMAEMA, the peel strength result of PET−PET film bonded with pGMA, the peel strength result and digital images of peel strength process with latex−PEN and latex−PET bonded by G−D1, digital images of latex-Al foil bonded by G-D1, the TGA results of G-D copolymer adhesive before and after thermal annealing from 25 to 500°C (PDF) Movie S1 (AVI) Movie S2 (AVI) Movie S3 (AVI) I

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DOI: 10.1021/acs.macromol.7b02102 Macromolecules XXXX, XXX, XXX−XXX