Light-Induced Reworkable Adhesives Based on ABA-type Triblock

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Applications of Polymer, Composite, and Coating Materials

Light-induced reworkable adhesives based on ABAtype triblock copolymers with azopolymer termini Shotaro Ito, Haruhisa Akiyama, Reiko Sekizawa, Miyuki Mori, Masaru Yoshida, and Hideyuki Kihara ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09319 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on September 3, 2018

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ACS Applied Materials & Interfaces

Light-induced reworkable adhesives based on ABA-type triblock copolymers with azopolymer termini Shotaro Ito,† Haruhisa Akiyama,*‡ Reiko Sekizawa,† Miyuki Mori,† Masaru Yoshida‡, and Hideyuki Kihara,*†

†Research Institute of Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST Chugoku) 3-11-32, Kagamiyama, Higashihiroshima, Hiroshima 739-0024, Japan ‡Research Institute of Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST) Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

Corresponding authors (H.A.) Email: [email protected], Phone: +81-29-861-4418, Fax: +81-29-861-4669 (H.K.) Email: [email protected], Phone: +81-29-861-4442, Fax: +81-82-420-8309

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KEYWORDS

photocurable; adhesives; block copolymers; azobenzene polymers; flexible

ABSTRACT

Photocurable adhesives based on polymers and resins are an integral part of different production processes because of their fast curing and local area bonding ability. Recently, dismantlable adhesives have attracted a lot of attention for recycling adherends or replacement of adhesion defects. However, adhesives that allow repeatable bonding and debonding solely by light irradiation, i.e., without heat activation, are lacking. Here, ABA-type triblock copolymers consisting of poly(meth)acrylates bearing an azobenzene moiety (A block) and 2-ethylhexyl (B block) side chains were synthesized and utilized as photocurable adhesives. In contrast to the azo homopolymers, the block copolymer structure and incorporation of the soft middle block actualized a low concentration of the azobenzene moiety and consequently, higher flexibility of the resultant copolymers. This enabled film formation of the azobenzene-based adhesives and light-induced bonding for the first time. Based on the


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photoisomerization of the azobenzene moiety, changes in their viscoelastic property, i.e., softening and hardening, were induced by UV irradiation at 365 nm (50–100 mW cm–2) and green light irradiation at 520 nm (40 mW cm–2), respectively. In fact, two glass substrates were bonded with the self-standing polymer film, which was sequentially softened and hardened upon UV and green light irradiations. They exhibited shear strengths of 1.5–2.0 MPa and UV irradiation lowered the adhesion strength to 0.5–0.1 MPa. Interestingly, the repeatable bonding and debonding ability of the polymers were accomplished without loss of the adhesion strength.

INTRODUCTION

Adhesives are widely used in the automotive, aerospace, construction, and biomedical industries, and they must meet various requirements such as heat resistance and/or long-term reliability. Nowadays, sustainability is also an important objective during the material life cycle.1 To meet these criteria, reworkable adhesives that enable repeatable bonding and debonding on demand have been developed for improving the efficiency of manufacturing processes and reducing wastes by replacing defective components and recycling adherends.2 However, there are very few reports of such adhesives,3-14 3 ACS Paragon Plus Environment


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although dismantlable adhesives for one-way debonding use have been reported with various mechanisms that are mainly based on heat activation.15-27 Among them, light-induced reworkable adhesives are of particular interest since light can activate the adhesive via an athermal process with focus on a local adhesion area. Thus, they are useful for biomedical applications and temporary fixing of adherends that are damaged by heat and are not covered by heat-activation-based adhesives. To the best of our knowledge, the light-induced phase transition behavior of liquid crystal (LC) molecules reported by us7,28-30 is the only reworkable adhesive system that can be activated by an athermal process. We have previously reported that the isothermal, photoinduced solid– liquid phase transition was realized for the first time based on the photoinduced trans– cis isomerization of azobenzene moieties of sugar-alcohol derivatives, and it was successfully

applied

to

reworkable

adhesives.7

Similarly,

azobenzene-based

poly(meth)acrylates with appropriate molecular designs showed repeatable softening and hardening upon photoirradiation28,30 because their glass transition temperatures of trans and cis-azobenzene forms lay above and below room temperature, respectively.31 Single lap shear tests using glass substrates revealed that their adhesion strength (~4 MPa) was comparable to commercially available hot melt adhesives and could be lowered to less than 0.2 MPa by ultraviolet (UV) light–irradiation for only 15 s (365


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nm, 50 mW cm–2).28,30 Thus, azobenzene-based polymers demonstrate strong bonding and fast debonding abilities. Nevertheless, despite their excellent aspects as a reworkable adhesive, their application still faces some limitations; owing to the high molar absorption coefficient of azobenzene moieties, upon UV irradiation, liquefaction proceeds slowly into the deep parts of the solid and it takes long time (~1 h) to completely liquefy it. In addition, the azopolymers are too brittle to be fabricated into self-standing thin films having a thickness of several tens of micrometers, although the azopolymer thin films as an adhesive form are thought to be effective for shortening the liquefaction time. Thus, the fabrication of the polymer films with a relatively low fraction of the azobenzene moiety in the polymers can presumably lead to the production of a reworkable adhesive that can be activated by light for the entire bonding process.

The abovementioned aspects may be comprehensively addressed, for example, by using either polymer blends of an azobenzene-based homopolymer with a general (meth)acrylic polymer or statistical or block copolymers of an azobenzene-based monomer and a (meth)acrylic monomer. However, polymer blends are generally macrophase-separated32 and ineffective as reworkable adhesive. Statistical copolymers with a general (meth)acrylic comonomer are also thought to be ineffective, as they do 5 ACS Paragon Plus Environment


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not have continuous azo (monomer) sequence, which has been necessary for light-induced solid-liquid phase transitions reported by us and H. Zhou et. al29–31 On the other hand, block copolymers of azobenzene-based polymers with liquid (meth)acrylic polymers were identified as promising candidates.33 These copolymers can be classified as thermoplastic elastomers (TPEs) as they possess hard and soft domains (azopolymer and liquid polymer, respectively), and the physical cross-links of hard domains can be weakened by light-induced softening of the azopolymer block.

In this study, azobenzene-based block copolymers have been developed as light-responsive reworkable adhesives that undergo both light-induced bonding and debonding processes. These block copolymers consisted of azobenzene-based poly(meth)acrylate

[poly(10-[4-(4-hexylphenylazo)phenoxy]decyl

methacrylate)

(PAzoM) and poly(10-[4-(4-hexylphenylazo)phenoxy]decyl acrylate) (PAzoA)] as photoresponsive units, and poly[2-ethylhexyl (meth)acrylate] (PEHA and PEHMA), which are liquid and colorless polymers. Incorporation of PEHA and PEHMA blocks imparted flexibility to the azobenzene-based polymers and kept the concentration of the azobenzene chromophore low, which helped with the adhesive film formation and quick photoinduced softening/hardening reaction. For the block copolymer structure, the ABA-type triblock copolymers were chosen. This is because it is known from the 6 ACS Paragon Plus Environment

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reported structure-property relationships of TPEs that the ABA-type triblock copolymers exhibit better mechanical properties than corresponding AB-type diblock copolymers.34 Since the photoresponsive property changes of the azo homopolymers are dependent on the polymer main chain structures, i.e., methacrylic and acrylic,30 all-acrylate-based and all-methacrylate-based ABA-type triblock copolymers were also synthesized. The photoisomerization behavior and viscoelastic property changes of the block copolymers, and film fabrication of the block copolymers and their application to a reworkable adhesive have been discussed in detail.

EXPERIMENTAL SECTION

Materials. All chemicals were purchased from either Tokyo Chemical Industry, Fujifilm Wako Pure Chemical Co., Ltd., or Sigma Aldrich Japan, and used without further purification unless otherwise stated. 2-Ethylhexyl (meth)acrylates and anisole were distilled over CaH2. AzoA and AzoM monomers were prepared following previously reported methods.21 CuBr and CuCl were washed with glacial acetic acid overnight, washed with acetone, and dried under vacuum.



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Synthesis of P2. Ethylene bis(2-bromoisobutylate) (EBBI) (36.0 mg, 0.100 mmol), EHMA

(2.00

g,

10.1

mmol),

CuBr

(14.3

mg,

0.100

mmol),

1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA) (23.0 mg, 0.100 mmol), and anisole (2.3 mL) were mixed under argon in a pre-dried Schlenk flask (30 mL) and degassed by freeze-pump-thaw cycles three times. The flask was placed in an oil bath preheated at 70 °C for 1 h. The conversion was determined to be 65% by 1H NMR. The solution was diluted with tetrahydrofuran (THF) and passed through an activated neutral alumina column before precipitating into MeOH. The collected polymer P1 was reprecipitated into MeOH and dried at 50 °C in vacuo overnight (1.08 g, 54% yield). Mn,GPC-LS = 16.7 kg mol–1, Mw/Mn = 1.14. For P2, a similar atom transfer radical transfer polymerization (ATRP) method35 using P1 as a macroinitiator was employed. The reagents used for the synthesis of P2 were as follows: P1 (0.246 g, 0.0147 mmol), AzoM (0.270 g, 0.534 mmol), CuCl (5.84 mg, 0.0590 mmol), HMTETA (13.6 mg, 0.0590 mmol), and anisole (1.5 mL). The block copolymerization was performed at 80 °C for 24 h. The monomer conversion was 97%. The resulting polymer was purified twice by precipitation into MeOH and freeze-dried from its benzene solution (0.33 g, 64% yield). Mn,GPC-LS = 45.6 kg mol–1, Mw/Mn = 1.29, Composition: PAzoM/PEHMA = 50/50 wt by 1H NMR.


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Synthesis of P4 and LMW-P4. Bifunctional PEHA P2 was prepared in a similar manner to P1. The following reagents were used: EBBI (36.0 mg, 0.100 mmol), EHA (2.00 g, 10.9 mmol), CuBr (14.3 mg, 0.100 mmol), HMTETA (23.0 mg, 0.100 mmol), and anisole (2.3 mL). The polymerization was performed at 80 °C for 19 h. The monomer conversion was 69% and the resulting polymer was purified in the same manner as P1 (1.02 g, 51% yield). Mn,GPC-LS = 15.9 kg mol–1, Mw/Mn = 1.14. The block copolymerization was similarly performed using the following reagents: P3 (0.265 g, 0.0167 mmol), AzoA (0.330 g, 0.664 mmol), CuBr (9.55 mg, 0.0666 mmol), 4,4′-dinonyl-2,2′-dipyridyl (54.4 mg, 0.0666 mmol), and anisole (0.33 mL). The polymerization was performed at 80 °C for 24 h. The monomer conversion was 90% and the resulting polymer was purified in the same manner as P2 (0.48 g, 81% yield). Mn,GPC-LS = 64.3 kg mol–1, Mw/Mn = 1.74, Composition: PAzoA/PEHA = 54/46 wt by 1H NMR. A part of P4 was separated by a preparative GPC to produce LMW-P4. Mn,GPC-LS = 44.6 kg mol–1, Mw/Mn = 1.26, Composition: PAzoA/PEHA = 52/48 wt by 1H NMR.

Measurements. All analyses by 1H NMR, differential scanning calorimetry (DSC), polarized optical microscopy (POM), gel permeation chromatography (GPC), ultraviolet-visible (UV-Vis) spectrometer, UV and green lights (light emitting diodes), rheometer, tensile testing machine were performed in the same manner as described in a 9 ACS Paragon Plus Environment


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previous report.30 The absolute molecular weight of the polymers was determined by GPC-LS and the polymer films were prepared on quartz plates for UV-Vis measurements according to the previously reported procedures.30 The details are given in the Supporting Information.

RESULTS AND DISCUSSION

Synthesis of ABA-Type Triblock Copolymers. Block copolymers using azobenzene-based methacrylate monomers have been previously synthesized by different polymerization methods.36-38 In this study, a typical two-step block copolymerization method using ATRP was chosen for the synthesis of ABA-type triblock copolymers (Figure 1).36 For the methacrylic block copolymer, the bifunctional macroinitiator Br-PEHMA-Br (P1 in Table S1) was first prepared in anisole at 80 °C using EBBI, CuBr, and HMTETA as the bifunctional initiator, catalyst, and ligand, respectively. Figure 2a shows the GPC trace of the resulting polymer with the relatively sharp and monomodal distribution. The absolute molecular weight was measured by GPC equipped with a multiangle laser light scattering detector (GPC-LS) and was found to be close to the calculated value based on the monomer/initiator feed ratio and


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monomer conversion (Table S1). The second monomer, AzoM, was then polymerized with the bifunctional macroinitiator P1 under similar conditions except for the use of CuCl instead of CuBr according to the literature method.36 The monomer conversion was 97% after 24 h and the GPC trace of the resulting polymer P2 was completely shifted to the higher molecular weight region (Figure 2a), suggesting quantitative block copolymer formation. The absolute molecular weight measured by GPC-LS was slightly higher than the calculated value because of the high molecular weight shoulder of the GPC trace, which was presumably produced by the radical recombination reaction at high monomer conversion.39 Figure S1 shows the 1H NMR spectrum of P2. The composition was determined by the peak area ratio of the characteristic signals derived from the trans-azobenzene moiety (7.7–8.0 ppm, 4H for the azo monomer unit) and the methylene protons adjacent to the ester groups for both blocks (3.7–4.1 ppm, 2H for EHMA unit and 4H for the azo monomer unit). The observed composition was almost identical to the theoretical value as listed in Table S1, suggesting the successful formation of the target methacrylic triblock copolymer.



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R O O Br O

O

O

Br

O

Br R

O

O

n

O

ATRP

O

R Br

O

n

O

O

O

P1: R = CH3 P3: R = H R O

O(CH2)10O ATRP

N N

R m

O

O

R

C6H13

O

O

n

O

O

R

O

O

R n

O

O

m

O

(CH2)10 O

N

O (CH2)10 O

N

P2: R = CH3 P4: R = H

C6 H13

N

N

C6H13

Figure 1. Synthesis of ABA-type triblock copolymers.


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Figure 2. GPC traces of (a) methacrylic triblock copolymer P2 (solid line) and its precursor polymer P1 (dashed line), and (b) acrylic triblock copolymer P4 (solid line), LMW-P4 (dotted line), and its precursor polymer P3 (dashed line).

To the best of our knowledge, block copolymerization of azobenzene-based acrylates has not been reported. Therefore, the ATRP method previously developed for the homopolymerization of azobenzene-based acrylates was applied to the block copolymerization for the synthesis of ABA-type acrylic triblock copolymer P4.30 Initially, a bifunctional macroinitiator Br-PEHA-Br P3 was prepared in a similar manner to P1. The obtained polymer possessed a relatively narrow molecular weight distribution and expected molecular weight (Table S1). The block copolymerization was then carried out by using the AzoA monomer in anisole solution at 80 °C. Although the second monomer conversion reached to 90%, a large shoulder of the GPC trace was in the higher molecular weight region (Figure 2b). Since the AzoA monomer concentration for polymerization was required to be high (in this case; 0.72 M) compared to AzoM monomer (0.27 M),30 more radical recombination products with high molecular weights were produced than those for P2. A supplementary polymerization that stopped at low monomer conversion (53%) did not afford an acrylic 13 ACS Paragon Plus Environment


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triblock copolymer free from the radical recombination products (Figure S2). Thus, P4 was used as-synthesized for analysis. In addition, the high-molecular-weight shoulder was

removed

by

preparative

GPC

(Figure

2

and

Table

S1)

to

afford

low-molecular-weight P4 (LMW-P4), which was also used for subsequent measurements.

Thermal Properties of ABA-Type Triblock Copolymers. DSC was used for the determination of the glass transition (Tg) and LC phase transition temperatures, which are listed in Table S1. The DSC curve for P2 showed large and small exothermic peaks at 98 °C and 55 °C (Figure 3a), which could be assigned to the isotropic (I)–smectic A (SmA) and SmC–SmF (or SmI) phase transitions of PAzoM block, respectively, based on the previous thermal analysis of the corresponding homopolymer PAzoM (Figure S3 shows polarized optical microscopy images of P2 and P4 at room temperature).30 The exothermic peak corresponding to SmA–SmC transition (~ 95 °C) was not detected because of its small transition heat. The acrylic triblock copolymer P4 similarly exhibited phase transitions at 101 °C and 76 °C derived from I–SmA and SmA–SmB transitions, respectively (Figure 3b). Because glass transitions of both blocks of P2 and


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P4 were not clearly observed by DSC, temperature-dependent dynamic viscoelastic behavior was investigated using a rheometer. Figure 4a shows the temperature dependence of storage modulus G', loss modulus G'', and loss tangent tan δ of P2. It is evident that the modulus changed at two different temperatures that corresponded to the I–SmA and SmC–SmF (or SmI) transitions. P2 behaved as a fluid above 85 °C, since

G'' exceeded G', while being elastic below 85 °C. The tan δ peak was observed at 50 °C and indicated glass transition of the PAzoM block. For the acrylic triblock copolymer P4, similar LC phase transitions and glass transition (at 53 °C) was observed (Figure 4b). Interestingly, LMW-P4 exhibited almost identical viscoelastic property changes as P4 (Figure S4). As a result, the azopolymer blocks of P2 and P4 exhibited the LC phase transition and glass transition temperatures that were close to those of corresponding homopolymers.30 Since the Tg values of middle blocks PEHMA and PEHA are typically around –10 °C and –60 °C,34,40 respectively, it is likely that two types of domains, namely, the hard domains of the azopolymer blocks and the soft domains of the PEHMA and PEHA blocks, were formed in the ABA-type triblock copolymers.33 Thus, the triblock copolymers were classified as a physically cross-linked TPE. In fact, P2 and P4 exhibited elastic behavior with G' ~10 MPa at room temperature, which corresponds to typical thermoplastic polymers, while they became fluidic with G' being less than 102



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Pa above 100 °C.41 Interestingly, the triblock copolymers were still elastic to some extent above the Tgs of the hard segments, since the liquid crystalline phases of the hard segments remained at a temperature between Tg and that for transition to the isotropic phase.

Figure 3. DSC thermograms of a) P2, b) P4, and c) LMW-P4.


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Figure 4. Temperature dependence of storage (G', solid line) and loss (G'', gray line) moduli, and that of the loss tangent (tan δ, dotted line) of (a) P2 and (b) P4 (cooling at 3 °C min–1 from 140 °C).

Light-Induced Viscoelastic Property Changes. Photoisomerization behavior of the azobenzene moiety in the triblock copolymers was investigated by ultraviolet– visible (UV–Vis) absorption spectroscopy. To study the photoswitching behavior in the



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solid state, thin films were prepared on quartz plates by spin-coating 4 wt% CHCl3 solution of P2 and annealing at 140 °C for 30 min. Initially, the spectrum showed an intense π–π* band in the UV region and a weaker n–π* band in the visible region, indicating the trans isomer of the azobenzene moiety (Figure 5a).42 The absorption maximum was blue-shifted compared to the corresponding CHCl3 solution (from 352 nm to 338 nm, Figure S5). This was attributed to H-aggregation, i.e., the parallel stacking of the azobenzene moiety, which is characteristic of smectic phases.43,44 Upon UV irradiation (365 nm), the π–π* band intensity decreased, while the n–π* band intensity increased concomitantly, suggesting the formation of the cis-azobenzene moiety (Figure 5a).42 Subsequent green light irradiation (520 nm) induced the opposite isomerization from cis- to trans-azobenzene (Figure 5b). 1H NMR measurements of the photoproducts showed that the cis- and trans-azobenzene moieties were mainly observed (~95%) in the photostationary state upon UV and green light irradiations, respectively (Figure S6). These experiments confirmed the photoisomerization behavior of the azobenzene moiety of P2, which was very similar to that of the corresponding homopolymer PAzoM.30 On the other hand, the photoisomerization behavior of P4 was different from that of the corresponding azohomopolymer PAzoA.30 The thin film of P4 similarly prepared and annealed on a quartz plate showed relatively small absorbance


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(Figure S5d). Subsequent irradiation of the film by UV and green light randomized the azobenzene alignment and the absorbance increased to some extent. Upon UV irradiation, the intensity of the π–π* band decreased and that of the n–π* band increased with the irradiation time, although the degree of the spectral changes was not as intense compared to those of P2 (Figure 5c). On the other hand, upon green light irradiation, the intensity of the π–π* band increased until an irradiation time of 5 s, while longer irradiation decreased the π–π* band intensity (Figure 5d). This was attributed to the azobenzene alignment-directional change during the irradiation, which was facilitated by the flexible acrylic P4 main chain. The higher flexibility of the acrylic P4 polymer chains compared to that of methacrylic P2 facilitated a more perpendicular alignment of its azobenzene moieties to the substrate in order to decrease its surface energy, and consequently resulted in the different photoisomerization behavior.45 The quantitative photoisomerization of the azobenzene moiety in P4 was confirmed by 1H NMR, which clearly showed the cis- and trans-isomers (~95%) after the UV and green light irradiations, respectively (Figure S6). Thus, photoisomerization behavior was observed for both methacrylic P2 and acrylic P4 triblock copolymers in their solid state.



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Figure 5. UV–Vis absorption changes of (a, b) P2 and (c, d) P4 thin films upon irradiation with (a, c) UV light at 20 mW cm–2 and (b, d) green light at 40 mW cm–2. The irregularity at 360 nm was caused by spectrometer light source switching.

Light-induced viscoelastic property changes, which occurred concomitantly with azobenzene photoisomerization, were studied by dynamic viscoelastic measurements under isothermal conditions (30 °C) (Figure S7 shows the temperature during measurements); the results are shown in Figure 6. About 1 min after UV light irradiation (365 nm, 100 mW cm–2), the G' value of P2 decreased from 10 to 1 MPa, while that of P4 decreased from 10 to 0.1 MPa. Since both azopolymers PAzoM and


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PAzoA exhibited a rapid decrease in the G' values from 100 to 0.01 MPa under similar conditions, the residual storage moduli were mainly attributable to the middle blocks, PEHMA and PEHA. Thus, the more flexible acrylic P4 showed a lower G' value. Although the change in the G' value of LMW-P4 was apparently similar to that of P4, G'' exceeded G' during irradiation (Figure S8) and suggested fluidic behavior, which could not be observed for P2 and P4. In summary, the removal of high molecular weight polymer in P4 resulted in a lower viscosity under UV irradiation. All the UV-irradiated triblock copolymers were found to exhibit similar viscoelasticity in pressure-sensitive adhesives, since their G' value lay in the range of 103 to 106 Pa.46,47 Note that the G', G'', and tan δ values reflected the most softened part near the irradiation surface, while the rest remained solid.



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Figure 6. Time-dependent changes of G' (solid line), G'' (gray line), and tan δ (dotted line) of (a) P2 and (b) P4 upon irradiation with UV light (365 nm, 100 mW cm–2) at 30 °C. UV irradiation was started at 2 min.

To study the degree of the photoisomerization that proceeded in the depth direction, P2 and P4 were placed between two glass slides and the thicknesses were set to ca. 30 µm by thermal spreading treatment. After UV irradiation (50 mW cm–2, 5 min), the


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samples were dissolved in CDCl3 in the dark and characterized by 1H NMR, with the fractions of the cis-azobenzene moiety listed in Table 1. The results showed that about half of the trans-azobenzene moieties were photoisomerized under the employed reaction conditions with 30 µm-thick samples. Thus, photoisomerization occurred from the irradiation surfaces and the differences in the polymer main chain structures of P2 and P4 did not affect the degree of photoisomerization in the depth direction, which was different from the case of the azo homopolymers PAzoM and PAzoA.30

Table 1. Fractions of cis-Azobenzene Moiety Observed during Thermal and Photochemical Treatments. Fraction of cis-azobenzene moieties (%) a

Thermal treatment b

UV irradiation Subsequent

green

P2

P4

0

0

57 ± 3.6

56 ± 2.1

light 6

5

c

irradiation a

Rapid cooling to room temperature from 150 °C. b Irradiation with UV light (365 nm,

50 mW cm–2) for 5 min. The reported values are averages of three measurements with standard deviations. c Sequential irradiation of UV and green light (40 mW cm–2) for 5 min each.



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Light-induced reworkable adhesive properties. At first, thermally bonded specimens, which were prepared by bonding two glass substrates with thermally melted and subsequently cooled polymers, were used for single lap shear tests (bonding area ~75 mm2, thickness ~20 µm). The adhesion strengths for P2 and P4 were 2.0 and 1.5 MPa, respectively (Figure 7a). Interestingly, LMW-P4 exhibited similar adhesion strength to P4, suggesting no significant effect of the molecular weight on the adhesion strength. The effect of cis-azobenzene fraction was also studied because the cis-azobenzene moiety has a large dipole moment because of its bent structure and allows stronger polymer-glass interactions. Sequential irradiation of the thermally bonded specimens with UV and green light produced polymers with about 5% of the cis-azobenzene fraction, while the thermally bonded polymers were free from the cis-azobenzene fraction. However, the adhesion strengths were not significantly influenced by the cis-azobenzene fraction (Figure 7b), which was different from the case of PAzoM and PAzoA homopolymers.30 The different results can be rationalized by considering the adhesion failure modes, namely, cohesive and interfacial failures. Since the azo homopolymers experienced interfacial failure or a complex of interfacial and cohesive failures, their adhesion strength was enhanced with the strong interfacial interaction of


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the cis-azobenzene–glass substrate. On the other hand, since the adhesion specimens in this study were deformed during testing, the cohesive force of the adhesive polymers mainly governed the adhesion strength. Consequently, the fraction of the cis-azobenzene moiety did not considerably influence the adhesion strength. The cohesive failure of the block copolymers was mainly due to the lower mechanical strength (G′ ~107 Pa at room temperature), compared to the azo homopolymers (G′ ~108 Pa).

Figure 7. Adhesion strengths of thermally bonded specimens with sample thickness of around 20 µm; (a) as is, (b) irradiated with UV (365 nm, 50 mW cm-2) and green light (520 nm, 40 mW cm-2), (c, d) irradiated with UV light (365 nm, 50 mW cm-2) for (c) 15 s and (d) 5 min.



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Adhesion strength phototuning was carried out by UV irradiation before the single lap shear test. With UV irradiation for only 15 s, critical stresses were lowered to 0.48 and 0.08 MPa for P2 and P4, respectively (Figure 7c). The residual strengths were very close to the adhesion strengths of the soft middle blocks (0.47 MPa for P1, 0.00 MPa for P3). Therefore, although the short-time irradiation softened the bonding polymers mainly near the irradiation surface, the adhesion strengths were effectively lowered. Prolonged irradiation (5 min) for P2 caused further decrease of the adhesion strength to 0.23 MPa, but it was still higher than that of P4.

Next, the light-induced bonding process was investigated. Polymer films of P2 and P4 were successfully prepared by spreading the thermally melted polymers between two poly(tetrafluoroethylene) (PTFE) films, followed by removal of the PTFE films after solidification. The block copolymer structures proved to be highly advantageous for the fabrication of azopolymer films, because the azo homopolymers could not be formed into thin films in a similar manner. Two glass substrates were used to sandwich the peeled P2 or P4 films and bound with bulldog clips before UV and green light irradiations for 5 min each. For P2, photochemical bonding was first attempted with UV light at 50 mW cm–2, but no adhesion occurred. In contrast to the photochemical debonding process, photochemical bonding requires a polymer film that was softened at 26 ACS Paragon Plus Environment

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both adhesive and substrate interfaces (top and bottom interfaces). Thus, a higher UV light intensity or longer irradiation time is required for the photochemical bonding process. In fact, photochemical bonding with P2 films was successful with higher UV light intensity (110 mW cm–2). The adhesion strengths were measured for films with two different thicknesses, 76 and 27 µm (Table 2). As already reported,48 the thinner adhesive film exhibited higher adhesion strength of 1.85 MPa (failure mode: adhesive deformation), which was close to that of the thermally bonded specimen. Thus, the adhesive-substrate interfaces were photochemically bonded as tightly as the thermally bonded specimen. On the other hand, P4 films could be bonded with weaker UV light intensity of 50 mW cm–2. Thus, the softer nature of P4 films under UV irradiation facilitated the adhesion of the polymer-substrate interfaces with lower UV light intensity. However, the adhesion strength of P4 (28 µm thickness) was low (0.81 MPa) compared to the thermally bonded one (1.44 MPa). This was attributed to the different microphase-separated structures formed by photochemical and thermal treatments, and are currently under investigation.

To determine the sample temperature during light-induced bonding, separate experiments were performed as shown in Figure S9, as the sample temperature during light-induced bonding could not directly measured. For the measurement, a P2 film (25 27 ACS Paragon Plus Environment


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µm thickness, ca. 15 mm diameter) prepared on an aluminum foil was attached to a temperature sensor, and was sequentially irradiated with 365 nm UV light (110 mW cm-2), 520 nm green light (40 mW cm-2), and 365 nm UV light (50 mW cm-2). As a result, the sample temperatures increased up to 45 °C (UV, 110 mW cm-2), 35 °C (UV, 50 mW cm-2), and 28 °C (green light, 40 mW cm-2), respectively, starting from 26 °C (room temperature 26 ° C). Thus, during the light-induced bonding using the P2 film, the sample temperature probably increased to around 45 °C. As G′ values of P2 at 45 °C (Figure 4) and upon UV irradiation (Figure 6) are comparable, i.e., between 106 and 107 Pa, both photochemical and thermal softening of P2 film contribute to the light-induced bonding. On the other hand, the temperature of P4 probably increased to ca. 35 °C during light-induced bonding (UV, 50 mW cm-2). As the G′ value of P4 upon UV irradiation (105 Pa) was lower than that of P4 at 35 °C (107 Pa), photochemical softening was thought to be main factor influencing the light-induced bonding of the P4 film.

Table 2. Adhesion Strength of Photochemically Bonded Specimens with Polymer Films. Polymer

Film thickness (µm)

Adhesion strength (MPa) 28

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P2a

76 ± 5

1.30 ± 0.19

P2

a

27 ± 3

1.85 ± 0.22

P4

b

28 ± 3

0.81 ± 0.16

The values are averages of three measurements with standard deviation. All the adhesion specimens were deformed during testing.

a

Photochemically bonded upon

irradiation with UV (110 mW cm–2, 5 min) and green (40 mW cm–2, 5 min) light.

b

Photochemically bonded upon irradiation with UV (50 mW cm–2, 5 min) and green (40 mW cm–2, 5 min) light.

Finally, the repeatability of bonding and debonding was studied using a P2 film (35 µm thickness). The results of these experiments are shown in Figure 8. The photochemically bonded specimens displayed almost constant adhesion strengths of 1.7 MPa, which decreased upon UV irradiation to 0.08 MPa. The loss of adhesion strength was negligible for three cycles of the bonding/debonding test. The residual adhesion strength was lower than that of thermally bonded specimen (Figure 7). One of the possible reasons for this observation is larger thickness of the P2 film (35 µm) compared to the thermally bonded specimen (about 20 µm). Thus, we have successfully demonstrated the reworkability of the azobenzene-based triblock copolymer adhesive.



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Figure 8. Repeatable bonding and debonding tests using a specimen photochemically bonded with a P2 film of 35 µm thickness; open circles show adhesion strengths of the specimens bonded with UV (365 nm, 110 mW cm–2, 5 min), and green (520 nm, 40 mW cm–2, 5 min) lights, while filled circles indicate adhesion strengths of those irradiated with UV light (365 nm, 50 mW cm–2, 5 min).

CONCLUSIONS Light is an attractive stimulus to control molecular systems in the solid state. The properties of azobenzene-based systems can be tuned by light because of their trans–cis photoisomerization behavior.49-51 Here, we have demonstrated the light-responsive behavior of azobenzene-based ABA-type triblock copolymers (methacrylic: P2, acrylic: P4) and investigated their reworkable adhesive properties based on the light-induced


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viscoelastic property change. Although P2 and P4 were solid before irradiation, UV irradiation induced photoisomerization and subsequent softening of the polymers with the G' changing from 107 to 106 and 105 Pa for P2 and P4, respectively. Specifically, the soft PEHA middle block of P4 led to a more significant decrease in G'. These values are typical of pressure sensitive adhesives46,47 and indicated that the synthesized polymers showed viscoelasticity upon UV irradiation. Single lap shear tests using glass substrates bonded with thermally melted polymers demonstrated the strong adhesion properties of P2 (2.0 MPa) and P4 (1.5 MPa), which are comparable to the commercially available hot melt adhesives.52 Upon UV irradiation for only 15 s, adhesion strengths lowered to 0.48 and 0.08 MPa for P2 and P4, respectively, thus confirming the debonding ability. The lower residual stress of P4 was rationalized by the lower G' value upon UV irradiation. Finally, the photochemical bonding process was performed using the polymer films. In contrast to the azobenzene-based homopolymers, incorporation of the soft middle block in the azopolymer system allowed fabrication of the polymer films with thickness of several tens of micrometers. In fact, ca. 30 µm-thick P2 films could be used for the photochemical bonding process and exhibited almost the same adhesion strength as the thermally bonded process (1.8 MPa). Furthermore, bonding and debonding were repeatable at least three times without loss of the adhesion strength.



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Thus, appropriate molecular design of the azobenzene-based block copolymers enabled film fabrication and photochemical bonding and debonding, leading to their utilization as novel light-induced reworkable adhesives. The molecular weight, composition, and microphase-separated structure of azo block copolymers effective for light-induced bonding/debonding are under investigation.

ASSOCIATED CONTENT

Supporting Information. Measurements, molecular and thermal characterization results of the synthesized block copolymers, 1H NMR spectrum of P2, GPC traces for the synthesis acrylic triblock copolymer (supplementary experiment), POM images of P2 and P4, temperature dependent viscoelastic measurements of LMW-P4, UV-Vis absorption spectra of P2 and P4, 1H NMR spectra of P2 and P4 irradiated with UV light, temperature-dependent viscoelastic measurements with sample temperatures, time-dependent viscoelastic measurements of LMW-P4 under UV irradiation, and sample temperature measurements upon light irradiations (PDF). This material is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION 32 ACS Paragon Plus Environment

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Corresponding Author *(H.A.) Email: [email protected] *(H.K.) Email: [email protected] ORCID Shotaro Ito: 0000-0001-5212-7466

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

Funding Sources Sumitomo Electric Group CSR foundation (2017), JSPS KAKENHI JP18K14292

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT S.I. thanks the supports by the Sumitomo Electric Group CSR foundation and JSPS KAKENHI (Grant Number JP18K14292). The authors appreciate the support by the fundamental research fund of AIST.



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ABBREVIATIONS LC, liquid crystal; UV, ultraviolet; TPE, thermoplastic elastomer; PAzoM, poly(10-[4-(4-hexylphenylazo)phenoxy]decyl poly(10-[4-(4-hexylphenylazo)phenoxy]decyl acrylate];

PEHMA,

bis(2-bromoisobutylate);

poly(2-ethylhexyl HMTETA,

methacrylate; acrylate;

PEHA,

methacrylate);

PAzoA, poly(2-ethylhexyl

EBBI,

ethylene

1,1,4,7,10,10-hexamethyltriethylenetetramine;

THF, tetrahydrofuran; ATRP, atom transfer radical polymerization; DSC, differential scanning calorimetry; POM, polarized optical microscopy; GPC, gel permeation chromatography; UV-Vis, ultraviolet-visible; GPC-LS, GPC equipped with laser light scattering detector; Sm, smectic; G', storage modulus; G'', loss modulus; tanδ, loss tangent.

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