Article pubs.acs.org/Macromolecules
Organotellurium-Mediated Living Radical Polymerization (TERP) of Acrylates Using Ditelluride Compounds and Binary Azo Initiators for the Synthesis of High-Performance Adhesive Block Copolymers for On-Demand Dismantlable Adhesion Tadashi Inui,† Keisuke Yamanishi,† Eriko Sato,†,* and Akikazu Matsumoto‡,* †
Department of Applied Chemistry and Bioengineering, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan ‡ Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai-shi, Osaka 599-8531, Japan
ABSTRACT: We report the organotellurium-mediated living radical polymerization (TERP) using diphenylditelluride (DT-Ph) and di-n-butylditelluride (DT-Bu) in the presence of a binary azo initiator system consisting of 2,2′-azobis(isobutyronitrile) (AIBN) and 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile) (AMVN) with different decomposition rates for the facile synthesis of high-molecular-weight block copolymers containing a polar side group. The block copolymers containing the poly(tert-butyl acrylate) (PtBA) sequence as the reactive segment and the random copolymer sequences of n-butyl acrylate (nBA) or 2-ethylhexyl acrylate (2EHA) with 2-hydroxyethyl acrylate (HEA) as the adhesive segment were synthesized. The concurrent use of the binary initiators was revealed to effectively increase both the polymerization reactivity and the molecular weight of the polymers along with a narrow molecular weight distribution. The produced block copolymers exhibited high performance for the dismantlable adhesion responsible for the dual external stimuli consisting of photoirradiation and postbaking in the presence of a photoacid generator.
■
INTRODUCTION Dismantlable adhesion is a new adhesion technique satisfying both the reliable adhesive strength during use and facile debonding after use. Recently, dismantlable adhesion techniques have attracted attention in various application fields due to their advantages for saving resources, materials, and energies by reuse and recycling systems, as well as the rework or repair for the manufacturing process of products.1−11 Several processes for dismantlable adhesion systems have already been adopted for practical uses; for example, induction heating combined with hot-melt polymers,12 systems using heat-expansive microcapsules and heat-forming,13,14 UV-responsible tapes for the manufacturing of Si wafers,15−17 etc. Adhesive materials are required to quickly change their adhesive property in response to external stimuli for dismantling, such as heating, UV irradiation, and induction heating, but the most serious issue of the dismantlable adhesion systems has been the simultaneous achievement of stability during use and on-demand easy debonding. The dismantling systems use a change in the © 2013 American Chemical Society
physical properties of the adhesive materials for debonding upon heating or during photoirradiation. In our previous paper,11 we proposed a new type of adhesive material using the block copolymers consisting of poly(tert-butyl acrylate) (PtBA) and poly(2-ethylhexyl acrylate) (P2EHA) segments as the reactive and adhesive polymer sequences, respectively, which were produced by an atom transfer radical polymerization (ATRP) technique.18 We demonstrated that the adhesive strength rapidly decreased upon heating in the presence of an acidic catalyst due to the side-group reaction of PtBA. We further proposed the use of a photoacid generator19 (PAG) that responds to UV irradiation and postbaking for the on-demand control of the adhesive properties. At the same time, we noticed that the peel strength of the adhesive polymers sensitively depended on their composition and molecular weight due to Received: July 29, 2013 Revised: August 23, 2013 Published: September 30, 2013 8111
dx.doi.org/10.1021/ma401595w | Macromolecules 2013, 46, 8111−8120
Macromolecules
Article
Scheme 1
the low cohesive force of the adherent polymers.11 Consequently, high performance appeared only in a narrow range of composition ratios of the reactive and adhesive segments of the block copolymers. An increase in the molecular weight of the copolymers and the introduction of polar repeating units were considered to be the most valid approach for manufacturing high-strength adhesion materials. In this paper, we report the facile synthetic procedure of high-molecular-weight acrylic block copolymers containing polar repeating units and the application as well-designed dismantlable adhesive materials with a reliable performance. The organotellurium-mediated living radical polymerization (TERP) is the most useful method for the synthesis of a variety of polyacrylates with a functional group as well as the precise control of the molecular weight and its distribution.20−24 An initiating system using organomonotellurium compounds (MT) is used for controlling the living radical polymerization of acrylates25,26 (Scheme 1a). Because the MTs are sensitive to oxygen, all the manipulations for the polymerization must be carefully carried out under an inert atmosphere. Yamago et al. previously reported a polymerization system using chain transfer agents generated in situ from organoditelluride compounds (DT) and an azo initiator as the practical protocols for TERP based on the fact that the DTs are more stable than the MTs under atmospheric conditions.27,28 However, no report has been seen in the literature for the synthesis of highmolecular-weight polyacrylate block copolymers containing polar repeating units, such as 2-hydroxyethyl acrylate (HEA) as one of typical acrylic monomers used for the design of adhesive materials, by the TERP methods.29−39 In this study, we used diphenylditelluride (DT-Ph) and di-n-butylditelluride (DT-Bu) in the presence of binary azo initiators with different decomposition rates, that is, 2,2′-azobis(isobutyronitrile) (AIBN) and 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile) (AMVN) (Scheme 2), for the synthesis of high-molecularweight block copolymers in high yields (Scheme 1b).40 Typical
Scheme 2
acrylate monomers, n-butyl acrylate (nBA), 2-ethylhexyl acrylate (2EHA), and HEA, were used as the source monomers for the adhesive polymer sequence of the block copolymers (Scheme 1c). The PtBA in response to the photoirradiation and postbaking as dual external stimuli was used as the reactive polymer segment.11 We demonstrated the validity of using the binary azo initiator system for the preparation of highperformance block copolymers, which show excellent properties for on-demand and quick dismantling processes.
■
EXPERIMENTAL SECTION
General Procedure. The NMR spectra were recorded using a Bruker AV300 spectrometer in chloroform-d or acetone-d6 as the solvents. The FT-IR spectra were recorded using a JASCO FT/IR 430 spectrometer. The number- and weight-average molecular weights (Mn and Mw) and the polydispersity (Mw/Mn) were determined by size exclusion chromatography (SEC) in tetrahydrofuran as the eluent using a Tosoh CCPD RE-8020 system and calibration with standard polystyrenes. The theoretical Mn values (Mn,th) for PtBA were calculated using the following equation: Mn,th = ([tBA] × 128.2 × % conversion)/(2[DT] × 100) + 140.2 + MWTeR, where MWTeR is the molecular weights of the ω chain-end groups. It was assumed that the α chain-end included an AMVN fragment and the used DTs were entirely consumed during the polymerization. The differential 8112
dx.doi.org/10.1021/ma401595w | Macromolecules 2013, 46, 8111−8120
Macromolecules
Article
2.58 × 105 and 1.49, respectively. Other block copolymers and a random copolymer were similarly prepared. 180° Peel Test. The adhesion tests were performed according to the standard test method for the peel adhesion of pressure-sensitive tape (ASTM D3330) using a Tokyo Testing Machine (TTM) universal testing machine, LSC-1/30, with a 1 kN (at maximum) load cell. The adhesive polymers (15 wt %) with a determined amount of PAG in toluene or acetone were coated to a thickness of 200 μm on a poly(ethylene terephthalate) (PET) film (50 μm thickness) using a film applicator, then dried overnight under reduced pressure at room temperature. A strip of the PET film (2-cm width) coated with the adhesive polymers was placed on a stainless steel plate (SUS430, 50 mm × 150 mm × 0.5 mm), then pressed using a 2-kg hand roller. The UV irradiation, heating, and 180° peel test were carried out after the specimen was left to stand for over 30 min at room temperature. For the UV irradiation, the test piece was placed at a distance of 10 cm from the UV source (Toshiba SHL-100UVQ-2) at room temperature. For the thermal treatment, the test piece was placed in a preheated oven for a predetermined time, removed from the oven, then naturally cooled to room temperature. All the adhesion tests were performed at 23 °C. The average value of three measurements was typically recorded.
scanning calorimetric (DSC) analysis was performed using a Seiko EXSTAR6200 at the heating rate of 10 °C/min. The atomic force microscopy (AFM) images were taken using a NanoScope IIIa system (Digital Instruments/Veeco) with a cantilever (OMCLAC240TS-C2, Olympus, spring constant 2 N/m, resonant frequency 70 kHz) in the height and phase modes. The samples for the AFM measurements were prepared on a release paper on which the adhesive polymer in acetone was coated, then dried overnight under reduced pressure at room temperature. The scanning electron microscopy (SEM) observations were carried out using a Keyence VE9800 with the accelerating voltage source of 1 kV. The sample was coated with gold using an MSP-1S Vacuum Device magnetron sputter for 40 s. Materials. The acrylate monomers, tBA, nBA, and 2EHA, were purchased from Tokyo Chemical Industry Co., Ltd., and distilled before use. AIBN and AMVN were purchased from Wako Pure Chemicals Co., Ltd., and recrystallized from methanol. The commercially available N-hydroxynaphthalimide triflate41−43 (NIT, 99%, Sigma-Aldrich Co.) and DT-Ph (98%, Tokyo Chemical Industry Co., Ltd.) were used as received. All the solvents were distilled before use. Ethyl 2-(n-butyltellanyl)-2-methylpropionate (MT-Bu) and DTBu were synthesized according to the methods described in the literature.25,28 Polymerization. Typical procedures for the polymerization are as follows. Synthesis of PtBA Using DT-Ph. To a 10-mL glass tube were added tBA (2.31 g, 18 mmol), AMVN (3.89 mg, 13 × 10−6 mol), and DT-Ph (3.84 mg, 9.4 × 10−6 mol) in 2.31 g of anisole, and the solution was stirred with argon bubbling at 0 °C for 30 min. After the polymerization was carried out at 60 °C for 5 h, the solution was cooled in a dry ice−methanol bath. A small amount of chloroform was added, then the solution was poured into a large amount of a methanol−water mixture (90/10 in volume ratio) in order to precipitate the resulting PtBA. The polymer was separated by decantation, then dried at 40 °C under reduced pressure overnight. The conversion and Mn and Mw/Mn values were determined by NMR and SEC, respectively; conversion =87.7%, Mn = 1.02 × 105, Mw/Mn = 1.38. Synthesis of PtBA-b-PnBA Using DT-Ph. The block copolymers were also prepared using tBA (1.04 g, 8.1 mmol), AIBN (1.33 mg, 8.1 × 10−6 mol), AMVN (3.51 mg, 11 × 10−6 mol), DT-Ph (3.47 mg, 8.5 × 10−6 mol), and anisole (1.04 g) by a similar procedure. After the polymerization of tBA at 60 °C for 4.5 h (conversion =96.5%, Mn = 5.52 × 104, Mw/Mn = 1.42), nBA (3.13 g, 0.024 mol) was added to the solution. The block copolymerization was carried out at 60 °C for 3 h. The block copolymer was separated using methanol−water mixture (90/10 in volume ratio) as the precipitant by decantation. The total conversion of tBA was 97.9% and the conversion of nBA was 74.7%. The Mn and Mw/Mn values of the block copolymer were 1.69 × 105 and 1.47, respectively. Other block copolymers were similarly prepared. Synthesis of PtBA Using MT-Bu. To a 10-mL glass tube, were added tBA (1.44 g, 11 mmol) and AMVN (0.347 mg, 1.1 × 10−6 mol) in 1.44 g of ethyl acetate, and the solution was stirred with argon bubbling at 0 °C for 30 min, followed by the addition of MT-Bu (1.29 μL, 5.6 × 10−6 mol) with a syringe. After the polymerization was carried out at 50 °C for 5 h, the solution was cooled in a dry icemethanol bath. A small amount of chloroform was added, then the solution was poured into a large amount of a methanol−water mixture (90/10 in volume ratio) in order to precipitate the resulting PtBA. The polymer was separated by decantation, then dried at 40 °C under reduced pressure overnight. The conversion was 75.9%. The Mn and Mw/Mn values were 9.32 × 104 and 1.30, respectively. Synthesis of PtBA-b-P(2EHA-co-HEA) Using MT-Bu. The polymerization of tBA was carried out, then 2EHA and HEA were added to the solution. The block copolymerization was carried out at 50 °C for 4 h. The copolymer was separated using a methanol−water mixture (90/10 in volume ratio) as the precipitant. The total conversion of tBA was 86.7% and the conversions of 2EHA and HEA were 45.8 and 52.2%, respectively. The Mn and Mw/Mn values of the block copolymer were
■
RESULTS AND DISCUSSION TERP Using Binary Azo Initiators. The living radical polymerization of tBA was carried out by the TERP method using the DTs in the presence of the binary azo initiators, AMVN and AIBN. The results of the polymerization are shown in Table 1. Only a trace amount of polymer was produced during the polymerization using AIBN as the single initiator at 60 °C for 5 h due to a low decomposition rate of the AIBN; kd [s−1] = 1.58 × 1015 exp(−128.9[kJ/mol]/RT), t1/2 = 19.8 h at 60 °C,44 and the polymerization retardation by the presence of Table 1. Synthesis of PtBA by TERP Using the DTs Combined with AMVN and AIBNa initiatorsb DT
AMVN
AIBN
temp (°C)
conversion (%)
Mn/104
Mw/Mn
DT-Ph
0 0 0 0 1.2 1.3 1.4 1.2 1.2 1.2 1.3 1.3 1.3 1.4 1.5 1.1 1.1 1.1 1.2 1.2 1.2 1.2
3.0 1.0 1.4 1.5 0 0 0 0.2 0.5 1.0 0.2 0.5 1.0 0.2 0.2 0 0.5 1.0 0 0.2 0.5 1.0
60 95 95 95 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60
trace 9.3 45.5 75.9 19.0 74.1 88.5 22.7 49.5 84.1 83.6 88.2 92.7 87.7 93.1 22.6 38.2 89.1 60.1 85.1 87.9 92.9
− 1.19 3.19 4.42 1.61 6.42 7.61 2.12 5.17 8.82 8.61 8.56 8.97 10.2 8.92 3.00 4.22 9.24 6.84 8.72 9.58 10.2
− 1.20 1.39 1.62 1.18 1.43 1.46 1.20 1.19 1.31 1.39 1.40 1.50 1.38 1.53 1.20 1.22 1.31 1.26 1.25 1.28 1.35
DT-Bu
a
Polymerization conditions: [tBA]/[DT] = 2000, tBA/anisole =1/1 in weight, polymerized for 5 h. bMolar ratio related to the DTs.
8113
dx.doi.org/10.1021/ma401595w | Macromolecules 2013, 46, 8111−8120
Macromolecules
Article
Figure 1. Results of TERP using (a) DT-Ph and (b) DT-Bu in the presence of binary azo initiators in anisole at 60 °C: [tBA]/[DT] = 2000/1, tBA/ anisole =1/1 in weight. The [AMVN]/[AIBN] ratios shown in the figures are related to [DT] (see also Table 1). The line in the right figures indicated theoretical Mn values.
the in situ production of the MT species to simultaneously achieve both a high conversion and well-controlled molecular weight distribution during the polymerization. The formation of the free polymers by an excess supply of the free radicals from the azo initiator should be avoided. In this study, we tried to concurrently use both AMVN and AIBN as the radical sources in order to control the polymerization reaction at a high conversion. We expected that the MT species would be effectively produced from the DTs by the fast decomposition of AMVN, and the propagation would be persistently promoted by the slow and continuous supply of radical species from AIBN during the entire period of the polymerization. Actually, the addition of AIBN accelerated the progress of polymerization without disturbing the controlled propagation of the polymer chains, and we consequently succeeded in the simultaneous achievement of high conversions (over 80%) and high Mn values (ca. 1 × 105) with a narrow molecular weight distribution (Mw/Mn = 1.3− 1.4) under the conditions using a low excess amount of AMVN toward DT-Ph; i.e., [AMVN]/[DT-Ph] = 1.2−1.4 (Table 1 and Figure 1a). The polymerization rate (polymer yield) significantly depended on the amounts of AIBN and AMVN. The higher the AIBN concentration, the higher the onversion, under the condition of [AMVN]/[DT-Ph] = 1.2. The conversion varied from 22.7 to 84.1% at [AIBN]/[DT-Ph] = 0.2−1.0. Of course, the conversion increased with the polymerization time and the Mn value increased in proportion to the conversion, as shown in Figure 1. The deviation from the theoretical line observed in the relationship between the conversion and the Mn value is due to occurrence of the partial bimolecular termination at a high conversion. It is noted that the AMVN/
DT-Ph. Yamago et al. previously reported that a polymer with Mn = 4.9−7.8 × 103 and Mw/Mn = 1.06−1.24 was produced in 3−89% yield during the polymerization with DT-Ph and dimethylditelluride (DT-Me) in the presence of AIBN at 100 °C for 24−48 h, in which the ratio of acrylate/DT/AIBN was 100/1/1.28 In this study, we used a higher amount of the charged monomer and a low excess amount of AIBN toward the DTs, i.e., tBA/DT/AIBN = 2000/1/1.0−1.5 at 95 °C in order to obtain polymers with a higher molecular weight in a high yield. Almost all the AIBNs decomposed during the adopted polymerization period (5 h) because of its short halflife (t1/2 = 14 min at 95 °C). As a result, the conversion and the Mn value increased from 9.3 to 75.9% and 1.19 to 4.42 × 104, respectively, with the increased amount of AIBN used. The large excess amount of AIBN toward DT-Ph ([AIBN]/[DTPh] ≥ 1.4) was required for the consumption of DT-Ph and the production of a high-molecular-weight polymer in a high yield, being due to the low initiator efficiency ( f ∼ 0.6) of AIBN and some side reactions out of the cage, such as a primary radical termination and the radical combination. The Mw/Mn value simultaneously increased from 1.20 to 1.62 as a result of the less control of the polymerization by the occurrence of a bimolecular termination producing dead polymer chains. Similar results were obtained during the polymerization at 60 °C using AMVN, of which the t1/2 value was 18 min; kd [s−1] = 1.36 × 1016exp(−123.3 [kJ/mol]/RT).45 The use of a larger amount of AMVN ([AMVN]/[DT-Ph] ≥ 1.3) led to increased Mn and Mw/Mn values as well as the conversion. At the same time, however, the Mw/Mn values also increased to 1.46. These results suggested the difficulty in the determination of the appropriate concentrations of an azo initiator and the DTs for 8114
dx.doi.org/10.1021/ma401595w | Macromolecules 2013, 46, 8111−8120
Macromolecules
Article
Table 2. Synthesis of Block Copolymers by TERP Using the DTs and MT-Bu in the Presence of Azo Initiatorsa first-stage polymerization
second-stage polymerization convn (%)
catalyst DT-Ph DT-Bu MT-Bu
AMVN/ AIBNb 1.4/1.0 1.4/1.0 1.2/1.0 1.2/1.0 0.2/0 0.2/0 0.2/0 0/1.0
tBAb 890 1000 1240 1000 890 1000 620 500
time (h)
convn (%)
5 4.5 3 5 2 2 2 4
88.5 96.5 80.6 87.9 75.9 87.0 74.1 97.1
Mn/104 Mw/Mn 6.82 5.52 6.15 4.39 9.32 9.75 6.86 6.56
1.36 1.42 1.32 1.16 1.30 1.24 1.32 1.37
acrylateb 2EHA(2050) nBA(3000) nBA(4840) 2EHA(3000) 2EHA(2050) 2EHA(1700) nBA(2420) nBA(1500)
HEA 130 0 220 0 130 0 110 0
time (h)
tBA/RAc/HEA
Mn/105
Mw/Mn
Tg (°C)
4 3 5 5 4 8 12 2
94.3/62.6/70.4 97.9/74.7/− 86.1/71.8/63.0 92.1/53.3/− 86.7/45.8/52.2 95.0/72.3/− 89.5/59.8/62.0 98.2/70.9/−
1.59 1.69 2.73 1.61 2.58 2.11 2.39 1.63
1.69 1.47 1.95 1.43 1.49 1.43 1.48 1.38
−57, 43 −47, −67, −61, −67, −46,
39 38 43 42 43
Polymerization conditions: tBA/solvent =1/1 in weight in anisole at 60 °C for the DTs and in ethyl acetate at 50 °C for MT-Bu. The polymerization of tBA was carried out for 2−5 h at the first stage, and then nBA, 2EHA, and HEA were added (monomers/solvent =1/1 in weight). b Molar ratio to the DTs and MT-Bu. cnBA or 2EHA. a
Figure 2. SEC traces of the precursor PtBA (dotted curves) and the block copolymers (solid curves) synthesized by TERP using the DTs with the binary azo initiators and the MT with the single azo initiator. (a) PtBA44.7-b-P(tBA3.0-co-2EHA49.1-co-HEA3.3) prepared with DT-Ph, (b) PtBA22.3-bP(tBA1.6-co-nBA71.9-co-HEA4.2) prepared with DT-Bu, (c) PtBA33.5-b-P(tBA1.6-co-2EHA64.9) prepared with DT-Bu, and (d) PtBA37.8-b- P(tBA5.4-co2EHA52.5-co-HEA4.3) prepared with MT-Bu.
AIBN ratio has to be finely tuned for controlling the polymerization. A significantly high excess amount of AMVN ([AMVN]/[DT-Ph] > 1.5) led to the less-controlled polymerization results even for the conditions in which a low amount of AIBN was used ([AIBN]/[DT-Ph] = 0.2), as shown in Table 1. The use of DT-Bu led to a similar polymerization result, but the polymers were produced in a higher yield during the polymerization using DT-Bu than that with DT-Ph. While the single use of AMVN induced a dead-end polymerization due to the depletion of AMVN before the complete utilization of tBA, the binary system of AMVN and AIBN readily achieved a high conversion. The single use of AMVN achieved 60% yield under the condition of [AMVN][DT-Bu] = 1.2, whereas the yield leveled off at 20% under a similar condition using DT-Ph (Figure 1). In the binary initiator systems, the addition of 0.2 equiv of AIBN was enough to achieve a higher yield (85%) during the polymerization using DT-Bu, whereas DT-Ph required the 1.0 equiv of AIBN. The bond dissociation energies of the MT compounds were reported to be 105 and 120 kJ/ mol for the C−Te bonds of (CH3)2C(CN)TePh and (CH3)2C(CN)TeCH3, respectively, in the literature.28 The faster degenerative transfer using a polymer with the Te-Ph terminal group than that with the Te-Bu group was also
reported. The rate constants of a degenerative transfer were 9.6 and 3.5 × 103 mol/L s for the polystyrene transfer agents endcapped with the TePh and TeBu groups, respectively.32 The lower Mw/Mn values were obtained during the polymerization using DT-Ph in the absence of AIBN in the present study and the literature,32 being due to more frequent chain transfer characteristics in the DT-Ph system. High-molecular-weight block copolymers containing a polar HEA repeating unit in the soft segment were synthesized by the TERP methods using the DTs combined with the binary initiators. The polymerization system using MT and a single initiator was also carried out using the typical TERP procedure. The polymerization results are summarized in Table 2. The SEC traces of the precursor PtBA and the block copolymers produced from each precursor are shown in Figure 2. The polymerization of tBA was performed for 2−5 h to obtain the precursor PtBA. The monomer conversion to the polymer was 74−97%. nBA or 2EHA in the presence or absence of a low amount of HEA (110−220 equivalent against DT or MT) was then added to the solution without isolating the produced PtBA after the first-stage polymerization. The conversion of the second monomers was 46−75%, and the tBA conversion finally reached 86−98%. The second sequences are the random 8115
dx.doi.org/10.1021/ma401595w | Macromolecules 2013, 46, 8111−8120
Macromolecules
Article
Figure 3. Representative peel strength-displacement curves and photographs of the specimens used in the 180° peel test of (a) PtBA37.8-b-P(tBA5.4co-2EHA52.5-co-HEA4.3), (b) PtBA24.1-b-P(tBA5.0-co-nBA67.0-co- HEA3.9), and (c) P(tBA35.5-co-nBA60.8-co-HEA3.7) in the presence of 0.4 mol % of NIT. Original (A), after heating at 100 °C for 1 h (B), after UV irradiation for 1 h (C), and after UV irradiation for 1 h and postbaking at 100 °C for 1 h (D).
Table 3. Peel Strength of the High-Molecular-Weight Block Copolymers Synthesized by TERP with DT-Ph or MT-Bua stimuli polymer
UV
PtBA44.7-b-P(tBA3.0-co-2EHA49.1-co-HEA3.3) Mn = 1.59 × 105, Mw/Mn = 1.69
none none 1h 1h none none 1h 1h none none 1h 1h none none 1h 1h none none 1h 1h none none 1h 1h none none 1h 1h
PtBA22.3-b-P(tBA1.6-co-nBA71.9-co-HEA4.2) Mn = 2.73 × 105, Mw/Mn = 1.95
PtBA37.8-b-P(tBA5.4-co-2EHA52.5-co-HEA4.3) Mn = 2.58 × 105, Mw/Mn = 1.49
PtBA24.1-b-P(tBA5.0-co-nBA67.0-co-HEA3.9) Mn = 2.39 × 105, Mw/Mn = 1.48
PtBA39.4-b-P(tBA3.6-co-2EHA57.0) Mn = 2.11 × 105, Mw/Mn = 1.43
PtBA35.9-b-P(tBA1.3-co-2EHA62.8) Mn = 1.60 × 105, Mw/Mn = 1.31
P(tBA35.5-co-nBA60.8-co-HEA3.7) Mn = 2.42 × 105, Mw/Mn = 1.67
a
heating none 100 °C, none 100 °C, none 100 °C, none 100 °C, none 100 °C, none 100 °C, none 100 °C, none 100 °C, none 100 °C, none 100 °C, none 100 °C, none 100 °C, none 100 °C, none 100 °C,
1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h 1h
peel strength (N/m)
relative value
failure mode
271 ± 38 457 ± 16 269 ± 14 2.9 ± 1.5 359 ± 21 543 ± 39 404 ± 52 15.6 ± 6.6 302 ± 18 352 ± 58 443 ± 41 17.8 ± 9.4 448 ± 45 923 ± 132 515 ± 112 3.7 ± 1.2 63.3 60.3 50.7 39.5 15.7 9.6 13.0 18.2 347 ± 36 843 ± 186 392 ± 17 stick slip
1 1.69 0.99 0.01 1 1.51 1.13 0.04 1 1.17 1.47 0.06 1 2.06 1.15 0.01 1 0.95 0.80 0.62 1 0.61 0.83 1.15 1 2.43 1.13 −
SUS−interfacial cohesive SUS−interfacial cohesive SUS−interfacial cohesive SUS−interfacial PET−interfacial SUS−interfacial cohesive SUS−interfacial cohesive SUS−interfacial cohesive SUS−interfacial PET−interfacial cohesive cohesive cohesive cohesive cohesive cohesive cohesive cohesive SUS−interfacial cohesive SUS−interfacial
NIT 0.4 mol % toward the tBA repeating units. Peel rate, 30 mm/min.
copolymers consisting of the repeating unit of 2EHA or nBA including a low amount of HEA and tBA. The compositions of the whole block copolymers are 2EHA or nBA = 49−53 mol %, HEA = 3−4 mol %, and tBA = 29−48 mol %. The 3−5 mol %
of the tBA units were included in the second sequences in the block copolymer chains. The exact structures of the produced block copolymers are expressed as PtBAn-b-P(tBAx-co-2EHAyco-HEAz) and PtBAn-b-P(tBAx-co-nBAy-co-HEAz), where sub8116
dx.doi.org/10.1021/ma401595w | Macromolecules 2013, 46, 8111−8120
Macromolecules
Article
scripts n, x, y, and z are the mol % for each component in the copolymers. The prolonged polymerization of the second stage in the presence of HEA resulted in the formation of partial insoluble polymers due to the chain transfer to the side chain of the polymers including a functional group. The Mw/Mn values tended to be greater for the polymerization systems in the presence of HEA. When the content of HEA was increased, the chain transfer reactions frequently occurred, leading to the formation of a block copolymer with a higher Mw/Mn value. In conclusion, the composition and structures of the obtained block copolymers using the DTs and the MT were sufficient to be used as the adhesive polymer materials for the dismantlable pressure-sensitive adhesion in this study. Adhesive Properties. We carried out the 180° peel test using the block copolymers including the HEA repeating units as the adhesives in the presence of NIT (0.4 mol %) as the PAG. The peel strength was determined using a strip of the PET film (2-cm width) coated with the adhesive polymers and a stainless steel plate as the substrate for adhesion. The obtained peel strength−displacement curves are shown in Figure 3. The results of the peel test are summarized in Table 3. The strengths of the adhesion tapes using the HEAcontaining 2EHA block copolymers were 270−300 N/m, being higher than those previously reported using the nonpolar PtBA-b-P2EHAs, of which the strengths were 4−310 N/m and significantly depended on the sequence composition of the hard and soft segments in the copolymers.11 In contrast to the previous results, the high and constant strength values were obtained using the polar block copolymers with a high cohesive strength in this study. No difference in the adhesion property was observed for the block copolymers synthesized by the polymerization systems with DT-Ph and MT-Bu. A greater strength value was obtained for the system using PtBA-bP(tBA-co-nBA-co-HEA) (448 N/m). This reflects the difference in the Tg values of the soft segments; −52 and −70 °C for the PnBA and P2EHA, respectively, although the peel test was carried out at a temperature much higher than the Tg values.46 We also checked the adhesion strength of the high-molecularweight PtBA-b-P(tBA-co-2EHA) as the adhesives including no polar HEA repeating unit (2EHA content = 57−63 mol %, Mn = 1.6−2.1 × 105), but the peel strength value was still low at 16−63 N/m. The block copolymer including a lower amount of the 2EHA unit (42 mol %) showed no adhesion property because it is too hard to be used as a pressure-sensitive adhesive material. On the other hand, the introduction of the HEA units definitely led to the significant enhancement in the adhesion properties, as has already been shown. The observed peeling strengths were higher than that for a commercially available pressure-sensitive adhesion tape under similar peel conditions, e.g., 180 N/m for a commercial adhesive tape with 18 mmwidth. The peel strength and failure mode were determined by several features of the adhesives, such as the Tg values of the segments, the hard and soft segment ratios, and the peel rate. An increase in the adhesion strength was observed during the heating treatment. This was due to the formation of a closer contact of the adhesive layer with the stainless steel plate surface by the reorganization of the polymer chains during the heating. A significant increase in the strength was observed for the nBA copolymers with a higher Tg value. As the result of the thermal annealing, the interfacial failure mode changed to a cohesive failure mode because of an enhanced dipolar interaction at the metal−adhesive interface by the close contact. The peel strengths at the cohesive failure corresponded
to the cohesive force of the polymer materials, i.e., 350−450 and 920 N/m for the 2EHA and nBA block copolymers, respectively. The NIT produces trifluoromethanesulfonic acid as a strong acid under UV irradiation by the action of water or any other proton donor contained in the adhesion systems (Scheme 3).41−43 However, no changes in the strength and the failure Scheme 3
mode were observed during the UV irradiation without heating. This is due to the slow diffusion of the protons produced from NIT in the PtBA domain at a temperature below the Tg. On the other hand, the postbaking after UV irradiation expectedly resulted in a significant change in the adhesive property, being largely different from the results after a single stimulus either by heating or UV irradiation. The adhesion strength drastically decreased to only 4−18 N/m after the dual stimuli, and the adhesion tapes were spontaneously peeled along with the help of the isobutene gas evolution at the interface of the adhesives and the substrates. The transformation of the PtBA segment to the poly(acrylic acid) segment accompanying the isobutene evolution was confirmed based on the results of the IR, NMR, and DSC measurements of the adhesive polymers.11 The use of the block copolymers was undoubtedly superior to the random copolymer resulting in stick−slip failure for dismantling, as shown in Table 3 and Figure 3c. For the block copolymers containing the polar HEA units, the occurrence of stick−slip failure was seldom observed and the dependence of the peel strength on the peel rate was small. The microphase-separated structure of the block copolymers were confirmed by the observation of both Tgs for the hard and soft segments by DSC and AFM observations (Table 2 and Figure 4). For example, the Tg transitions were observed at the temperature range of 38 to 43 °C for the hard PtBA domain and at the range of −61 to −46 °C for the soft domains. In contrast, a single Tg transition was observed at −25 °C for the random copolymer, P(tBA35.5co-nBA60.8-co-HEA3.7). Figure 4a shows the AFM images of the tBA−nBA/HEA block copolymer adhesives in a phase mode before and after the dual external stimuli. The phase-separated PtBA domains were observed as the dark island in the low-Tg matrix of the PnBA domain including a low amount of tBA and HEA repeating units. After the UV irradiation and postbaking, the island structure was more clearly observed because of the increase in the Tg value of the hard segment of the block copolymers by a change in the side-chain structrue of PtBA to poly(acrylic acid) (PAA). The produced isobutene molecules can immidiately diffuse in the low-Tg PnBA domian and reach the adhesive surface. Consequently, isobutene gas was mainly evolved at the interfaces between the adhesive layer and the substrates, the SUS plate or the PET film. In contrast, no phase separation was observed for the random copolymer and the 8117
dx.doi.org/10.1021/ma401595w | Macromolecules 2013, 46, 8111−8120
Macromolecules
Article
Figure 4. AFM and SEM images of the surface structures of the thin films of (a) block and (b) random copolymers before and after the UV irradiation and postbaking. Illustrations indicate models for the change in the structures of the adhesive polymer layers and the interfaces after the UV irradiation and postbaking during the dismantling process.
Figure 5. Comparison of the dismantlable adhesion property of the block and random copolymers for quick response to external stimuli. (a) Peel strength−displacement curves before and after the UV irradiation for 2.5 min and postbaking at 150 °C for 5 min. (b) Change in the relative peel strength after the UV irradiation for 1−10 min and postbaking at 150 °C for 5 min.
random copolymers because of the high segment density of the tert-butyl ester groups in the polymer. No change was observed in the IR and NMR spectra and the DSC traces of the adhesive polymers after photoirradiation, but almost all the polymers were insoluble after the photoirradiation and subsequent postbaking. The SEC analysis using a model reaction revealed the occurrence of cross-linking during the UV irradiation and the subsequent heating in the presence of PAG by the frequent transesterification. These results are shown in Figure 6. The same amount of highmolecular-weight PnBA (Mn = 1.47 × 105, Mw/Mn = 1.19) and low-molecular-weight P(nBA93.2-co-HEA6.8) (Mn = 8.90 × 103, Mw/Mn = 1.07) were irradiated for 1 h in the bulk in the presence of 1 wt % NIT, then heated at 100 °C for 10−40 min. As a result, an additional peak was observed at the elution time corresponding to twice the original P(nBA-co-HEA). Further heating induced a change in the intensity and broadening of the new peak along with a decrease in the intensity of the original peak due to P(nBA-co-HEA). A change was observed in the elution curve due to the higher-molecular weight PnBA (curve D in Figure 6a). The 40-min heating resulted in cross-linking of
photoirradiation, and the subsequent heating resulted in the gas evolution over the entire adhesive layers, as shown in the SEM image. This is due to the isobutene formation in the entire adhesive polymer with the single Tg value, resulting in foaming in the adhesive layers containing no phase-separated domain (Figure 4b). The block copolymers exhibited superior characteristics for a quick response to the UV irradiation and postbaking for only several minutes in the dismantling process. Figure 5a shows the dismantling property of the block and random copolymers under the conditions of UV irradiation for 2.5 min and the postbaking at 150 °C for 5 min. The adhesive tape using the block copolymers spontaneously peeled during the total dismantling process within 10 min, while a half magnitude of the strength remained for the random copolymers even after the dual stimuli. The adhesive strength of the random copolymer adhesives did not reach a value sufficient for spontaneous peeling even after a 10-min irradiation (Figure 5b). The rate of the chemically amplified transformation of PtBA to PAA accompanying the isobutene elimination may be accelerated in the block copolymer sequence rather than in the 8118
dx.doi.org/10.1021/ma401595w | Macromolecules 2013, 46, 8111−8120
Macromolecules
Article
Figure 6. (a) SEC elution curves (RI response) for the mixture of PnBA and P(nBA-co-HEA) in the presence 1 wt % NIT before UV irradiation and heating (A, blue), after UV irradiation for 1 h (B, purple), after the UV irradiation for 1 h and postbaking at 100 °C for 10 min (C, green), and after the UV irradiation for 1 h and postbaking at 100 °C for 20 min (D, red), and difference in the elution curves D and A.
■
all the polymers. The quantitative change in the molecular weight of the polymers was clearly shown in the differential elution curve of D and A; The amount of the original P(nBAco-HEA) and PnBA was reduced and the new polymers with a double or higher molecular weight of the original one formed during the heating. The P(nBA-co-HEA) chains underwent intermolecular transesterification between the hydroxy group and the n-butyl ester group in the presence of an acidic catalyst to produce a two-times higher molecular-weight P(nBA-coHEA) during the initial stage of the reaction (curve C in Figure 6a). The reaction between the hydroxy group of P(nBA-coHEA) and the n-butyl ester group of PnBA is also possible, but the change in the molecular weight was too small to be detected as a change in the SEC elution curve because of the significant difference in the molecular weight of these polymers. The transesterification were frequently repeated and led to the broad peak appearance in the higher-molecular-weight regions. A schematic illustration for the proposed reactions is shown in Figure 6d. In contrast to no change in the SEC traces by the RI detector after the photoirradiation, the peak intensity in the SEC elution curves monitored by a UV detector increased (Figure 6b), suggesting a reaction of NIT with the polymer chains. The intensity further increased during the postbaking as shown in Figure 6c. The possible reaction mechanism is as follows. The naphthalimide cations formed by the photodecomposition of NIT may directly react with a hydroxy group in the polymer side chain as HX in Scheme 3. A strong trifluoromethanesulfonic acid possibly promotes the transesterification of PnBA and the produced N-hydroxynaphthalimide. These reactions accompany the introduction of a naphthalimide moiety into the polymer side group, leading to an unusual change in the peak intensity of the UV-detected SEC elution curves.
CONCLUSION
The TERP technique is one of the living radical polymerizations used to synthesize high-molecular-weight acrylic polymers containing polar repeating units. Because organomonotellurides (MT) used as the chain transfer agents during the conventional TERP method are sensitive to oxygen, the in situ formation of chain transfer agents using oxygen-stable organoditellurides (DT) during polymerization has already been developed. In this study, we proposed the facile synthesis of the precisely controlled high-molecular-weight acrylic polymers containing polar repeating units by the modified TERP using DT and binary azo initiators, AMVN and AIBN, having different decomposition rate constants in order to accelerate the polymerization while maintaining control of the polymerization. The produced block copolymers exhibited a high performance for the dismantlable adhesion responding to the dual external stimuli of photoirradiation and postbaking in the presence of a photoacid generator. The change in the peel strength before and after the UV irradiation and postbaking was investigated and the mechanism of the superior adhesion and dismantling properties of the block copolymer adhesives was revealed based on the results of the peel test, DSC and AFM analyses of the adhesives, as well as the model reactions of the cross-linking formation by transesterification. We have demonstrated that the precisely controlled living radical polymerization technique is important for the fabrication of high-performance adhesive polymer materials with high molecular weights and a controlled molecular weight distribution containing polar repeating units. The dismantling adhesive systems will be expected to be used in a wide range of applications in the near future. 8119
dx.doi.org/10.1021/ma401595w | Macromolecules 2013, 46, 8111−8120
Macromolecules
■
Article
(31) Sugihara, Y.; Kagawa, Y.; Yamago, S.; Okubo, M. Macromolecules 2007, 40, 9208−9211. (32) Kayahara, E.; Yamago, S.; Kwak, Y.; Goto, A.; Fukuda, T. Macromolecules 2008, 41, 527−529. (33) Okubo, M.; Sugihara, Y.; Kitayama, Y.; Kagawa, Y.; Minami, H. Macromolecules 2009, 42, 1979−1986. (34) Nakamura, Y.; Kitada, Y.; Kobayashi, Y.; Ray, B.; Yamago, S. Macromolecules 2011, 44, 8388−8397. (35) Nakamura, Y.; Arima, T.; Tomita, S.; Yamago, S. J. Am. Chem. Soc. 2012, 134, 5536−5539. (36) Mishima, E.; Tamura, T.; Yamago, S. Macromoelcules 2012, 45, 8998−9003. (37) Feng, S.; Xu, W.; Nakanishi, K.; Yamago, S. ACS Macro Lett. 2012, 1, 146−149. (38) Kumar, S.; Changez, M.; Murthy, C. N.; Yamago, S.; Lee, J. -S. Macromol. Rapid Commun. 2011, 32, 1576−1582. (39) Kitayama, Y.; Kishida, K.; Okubo, M. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 716−723. (40) Preliminary results: Yamanishi, K.; Sato, E.; Matsumoto, A. J. Photopolym. Sci. Technol. 2013, 26, 239−244. (41) Ortica, F.; Scaiano, J. C.; Pohlers, G.; Cameron, J. F.; Zampini, A. Chem. Mater. 2000, 12, 414−420. (42) Okamura, H.; Sakai, K.; Tsunooka, M.; Shirai, M.; Fujiki, T.; Kawasaki, S.; Yamada, M. J. Photopolym. Sci. Technol. 2003, 16, 87−90. (43) Malval, J. -P.; Suzuki, S.; Morlet-Savary, F.; Allomas, X.; Fonassier, J. -P.; Takahara, S.; Yamaoka, T. J. Phys. Chem. A 2008, 112, 3879−3885. (44) Van Hook, J. P.; Tobolsky, A. V. J. Am. Chem. Soc. 1958, 80, 779−782. (45) Yamada, B.; Matsumoto, A.; Otsu, T. Macromol. Chem. 1991, 192, 1921−1929. (46) Everaerts, A. I.; Clemens, L. M. In Adhesion Science and Engineering 2, Surfaces, Chemistry & Applications; Chaudhury, M.; Pocius, A. V., Eds.; Elsevier: Amsterdam, 2002, Chapter 11, pp 465− 534.
AUTHOR INFORMATION
Corresponding Authors
*(A.K.) Fax: +81-72-254-9292. E-mail: matsumoto@chem. osakafu-u.ac.jp. *(E.S.) Fax: +81-6-6605-2981. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Yang, S.; Chen, J. -S.; Körner, H.; Breiner, T.; Ober, C. K. Chem. Mater. 1998, 10, 1475−1482. (2) Ogino, K.; Chen, J. -S.; Ober, C. K. Chem. Mater. 1998, 10, 3833−3838. (3) Wang, L. J.; Li, H. Y.; Wong, C. P. J. Polym. Sci., Part A, Polym. Chem. 2000, 38, 3771−3782. (4) Chen, J. -S.; Ober, C. K.; Poliks, M. D. Polymer 2002, 43, 131− 139. (5) Malik, J.; Clarson, S. J. Int. J. Adhes. Adhes. 2002, 22, 283−289. (6) Shirai, M. Prog. Org. Coat. 2007, 58, 158−165. (7) González, L.; Ferrando, F.; Ramis, X.; Salla, J. M.; Mantecón, A.; Serra, A. Prog. Org. Coat. 2009, 65, 175−181. (8) Hutchinson, A. R.; Winfield, P. H.; McCurdy, R. H. Adv. Eng. Mater. 2010, 12, 646−652. (9) Sato, E.; Tamura, H.; Matsumoto, A. ACS Appl. Mater. Inter. 2010, 2, 2594−2601. (10) Sato, E.; Hagihara, T.; Matsumoto, A. ACS Appl. Mater. Inter. 2012, 4, 2057−2064. (11) Inui, T.; Sato, E.; Matsumoto, A. ACS Appl. Mater. Inter. 2012, 4, 2124−2132. (12) Sekine, T.; Tomita, H.; Obata, S.; Saito, Y. Electr. Eng. Jpn. 2009, 168, 32−39. (13) Nishiyama, Y.; Uto, N.; Sato, C.; Sakurai, H. Int. J. Adhes. Adhes. 2003, 23, 377−382. (14) Ishikawa, H.; Seto, K.; Shimotuma, S.; Kishi, N.; Sato, C. Int. J. Adhes. Adhes. 2005, 25, 193−199. (15) Ozawa, T.; Ishikawa, S.; Kano, Y.; Kasemura, T. J. Adhes. 2000, 72, 1−16. (16) Ebe, K.; Seno, H.; Horigome, K. J. Appl. Polym. Sci. 2003, 90, 436−441. (17) Lee, S. -W.; Park, J. -W.; Lee, Y. -H.; Kim, H. -J.; Rafailovich, M.; Sokolov, J. J. Adhes. Sci. Technol. 2012, 26, 1629−1643. (18) Tsarevsku, N. V.; Matyjaszewski, K., In Fundamentals of Controlled/Living Radical Polymerization; Tsarevsku, N. V., Sumerlin, S. B. S., Eds.; RSC Publishing, The Royal Society of Chemistry: Dorchester, U.K., 2013; pp 287−357. (19) Ito, H. Adv. Polym. Sci. 2005, 172, 37−245. (20) Yamago, S. J. Polym. Sci., Part A, Polym. Chem. 2006, 44, 1−12. (21) Yamago, S. Chem. Rev. 2009, 109, 5051−5068. (22) Yamago, S.; Nakamura, Y. In Polymer Science: A Comprehensive Reference; Matyjaszewski, K., Möller, M., Eds.-in-Chief; Coates, G. W., Sawamoto, M., Eds.; Elsevier: Amsterdam, 2012; vol. 3, pp 227−248. (23) Yamago, S.; Nakamura, Y. Polymer 2013, 54, 981−994. (24) Zetterlund, P. B.; Kagawa, Y.; Okubo, M. Chem. Rev. 2008, 108, 3747−3794. (25) Yamago, S.; Iida, K.; Yoshida, J. J. Am. Chem. Soc. 2002, 124, 2874−2875. (26) Yamago, S.; Iida, K.; Yoshida, J. J. Am. Chem. Soc. 2002, 124, 13666−13667. (27) Takagi, K.; Soyano, A.; Kwon, T. S.; Kunisada, H.; Yuki, Y. Polym. Bull. 1999, 4, 143−150. (28) Yamago, S.; Iida, K.; Nakajima, M.; Yoshida, J. Macromolecules 2003, 36, 3793−3796. (29) Goto, A.; Kwak, Y.; Fukuda, T.; Yamago, S.; Iida, K.; Nakajima, M.; Yoshida, J. J. Am. Chem. Soc. 2003, 125, 8720−8721. (30) Yusa, S.; Yamago, S.; Sugahara, M.; Morikawa, S.; Yamamoto, T.; Morishima, Y. Macromolecules 2007, 40, 5907−5915. 8120
dx.doi.org/10.1021/ma401595w | Macromolecules 2013, 46, 8111−8120
