Research Article www.acsami.org
Dismantlable Thermosetting Adhesives Composed of a CrossLinkable Poly(olefin sulfone) with a Photobase Generator Takeo Sasaki,* Shouta Hashimoto, Nana Nogami, Yuichi Sugiyama, Madoka Mori, Yumiko Naka, and Khoa V. Le Department of Chemistry, Faculty of Science, Tokyo University of Science 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan S Supporting Information *
ABSTRACT: A novel photodetachable adhesive was prepared using a photodepolymerizable cross-linked poly(olefin sulfone). A mixture of a cross-linkable poly(olefin sulfone), a cross-linking reagent, and a photobase generator functioned as a thermosetting adhesive and exhibited high adhesive strength on quartz plates comparable to that obtained for commercially available epoxy adhesives. The cured resin was stable in the absence of UV light irradiation but completely lost its adhesive strength upon exposure of glued quartz plates to UV light in conjunction with heating to 100 °C. KEYWORDS: dismantlable adhesives, thermosetting adhesives, photoinduced depolymerization, poly(olefin sulfone)s, photobase generator
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INTRODUCTION Poly(olefin sulfone)s containing photobase generators (PBG) are known to exhibit photoinduced depolymerization.1−4 A poly(olefin sulfone) is a 1:1 alternating copolymer of an olefin monomer and sulfur dioxide,5,6 and the protons on the carbons adjacent to the sulfonyl groups in these polymers are readily abstracted by bases.7 This abstraction results in a depolymerization chain reaction, and so poly(olefin sulfone)s incorporating a photobase-generating chromophore will undergo a photoinduced unzipping reaction. In this reaction, the primary chain of the poly(olefin sulfone) is depolymerized to regenerate the original olefin monomer together with sulfur dioxide (Figure 1). For this reason, these polymers have been considered for a wide variety of applications, including stereolithography, printable microcircuit fabrication, and dismantlable adhesives. Dismantlable adhesives have attracted significant interest with regard to the development of recyclable products and reworkable systems.8−12 Adhesives are essential materials in modern society, and a number of strong adhesives that can be employed in severe environments have been developed.13 However, the majority of high-performance adhesives are overly difficult to remove and so cannot be utilized in recyclable materials or in reworking processes. As such, glues capable of firmly bonding materials together but that can also be easily detached are required. The strength of an adhesive bond depends strongly on surface interactions between the substrate and the adhesive material, and so if the chemical structure of the adhesive material is changed after adhesion, the adhesion strength will therefore also change. There have been studies on the adhesive strength of substrates glued using degradable polymers.9,10,12 These studies have shown that using a depolymerizable polymer as an adhesive allows the substrate to be detached in some cases. © XXXX American Chemical Society
Figure 1. Photoinduced depolymerization of poly(olefin sulfone)s containing photobase generators and a sequence showing a photodetachable thermosetting adhesive.
In the present study, a poly(olefin sulfone) composed of a volatile olefin monomer and a second olefin monomer possessing a cross-linkable moiety was synthesized. This polymer was expected to act as a dismantlable adhesive, as shown in Figure 1. If a mixture of this poly(olefin sulfone) and a cross-linking reagent is sandwiched between glass plates and cured, the plates will be glued together. Subsequently, Received: October 23, 2015 Accepted: February 12, 2016
A
DOI: 10.1021/acsami.5b10110 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces irradiating the glued plates with UV light in conjunction with heating will separate the plates. Thus, a mixture of the poly(olefin sulfone), a cross-linking agent, and a photobase generator was prepared, and the adhesion properties before and after photoirradiation were investigated. It was found that the poly(olefin sulfone) adhesive prepared in this study exhibits high adhesion strength on glass plates and detaches when exposed to UV light and heated to 100 °C.
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Table 1. Molecular Weights, Glass Transition Temperatures, and Decomposition Temperatures for the Polymers Used in This Study TPAS-3 TPAS-11 TPAS-23 PMPS PCD polypropylene
EXPERIMENTAL SECTION
Samples. The structures of the two poly(olefin sulfone)s, the cross-linker, and the photobase generator used in this study are shown in Figure 2. Poly(2-methyl-1-pentene sulfone) (PMPS) and the
a
Mn
Mw/Mn
Tg (°C)
Tda (°C)
114000 86000 54000 206000 4000 75000
2.2 2.0 1.8 3.5 2.8 2.3
83 86 88 87 70 0
145 139 118 139 400 330
TGA curves of TPAS-y are shown in Figure S1.
generator. In preparation for the adhesion trials, one of the poly(olefin sulfone)s together with PCD and the photobase generator were dissolved in chloroform and then dried under vacuum. Synthesis of 4-Methyl-4-pentenoic Acid. Methyltriphenylphosphonium bromide 20.0 g (56 mmol) was dissolved in dry tetrahydrofuran (THF, 150 mL) under dry N2 atmosphere and cooled to 0 °C. Hexane solution of buthyllitium (2.6 M, 20 mL) was added dropwise to the THF solution. After the color of the solution turned to reddish brown, 6.5 g of levulinic acid dissolved in 15 mL of dry THF was slowly added. The solution was stirred at 0 °C for 1 h and kept stirring for 2 days at room temperature. Aqueous solution of hydrochloric acid (1 N, 100 mL) was added to the solution. The THF solvent was evaporated, and the product was extracted by ethyl acetate from the aqueous solution. After the ethyl acetate solution was dried over magnesium sulfate, the solvent was evaporated. The product was purified by column chromatography on silica gel (eluent: ethyl acetate). 4-Methyl-4-pentenoic acid was obtained as a transparent and colorless liquid. Yield = 53%. 1H NMR (CDCl3): δ 4.79 (s, 1H, CCH2), 4.71 (s, 1H, CCH2), 2.52 (t, 2H, CCH3 CH2), 2.35 (t, 2H, HOOCCH2), 1.75 (s, 3H, CCH3 CH2). Synthesis of TPAS-23. 2-Metyl-1-pentene and 4-methyl-4pentenoic acid were dissolved in liquefied SO2 (15 g) and polymerized with tert-butylperoxide (tBuOOH) as an initiator. tBuOOH acted as a redox initiator with SO2, producing a tert-butyloxy radical. The initiator (0.5 mmol), 2-methyl-1-pentene (0.42 g, 5.00 mmol), 4methyl-4-pentenoic acid (0.58 g, 5.08 mmol), and SO2 (15 g) were added to a glass tube at −196 °C. When the temperature of the tube was raised above −70 °C, the frozen solution fused and polymerization started. The tube was maintained at −13 °C for 1 h. After the polymerization, the polymer was purified by precipitation from methanol then washed several times with methanol and dried under vacuum at room temperature. The presence of SO2 in the polymer was confirmed by FT-IR (1311 and 1130 cm−1) and 1H NMR. Molecular weights and thermal properties of the polymers obtained are listed in Table 1. The polymers were soluble in chloroform, tetrahydrofuran, N,N-dimethylformamide, dimethylacetamide, and dimethyl sulfoxide. The copolymerization ratio was determined by the 1H NMR measurement. 1H NMR (500 MHz, THF-d8) δ ppm: 3.70 (s, br, 2H, SO2−CH2−C), 3.64 (s, br, 2H, SO2−CH2−C), 2.68 (m, br, 2H, −CH2−CH2−COOH), 2.49 (m, br, −CH2−CH2−COOH), 2.07 (m, br, 2H, C−CH2−C2H5), 1.78−1.59 (m, br, 5H, −CH2−CH2−CH3, 3H, CH3−C), 1.59 (s, br, 3H, CH3−C), 0.95 (t, br, 3H, CH3−CH2−). IR (KBr): 1308 (aym, SO2, str), 1133 (sym, SO2, str), 2966 cm−1 (C− H, str), 1735 (CO, str), 3310 cm−1 (−O−H, str) Synthesis of Polycarbodiimide (PCD). Tolylene-2,4-diisocyanate (5.00 g, 28 mmol) and 4-methyl-1-phenyl-2,3-dihydrophosphol1-one (50 mg, 28 × 10−2 mmol) were dissolved in THF(20 mL) under N2 atmosphere. The solution was refluxed for 2 h. The solution was cooled to room temperature, and the solvent was evaporated. The residual was dissolved in chloroform and added dropwise to pure hexane twice. Yield = 27%. 1H NMR (500 MHz, CDCl3) δ ppm: 7.08 (s, 1H, Ar−H), 6.92 (s, 1H, Ar−H), 6.87 (s, 1H, Ar−H), 2.29 (s, 3H, Ar−CH3). IR (KBr): 2130 (s, NCN, str), 2920 (s, ar C−H,
Figure 2. Structures of the poly(olefin sulfone)s (PMPS and TPAS-y), photobase generator (L-ANC2) and cross-linker (PCD) employed in the present study and cross-linking reaction of the carbodiimide PCD and a carboxylic acid.. random copolymer poly[(2-methyl-1-pentene sulfone)-(4-methyl 4pentanoic acid sulfone)] (TPAS-y) were employed as the host poly(olefin sulfone)s. The y in TPAS-y indicates the polymerization ratio (%) of the carboxylic acid moiety as determined by 1H NMR measurements. Both PMPS and TPAS-y were prepared by the radical polymerization of 2-methylpentene and 3-methyl pentenoic acid in liquefied sulfone dioxide, using tert-butyl hydroperoxide as an initiator at −30 °C. Polycarbodiimide (PCD) was used as the cross-linker and was prepared by the polymerization of tolylene-2,3-diisocyanate in a THF solution with 4-methyl-1-phenyl-2,3-dihydrophoshol-1-one as the catalyst.14 PCD reacts with the carboxylic acid groups of TPAS to form a network structure (Figure 2). The molecular weights and thermal properties of the polymers used in this study are listed in Table 1. In this work, 4,4′-[bis[[methyl(2-nitrobenzyl)oxy]carbonyl]trimethylene]dipiperidine] (L-ANC2) was used as the photobase B
DOI: 10.1021/acsami.5b10110 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces weak), 1457−1597 cm−1 (m, ar C−C). Mn = 4000, Mw = 11000, Mw/ Mn = 2.8. Measurements. Fourier-transform infrared (FT-IR) spectra were obtained with a Jeol JIR-5500 spectrophotometer. The weight-average molecular weight (Mw) of each polymer was determined by gel permeation chromatography (GPC; Tosoh HLC-8220 with a Super Multipore HZ-M column, using tetrahydrofuran as the eluent), and glass transition temperature (Tg) was obtained by differential scanning calorimetry (DSC; Mettler, DSC822e). The decomposition temperature for each adhesive mixture was measured by thermogravimetric analysis (TGA, TA Instruments, Hi-Res TGA2950). 1H NMR spectra were acquired using a Jeol EX-400 spectrometer (500 MHz). During adhesive degradation trials, the polymer film was irradiated using a 250 W superhigh-pressure mercury lamp (Ushio, SX-UI250HQ). The intensity of the UV light was maintained at 23 mW/cm2 (at 254 nm), and a color filter (Toshiba, UVD-36C) and an interference filter were used to obtain monochromatic radiation. The adhesion strength was measured using a tensile test apparatus (INSTRON 3340 single column system 3342). Samples intended for adhesion strength measurements were prepared using 1 mm-thick quartz plates as substrates.
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RESULTS AND DICUSSION Cross-Linking and Photoinduced Depolymerization of a Mixture of TPAS-y, PCD, and Photobase Generator. The cross-linking reaction of a mixture of TPAS-y and PCD was initially investigated. In these trials, TPAS-5 was mixed with PCD (5 wt %) and L-ANC2 (5−20 wt %) in chloroform, such that the concentration of the solute in the resulting solution was 5 wt %. The solution was then heated to 100 °C for 30 min and cooled to room temperature, after which more chloroform was added. The mixture was soluble in chloroform prior to heating but insoluble after heating. This mixture of TPAS-23, PCD (5 wt %), and L-ANC2 (20 wt %) was coated on a KBr plate and FT-IR spectra were obtained (Figure 3). The spectra of the mixture did not exhibit any changes after storing the film at room temperature (24−26 °C) for as long as 7 days. However, the absorption peak resulting from the NC N moiety (2130 cm−1) was found to decrease rapidly when the mixture was kept at 80−100 °C. The cross-linking ratios at each temperature were calculated from the IR absorption spectra. cross linking ratio = 1 −
Figure 3. FT-IR spectra of films of mixed TPAS-23 and PCD after aging for varying intervals at (a) room temperature (26 °C), (b) 50, (c) 70, (d) 80, and (e) 100 °C.
mixture previously coated on a KBr plate following heating at 100 °C for various time intervals are shown in Figure 6. Prior to UV irradiation, no changes are seen in the spectra on heating, with the exception of a slight decrease in the absorption of the cross-linking moiety (2130 cm−1, NCN). However, following UV irradiation, a decrease in the absorption of the SO2 moiety (1130 and 1311 cm−1) is observed on heating, signifying the photochemical base-induced depolymerization of the poly(olefin sulfone) chain. The ratios of the residual SO2 after UV irradiation as functions of heating time at 100 °C are plotted in Figure 7. The decomposition of the poly(olefin sulfone) was evidently independent of the amount of crosslinking points. Adhesion Properties of the Cross-Linkable Poly(olefin Sulfone) Mixture. The adhesive strength of quartz plates bonded with the cross-linkable poly(olefin sulfone) was measured. The structure of the samples used in adhesive strength trials is shown in Figure 8. The bond strength of these test specimens was measured using a cross tensile test apparatus. In these trials, the polymer (TPAS-11, 20 mg), cross-linker (PCD, 1 mg), and photobase generator (L-ANC2, 4 mg) were dissolved in chloroform (200 μL). Precise 2.5 μL of the solution was put on two quartz substrates, and the solvent was evaporated. The two quartz plates were sandwiched together, with the polymer mixture between the two. A 50-μmthick Teflon sheet with a 3 mm-diameter hole in its center was placed between the two quartz plates to define the area of
(100 − Tt ) (100 − T0)
where the Tt is a transmittance of carbodiimide group (2130 cm−1) at t min and T0 is a transmittance of the carbodiimide group at 0 min. The cross-linking ratios of a film of TPAS-23 mixed with PCD are plotted as functions of heating time in Figure 4a. The cross-linking reaction was evidently accelerated at temperatures above 80 °C. The cross-linking extents of the TPAS-y obtained with different initial ratios of the carboxylic acid moiety are plotted as functions of heating time (100 °C) in Figure 4b. The TPAS polymer with a higher carboxylic acid ratio is seen to have cured more rapidly. Absorption spectra of the photobase generator L-ANC2 following exposure to various amounts of UV radiation are shown in Figure 5. The L-ANC2 exhibited a change in its absorption spectra when exposed to UV as the result of the base-generating reaction shown in the upper part of this figure, during which the nitro groups in the molecule transition to nitroso groups via a photochemical reaction.15−17 The photoinduced depolymerization of cross-linked samples was subsequently investigated. IR absorption spectra of a C
DOI: 10.1021/acsami.5b10110 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 6. FT-IR spectra of a film of mixed TPAS-11, PCD (5 wt %), and L-ANC2 (20 wt %) following heating for varying intervals at (a) 100 °C before and (b) after 254 nm irradiation at 3.0 J/cm2.
Figure 4. (a) Cross-linking ratios in films of mixed TPAS-23 and PCD as functions of the heating time. The heating temperatures are indicated in the figure. (b) Effect of the initial amount of the carboxylic acid moiety in TPAS-y on the cross-linking ratios in films of mixed TPAS-y and PCD as functions of the heating time at 100 °C. Figure 7. Residual SO2 moiety ratios in films of mixed TPAS-11, PCD (5 wt %), and L-ANC2 (20 wt %) as functions of heating time at 100 °C following 254 nm irradiation at 3.0 J/cm2.
Figure 8. Preparation of samples for adhesion strength measurements: (a) a mixture of TPAS-11, PCD (5 wt %), and L-ANC2 (20 wt %) is placed on quartz plates, (b) the plates are sandwiched on either side of a 50 μm Teflon sheet with a 3 mm diameter hole and the assembly heated to 100 °C for 10 min, and (c) a finished sample ready for adhesive strength measurements (cross tensile test).
Figure 5. UV−vis absorption spectra of L-ANC2 in chloroform solution (4.36 × 10−5 mol/L) following irradiation by 254 nm UV light at varying intensities.
adhesion. The entire setup was then fastened with binder clips and heated to 100 °C for 5 min. Figure 9a shows the tensile strengths obtained from plates bonded with the cross-linked TPAS-11 and also with PMPS, a commercially available epoxy adhesive (Araldite rapid) and polypropylene. It is evident that the polypropylene did not produce any adhesion between the quartz plates, while the cross-linked TPAS-11 produced a
tensile strength greater than those obtained from the Araldite and PMPS. The superior bonding strength of the TPAS-11 is considered to result from the highly polar poly(olefin sulfone) main chain and from hydrogen bonding between the carboxylic acid groups and the N-acylurea groups at the cross-linking sites. Since PMPS is a thermoplastic resin, the substrates will detach D
DOI: 10.1021/acsami.5b10110 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 10. Photographs of a mixture of TPAS-11, PCD (5 wt %), and L-ANC2 (20 wt %): (a) after heating at 100 °C for 30 min without UV irradiation, (b) after irradiation with 254 nm UV light (3.0 J/cm2), (c) after UV irradiation and heating at 100 °C for 5 min, (d) after UV irradiation and heating at 100 °C for 15 min, (e) after UV irradiation and heating at 100 °C for 30 min, and (f) after UV irradiation and heating at 100 °C for 60 min.
adhesive strength is plotted in Figure 11. The bond strength dropped rapidly at temperatures higher than 60 °C and fell to
Figure 9. (a) Adhesive strengths of quartz plates bonded with polypropylene (PP), Araldite rapid, PMPS, and cross-linked TPAS-11 (with 5 wt % of PCD). Both the PMPS and cross-linked TPAS contained 20 wt % L-ANC2. The maximum tensile strength that could be measured (7.2 N/mm2) is indicated by a dashed line. The tensile test was conducted 5× in each measurement. (b) Adhesive strengths of quartz plates bonded with a mixture of TPAS-11, PCD (5 wt %) and L-ANC2 (20 wt %): (1) immediately after sample preparation, (2) after heating at 100 °C for 60 min, (3) after irradiation with 254 nm UV light (3.0 J/cm2), (4) after irradiation with UV and heating at 100 °C for 5 min, (5) after irradiation with UV and heating at 100 °C for 15 min, (6) after irradiation with UV and heating at 100 °C for 30 min, and (7) after irradiation with UV and heating at 100 °C for 60 min. The tensile test was conducted 5× in each measurement.
Figure 11. Effect of the post exposure bake temperature on the adhesive bond of quartz plates glued with a mixture of TPAS-11, PCD (5 wt %), and L-ANC2 (20 wt %). The tensile test was conducted 5× in each measurement.
zero within 60 min when heating at temperatures above 80 °C. When the sample was heated at 100 °C, the adhesive strength decreased to almost zero within 15 min. As shown in Figure 11, the tensile strength increased after heating at 60 °C for 15 min. At 60 °C, the temperature is not sufficiently high enough to complete the depolymerization reaction of the poly(olefin sulfone). The poly(olefin sulfone) chain was partially depolymerized at this condition and the polymer was softened. The tensile strength increased because of the reattachment of the softened polymer.
when heated above the glass transition temperature of the polymer. In contrast, TPAS is a thermoset resin, such that stable adhesion was obtained. Subsequently, the photoinduced change in the adhesive strength of the cross-linked TPAS-11 was investigated. Figure 9b summarizes the tensile strength of the cross-linked TPAS-11 sample as prepared and after heating at 100 °C for 60 min and irradiation with 254 nm UV light at 3.0 J/cm2. The bond strength did not change as the result of heating before UV irradiation, although heating after UV irradiation caused the adhesive strength to rapidly decrease to zero. Photographs of the adhesion samples are shown in Figure 10, in which it can be seen that the cross-linked TPAS-11 was both colorless and transparent. The absorption spectra of the TPAS-11 film is shown in Figure S2. Following irradiation with UV light, the resin became slightly yellow because of the production of nitrosobenzaldehyde via photodecomposition of L-ANC2. Heating the sample to 100 °C also generated gaseous products because of depolymerization of the poly(olefin sulfone), allowing the quartz plates to be detached. The effect of the heating temperature applied following UV irradiation on the
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CONCLUSION A poly(olefin sulfone) incorporating carboxylic acid moieties was synthesized and mixed with a polycarbodiimide cross-linker and a photobase generator. The mixture worked as a thermosetting adhesive and exhibited high adhesive strength on quartz plates, comparable to the bond strengths obtained with commercially available epoxy adhesives. When the bonded quartz plates were exposed to 254 nm UV light and subsequently heated to 100 °C, the poly(olefin sulfone) depolymerized and the plates could be detached. Although the depolymerization process releases gaseous species (i.e., sulfur dioxide, olefin monomers), the drastic change in adhesion strength induced by light must be attractive for utilization in production site with adequate exaust equipment. E
DOI: 10.1021/acsami.5b10110 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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(15) Urankar, J. E.; Fréchet, J. M. J. Photogenerated Base in Polymer Curing and Imaging: Cross-Linking of Base-Sensitive Polymers Containing Enolizable Pendant Groups. Chem. Mater. 1997, 9, 2861−2868. (16) Urankar, J. E.; Brehm, I.; Niu, J. Q.; Fréchet, J. M. J. BaseSensitive Polymers as Imaging Materials: Radiation-Induced βElimination To Yield Poly(4-hydroxystyrene). Macromolecules 1997, 30, 1304−1310. (17) Harkness, R. B.; Takeuchi, K.; Tachikawa, M. Photopatternable Thin Films from Silyl Hydride Containing Silicone Resins and Photobase Generators. Polym. Adv. Technol. 1999, 10, 669−677.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10110. TGA curbs of TPAS-y and absorption spectrum of TPAS-11, PCD, and L-ANC2 film (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
All authors contributed equally. Notes
The authors declare no competing financial interest.
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REFERENCES
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