**8. Application of photo‐depolymerizable poly (olefin sulfone)s to photo‐detachable adhesives**

**7. Poly(olefin sulfone)s composed of volatile monomers**

**Figure 18.** Decomposition ratio of BA1 (○) and polymer 10 (●) as a function of the irradiation energy.

nanocircuit fabrication.

136 Alkenes

The photoinduced depolymerization of poly(olefin sulfone)s composed of volatile olefins was investigated [18]. The polymers were converted into volatile compounds by photoinduced depolymerization. A poly(olefin sulfone) film containing a photobase generator (ANC2) was irradiated with 254‐nm light at ambient temperature through a photomask. While very little change occurred in the film just after irradiation, the irradiated area of the film vaporized upon heating to 110**°**C, forming a mask pattern on the film (**Figure 19**). The surface of the film was also observed by AFM after heat treatment. The results confirmed that the irradiated area was removed, leaving a bare surface. This enables a wide variety of applications, such as stereolithography without the use of solvents, photo‐detachable adhesives, and printable Detachable adhesives have attracted great interest due to their potential applications in reusable products and reworkable systems [22–26]. Adhesives used in many applications, and a variety of strong adhesives that can be employed in extreme environments have been developed [27]. However, most high‐performance adhesives are strongly adhesive, making them difficult to remove and therefore cannot be used in recyclable materials or in reworking processes. Therefore, a need exists for glues that provide firm bonding but also can be easily detached. The strength of an adhesive bond essentially depends on surface interactions between the adhesive material and the substrate. Therefore, if the chemical structure of the adhesive material can be changed after adhesion, the adhesion strength may also change. Studies have been reported on the adhesive strength of substrates fastened using degradable polymers [23, 24, 26]. The results showed that depolymerizable polymers can act as detachable adhesives.

The adhesive strength of a poly(olefin sulfone) composed of a volatile olefin monomer and a second olefin monomer possessing a crosslinkable moiety has been evaluated [28]. This polymer was expected to act as a detachable adhesive, as illustrated in **Figure 20**. When a mixture of this poly(olefin sulfone) and a crosslinking reagent was embedded between two glass plates and cured, the plates remained glued together. Subsequently, the glued plates could be separated upon irradiation with UV light and heating. For further investigation, a mixture of the poly(olefin sulfone), a cross‐linking agent, and a photobase generator were prepared, and the adhesive strengths before and after photoirradiation were examined. **Figure 21** shows the chemical structures of the samples. The bond strength of the test samples was measured using a cross‐tensile test apparatus, shown in **Figure 22**. **Figure 23(a)** shows the tensile strengths obtained from plates bonded with various adhesives: cross‐linked TPAS‐11, PMPS, a commercially available epoxy adhesive (Araldite Rapid), and polypropylene. The polypropylene did not result in adhesion between the quartz plates, while the cross‐linked TPAS‐11 clearly possessed a tensile strength greater than those obtained either with the Araldite Rapid or PMPS. The superior bonding strength of the cross‐linked TPAS‐11 was thought to be due to the highly polar poly(olefin sulfone) main chain and hydrogen bond-

**Figure 21.** Structures of the poly(olefin sulfone)s (PMPS and TPAS‐11), photobase generator (L‐ANC2) and crosslinker (PCD) employed in the present study and crosslinking reaction of the carbodiimide PCD and a carboxylic acid.

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**Figure 22.** 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 m 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 20.** Photoinduced depolymerization of poly(olefin sulfone)s containing photobase generators and a sequence showing a photodetachable thermosetting adhesive.

The adhesive strength of a poly(olefin sulfone) composed of a volatile olefin monomer and a second olefin monomer possessing a crosslinkable moiety has been evaluated [28]. This polymer was expected to act as a detachable adhesive, as illustrated in **Figure 20**. When a mixture of this poly(olefin sulfone) and a crosslinking reagent was embedded between two glass plates and cured, the plates remained glued together. Subsequently, the glued plates could be separated upon irradiation with UV light and heating. For further investigation, a mixture of the poly(olefin sulfone), a cross‐linking agent, and a photobase generator were prepared, and the adhesive strengths before and after photoirradiation were examined. **Figure 21** shows the chemical structures of the samples. The bond strength of the test samples was measured using a cross‐tensile test apparatus, shown in **Figure 22**. **Figure 23(a)** shows the tensile strengths obtained from plates bonded with various adhesives: cross‐linked TPAS‐11, PMPS, a commercially available epoxy adhesive (Araldite Rapid), and polypropylene. The polypropylene did not result in adhesion between the quartz plates, while the cross‐linked TPAS‐11 clearly possessed a tensile strength greater than those obtained either with the Araldite Rapid or PMPS. The superior bonding strength of the cross‐linked TPAS‐11 was thought to be due to the highly polar poly(olefin sulfone) main chain and hydrogen bond-

138 Alkenes

**Figure 20.** Photoinduced depolymerization of poly(olefin sulfone)s containing photobase generators and a sequence

showing a photodetachable thermosetting adhesive.

**Figure 21.** Structures of the poly(olefin sulfone)s (PMPS and TPAS‐11), photobase generator (L‐ANC2) and crosslinker (PCD) employed in the present study and crosslinking reaction of the carbodiimide PCD and a carboxylic acid.

**Figure 22.** 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 m 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).

light at 3.0 J/cm<sup>2</sup>

**9. Conclusions**

**Author details**

, with or without heating for different durations. The results demonstrate

), (c) after UV irradiation and heating at

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that heating before UV irradiation did not weaken the bond strength, while heating after UV irradiation caused the adhesive strength to decrease to nearly zero. **Figure 24** shows photographs of the adhered samples, indicating that the cross‐linked TPAS‐11 was both colorless and transparent. 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

**Figure 24.** Photographs of a mixture of TPAS‐11, PCD (5 wt%) and L‐ANC2 (20 wt%): (a) after heating at 100**°**C for 30

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

Photoinduced depolymerization of poly(olefin sulfone)s containing photobase generators was summarized. Since poly(olefin sulfone)s are copolymers of olefins and sulfur dioxide, the sulfur dioxide produced through the depolymerization evaporates from the system. Thus, the depolymerization reaction proceeds only in one direction, and the polymer is converted into consistent olefins. The photoinduced depolymerization of poly(olefin sulfone)s has been investigated for a wide variety of applications, including stereolithography, printable micro-

of the poly(olefin sulfone), allowing the quartz plates to detach.

min without UV irradiation, (b) after irradiation with 254‐nm UV light (3.0 J/cm<sup>2</sup>

for 30 min, and (f) after UV irradiation and heating at 100**°**C for 60 min.

circuit fabrication, and detachable adhesives.

Takeo Sasaki\*, Khoa Van Le and Yumiko Naka

\*Address all correspondence to: sasaki@rs.kagu.tus.ac.jp Tokyo University of Science, Shinjuku‐ku, Tokyo, Japan

**Figure 23.** (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/mm<sup>2</sup> ) is indicated by a dashed line. The tensile test was conducted for 5 times 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/cm<sup>2</sup> ), (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 for 5 times in each measurement.

ing between the carboxylic acid groups and the N‐acylurea groups at the cross‐linking sites (**Figure 21**). Note that PMPS‐glued plates detached upon heating to temperatures above the glass transition temperature of PMPS because it is a thermoplastic resin. In contrast, cross‐ linked TPAS acted as a thermoset resin and resulted in stable adhesion. The change in the adhesive strength of the cross‐linked TPAS‐11 upon light irradiation and/or heating was also investigated. **Figure 23(b)** shows the tensile strength of the cross‐linked TPAS‐11 sample as prepared and after heating at 100**°**C for 60 min, and upon irradiation with 254‐nm UV

**Figure 24.** 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/cm<sup>2</sup> ), (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.

light at 3.0 J/cm<sup>2</sup> , with or without heating for different durations. The results demonstrate that heating before UV irradiation did not weaken the bond strength, while heating after UV irradiation caused the adhesive strength to decrease to nearly zero. **Figure 24** shows photographs of the adhered samples, indicating that the cross‐linked TPAS‐11 was both colorless and transparent. 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 detach.
