Azumi
Fukuyoshi
a,
Shogo
Oshiro
a,
Yuya
Seino
a,
Yosuke
Uchida
a,
Takashi
Aketa
b,
Toshiyuki
Ozai
b,
Hideo
Nakagawa
c and
Yoshiro
Kaneko
*a
aGraduate School of Science and Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan. E-mail: ykaneko@eng.kagoshima-u.ac.jp; Fax: +81 99 285 7794; Tel: +8199 285 7794
bSilicone-Electronics Materials Research Center, Shin-Etsu Chemical Co., Ltd, Japan
cResearch & Development Department, Shin-Etsu Chemical Co., Ltd, Japan
First published on 18th June 2025
In this study, polysiloxane-based adhesives were prepared by introducing a catechol component into the side chains of copolymers comprising polydimethylsiloxane (PDMS) and ammonium-functionalized polysiloxane components via urea bonds. Aluminum plates were adhered using these polysiloxanes containing PDMS and catechol-functionalized polysiloxane (PS-Ph(OH)2) components at the compositional ratios of 9:
1, 8
:
2, 7
:
3, 6
:
4 and 5
:
5 (PDMS-PS-Ph(OH)2 (9
:
1, 8
:
2, 7
:
3, 6
:
4 and 5
:
5), respectively), and adhesion was evaluated via tensile shear tests at room temperature. The aluminum plates adhered using PDMS-PS-Ph(OH)2 (8
:
2, 7
:
3, 6
:
4 and 5
:
5) exhibited high adhesive strength, withstanding tensile shear stresses of 12.7–21.7 MPa. Moreover, PDMS-PS-Ph(OH)2 (8
:
2 and 7
:
3) maintained strong adhesion under impact loads. Furthermore, when a 5 kg weight was suspended from the aluminum plates adhered using PDMS-PS-Ph(OH)2 (8
:
2 and 7
:
3) and the entire assembly was subsequently heated in an oven, the plates did not peel off until the temperature reached 180 °C and 190 °C, respectively, indicating that adhesion was maintained even at relatively high temperatures. Additionally, adherend plates possessing different thermal expansion coefficients, such as aluminum and stainless-steel plates, were adhered using PDMS-PS-Ph(OH)2 (8
:
2 and 7
:
3). Thereafter, a 5 kg weight was suspended from the plates, and the temperature was cycled between room temperature and 150 °C. Consequently, the plates did not peel off, suggesting that PDMS-PS-Ph(OH)2 (8
:
2 and 7
:
3) could effectively achieve adhesion between dissimilar materials.
In recent years, particularly in the automobile industry, multimaterial technology has gained attention as an effective approach for achieving low fuel consumption by reducing the weight of the car body structure.20–22 In this technology, conventionally used steel materials are combined with lightweight materials, such as aluminum and carbon fiber–reinforced plastic. Therefore, new joining methods that can accommodate these material changes are necessary, and adhesion-based joining is attracting attention. Because adhesive bonding is a surface technique, stress can be uniformly dispersed. Furthermore, as materials with different characteristics can be freely selected as the adhesive layer, stress can be efficiently transmitted between adherends, enhancing the strength of the entire adhering system. However, the durability of current organic polymer-based adhesives raises concerns during their long-term usage. Furthermore, temperature fluctuations result in strain between adherends possessing different thermal expansion coefficients, necessitating the use of adhesives that can relieve the stress caused by the aforementioned strain. Therefore, the adhesive should possess rigidity to maintain the strength of the entire adhering system as well as flexibility to accommodate the thermal strain due to temperature fluctuations.
Polysiloxane is useful as a base polymer, offering flexibility to address the latter issue. Because the Si–O bond in the polysiloxane main chain has a higher bond energy than the C–C bond in organic polymers, polysiloxane exhibits superior durability, heat and ultraviolet resistances and other advantages, rendering it suitable for long-term applications, such as those in the automobile industry. Moreover, because the Si–O bond distance is longer than the C–C bond distance and the Si–O–Si bond angle is larger than the C–C–C bond angle, polysiloxane possesses a flexible main chain. Therefore, it is a suitable base polymer for adhesives targeting adherends possessing different thermal expansion coefficients because it can relieve the stress resulting from thermal strain due to temperature fluctuations.
Based on the abovementioned research background, several polysiloxane-based adhesives containing catechol components have been developed.23,24 These polymers may exhibit potential as adhesives for dissimilar adherends because they have flexible siloxane main chains. However, because the strength of pure polysiloxane is low, the lap shear strength of the aluminum plates adhered using this catechol-containing polysiloxane is 62.6 kgf cm−2 (∼6.13 MPa) under optimal conditions.24 Nevertheless, structural adhesives for automotive applications may require higher adhering strengths.
In this study, we aimed to develop polysiloxane-based adhesives containing catechol components to achieve the rigidity required to maintain the strength of the entire adhering system as well as the flexibility needed to accommodate the thermal strain due to temperature fluctuations. Two key steps were employed to achieve these objectives. The first step was the introduction of a catechol component into the polysiloxane main chain through a urea bond. This was expected to enhance the cohesive strength between the polymer chains via hydrogen bonding through the urea bonds. The second step was increasing the curing temperature during adhesion. At high temperatures, catechol-containing polysiloxane could form a cross-linked structure via the radical coupling of the catechol components. The aforementioned two strategies were expected to contribute to the strength of the cured adhesive.
Therefore, we prepared polysiloxanes by introducing a catechol component into the side chains via urea bonds. Polysiloxanes containing polydimethylsiloxane (PDMS) and catechol-functionalized polysiloxane (PS-Ph(OH)2) components were labelled PDMS-PS-Ph(OH)2 (x:
y), wherein “x
:
y” represents the compositional ratio of the PDMS (x) and PS-Ph(OH)2 (y) components. Aluminum plates adhered using PDMS-PS-Ph(OH)2 demonstrated high adhesive strength, withstanding tensile shear stresses of 12.7–21.7 MPa, and maintained strong adhesion even under impact loads. Furthermore, when a 5 kg weight was suspended from the adhered aluminum plates and the entire assembly was heated in an oven, no peeling occurred until the temperature reached 180 °C–190 °C, confirming that adhesion was maintained under high-temperature conditions. Additionally, when aluminum and stainless-steel plates, which possess different thermal expansion coefficients, were adhered using PDMS-PS-Ph(OH)2 and subsequently subjected to temperature cycling between room temperature and 150 °C after suspending a weight from them, the plates did not peel off, demonstrating that PDMS-PS-Ph(OH)2 could achieve effective adhesion between dissimilar materials.
When the DMDMS:
APDMMS feed molar ratios were in the range of 8
:
2–5
:
5, the compositional ratio of the PDMS and PS-NH3Cl components in the resulting copolymer closely corresponded with the feed ratio. However, when preparing a copolymer possessing a compositional ratio of 9
:
1 using the same DMDMS
:
APDMMS feed molar ratio, the desired compositional ratio could not be achieved. Increasing the DMDMS
:
APDMMS feed molar ratio to 11
:
1 successfully yielded a copolymer possessing the PDMS and PS-NH3Cl components in a compositional ratio of 9
:
1. This discrepancy is considered to result from the cyclization of DMDMS during the reaction, forming low-boiling compounds that may volatilize.
In the 1H nuclear magnetic resonance (NMR) spectra of PDMS-PS-NH3Cl (x:
y), broad signals derived from the copolymers were observed. The compositional ratios of the PDMS component (x) to the PS-NH3Cl component (y) in PDMS-PS-NH3Cl (8
:
2–5
:
5) closely mirrored the DMDMS
:
APDMM feed molar ratios (Fig. S1b–e†). However, to achieve a PDMS
:
PS-NH3Cl compositional ratio of 9
:
1 (Fig. S1a†), the DMDMS
:
APDMMS feed molar ratio should be slightly increased to 11
:
1.
Additionally, to determine the average molecular weight of PDMS-PS-NH3Cl (x:
y), the primary ammonium groups on the polymer side chains were protected through a reaction with lauroyl chloride in the presence of triethylamine (Et3N) (Scheme S1†). The introduction of the lauroyl groups was confirmed via1H NMR measurements (Fig. S2†). Based on the gel permeation chromatography (GPC) measurements of the resulting products, the weight-average molecular weights (Mw) were estimated as 2.30 × 104–4.00 × 104 (molecular weight distributions (Mw/number-average molecular weight (Mn)): 1.32–1.76) (Fig. S3†).
In the Fourier transform infrared (FT-IR) spectra of PDMS-PS-Ph(OH)2 (x:
y), absorption peaks corresponding to the urea bond were observed at approximately 1630 and 1560 cm−1 (Fig. 1). Additionally, the 1H NMR spectra of PDMS-PS-Ph(OH)2 (x
:
y) exhibited signals h attributable to the aromatic ring (Fig. 2), confirming the introduction of the catechol component via covalent bonds. Furthermore, the compositional ratios of the PDMS component (x) to the PS-Ph(OH)2 component (y), calculated using the integral ratio of the signals a of the methyl protons adjacent to the silicon atom and the signals h of the aromatic ring, were approximately equivalent to those of the PDMS-PS-NH3Cl (x
:
y) precursors.
![]() | ||
Fig. 1 FT-IR spectra of PDMS-PS-Ph(OH)2 (x![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
![]() | ||
Fig. 2
1H NMR spectra in DMSO-d6 of PDMS-PS-Ph(OH)2 (x![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
![]() | ||
Fig. 3 Method of adhesion between the adherends (e.g., aluminum plates) using PDMS-PS-Ph(OH)2 (x![]() ![]() |
![]() | ||
Fig. 4 Stress–strain curves obtained via the tensile shear tests of the aluminum plates adhered using PDMS-PS-Ph(OH)2 (x![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Their failure modes were predominantly identified as cohesive fractures based on visual observation. In this assessment, aluminum plates adhered using PDMS-PS-Ph(OH)2 (7:
3) were selected as a representative example (Fig. S4†). Furthermore, scanning electron microscopy (SEM) images revealed adhesive residue remaining on the fractured surfaces of the aluminum plates (Fig. S5†), providing additional support for the occurrence of cohesive fracture. Energy-dispersive X-ray spectroscopy (EDX) detected silicon atoms at spots 1–3 on the fractured surfaces in the SEM image shown in Fig. S5 (Table S1†), offering further evidence of cohesive failure. These SEM and EDX results collectively confirm that the failure mode was cohesive in nature.
For comparison, tensile shear tests were performed using the PDMS-PS-NH3Cl (x:
y) precursor copolymers. The tensile shear stresses required for peeling off the plates were 0.79, 1.49, 4.09, 2.95 and 2.37 MPa for PDMS-PS-NH3Cl (9
:
1) (Fig. S6a†), PDMS-PS-NH3Cl (8
:
2) (Fig. S6b†), PDMS-PS-NH3Cl (7
:
3) (Fig. S6c†), PDMS-PS-NH3Cl (6
:
4) (Fig. S6d†) and PDMS-PS-NH3Cl (5
:
5) (Fig. S6e†), respectively. All these values were lower than those obtained for PDMS-PS-Ph(OH)2 (x
:
y). These results demonstrate that the catechol component in the side chains of PDMS-PS-Ph(OH)2 (x
:
y) plays a crucial role in enhancing adhesion.
![]() | ||
Fig. 5 Evaluation of adhesiveness under impact loads for the aluminum plates adhered using PDMS-PS-Ph(OH)2 (x![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
The aforementioned results emphasize the importance of the flexible PDMS component for the impact load resistance of PDMS-PS-Ph(OH)2 (x:
y). This is plausibly because of the mitigation of the stress due to the impact-induced strain by the PDMS component. However, the plates adhered using PDMS-PS-Ph(OH)2 (9
:
1) peeled off upon the first impact, indicating that an excess of the PDMS component reduces the adhesive strength under impact loads. These findings indicate that the balance between the PDMS and PS-Ph(OH)2 components plays a critical role in determining the mechanical performance of the adhesives, particularly in achieving flexibility provided by the PDMS component and sufficient cohesive and adhesive strength contributed by the PS-Ph(OH)2 component. Therefore, these results clearly demonstrate that an optimal compositional ratio of the PDMS and PS-Ph(OH)2 components is crucial for achieving a high adhesive performance in both the tensile shear test and impact load test.
![]() | ||
Fig. 6 Evaluation of adhesiveness at high temperatures for the aluminum plates adhered using PDMS-PS-Ph(OH)2 (x![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
To evaluate the thermal stability of the cured material, thermogravimetric analysis (TGA) was performed on the PDMS-PS-Ph(OH)2 (7:
3) sample after heat treatment at 150 °C for 12 h. The cured material exhibited an initial decomposition temperature of 218 °C and a 10% weight loss temperature (Td10) of 293 °C (Fig. S7†). Accordingly, the adhesive failure observed at 200 °C is likely attributable to the onset of thermal degradation, as indicated by the TGA results.
Although PDMS-PS-Ph(OH)2 (8:
2 and 7
:
3) were polymers composed of flexible siloxane main chains, they retained their adhesive properties at relatively high temperatures (180 °C–190 °C). This could be attributed to the radical coupling reaction between the catechol components in PDMS-PS-Ph(OH)2 (8
:
2 and 7
:
3) upon heating, which formed a network (cross-linked) structure. To investigate structural changes associated with reactions between catechol groups, ultraviolet–visible (UV–Vis) absorption spectroscopy was performed on coatings prepared by applying the precursor (e.g., PDMS-PS-Ph(OH)2 (7
:
3)) to a glass plate and heating it at 150 °C for 12 h. Before heating, the coating was colorless and transparent, with almost no absorption observed above 310 nm (Fig. S8a†). After heating, however, the coating turned yellow-brown and exhibited a red shift in the absorption edge, extending to ∼500 nm (Fig. S8b†). This change in absorption characteristics suggests an extension of the π-conjugated system due to reactions involving catechol groups, supporting the occurrence of inter- or intramolecular coupling reactions during heat treatment. These findings indicate the formation of a network (cross-linked) structure. Consequently, softening—which would considerably reduce adhesion—was prevented even at high temperatures.
For both PDMS-PS-Ph(OH)2 (8:
2 and 7
:
3), the weights did not drop when heated to 150 °C (Fig. 7), and adhesion was maintained upon returning to room temperature. This was plausibly because of the flexible PDMS component, which alleviated the stress caused by the thermal strain resulting from the difference in the thermal expansion coefficients of aluminum and stainless steel. Indeed, differential scanning calorimetry (DSC) of the cured PDMS-PS-Ph(OH)2 (7
:
3) sample (heat-treated at 150 °C for 12 h) revealed a glass transition temperature (Tg) of 45 °C (Fig. S9†), suggesting that the cured material exhibits rubbery behavior at temperatures above Tg. Furthermore, the formation of a cross-linked structure through the radical coupling reaction between the catechol components prevented flow at high temperatures, ensuring stable adhesion.
![]() | ||
Fig. 7 Evaluation of adhesiveness at high temperatures (150 °C) between the aluminum and stainless-steel plates adhered using PDMS-PS-Ph(OH)2 (x![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Impact load tests were performed on the aluminum plates adhered using PDMS-PS-Ph(OH)2 (8:
2 and 7
:
3), and peeling off was not observed even after multiple hammer impacts. However, the aluminum plates adhered using PDMS-PS-Ph(OH)2 (6
:
4 and 5
:
5) peeled off after multiple hammer strikes, suggesting that an excessive PS-Ph(OH)2 component would lead to brittleness and a reduced impact resistance. These results demonstrate that the balance between the PDMS and PS-Ph(OH)2 components is critical for achieving both flexibility and mechanical strength, particularly flexibility from the PDMS component and cohesive and adhesive strength from the PS-Ph(OH)2 component.
Furthermore, when a 5 kg weight was suspended from the aluminum plates adhered using PDMS-PS-Ph(OH)2 (8:
2 and 7
:
3) and the system was subsequently heated in an oven, the plates maintained adhesion up to 180 °C and 190 °C, respectively, without peeling off, indicating adhesive stability at high temperatures. This thermal stability is attributed to the formation of cross-linked structures through radical coupling between the catechol components during heating.
Additionally, PDMS-PS-Ph(OH)2 (8:
2 and 7
:
3) were used to adhere plates possessing different thermal expansion coefficients, such as aluminum and stainless-steel plates. When a 5 kg weight was suspended from the adhered plates and the temperature was cycled between room temperature and 150 °C, no peeling off was observed. This suggests that PDMS-PS-Ph(OH)2 (8
:
2 and 7
:
3) are effective for adhering dissimilar materials. The formation of cross-linked structures through radical coupling between the catechol components maintains adhesion at high temperatures, whereas the flexibility of polysiloxane plausibly alleviates the stress caused by the temperature-induced thermal expansion.
Future research will focus on the application of catechol-functionalized siloxane polymers for the adhesion of dissimilar materials, particularly those with considerably different thermal expansion coefficients, such as resins and metals. This approach aims to accelerate the practical implementation of multimaterial bonding using adhesives and broaden its applications across various industrial fields.
Footnote |
† Electronic supplementary information (ESI) available: Experimental section, 1H NMR spectra of PDMS-PS-NH3Cl and lauroyl-protected PDMS-PS-NH3Cl, GPC curves of lauroyl-protected PDMS-PS-NH3Cl, fracture surface information (photographs, SEM and EDX) of aluminum plates adhered using PDMS-PS-Ph(OH)2 (7:3), stress–strain curves of PDMS-PS-NH3Cl, TGA and DSC data of the cured PDMS-PS-Ph(OH)2 (7:3) and UV–Vis spectra of the PDMS-PS-Ph(OH)2 (7:3) coating. See DOI: https://doi.org/10.1039/d5py00442j |
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