Reversing Adhesion: A Triggered Release Self‐Reporting Adhesive

Here, the development of an adhesive is reported – generated via free radical polymerization – which can be degraded upon thermal impact within minutes. The degradation is based on a stimuli responsive moiety (SRM) that is incorporated into the network. The selected SRM is a hetero Diels‐Alder (HDA) moiety that features three key properties. First, the adhesive can be degraded at relatively low temperatures (≈80 °C), second the degradation occurs very rapidly (less than 3 min), and third, the degradation of the network can readily be analyzed and quantified due to its self‐reporting nature. The new reversible self‐reporting adhesion system is characterized in detail starting from molecular studies of the retro HDA reaction. Moreover, the mechanical properties of the network, as well as the adhesion forces, are investigated in detail and compared to common methacrylate‐based systems, demonstrating a significant decrease in mechanic stability at elevated temperatures. The current study thus represents a significant advance of the current state of the art for debonding on demand adhesives, making the system interesting for several fields of application including dental adhesives.

be incorporated into a crosslinker, which is typically a di-or polyvinylic monomer.
In recent years several attempts have been made to prepare degradable networks. [ 6 ] However, degradable systems known today require relatively high temperatures [ 7 ] and/or many hours [ 8 ] or days [ 5a ] to cleave, which in most cases limits their applicability drastically.
Inspired by the goal to overcome these limitations, we present a DoD adhesive that can be degraded in minutes when heated to 80 °C (see Figure 1 ).
The new adhesive is based on a polymer network formed via free radical polymerization of a dimethacrylate crosslinker including two thermally sensitive hetero Diels-Alder (HDA) moieties (DiHDA-linker). A methacrylate species was chosen due to its low toxicity, making the system also interesting for biomedical applications. The HDA groups, which are incorporated in the linker can be cleaved due to the retro HDA reaction, which occurs upon heating (refer to Figure 1 b). The cleavage leads to the desired degradation and debonding of the network and the concomitant formation of highly colored (red) dithioester species (refer to Figure 1 d), indicating the release of the adhesive in a simple visual inspection system. In order to increase the cleavage effi ciency within the network, two HDA groups are incorporated into one linker, since a cleavage of only one HDA functionality would already lead to a cleavage of the crosslinks between the polymeric chains. The synthetic protocol for the dimethacrylate crosslinker (DiHDA-linker) and the prepared polymeric networks as well as the detailed reaction procedures can be found in the Supporting Information.
Prior to the adhesion and network release studies, the SRM has to be characterized in detail in order to confi rm and quantify the debonding performance of the molecular system. Therefore, the HDA-moiety was analyzed with high temperature (HT)-NMR and UV/Vis spectroscopy. In order to prevent an undesired polymerization at elevated temperatures, a precursor (DiHDA-core), instead of the vinyl functional linker itself was investigated (refer to Figure 1 b). Since the substitution close to the HDA-moiety is identical in both molecules, the DiHDA-linker displays the same behavior as the DiHDA-core. HT-NMR spectroscopic measurements of the DiHDA-core reveal that the HDA equilibrium is completely shifted to the side of the HDA product at temperatures below 25 °C. When the HDA moiety is heated, the concentration of retro HDA products increases steadily until complete conversion is reached at 120 °C (Figure 1 c).
Due to the absorption of the formed C S double bond in the visible light range (535 nm), the retro HDA reaction induces a color change, giving the adhesive release system its self-reporting nature. The change in color upon heating can readily be seen with the naked eye, making the system ideally suited for a range of applications, as it provides the user with a visible inspection system for the degree of debonding (refer to Figure 1 d).
UV/Vis spectroscopic measurements can quantify the visibly observable color change. For networks based on a DiHDAlinker, the measurement can be carried out when the network is directly formed in a UV/Vis cuvette. The amount of retro HDA product can be deduced directly at any given temperature as the start of the retro reaction (no absorption) and complete conversion (maximum absorption) can be determined during the measurement (refer to Figure 1 e and the Supporting Information). The start and end temperature of the retro HDA reaction assigned in the UV/Vis spectroscopic measurement correlates with the values determined in the HT-NMR analysis. When compared to the core in solution, the linker network shows very similar behavior. However, a higher degree of debonding of the DiHDA-linker network can be detected at temperatures below 65 °C. Such an observation is consistent with the fact that a detachment in a network leads to a higher entropy release compared to a detachment from a molecule in solution. A more detailed study of the entropic effect operational in cleaving macromolecular systems was carried out by our research group in a series of previous studies. [ 9 ] In addition to the spectroscopic studies, the degradation as a function of time was determined as well. The time required for the degradation of the adhesive is a crucial point if the adhesive release system is to be applicable in a practical context. However, most of the degradable networks known today require several hours or days to debond completely. [ 5a , c ] As the retro HDA reaction is known to be quite rapid in the current system, this limitation should be overcome with the invented adhesive. Therefore, the kinetics of the retro HDA reaction in the polymeric network were investigated (refer to Figure 1 f). At 100 °C, the equilibrium is reached in less than 3 min, demonstrating that the debonding takes place very rapidly, making the HDA system highly interesting for real life applications.
In order to evidence that the fast cleavage of the employed SRM can lead to an effi cient degradation of a polymeric network, its mechanical properties upon application of the degradation trigger were carefully studied. If the degradation is successful, a clear decrease in mechanical stability should be detectable, with the same change not evident in a reference systems containing a noncleavable linker.
Thus, rheological measurements of the prepared materials were carried out (for more details, refer to the Supporting Information). When compared to a typical, nondegradable polymer network based on urethane dimethacrylate (UDMA), the differences in the mechanical properties are clearly visible (see Figure 2 a). Here, the storage modulus G′ associated with the elastic contribution of the stress response of the sample and therefore its stability is plotted against the temperature. As expected for the nondegradable reference network, the storage modulus of the UDMA network decreases only slightly when the temperature increases. For the DiHDA-linker based network, a drastic loss in G′ by over two orders of magnitude (from 8.5 × 10 8 Pa at 25 °C to 5.5 × 10 6 Pa at 120 °C) and therefore in the stability of the network is observed upon heating, evidencing that substantial degradation of the adhesive takes place when the sample is heated.
The temperature at which G′ commences to decrease drastically can also be tuned by employing a comonomer, enabling the system to be adapted to different applicational requirements. In the example shown in Figure 2 b, isobornyl methacrylate ( i BoMA) is used as comonomer to form the degradable network and to shift the onset temperature for the decrease in G′ from close to 30 °C for the pure DiHDA-linker network to close to 50 °C for the copolymer network ( i BoMA + 0.2 equation DiHDA-linker). Thus, the system can be fi ne-tuned,   At ambient temperature, the network is rigid and cannot be bend (i). At 100 °C, the network can easily be bended by using tweezers (ii) and cut into pieces (iii). * 1,10-Decandiol dimethacrylate.
making it interesting also for applications that require a different debonding temperature. The drastic decrease in mechanic stability can also be detected by eye as seen in Figure 2 c. At ambient temperature, the sample is rigid and cannot be bent or deformed. However, when heated to 100 °C, the sample can easily be bent and cut in half. As the catalyst required for the HDA reaction is removed prior to the network formation, the back reaction to the HDA product and therefore the back formation of the network is disabled, resulting in a permanent cleavage of the network.
To demonstrate the practical applicability of the invented system, adhesion tests were carried out. Therefore, dental crowns were cemented to an implant-abutment and the pull-off force was measured at 23 and 80 °C (refer to Figure 3 a,b).
80 °C can temporarily be employed in the oral cavity, as teeth are known to be poor heat-conductors. [ 10 ] In the test, the DiHDA-linker or a dimethacrylate crosslinker (bisGMA), which is commonly used in dental materials, were copoly merized with n -butyl methacrylate ( n BMA, T g of poly( n BMA) = 20 °C). [ 11 ] More details regarding the test system and the self-curing monomer mixture can be found in the Supporting Information. As inspection of Figure 3 c indicates, both systems show strong adhesion at 23 °C. However, when heated to 80 °C, only the DiHDA-linker system shows a drastic decrease in the pulloff force from 663 N to merely 42 N, resulting in a radical loss of adhesion stability of 94%, compared to only 42% for the common dental adhesion system. When the ratio of the pulloff force for the two systems at 23 and 80 °C is compared, the drastic difference becomes even clearer (refer to Figure 3 c).
To the best of our knowledge, the presented degradable adhesion system is a major improvement over the state of the art, as it is the only adhesive known to combine a high adhesion strength with a fast and easy removability at slightly elevated temperatures.
By designing an adhesive that can be degraded within minutes, we have introduced a polymeric network, which features the properties of a typical adhesive, yet can be destructed on demand. Importantly, the degradation can also be quantifi ed via spectroscopic measurements or even by a simple visual inspection system. By employing different comonomers and/or by using a different HDA-pair as SRM, [ 12 ] the debonding temperature can be fi ne-tuned, making the system interesting for several fi elds of applications.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.