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Kim K. Oehlenschlaeger, Jan O. Mueller, Josef Brandt, Stefan Hilf, Albena Lederer, Manfred Wilhelm, Robert Graf, Michelle L. Coote, Friedrich G. Schmidt, and Christopher Barner-Kowollik* Over the last decade, materials scientists have focused strongly on the design and development of smart materials supporting daily life. In particular, the fabrication and life-time of polymeric materials as well as composites have become a topic of considerable interest, with self-healing materials gaining significant attention. A multitude of researchers have tackled the problem of restoring the mechanical properties of damaged materials by following different approaches.[1] The two main strategies are either based on networks with embedded microcapsules containing a healing agent, which is released into the network when the shell of a microcapsule is damaged or
K. K. Oehlenschlaeger, J. O. Mueller, Prof. C. Barner-Kowollik Preparative Macromolecular Chemistry Institut für Technische Chemie und Polymerchemie Karlsruhe Institute of Technology (KIT) Engesserstr. 18, 76131 Karlsruhe, Germany und Institut für Biologische Grenzflächen Karlsruhe Institute of Technology (KIT) Hermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopoldshafen, Germany E-mail: [email protected] J. Brandt, Dr. A. Lederer Leibniz-Institut für Polymerforschung Dresden Hohe Strasse 6, 01069 Dresden, Germany Universität Dresden 01062 Dresden, Germany Prof. M. Wilhelm Institut für Technische Chemie und Polymerchemie Karlsruhe Institute of Technology (KIT) Engesserstr. 18, 76131 Karlsruhe, Germany Dr. R. Graf Max-Planck Institut für Polymerforschung Ackermannweg 10 55128 Mainz, Germany Prof. M. L. Coote ARC Centre of Excellence for Free-Radical Chemistry and Biotechnology Research School of Chemistry Australian National University Canberra ACT 0200, Australia Dr. S. Hilf Evonik Industries AG Rodenbacher Chaussee 4 63457 Hanau-Wolfgang, Germany Dr. F. G. Schmidt Evonik Industries AG Paul-Baumann-Strasse 1 45764 Marl, Germany
DOI: 10.1002/adma.201306258
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Adaptable Hetero Diels–Alder Networks for Fast Self-Healing under Mild Conditions
on the use of reversible reactions to construct adaptable networks. In this latter approach, the network linkages are recycled to achieve healing via a metathesis mechanism of the crosslinking groups. Both approaches have advantages and disadvantages, and the employed self-healing technique has to be chosen judiciously with respect to the targeted application of the material. For example, embedded microcapsule networks display autonomous self-healing only for a limited number of times, whereas adaptable networks can theoretically be healed infinitely. On the other hand – and in contrast to the microcapsule approach – adaptable networks require external triggering of a stimulus for inducing the healing process. In an effort to tune and tailor the existing approaches for the desired applications, variable healing agents, encapsulated network systems, reversible cross-linking reactions, and stimuli are being considered.[1] One of the most remarkable embedded microcapsule network systems was published by White et al. who reported autonomous self-healing via the ring opening metathesis polymerization of encapsulated dicyclopentadiene which was released into an epoxy based network loaded with a Grubbs I catalyst.[2] The same authors later described refined networks with incorporated vascular systems, which permanently supply the network with the encapsulated healing agent, enabling the network to self-heal an unlimited number of times.[3] In the same year, groundbreaking work in adaptable network design was reported by Wudl and coworkers, reporting an adaptable network featuring self-healing properties under heat treatment through a Diels–Alder/retro Diels–Adler process.[4] Although other successful healing systems triggered by variable stimuli such as light,[5] pH-value,[6] redox agents[7] or various heat triggered approaches[8] have been reported, Diels–Alder (DA) adaptable networks have become one of the most studied systems in the field of self-healing materials.[9] Among its advantages for establishing industrial products is the tolerance of the Diels–Alder/retro Diels–Alder process to oxygen and moisture, as well as its hysteresis-free cycling between the bonded and debonded state.[10] However, the number of suitable DA pairings which have proven to be cycled in a reasonable temperature range (i.e. below the materials’ degradation temperature) are scarce. To date, only a few DA systems are used in self-healing materials offering cyclability at mild temperatures in a reasonable timeframe. The most prominent system is by far the N-maleimide/ furane DA pair pioneered by Wudl and coworkers in their aforementioned work.[4,11] Alternative DA pairings which are suitable for self-healing applications were reported by Murphy et al.[12]
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and the group of Lehn.[13] Our group has recently designed a new low temperature reversible system based on covalent chemistry, utilizing acid activated dithioesters as dienophiles in hetero Diels–Alder (HDA) reactions with cyclopentadiene (Cp).[14] However, a disadvantage of these materials is their slow self-healing kinetics in the absence of a catalyst. To address this limitation, we recently reported a novel HDA system, consisting of a new cyanodithioester (CDTE) compound and Cp.[15] The new system does not require a catalyst while offering hysteresis-free repetitive cyclability between 40–120 °C in a very rapid fashion (i.e., less than 5 min) as evidenced by polymerizing/depolymerizing a HDA based step growth polymer. In the present work, we utilize this CDTE/Cp HDA pair to design and characterize a novel self-healing material. By employing CDTE/Cp multi-functional building blocks, we show that healing is achieved in a very short timeframe at relatively low temperatures (120 °C). The cross-linked character of the material is qualitatively analyzed by a swelling test, whereas the adaptable character of the obtained network is demonstrated by temperature dependent complex viscosity cycling between 40 °C and 120 °C. We unambiguously demonstrate that the characteristic transitions of the storage (G’) and loss (G”) moduli point firmly to a cross-linked network revealing a glassy state, the glass transition state, the rubber plateau, and after release of the cross-linking points to a liquid state. Tensile tests of original and healed samples were conducted to assess the healing efficiency after each cycle, while temperature dependent solid-state NMR was utilized to explore the chemical structure of the material in its debonded state. Recently, we described the successful synthesis of a CDTE functional di-linker, suppressing the possible dimerization reaction of the CDTE group by protecting the C=S-double bond through in situ DA trapping with Cp.[15] A similar strategy was followed for the synthesis of a tetra-functional linker moiety featuring pentaerythritol as its basic structure. Subsequent esterification of the tetra alcohol with bromoacetyl-bromide, a nucleophile substitution of the bromine against the cyanodithioate nucleophile was carried out in the presence of Cp (Scheme 1a). It is important to note that the nucleophilic substitution of the bromine with the cyano-dithioate nucleophile (Scheme 1a)
proceeds to almost quantitative conversion, which simplifies the purification procedure due to the lack of side products. With the CDTE tetra-linker (1) in hand, the formation of an HDA based network was targeted with a readily available Cp2polymer (P(iBoA-nBA), Mn = 13 000 g mol−1, ÐM = 1.4, 2). A homogeneous solution of 1 and 2 was prepared in anisole with a molar ratio of 1:2 and a molar concentration of 0.01 mol L−1 of 1. The viscous solution was subsequently introduced into a 120 °C heated template (for images of the template refer to Figure S1a) and kept at 120 °C for 2 h. After cooling to ambient temperature, the desired network 3 was obtained. Elevated temperature is essential for the synthesis of the cross-linked material as it triggers the retro DA reaction of linker 1, necessary to remove the Cp protecting group. The unprotected linker species (i.e. the cyano dithioester) reacts upon cooling with the Cp end groups of polymer 2 to form network 3 (Scheme 1b). The use of anisole as high boiling solvent, which evaporates slowly at 120 °C, supports the immediate vaporization of the released Cp. Thus, the DA equilibrium cascade is driven towards the unprotected linker species, resulting in high conversions (84 wt% gel) compared with cross-linking reactions performed neat (34 wt% gel). At this stage, a weak entangled network of polymer chains is already formed. During the cooling process, the network density increases with proceeding HDA reactions between free CDTE and Cp end groups in the network. To qualitatively assess the extent of cross-linking, a swelling test was performed with the obtained material 3. Polymer 2 served as a reference and was treated in the same fashion as the 1:2 mixtures of 1 and 2. The control sample dissolved in toluene within 10 min of shaking, whereas upon similar treatment of network 3, swelling was observed, proving the cross-linking reaction to be successful (for the calculation of the degree of swelling (Ds) and the associated images, see Figure S2). After 24 h in toluene, a degree of swelling (Ds) of 338 wt% was found. Taking into account the molecular weight of polymer 2 (Mn = 13 000 g mol−1) and the resulting minimum mesh length of 3–4 nm (see Figure S3 for the calculation), the swelling behavior of network 3 appears to be reasonable. For a more detailed analysis of the network properties, temperature dependent rheology experiments were performed with
Scheme 1. a) Reaction scheme for the preparation of the CDTE based tetra-linker 1. b) Network formation of 3 via DA/rDA chemistry during the deprotection reaction and the cycling process. c) Pressed pellet of 3. d) Broken specimen after rheology experiments. e) Healed specimen with identical mechanical properties as the original one.
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again the dominant modulus leading to a state where the material has strong elastomeric properties, representing the rubber state. The elastic rubber state turns into the liquid state with a strong moduli and viscosity decrease at temperatures higher than 150 °C. All these states of cross-linked materials are well known and described in the literature for variable network systems.[16] However, the given network examples in the literature describe either permanently crosslinked networks, or adaptable networks with a high network density and consequently a small mesh length. Compared with the reversible network 3, weak cross-linked permanent networks show a glassy state, glass transition, and a rubber state, yet degradation (rather than a liquid state) is observed at higher temperatures due to their permanent linkages. In the case of adaptable networks with a small mesh length (as in the present case), only two significant states – a hard glassy and a more viscous liquid state – are detected, due to suppression of the network transition states (glass transition and rubber state) by the high network density. In other words, the HDA network 3 is not only able to heal damage by heat treatment, but is also able to specifically adjust the mechanical properties of the network by adjusting the temperature. This makes network 3 unique and distinguishes it from the dynamic covalent systems that have been reported to date. Next, a repetitive tensile test was performed to study the healing efficiency after Figure 1. (a) Evolution of the modulus G’ and G” from network 3 as a function of the tem- repetitive cycles of breaking and repairing the perature (γ0 = 1%, 8 mm geometry, ω/2π = 1 Hz). The applied heating rate was 2.5 °C min−1. material. A rod shaped sample was stretched b) Repetitive tensile tests of a rod sample of 3. till breakage and consecutively healed in the heat press for 10 min at 120 °C and 1 kN. Although the healing should already proceed in a reasonable an 8 mm plate-plate geometry on an ARES G2 rheometer in time frame above 70 °C,[15] at the beginning of the rubber state, the oscillating shear mode. Therefore, a pellet of 3 was produced in a heatable press (for pictures of the press refer to a healing temperature of 120 °C was selected based on the G’ Figure S1b) at 120 °C and 1 kN for 5 min, matching perfectly and G” moduli traces depicted in Figure 1a. At 120 °C, the with the 8 mm plate geometry. The rheology experiments were network is opened further resulting in a lower value of the G’ conducted with a fixed strain amplitude of 1 %, a frequency of modulus, thus providing more rapid healing while still being 1 Hz, and a heating rate of 2.5 °C min−1. With such a rheology sufficiently mild to tolerate most functional groups. Figure 1b depicts the obtained load vs. displacement curves for the origset up, measurements below 30 °C could not be performed, inal and the healed specimens. All samples display a small due to the high modulus of the sample causing an overload of decrease in force while displacing the sample over time, so the torque detection, which can be attributed to the hardness called creep. Creep is a well-known deformation phenomenon of the sample and insufficient adhesion at low temperatures. in material science appearing for example in metals and glassy Inspection of Figure 1a (which depicts G’ and G” as a funcpolymer materials.[17,18] As the tensile test was performed at tion of temperature) suggests a simple explanation for such a behavior. ambient temperature, such a behavior confirms the finding of the temperature dependent G moduli measurement, and Below 31 °C, the G’ modulus becomes higher than the value reveals a transition to the glassy state below 31 °C. of G”, indicating the transition to the hard and brittle glassy Regarding the healing, a higher force was required to disstate, thus causing the measurement problem with the plateplace the healed specimens compared to the original. Keeping plate geometry. Above 31 °C, the material reaches the so-called in mind the reversible nature and the synthesis of network 3, it glass transition or leather state, where the network can be plastiis not surprising that a healed (cycled) sample shows – after the cally reshaped although it remains fragile. At 72 °C, G’ becomes
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complete debonding in this temperature range. This is first cycle –slightly higher toughness properties than the origsomewhat lower than the temperature predicted for cominal one. Through the healing process the chemical structure plete debonding of the low molecular weight Cp-dilinker 5 of the network is refurbished leading to a different macroscopic in ref [15], due to the significantly greater entropic gain on order and a slightly higher cross-linking density. Such a process is underpinned by the smaller displacement, which is needed debonding the networked system compared with the small to break the healed specimens. As a higher cross-linking molecule. The repetitive 5 cycles exhibit a perfect reproducdensity is obtained, the more restricted movement of the ibility of the measurements with a viscosity of approximately chains results in a smaller deformation of the material before 2000 Pa·s at 120 °C and a viscosity that is ten times higher breakage. According to the equations used by White and cowthan the reference sample at 40 °C without any hysteresis. orkers, a healing efficiency of 106 % for the first healing cycle The reproducibility of the low temperature values after each and 96 % for the second one are obtained, being in excellent cycle suggests that complete re-bonding occurs during the agreement with the published data of White.[17] However, these cooling periods. Considering the relatively fast heating rate of 2.5 °C min−1, very fast reaction kinetics can be assumed, healing efficiency values should not be considered as absolute numbers, as the tensile test can easily deviate with fluctuations comparable to those found for a linear step growth polymer of inner stress or fabrication errors of the sample and the temsystem employing the identical reactive groups.[15] Thus, perature. Nonetheless, a very high healing efficiency is certain the on-line viscosity measurements are indicative of very for the system. In order to further demonstrate the regulating properties of the material, additional on-line analyses were carried out. Since the flow of a network is directly dependent on its cross-linking density, on-line complex viscosity measurements were considered to be a suitable tool for the inspection of the cyclability of the reversible HDA linkages. Consequently, a pressed pellet was cycled in the rheometer between 40 °C and 120 °C, with the same set-up as used for the G modulus measurements, while recording the complex viscosity of the system. As can be seen in Figure 2a, the absolute value of the complex viscosity of network 3 is close to a factor of ten higher than the reference sample, containing the Cp2-polymer only, at the starting temperature of 40 °C. However, with increasing temperature this factor increases strongly. At 110 °C, for instance, the viscosity of the reference sample decreases below 50 Pa·s to a region where the 8 mm plate-plate rheology set up does not provide accurate measurements. In contrast to the behavior of the reference sample, 3 still shows a reproducible viscosity above 2000 Pa·s at 120 °C, which is not surprising considering the G modulus traces from Figure 1a. At a temperature of 120 °C, the material displays elastomeric behavior, implying that a few cross-linking points are still present in the material. Nonetheless, the significant viscosity decrease (by a factor of 500) during heating for network 3 points to a strong loss of cross-linking density and a high percentage of debonding. In order to identify the temperature at which complete debonding of network 3 occurs, a temperature ramp to 180 °C was Figure 2. (a) Complex viscosity of network 3 (solide curve) and the reference sample (doted curve) during the temperature cycling between 40 °C and 120 °C (γ0 = 1%, 8 mm geomapplied (see Figure S4). The temperature etry, ω/2π = 1 Hz). (b) 1H MAS-NMR spectra of polymer 2 and network 3 at 120 °C (νMAS = ramp revealed that the viscosity reached 25 kHz, ν 1 1H Larmor = 700 MHz, upper and middle spectra) and H-solution NMR spectrum the value measured for the reference (at (400 MHz) of the step growth polymer 6 from ref. 15, recorded in toluene-d8 at 120 °C (lower 110 °C), just above 170 °C, indicating spectrum).[15]
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements C.B.-K. is grateful for continued support from and the excellent collaboration with Evonik Industries. Additional funding from the Ministry of Science and Arts of the state of Baden-Württemberg as well as the STN and BioInterfaces program of the Helmholtz association is gratefully acknowledged.
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fast reversible chemistry showing no hysteresis during the cycling of the bulk material. For a detailed structural analysis of the debonded network material, high temperature solid-state NMR spectra were recorded. Inspection of the 1H MAS-NMR spectrum of network 3, taken at 120° C with 25 kHz MAS spinning and 700 MHz 1H Larmor frequency (see Figure 2b, middle spectrum), revealed a set of resonances between 6.5 and 6.1 ppm. A comparison with the MAS-NMR spectrum of polymer 2 (Figure 2b, top spectrum) recorded under the same conditions suggests that the new set of resonances can be attributed to Cp-polymer end groups generated by the retro-HDA reaction of the network linkages as the shift and the signal splitting of both MAS-NMR spectra between 6.5 and 6.1 ppm matches perfectly. To further corroborate the hypotheses of the responses between 6.5 and 6.1 ppm in the MAS-NMR spectrum of 3 being Cp-polymer end group signals, the spectrum of the step growth polymer 6 from reference 15 (Figure 2b, bottom spectrum),[15] recorded in toluene-d8 at 120° C, was considered. The liquid state NMR spectrum of 6 displays the same shift of the Cp-polymer end group responses to the region between 6.5 and 6.1 ppm, evidencing the assignment of the MAS-NMR spectrum of 3. In the liquid state NMR spectrum of step growth polymer 6 (from reference 15) the resonances between 6.0 and 5.4 ppm were assigned to the HDA product isomers. Importantly, the same resonances between 6.0 and 5.4 are also present in the MAS-NMR spectrum of 3, which supports on the one hand the correct assignment of the responses between 6.5 and 6.1 to the Cp-polymer end groups and on the other hand the presence of the HDA product isomers in the network. To conclude, a novel self-healing material based on the HDA chemistry of a CDTE with Cp has been developed. The success of the cross-linking process was assessed by a swelling test as well as plate-plate rheology measurements, while the chemical structures of the network debonding were confirmed via high temperature 1H MAS-NMR analysis. Temperature dependent on-line viscosity data demonstrated the successful cross-linking of the material and confirmed the extremely fast reaction kinetics for the de- and re-bonding of the network system. The recorded temperature dependent traces of G’ and G” display the typical properties of a network with long mesh lengths, pointing to a material with excellent self-regulating qualities. Thus, in contrast to previously reported dynamic covalent systems, the current material is not only able to heal damage by heat treatment, but is also able to specifically adjust the mechanical properties of the network by adjusting the temperature, making it highly appealing for multiple reversible bonding applications. Not only can the properties of the network be set specifically by controlling the operating temperature in a very mild range, but also the changes of the settings occur within a very short time. Such properties make the designed network very useful for application such as rapid prototyping, adhesives, composite materials, coatings and paints as well as sealents. Moreover, it should be noted that the nature of the utilized Cp2-polymer has a significant impact on the materials properties, giving an additional opportunity to tune the self-healing material properties.
Received: December 23, 2013 Revised: January 24, 2014 Published online: March 21, 2014
[1] a) S. D. Bergman, F. Wudl, J. Mater. Chem. 2008, 18, 41; b) S. Billiet, X. K. D. Hillewaere, R. F. A. Teixeira, F. E. Du Prez, Macromol. Rapid Commun. 2013, 34, 290; c) N. K. Guimard, K. K. Oehlenschlaeger, J. Zhou, S. Hilf, F. G. Schmidt, C. Barner-Kowollik, Macromol. Chem. Phys. 2012, 213, 131. [2] S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler, S. R. Sriram, E. N. Brown, S. Viswanathan, Nature 2001, 409, 794. [3] a) K. S. Toohey, C. J. Hansen, J. A. Lewis, S. R. White, N. R. Sottos, Adv. Funct. Mater. 2009, 19, 1399; b) K. S. Toohey, N. R. Sottos, J. A. Lewis, J. S. Moore, S. R. White, Nat Mater 2007, 6, 581. [4] X. Chen, M. A. Dam, K. Ono, A. Mal, H. Shen, S. R. Nutt, K. Sheran, F. Wudl, Science 2002, 295, 1698. [5] a) G. L. Fiore, S. J. Rowan, C. Weder, Chem. Soc. Rev. 2013, 42, 7278; b) D. Habault, H. Zhang, Y. Zhao, Chem. Soc. Rev. 2013, 42, 7244. [6] a) N. Holten-Andersen, M. J. Harrington, H. Birkedal, B. P. Lee, P. B. Messersmith, K. Y. C. Lee, J. H. Waite, Proceedings of the National Academy of Sciences 2011, 108, 2651; b) D. C. Tuncaboylu, M. Sari, W. Oppermann, O. Okay, Macromolecules 2011, 44, 4997. [7] a) Q. Yan, A. Feng, H. Zhang, Y. Yin, J. Yuan, Polymer Chemistry 2013, 4, 1216; b) L.-P. Lv, Y. Zhao, N. Vilbrandt, M. Gallei, A. Vimalanandan, M. Rohwerder, K. Landfester, D. Crespy, J. Am. Chem. Soc. 2013, 135, 14198; c) A. P. Vogt, B. S. Sumerlin, Soft Matter 2009, 5, 2347. [8] a) M. Zhang, D. Xu, X. Yan, J. Chen, S. Dong, B. Zheng, F. Huang, Angew. Chem. Int. Ed. 2012, 51, 7011; b) Z. P. Zhang, M. Z. Rong, M. Q. Zhang, C. E. Yuan, Polymer Chemistry 2013, 4, 4648; c) S. Burattini, H. M. Colquhoun, J. D. Fox, D. Friedmann, B. W. Greenland, P. J. F. Harris, W. Hayes, M. E. Mackay, S. J. Rowan, Chem. Commun. 2009, 6717. [9] a) Y.-L. Liu, T.-W. Chuo, Polymer Chemistry 2013, 4, 2194; b) C. N. Bowman, C. J. Kloxin, Angew. Chem. Int. Ed. 2012, 51, 4272. [10] a) C. Barner-Kowollik, A. J. Inglis, L. Nebhani, F. G. Schmidt, S. Fengler, S. Hilf, S. Krause, A. Henning, International Patent PCT, WO/2011/101175; b) C. Barner-Kowollik, A. J. Inglis, L. Nebhani, O. Altintas, F. G. Schmidt, S. Fengler, S. Hilf, S. Krause, A. Henning, International Patent PCT, WO/2011/101176; c) C. BarnerKowollik, F. G. Schmidt, S. Hilf, J. Zhou, International Patent PCT, WO/2012/065786. [11] X. Chen, F. Wudl, A. K. Mal, H. Shen, S. R. Nutt, Macromolecules 2003, 36, 1802. [12] E. B. Murphy, E. Bolanos, C. Schaffner-Hamann, F. Wudl, S. R. Nutt, M. L. Auad, Macromolecules 2008, 41, 5203.
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[13] P. Reutenauer, E. Buhler, P. J. Boul, S. J. Candau, J. M. Lehn, Chemistry – A European Journal 2009, 15, 1893. [14] a) N. K. Guimard, J. Ho, J. Brandt, C. Y. Lin, M. Namazian, J. O. Mueller, K. K. Oehlenschlaeger, S. Hilf, A. Lederer, F. G. Schmidt, M. L. Coote, C. Barner-Kowollik, Chemical Science 2013, 4, 2752; b) A. J. Inglis, L. Nebhani, O. Altintas, F. G. Schmidt, C. Barner-Kowollik, Macromolecules 2010, 43, 5515; c) J. Zhou, N. K. Guimard, A. J. Inglis, M. Namazian, C. Y. Lin, M. L. Coote, E. Spyrou, S. Hilf, F. G. Schmidt, C. Barner-Kowollik, Polymer Chemistry 2012, 3, 628.
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[15] K. K. Oehlenschlaeger, N. K. Guimard, J. Brandt, J. Mueller, C. Y. Lin, S. Hilf, A. Lederer, M. L. Coote, F. Schmidt, C. BarnerKowollik, Polymer Chemistry 2013, 4, 4348. [16] L. H. Sperling, Introduction to Physical Polymer Science, Wiley-VCH, Weinheim, Germany 2006, fourth edition, Ch. 8. [17] M. R. Kessler, N. R. Sottos, S. R. White, Composites Part A: Applied Science and Manufacturing 2003, 34, 743. [18] a) G. W. Greenwood, Materials Science and Engineering 1976, 25, 241; b) G. J. Weng, Materials Science and Engineering 1983, 57, 127; c) R. W. Penn, G. E. Fulmer, Rev. Sci. Instrum. 1965, 36, 1230.
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Kim K. Oehlenschlaeger, Jan O. Mueller, Josef Brandt, Stefan Hilf, Albena Lederer, Manfred Wilhelm, Robert Graf, Michelle L. Coote, Friedrich G. Schmidt, and Christopher Barner-Kowollik* Over the last decade, materials scientists have focused strongly on the design and development of smart materials supporting daily life. In particular, the fabrication and life-time of polymeric materials as well as composites have become a topic of considerable interest, with self-healing materials gaining significant attention. A multitude of researchers have tackled the problem of restoring the mechanical properties of damaged materials by following different approaches.[1] The two main strategies are either based on networks with embedded microcapsules containing a healing agent, which is released into the network when the shell of a microcapsule is damaged or
K. K. Oehlenschlaeger, J. O. Mueller, Prof. C. Barner-Kowollik Preparative Macromolecular Chemistry Institut für Technische Chemie und Polymerchemie Karlsruhe Institute of Technology (KIT) Engesserstr. 18, 76131 Karlsruhe, Germany und Institut für Biologische Grenzflächen Karlsruhe Institute of Technology (KIT) Hermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopoldshafen, Germany E-mail: [email protected] J. Brandt, Dr. A. Lederer Leibniz-Institut für Polymerforschung Dresden Hohe Strasse 6, 01069 Dresden, Germany Universität Dresden 01062 Dresden, Germany Prof. M. Wilhelm Institut für Technische Chemie und Polymerchemie Karlsruhe Institute of Technology (KIT) Engesserstr. 18, 76131 Karlsruhe, Germany Dr. R. Graf Max-Planck Institut für Polymerforschung Ackermannweg 10 55128 Mainz, Germany Prof. M. L. Coote ARC Centre of Excellence for Free-Radical Chemistry and Biotechnology Research School of Chemistry Australian National University Canberra ACT 0200, Australia Dr. S. Hilf Evonik Industries AG Rodenbacher Chaussee 4 63457 Hanau-Wolfgang, Germany Dr. F. G. Schmidt Evonik Industries AG Paul-Baumann-Strasse 1 45764 Marl, Germany
DOI: 10.1002/adma.201306258
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Adaptable Hetero Diels–Alder Networks for Fast Self-Healing under Mild Conditions
on the use of reversible reactions to construct adaptable networks. In this latter approach, the network linkages are recycled to achieve healing via a metathesis mechanism of the crosslinking groups. Both approaches have advantages and disadvantages, and the employed self-healing technique has to be chosen judiciously with respect to the targeted application of the material. For example, embedded microcapsule networks display autonomous self-healing only for a limited number of times, whereas adaptable networks can theoretically be healed infinitely. On the other hand – and in contrast to the microcapsule approach – adaptable networks require external triggering of a stimulus for inducing the healing process. In an effort to tune and tailor the existing approaches for the desired applications, variable healing agents, encapsulated network systems, reversible cross-linking reactions, and stimuli are being considered.[1] One of the most remarkable embedded microcapsule network systems was published by White et al. who reported autonomous self-healing via the ring opening metathesis polymerization of encapsulated dicyclopentadiene which was released into an epoxy based network loaded with a Grubbs I catalyst.[2] The same authors later described refined networks with incorporated vascular systems, which permanently supply the network with the encapsulated healing agent, enabling the network to self-heal an unlimited number of times.[3] In the same year, groundbreaking work in adaptable network design was reported by Wudl and coworkers, reporting an adaptable network featuring self-healing properties under heat treatment through a Diels–Alder/retro Diels–Adler process.[4] Although other successful healing systems triggered by variable stimuli such as light,[5] pH-value,[6] redox agents[7] or various heat triggered approaches[8] have been reported, Diels–Alder (DA) adaptable networks have become one of the most studied systems in the field of self-healing materials.[9] Among its advantages for establishing industrial products is the tolerance of the Diels–Alder/retro Diels–Alder process to oxygen and moisture, as well as its hysteresis-free cycling between the bonded and debonded state.[10] However, the number of suitable DA pairings which have proven to be cycled in a reasonable temperature range (i.e. below the materials’ degradation temperature) are scarce. To date, only a few DA systems are used in self-healing materials offering cyclability at mild temperatures in a reasonable timeframe. The most prominent system is by far the N-maleimide/ furane DA pair pioneered by Wudl and coworkers in their aforementioned work.[4,11] Alternative DA pairings which are suitable for self-healing applications were reported by Murphy et al.[12]
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and the group of Lehn.[13] Our group has recently designed a new low temperature reversible system based on covalent chemistry, utilizing acid activated dithioesters as dienophiles in hetero Diels–Alder (HDA) reactions with cyclopentadiene (Cp).[14] However, a disadvantage of these materials is their slow self-healing kinetics in the absence of a catalyst. To address this limitation, we recently reported a novel HDA system, consisting of a new cyanodithioester (CDTE) compound and Cp.[15] The new system does not require a catalyst while offering hysteresis-free repetitive cyclability between 40–120 °C in a very rapid fashion (i.e., less than 5 min) as evidenced by polymerizing/depolymerizing a HDA based step growth polymer. In the present work, we utilize this CDTE/Cp HDA pair to design and characterize a novel self-healing material. By employing CDTE/Cp multi-functional building blocks, we show that healing is achieved in a very short timeframe at relatively low temperatures (120 °C). The cross-linked character of the material is qualitatively analyzed by a swelling test, whereas the adaptable character of the obtained network is demonstrated by temperature dependent complex viscosity cycling between 40 °C and 120 °C. We unambiguously demonstrate that the characteristic transitions of the storage (G’) and loss (G”) moduli point firmly to a cross-linked network revealing a glassy state, the glass transition state, the rubber plateau, and after release of the cross-linking points to a liquid state. Tensile tests of original and healed samples were conducted to assess the healing efficiency after each cycle, while temperature dependent solid-state NMR was utilized to explore the chemical structure of the material in its debonded state. Recently, we described the successful synthesis of a CDTE functional di-linker, suppressing the possible dimerization reaction of the CDTE group by protecting the C=S-double bond through in situ DA trapping with Cp.[15] A similar strategy was followed for the synthesis of a tetra-functional linker moiety featuring pentaerythritol as its basic structure. Subsequent esterification of the tetra alcohol with bromoacetyl-bromide, a nucleophile substitution of the bromine against the cyanodithioate nucleophile was carried out in the presence of Cp (Scheme 1a). It is important to note that the nucleophilic substitution of the bromine with the cyano-dithioate nucleophile (Scheme 1a)
proceeds to almost quantitative conversion, which simplifies the purification procedure due to the lack of side products. With the CDTE tetra-linker (1) in hand, the formation of an HDA based network was targeted with a readily available Cp2polymer (P(iBoA-nBA), Mn = 13 000 g mol−1, ÐM = 1.4, 2). A homogeneous solution of 1 and 2 was prepared in anisole with a molar ratio of 1:2 and a molar concentration of 0.01 mol L−1 of 1. The viscous solution was subsequently introduced into a 120 °C heated template (for images of the template refer to Figure S1a) and kept at 120 °C for 2 h. After cooling to ambient temperature, the desired network 3 was obtained. Elevated temperature is essential for the synthesis of the cross-linked material as it triggers the retro DA reaction of linker 1, necessary to remove the Cp protecting group. The unprotected linker species (i.e. the cyano dithioester) reacts upon cooling with the Cp end groups of polymer 2 to form network 3 (Scheme 1b). The use of anisole as high boiling solvent, which evaporates slowly at 120 °C, supports the immediate vaporization of the released Cp. Thus, the DA equilibrium cascade is driven towards the unprotected linker species, resulting in high conversions (84 wt% gel) compared with cross-linking reactions performed neat (34 wt% gel). At this stage, a weak entangled network of polymer chains is already formed. During the cooling process, the network density increases with proceeding HDA reactions between free CDTE and Cp end groups in the network. To qualitatively assess the extent of cross-linking, a swelling test was performed with the obtained material 3. Polymer 2 served as a reference and was treated in the same fashion as the 1:2 mixtures of 1 and 2. The control sample dissolved in toluene within 10 min of shaking, whereas upon similar treatment of network 3, swelling was observed, proving the cross-linking reaction to be successful (for the calculation of the degree of swelling (Ds) and the associated images, see Figure S2). After 24 h in toluene, a degree of swelling (Ds) of 338 wt% was found. Taking into account the molecular weight of polymer 2 (Mn = 13 000 g mol−1) and the resulting minimum mesh length of 3–4 nm (see Figure S3 for the calculation), the swelling behavior of network 3 appears to be reasonable. For a more detailed analysis of the network properties, temperature dependent rheology experiments were performed with
Scheme 1. a) Reaction scheme for the preparation of the CDTE based tetra-linker 1. b) Network formation of 3 via DA/rDA chemistry during the deprotection reaction and the cycling process. c) Pressed pellet of 3. d) Broken specimen after rheology experiments. e) Healed specimen with identical mechanical properties as the original one.
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again the dominant modulus leading to a state where the material has strong elastomeric properties, representing the rubber state. The elastic rubber state turns into the liquid state with a strong moduli and viscosity decrease at temperatures higher than 150 °C. All these states of cross-linked materials are well known and described in the literature for variable network systems.[16] However, the given network examples in the literature describe either permanently crosslinked networks, or adaptable networks with a high network density and consequently a small mesh length. Compared with the reversible network 3, weak cross-linked permanent networks show a glassy state, glass transition, and a rubber state, yet degradation (rather than a liquid state) is observed at higher temperatures due to their permanent linkages. In the case of adaptable networks with a small mesh length (as in the present case), only two significant states – a hard glassy and a more viscous liquid state – are detected, due to suppression of the network transition states (glass transition and rubber state) by the high network density. In other words, the HDA network 3 is not only able to heal damage by heat treatment, but is also able to specifically adjust the mechanical properties of the network by adjusting the temperature. This makes network 3 unique and distinguishes it from the dynamic covalent systems that have been reported to date. Next, a repetitive tensile test was performed to study the healing efficiency after Figure 1. (a) Evolution of the modulus G’ and G” from network 3 as a function of the tem- repetitive cycles of breaking and repairing the perature (γ0 = 1%, 8 mm geometry, ω/2π = 1 Hz). The applied heating rate was 2.5 °C min−1. material. A rod shaped sample was stretched b) Repetitive tensile tests of a rod sample of 3. till breakage and consecutively healed in the heat press for 10 min at 120 °C and 1 kN. Although the healing should already proceed in a reasonable an 8 mm plate-plate geometry on an ARES G2 rheometer in time frame above 70 °C,[15] at the beginning of the rubber state, the oscillating shear mode. Therefore, a pellet of 3 was produced in a heatable press (for pictures of the press refer to a healing temperature of 120 °C was selected based on the G’ Figure S1b) at 120 °C and 1 kN for 5 min, matching perfectly and G” moduli traces depicted in Figure 1a. At 120 °C, the with the 8 mm plate geometry. The rheology experiments were network is opened further resulting in a lower value of the G’ conducted with a fixed strain amplitude of 1 %, a frequency of modulus, thus providing more rapid healing while still being 1 Hz, and a heating rate of 2.5 °C min−1. With such a rheology sufficiently mild to tolerate most functional groups. Figure 1b depicts the obtained load vs. displacement curves for the origset up, measurements below 30 °C could not be performed, inal and the healed specimens. All samples display a small due to the high modulus of the sample causing an overload of decrease in force while displacing the sample over time, so the torque detection, which can be attributed to the hardness called creep. Creep is a well-known deformation phenomenon of the sample and insufficient adhesion at low temperatures. in material science appearing for example in metals and glassy Inspection of Figure 1a (which depicts G’ and G” as a funcpolymer materials.[17,18] As the tensile test was performed at tion of temperature) suggests a simple explanation for such a behavior. ambient temperature, such a behavior confirms the finding of the temperature dependent G moduli measurement, and Below 31 °C, the G’ modulus becomes higher than the value reveals a transition to the glassy state below 31 °C. of G”, indicating the transition to the hard and brittle glassy Regarding the healing, a higher force was required to disstate, thus causing the measurement problem with the plateplace the healed specimens compared to the original. Keeping plate geometry. Above 31 °C, the material reaches the so-called in mind the reversible nature and the synthesis of network 3, it glass transition or leather state, where the network can be plastiis not surprising that a healed (cycled) sample shows – after the cally reshaped although it remains fragile. At 72 °C, G’ becomes
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complete debonding in this temperature range. This is first cycle –slightly higher toughness properties than the origsomewhat lower than the temperature predicted for cominal one. Through the healing process the chemical structure plete debonding of the low molecular weight Cp-dilinker 5 of the network is refurbished leading to a different macroscopic in ref [15], due to the significantly greater entropic gain on order and a slightly higher cross-linking density. Such a process is underpinned by the smaller displacement, which is needed debonding the networked system compared with the small to break the healed specimens. As a higher cross-linking molecule. The repetitive 5 cycles exhibit a perfect reproducdensity is obtained, the more restricted movement of the ibility of the measurements with a viscosity of approximately chains results in a smaller deformation of the material before 2000 Pa·s at 120 °C and a viscosity that is ten times higher breakage. According to the equations used by White and cowthan the reference sample at 40 °C without any hysteresis. orkers, a healing efficiency of 106 % for the first healing cycle The reproducibility of the low temperature values after each and 96 % for the second one are obtained, being in excellent cycle suggests that complete re-bonding occurs during the agreement with the published data of White.[17] However, these cooling periods. Considering the relatively fast heating rate of 2.5 °C min−1, very fast reaction kinetics can be assumed, healing efficiency values should not be considered as absolute numbers, as the tensile test can easily deviate with fluctuations comparable to those found for a linear step growth polymer of inner stress or fabrication errors of the sample and the temsystem employing the identical reactive groups.[15] Thus, perature. Nonetheless, a very high healing efficiency is certain the on-line viscosity measurements are indicative of very for the system. In order to further demonstrate the regulating properties of the material, additional on-line analyses were carried out. Since the flow of a network is directly dependent on its cross-linking density, on-line complex viscosity measurements were considered to be a suitable tool for the inspection of the cyclability of the reversible HDA linkages. Consequently, a pressed pellet was cycled in the rheometer between 40 °C and 120 °C, with the same set-up as used for the G modulus measurements, while recording the complex viscosity of the system. As can be seen in Figure 2a, the absolute value of the complex viscosity of network 3 is close to a factor of ten higher than the reference sample, containing the Cp2-polymer only, at the starting temperature of 40 °C. However, with increasing temperature this factor increases strongly. At 110 °C, for instance, the viscosity of the reference sample decreases below 50 Pa·s to a region where the 8 mm plate-plate rheology set up does not provide accurate measurements. In contrast to the behavior of the reference sample, 3 still shows a reproducible viscosity above 2000 Pa·s at 120 °C, which is not surprising considering the G modulus traces from Figure 1a. At a temperature of 120 °C, the material displays elastomeric behavior, implying that a few cross-linking points are still present in the material. Nonetheless, the significant viscosity decrease (by a factor of 500) during heating for network 3 points to a strong loss of cross-linking density and a high percentage of debonding. In order to identify the temperature at which complete debonding of network 3 occurs, a temperature ramp to 180 °C was Figure 2. (a) Complex viscosity of network 3 (solide curve) and the reference sample (doted curve) during the temperature cycling between 40 °C and 120 °C (γ0 = 1%, 8 mm geomapplied (see Figure S4). The temperature etry, ω/2π = 1 Hz). (b) 1H MAS-NMR spectra of polymer 2 and network 3 at 120 °C (νMAS = ramp revealed that the viscosity reached 25 kHz, ν 1 1H Larmor = 700 MHz, upper and middle spectra) and H-solution NMR spectrum the value measured for the reference (at (400 MHz) of the step growth polymer 6 from ref. 15, recorded in toluene-d8 at 120 °C (lower 110 °C), just above 170 °C, indicating spectrum).[15]
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements C.B.-K. is grateful for continued support from and the excellent collaboration with Evonik Industries. Additional funding from the Ministry of Science and Arts of the state of Baden-Württemberg as well as the STN and BioInterfaces program of the Helmholtz association is gratefully acknowledged.
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fast reversible chemistry showing no hysteresis during the cycling of the bulk material. For a detailed structural analysis of the debonded network material, high temperature solid-state NMR spectra were recorded. Inspection of the 1H MAS-NMR spectrum of network 3, taken at 120° C with 25 kHz MAS spinning and 700 MHz 1H Larmor frequency (see Figure 2b, middle spectrum), revealed a set of resonances between 6.5 and 6.1 ppm. A comparison with the MAS-NMR spectrum of polymer 2 (Figure 2b, top spectrum) recorded under the same conditions suggests that the new set of resonances can be attributed to Cp-polymer end groups generated by the retro-HDA reaction of the network linkages as the shift and the signal splitting of both MAS-NMR spectra between 6.5 and 6.1 ppm matches perfectly. To further corroborate the hypotheses of the responses between 6.5 and 6.1 ppm in the MAS-NMR spectrum of 3 being Cp-polymer end group signals, the spectrum of the step growth polymer 6 from reference 15 (Figure 2b, bottom spectrum),[15] recorded in toluene-d8 at 120° C, was considered. The liquid state NMR spectrum of 6 displays the same shift of the Cp-polymer end group responses to the region between 6.5 and 6.1 ppm, evidencing the assignment of the MAS-NMR spectrum of 3. In the liquid state NMR spectrum of step growth polymer 6 (from reference 15) the resonances between 6.0 and 5.4 ppm were assigned to the HDA product isomers. Importantly, the same resonances between 6.0 and 5.4 are also present in the MAS-NMR spectrum of 3, which supports on the one hand the correct assignment of the responses between 6.5 and 6.1 to the Cp-polymer end groups and on the other hand the presence of the HDA product isomers in the network. To conclude, a novel self-healing material based on the HDA chemistry of a CDTE with Cp has been developed. The success of the cross-linking process was assessed by a swelling test as well as plate-plate rheology measurements, while the chemical structures of the network debonding were confirmed via high temperature 1H MAS-NMR analysis. Temperature dependent on-line viscosity data demonstrated the successful cross-linking of the material and confirmed the extremely fast reaction kinetics for the de- and re-bonding of the network system. The recorded temperature dependent traces of G’ and G” display the typical properties of a network with long mesh lengths, pointing to a material with excellent self-regulating qualities. Thus, in contrast to previously reported dynamic covalent systems, the current material is not only able to heal damage by heat treatment, but is also able to specifically adjust the mechanical properties of the network by adjusting the temperature, making it highly appealing for multiple reversible bonding applications. Not only can the properties of the network be set specifically by controlling the operating temperature in a very mild range, but also the changes of the settings occur within a very short time. Such properties make the designed network very useful for application such as rapid prototyping, adhesives, composite materials, coatings and paints as well as sealents. Moreover, it should be noted that the nature of the utilized Cp2-polymer has a significant impact on the materials properties, giving an additional opportunity to tune the self-healing material properties.
Received: December 23, 2013 Revised: January 24, 2014 Published online: March 21, 2014
[1] a) S. D. Bergman, F. Wudl, J. Mater. Chem. 2008, 18, 41; b) S. Billiet, X. K. D. Hillewaere, R. F. A. Teixeira, F. E. Du Prez, Macromol. Rapid Commun. 2013, 34, 290; c) N. K. Guimard, K. K. Oehlenschlaeger, J. Zhou, S. Hilf, F. G. Schmidt, C. Barner-Kowollik, Macromol. Chem. Phys. 2012, 213, 131. [2] S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler, S. R. Sriram, E. N. Brown, S. Viswanathan, Nature 2001, 409, 794. [3] a) K. S. Toohey, C. J. Hansen, J. A. Lewis, S. R. White, N. R. Sottos, Adv. Funct. Mater. 2009, 19, 1399; b) K. S. Toohey, N. R. Sottos, J. A. Lewis, J. S. Moore, S. R. White, Nat Mater 2007, 6, 581. [4] X. Chen, M. A. Dam, K. Ono, A. Mal, H. Shen, S. R. Nutt, K. Sheran, F. Wudl, Science 2002, 295, 1698. [5] a) G. L. Fiore, S. J. Rowan, C. Weder, Chem. Soc. Rev. 2013, 42, 7278; b) D. Habault, H. Zhang, Y. Zhao, Chem. Soc. Rev. 2013, 42, 7244. [6] a) N. Holten-Andersen, M. J. Harrington, H. Birkedal, B. P. Lee, P. B. Messersmith, K. Y. C. Lee, J. H. Waite, Proceedings of the National Academy of Sciences 2011, 108, 2651; b) D. C. Tuncaboylu, M. Sari, W. Oppermann, O. Okay, Macromolecules 2011, 44, 4997. [7] a) Q. Yan, A. Feng, H. Zhang, Y. Yin, J. Yuan, Polymer Chemistry 2013, 4, 1216; b) L.-P. Lv, Y. Zhao, N. Vilbrandt, M. Gallei, A. Vimalanandan, M. Rohwerder, K. Landfester, D. Crespy, J. Am. Chem. Soc. 2013, 135, 14198; c) A. P. Vogt, B. S. Sumerlin, Soft Matter 2009, 5, 2347. [8] a) M. Zhang, D. Xu, X. Yan, J. Chen, S. Dong, B. Zheng, F. Huang, Angew. Chem. Int. Ed. 2012, 51, 7011; b) Z. P. Zhang, M. Z. Rong, M. Q. Zhang, C. E. Yuan, Polymer Chemistry 2013, 4, 4648; c) S. Burattini, H. M. Colquhoun, J. D. Fox, D. Friedmann, B. W. Greenland, P. J. F. Harris, W. Hayes, M. E. Mackay, S. J. Rowan, Chem. Commun. 2009, 6717. [9] a) Y.-L. Liu, T.-W. Chuo, Polymer Chemistry 2013, 4, 2194; b) C. N. Bowman, C. J. Kloxin, Angew. Chem. Int. Ed. 2012, 51, 4272. [10] a) C. Barner-Kowollik, A. J. Inglis, L. Nebhani, F. G. Schmidt, S. Fengler, S. Hilf, S. Krause, A. Henning, International Patent PCT, WO/2011/101175; b) C. Barner-Kowollik, A. J. Inglis, L. Nebhani, O. Altintas, F. G. Schmidt, S. Fengler, S. Hilf, S. Krause, A. Henning, International Patent PCT, WO/2011/101176; c) C. BarnerKowollik, F. G. Schmidt, S. Hilf, J. Zhou, International Patent PCT, WO/2012/065786. [11] X. Chen, F. Wudl, A. K. Mal, H. Shen, S. R. Nutt, Macromolecules 2003, 36, 1802. [12] E. B. Murphy, E. Bolanos, C. Schaffner-Hamann, F. Wudl, S. R. Nutt, M. L. Auad, Macromolecules 2008, 41, 5203.
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[13] P. Reutenauer, E. Buhler, P. J. Boul, S. J. Candau, J. M. Lehn, Chemistry – A European Journal 2009, 15, 1893. [14] a) N. K. Guimard, J. Ho, J. Brandt, C. Y. Lin, M. Namazian, J. O. Mueller, K. K. Oehlenschlaeger, S. Hilf, A. Lederer, F. G. Schmidt, M. L. Coote, C. Barner-Kowollik, Chemical Science 2013, 4, 2752; b) A. J. Inglis, L. Nebhani, O. Altintas, F. G. Schmidt, C. Barner-Kowollik, Macromolecules 2010, 43, 5515; c) J. Zhou, N. K. Guimard, A. J. Inglis, M. Namazian, C. Y. Lin, M. L. Coote, E. Spyrou, S. Hilf, F. G. Schmidt, C. Barner-Kowollik, Polymer Chemistry 2012, 3, 628.
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[15] K. K. Oehlenschlaeger, N. K. Guimard, J. Brandt, J. Mueller, C. Y. Lin, S. Hilf, A. Lederer, M. L. Coote, F. Schmidt, C. BarnerKowollik, Polymer Chemistry 2013, 4, 4348. [16] L. H. Sperling, Introduction to Physical Polymer Science, Wiley-VCH, Weinheim, Germany 2006, fourth edition, Ch. 8. [17] M. R. Kessler, N. R. Sottos, S. R. White, Composites Part A: Applied Science and Manufacturing 2003, 34, 743. [18] a) G. W. Greenwood, Materials Science and Engineering 1976, 25, 241; b) G. J. Weng, Materials Science and Engineering 1983, 57, 127; c) R. W. Penn, G. E. Fulmer, Rev. Sci. Instrum. 1965, 36, 1230.
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