Letter

pubs.acs.org/macroletters

Catalytic Control of the Vitrimer Transition Mathieu Capelot, Miriam M. Unterlass, Francoiş Tournilhac, and Ludwik Leibler*

Matierè Molle et Chimie, UMR 7167 ESPCI-CNRS, Ecole Superieuré de Physique et Chimie Industrielles de la Ville de Paris ESPCI ParisTech, 10 rue Vauquelin 75005 Paris, France

*S Supporting Information

ABSTRACT: Vitrimers, strong organic glass formers, are covalent networks that are able to change their topology through thermoactivated bond exchange reactions. At high temperatures, vitrimers can flow and behave like viscoelastic liquids. At low temperatures, exchange reactions are very long and vitrimers behave like classical thermosets. The transition from the liquid to the solid is reversible and is, in fact, a . By changing the content and nature of the catalyst, we can tune the transesterification reaction rate and show that the vitrimer glass transition temperature and the broadness of the transition can be controlled at will in -based vitrimers. This opens new possibilities in practical applications of thermosets such as healing or convenient processability in a wide temperature range.

hen an amorphous melt is cooled down, it freezing transition temperature Tv depends on the cooling rate. W undergoes the glass transition. In the vicinity of the glass Thermal expansion measurements on vitrimers confirmed the 10 transition temperature, Tg, the polymer hardens abruptly as its glass-like nature and rate dependence of the transition at Tv. increases by several orders of magnitude following a We would like to stress that the freezing topology transition non-Arrhenius, Williams−Landel−Ferry (WLF) law (Figure is not a viscous-to-elastic gel transition. When approaching the − 1A).1 4 Fragility describes how abrupt the temperature gelation transition, the number of bonds and the connectivity of − variation of viscosity near the glass transition is.5 the molecules increase.19 21 At the gel point, the system is like all other organic compounds are fragile glass-formers. composed of a broad distribution of linked clusters22 with one In striking contrast, silica, the archetype of glass, and few of them being “infinite” and percolating through the sample: other inorganic compounds undergo a very gradual Arrhenius- the system becomes elastic. In vitrimers, the connectivity does 6−9 like viscosity change near Tg. They are called strong glass- not change and the network is always “infinite”, that is, beyond formers, the term strong being coined to underline their unique the percolation threshold (gel point). Remarkably, although the fl behavior. Indeed, glass blowing or easy shaping by local heating network ows above Tv, the material is insoluble at all without need of precise temperature control or a mold is temperatures, since the number of bonds remains constant.23 possible because silica is a strong glass-former. Upon further cooling, below Tv, vitrimers undergo another Recently, Leibler and co-workers introduced and demon- much more abrupt transition from an elastic to a hard solid, a

Downloaded via KAOHSIUNG MEDICAL UNIV on September 18, 2018 at 05:27:38 (UTC). strated the concept of vitrimers, organic strong glass- glass with modulus of about 1 GPa. This transition from an 10,11 See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. formers. Vitrimers are made of atoms that are covalently elastic solid to a hard glass is analogous to Tg observed in bonded to form a network and the design principle is based on classical thermosets and elastomers. reversible network topology freezing. When the network is able Relying on thermal activation considerably limits the to change its topology through bond exchange reactions, the practical importance of vitrimer systems. Catalysis offers an − material relaxes stresses and flows,12 17 even though the total efficient way of controlling exchange reaction rate and, as a number of bonds stays constant in time and does not fluctuate. consequence, the broadness of the topology freezing transition. The material behaves like a viscoelastic melt. In a vitrimer, the Here, we demonstate this concept for the simple and practical exchange reactions are thermally activated. As a result, upon system of epoxy/acid networks and transesterification exchange cooling, the relaxation time and viscosity controlled by reactions. Transesterifications are equilibrium reactions, where exchange reactions rate decrease slowly and it follows an an and an alcohol are transformed into another ester and Arrhenius law (Figure 1B). At some temperature Tv, the another alcohol, via alkoxy interchange. Various catalysts, such mechanical relaxation time controlled by the exchange reaction as organic bases or inorganic salts, have been used in the fi 24 rate becomes longer than the experimental time scale and on literature to control transesteri cation reactions. All epoxy/ this time scale, the network topology is frozen. The material appears to be an elastic solid with the elastic modulus between Received: May 16, 2012 1 MPa and 100 MPa, depending on the cross-link density. Like Accepted: June 6, 2012 for a classical glass transition,18 the value of the topology Published: June 11, 2012

© 2012 American Chemical Society 789 dx.doi.org/10.1021/mz300239f | ACS Macro Lett. 2012, 1, 789−792 ACS Macro Letters Letter

− Figure 1. (A) Left: V T characteristics of a polymer. At T < Tg (Amorphous I), the physical state is conventionally referred to as glass, at T > Tg (Amorphous II) as liquid. Right: Above Tg, the viscosity of a thermoplastic polymer follows a WLF power law with the temperature. (B) − fl Left: V T characteristics of a vitrimer, thus, depicting two glass transitions: the classical Tg and Tv, a glass transition that re ects topology freezing upon cooling. Right: The viscosity follows an Arrhenius law in the vicinity of Tv.

Figure 2. (A) Shear stress relaxation experiments: normalized relaxation modulus as a function of time for three different concentrations of ° ° Zn(OAc)2 at 150 C. Relaxation times are measured for a 63% relaxation. (B) Elongational creep experiments at 150 C for a sample catalyzed at 10 ° σ mol % of Zn(OAc)2 at 70 and 150 C with a nominal stress = 0.1 MPa. Please note that because of thermal expansion, the strain at initial time is ff slightly di erent for two temperatures. (C) Arrhenius plot of the measured relaxation times for 1, 5, and 10 mol % of Zn(OAc)2. (D) Dilatometry experiments for 1, 5, and 10 mol % Zn(OAc)2 with a heating rate of 5 K/min. acid networks investigated here seem to behave like insoluble ring-opening.26 The stoichiometry was adjusted to one strong glass formers. We show that by changing the content per epoxy function, which leads to the formation β fi 25 and nature of the catalyst, the value of Tv and the broadness of of -hydroxyl-, prone to transesteri cations. All trans- the transition can be changed in a controlled way. esterification catalysts used in this study are also efficient Vitrimers were synthesized from diglycidyl ether of bisphenol catalysts regarding epoxy-acid polymerizations.27 The resulting A (DGEBA) and a mixture of tricarboxylic and dicarboxylic networks are elastomeric, since Tg is below room temperature fatty acids. The epoxy-acid reaction is rather complicated, (15 °C from DSC studies). because of several backside reactions occurring during the The effect of the catalyst concentration on the network , such as homopolymerization of the epoxy groups.25 dynamics was investigated. Previous studies on model Thus, catalysts are conventionally used to facilitate the epoxy molecules indicated that exchange reaction rate varies with

790 dx.doi.org/10.1021/mz300239f | ACS Macro Lett. 2012, 1, 789−792 ACS Macro Letters Letter

Figure 3. (A) Catalysts used in the study. The acetate ligands are replaced by the fatty acids in our systems. (B) Creep and recovery experiments at ° σ 200 C for samples catalyzed at 5 mol % by Zn(OAc)2 and PPh3 with an applied stress = 0.1 MPa. The stress was released after 60 min. (C) Arrhenius plot of the relaxation times for Zn(oAc)2, PPh3, and TBD at 5 mol %, measured from shear stress relaxation experiments. (D) Angell fragility plot, normalized at Tv for Zn(oAc)2, PPh3, and TBD at 5 mol %. the catalyst content.11 Networks were synthesized with without cross-links. In the absence of the Zn catalyst, the different concentrations of zinc acetate: 1 mol %, 5 mol %, transesterification reactions are very slow and the linear and 10 mol %. The exchange reactions and resulting topology expansion coefficient remains constant from 100 to 300 °C. rearrangements allow the networks to relax stresses. Shear This observation is consistent with the notion of a permanent stress relaxation experiments were conducted in the linear network. When a catalyst is added, exchange reactions are faster regime on the samples at different concentrations of catalyst at and an increase in the expansion coefficient is clearly observed ° temperatures varying between 100 and 280 C. Figure 2A at Tv. An increase in the catalyst concentration clearly shifts this shows shear stress relaxation curves obtained at the same transition to lower temperatures. Dilatometry also shows the ° ff temperature (150 C) for the 3 di erent concentrations for an other glass transition at a lower temperature, Tg, which is applied strain γ = 5%. As expected, the higher the catalyst identical for all samples. This reversible transition corresponds concentration, the faster the stress relaxation. By convention, to freezing-in local bond orientational motions and is usually we chose to determine relaxation times τ* from the curves as referred to as the “rubber-glass transition” for rubbers and the time necessary to relax 63% of the initial stress. tightly cross-linked networks. In our systems, the proper term Creep experiments confirm that materials flow. Figure 2B should be glass-to-glass transition. Indeed, a permanent shows the results from elongational creep experiments for the covalent network is nonergodic and can be viewed as a 29,30 networks catalyzed with 10 mol % of zinc acetate (Zn(OAc)2) glass. Therefore, the catalyst concentration controls the ° at 150 C. After the elastic response, the deformation varies transition temperature Tv, in a very similar way a plasticizer ff linearly with time, as expected for a viscoelastic liquid. a ects the classical glass transition Tg. Relaxation times τ* follow an Arrhenius law with the Numerous catalysts can be employed to control the τ τ fi 31 temperature (T)= 0 exp(Ea/RT) (Figure 2C). Convention- transesteri cation rate. Triphenylphosphine (PPh3)and 32 ally, the liquid-to-solid transition temperature (i.e., Tv for triazobicyclodecene (TBD) were studied and compared vitrimers) is considered as the point at which the viscosity with zinc acetate (see chemical structures in Figure 3A). becomes higher than 1012 Pa·s.2,3,6 The liquid-to-solid Indeed, the mechanisms of catalysis with organic compounds ff ff transition temperature, Tv, may therefore be determined from are very di erent to those of metal salts and strong e ects on the Arrhenius curves for relaxation times of 106 s.28 For 10 mol activation energies can thus be anticipated. Creep experiments ° ffi % of Zn(OAc)2, this transition temperature is about 75 C. allow for the determination of the e ciency of the catalysts at a Below this temperature, the material behaves like a conven- particular temperature. Indeed, the more efficient the catalyst is tional thermosetting network, as confirmed by a creep for the transesterification reactions, the faster the exchange experiment at 70 °C (Figure 2B). With smaller amounts of reactions and the lower the viscosity. Figure 3B depicts the catalyst, this transition temperature increases. Interestingly, for elongational creep experiments obtained for Zn(OAc)2 and ° all three catalyst concentrations, the same activation energy for PPh3, under identical conditions (200 C, applied stress of 0.1 ∼ the topology freezing is observed (Ea 90 kJ/mol). MPa for 1 h, amount of catalyst: 5 mol %). Interestingly, the Dilatometry experiments (Figure 2D) nicely confirm the two catalysts lead to a very different viscoelastic behavior. For existence of the glass transition at Tv in networks capable of example, after the stress release, the sample loaded with topology rearrangements. Indeed, a network with its degrees of Zn(OAc)2 exhibits a residual deformation of 35%; whereas, the freedom constrained by cross-links is expected to show a lower sample with PPh3 recovers its original length almost entirely. thermal expansion coefficient than a corresponding system We measured for different temperatures the relaxation times τ*

791 dx.doi.org/10.1021/mz300239f | ACS Macro Lett. 2012, 1, 789−792 ACS Macro Letters Letter via stress relaxation for the three catalysts at the same (13) (a) Osthoff, R. C.; Bueche, A. M.; Grubb, W. T. J. Am. Chem. concentration. The variations with the temperature follow Soc. 1954, 76, 4659−4663. (b) Zheng, P.; McCarthy, T. J. J. Am. Chem. − Arrhenius law. As expected, the more efficient transesterifica- Soc. 2012, 134, 2024 2027. tion catalysts, Zn(OAc) and TBD, yield lower relaxation times (14) (a) Scott, T. F.; Schneider, A. D.; Cook, W. D.; Bowman, C. N. 2 Science 2005, 308, 1615−1617. (b) Kloxin, C. J.; Scott, T. F.; Park, H. at high temperatures (Figure 3C). − ff Y.; Bowman, C. N. Adv. Mater. 2011, 23, 1977 1981. The three catalysts display di erent freezing topology (15) (a) Nicolay, R.; Kamada, J.; Wassen, A. V.; Matyjaszewski, K. temperatures Tv, whereas the glass-to-glass transition Tg does Macromolecules 2010, 43, 4355−4361. (b) Amamoto, Y.; Kamada, J.; not change (from DSC studies). The activation energies are Otsuka, H.; Takahara, A.; Matyjaszewski, K. Angew. Chem., Int. Ed. remarkably different for different catalysts, as emphasized by 2011, 50, 1660−1663. (c) Amamoto, Y.; Otsuka, H.; Takahara, A.; the “Angell fragility plot” in Figure 3D. This flexibility brought Matyjaszewski, K. ACS Macro Lett. 2012, 1, 478−481. by the catalyst could prove to be useful to tune vitrimer flow (16) Lu, Y.-X.; Tournilhac, F.; Leibler, L.; Guan, Z. J. Am. Chem. Soc. properties for different applications and service temperatures. 2012, 134, 8424−8427. Vitrimers constitute a new class of materials: they behave like (17)(a)Deng,G.;Tang,C.;Li,F.;Jiang,H.;Chen,Y. Macromolecules 2010, 43, 1191−1194. (b) Deng, G.; Li, F.; Yu, H.; strong organic liquids and display two glass transitions, Tg and T . The topology freezing transition, occurring at T , is solely Liu, F.; Liu, C.; Sun, W.; Jiang, H.; Chen, Y. 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Fundamentally, a Tv much Press: New York, 2003. lower than that of inorganic strong may help to carry − out physical studies to obtain a better understanding of the (22) Leibler, L.; Schosseler, F. Phys. Rev. Lett. 1985, 55, 1110 1113. physics of the glass transition. This concept could be readily (23) In contact with solvent, the material swells and reorganizes. 34 Some clusters can be dissolved, and the remaining network contains used for other exchange chemistries. less defects. (24) Otera, J. Chem. Rev. 1993, 93, 1449−1470. ■ ASSOCIATED CONTENT (25) Matejka, L.; Pokorny, S.; Dusek, K. Polym. Bull. 1982, 7, 123− *S Supporting Information 128. (26) (a) Montarnal, D.; Tournilhac, F.; Hidalgo, M.; Leibler, L. J. Experimental section. This material is available free of charge Polym. Sci., Polym. Chem. 2010, 48, 1133−1141. (b) Pire, M.; Norvez, via the Internet at http://pubs.acs.org. S.; Iliopoulos, I.; Le Rossignol, B.; Leibler, L. Polymer 2011, 52, 5243− 5249. 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