Letter to the Editor pubs.acs.org/journal/ascecg
Comment on “Turning Vulcanized Natural Rubber into a Self-Healing Polymer: Effect of the Disulfide/Polysulfide Ratio” Kay Saalwac̈ hter*,† and Dariush Hinderberger*,‡ †
Institut für Physik, Martin-Luther-Universität Halle-Wittenberg, Betty-Heimann-Str. 7, D-06120 Halle, Germany Institut für Chemie, Martin-Luther-Universität Halle-Wittenberg, von-Danckelmann-Platz 4, D-06120 Halle, Germany
‡
ACS Sustainable Chem. Eng. 2016, 4 (10), 5776−5784. DOI: 10.1021/acssuschemeng.6b01760 ACS Sustainable Chem. Eng. 2017, 5 (12). DOI: 10.1021/acssuschemeng.7b03647
A
recent paper of Hernández et al.1 claims evidence for the role of free sulfur radicals in the “self-healing” of conventional lowly vulcanized natural rubber samples on the basis of Raman spectroscopy observations of the disulfide/ polysulfide (D/P) ratio of the sulfur linkages, as well as electron-spin resonance (ESR) spectra. We here argue that none of the observations can be related to the healing mechanism, rather healing appears to be governed by chain interdiffusion that is possible at low cross-linking densities. In addition, the ESR results are unsuitable to make a claim of elucidating the role of sulfur-based radicals. The samples were prepared using a standard recipe using a benzothiazole sulfenamide accelerator and no special catalyst. The correlation relevant for the main conclusion is reproduced in Figure 1a and immediately raises two questions: (i) How significant is this correlation, considering that two sample pairs (i.e., four out of six samples in total) exhibit rather different healing efficiency but within error the same D/P ratio? (ii) Why should there be different correlations for two sample series differing by their curing characteristics (in terms of the percentage of maximal vulcameter torque after a certain vulcanization time)? Occam’s razor requires that out of various explanations the least complex one requiring the least number of additional assumptions is to be favoredunless of course additional direct
evidence unambiguously favors a more complex one. Figure 1b shows a statistically convincing correlation of the healing efficiency with the cross-linking density of the different samples, which obviously avoids doubts (i) and (ii), and can easily be rationalized by the higher potential of chain interdiffusion across a damage zone provided by the higher level of defect chains in lowly cross-linked samples. The sulfur-based hypothesis requires many more assumptions in order to explain the more complex (and statistically not well supported) correlation in Figure 1a. We stress that the potential dynamics of the sulfur bridges is not a requirement for an interdiffusion of dangling ends or sol chains. Besides, it is noted that the decrease in the D/P ratio with increasing curing degree evidenced in Figure 1a is a priori counterintuitive and not commented upon in ref 1. Sulfur bridges are expected to shorten in the later stages of vulcanization.2 The Raman-based observable will thus have to be discussed carefully in future work. Independent of the correlation argument, the ESR evidence presented in favor of a sulfur-based mechanism does not withstand closer scrutiny. It should first be stressed that even if the ESR observations were compatible with the presence of specific sulfur-based radicals, this would not constitute any direct evidence for the presence and relevance of sulfur-related exchange reactions. The anecdotal finding of a specific resonance in one sample from the investigated series at elevated temperature cannot be the basis of the central conclusion, claiming the “[demonstration] that the underlying disulfide metathesis healing mechanism is based on temperature-driven sulfur radical reactions.” Such a link can only be established on the basis of systematic variations and (semi)quantitative determination of the sulfur-based radical content and its correlation with, for instance, the healing behavior. As future readers may be tempted to use the shown ESR spectra as a starting point for further studies, we feel the need to go into more detail and explain why these spectra do not allow for the conclusions drawn from them. As a minor point that raised significant confusion, the shown x-axis scales (magnetic field in Gauss) are not compatible with the reported g values when using eq 3 of the paper with the given resonance frequency of 9.44 GHz. A first round of discussion with the authors revealed that the field ranges were plotted incorrectly
Figure 1. Correlation of the experimental healing efficiency with (a) the Raman-based disulfide/polysulfide (D/P) ratio, same as Figure 9 of ref 1, and (b) the equilibrium-swelling-based cross-linking density given in Table 3 of the same reference. The dotted regions in panel (a) mark sample pairs with practically the same D/P ratio but rather different healing efficiencies. The green dashed line in panel (b) is a guide to the eye. © 2017 American Chemical Society
Received: August 27, 2017 Revised: October 20, 2017 Published: November 3, 2017 11125
DOI: 10.1021/acssuschemeng.7b02965 ACS Sustainable Chem. Eng. 2017, 5, 11125−11126
ACS Sustainable Chemistry & Engineering
Letter to the Editor
systems at elevated temperatures in air.6 Taking together all evidence presented, the dominance of a defect-related interdiffusion mechanism without a relevant role of potentially labile sulfur links appears more likely for the given case of conventional sulfur-vulcanized natural rubber.
and have to be decreased by 350 G (correct range for all spectra in the main paper and the SI: 2800−3800 G). The indicated g values are correct, though. Now, the two spectra shown in Figure 8b and c of ref 1 taken at room temperature (RT) and 70 °C, respectively, raise serious questions. A largely dominating single and broad resonance, covering the whole range of interest, is observed in both cases. Give and take some uncertainty related to locating such a wide line, its center is found roughly at the same position as the truly minor feature at g = 2.048 that the authors observe at 70 °C and attribute to thermally activated polysulfanyl radicals. At RT, the main signal is compatible with an even higher g value of about 2.08. Arguably, the reported spectra cannot be interpreted without clarifying the origin of the main observed resonance. Addressing now the minor resonance only visible at 70 °C, its g value of 2.048 is associated with polysulphanyl species by comparison with results of Dondi et al.,3 who observed a rather weak and unusually wide signal only at 180 °C in a single sample (a sulfur−polybutadiene mixture), but not in a more similar vulcanized system (that also included a benzothiazole sulfenamide accelerator). Notably, they also state that the signal was not observable any more at 150 °C and below and explained their observations by insufficient thermal averaging of the g tensor. From this, one may conclude that at significantly lower temperature only the quasi-static spectrum consisting of a much broadened and anisotropic g tensor should be observable. But since only one minor peak is observed by Hernández et al., it is by no means possible to unambiguously conclude on the isotropic g value. Even if the minor signal at g = 2.048 were an isotropic value, its disappearance at RT would not be indicative of the absence of the associated radical, but possibly be due to intermediate- to slow-motion effects arising from incomplete thermal averaging and the ensuing line broadening. Finally, a very recent ref 2 featuring an expertly conducted quantative ESR study of radical species in natural rubber and polyisoprene systems with a variety of curing systems, including the relevant conventional one, has reported for a temperature of 150 °C one dominating and comparably narrow resonance located at g = 2.004, a minor one at g = 2.024, and a few more at even lower g. These observations are in line with those in ref 3 for sulfur−polybutadiene at 180 °C, but in contrast to ref 1, where at 70° and below there is for reasons not explained no detectable resonance around g = 2.004 but mainly the one discussed above, that is 10 times wider. It thus seems impossible to establish a link of the observations of Hernández et al. with previous literature results. We therefore conclude that the spectra reported in ref 1 cannot be interpreted without a more encompassing study involving spectra taken at significantly higher temperatures and careful line shape analyses. It may be advisible to resolve this issue by employing high-field/high-frequency EPR spectroscopy (e.g., at Q- or W-band frequencies, 35 and 94 GHz, respectively) to obtain spectra with better g resolution with higher sensitivity. In particular, discriminating the different radical species may then be possible with less ambiguity. In summary, while a sulfur-based mechanism cannot be excluded to be relevant on the basis of the data given in ref 1 (it seems to be operative for other systems studied in other works4,5), none of the data in ref 1 can be used to claim the correctness of the main claims made. We note that significant stress relaxation in natural rubber systems was much earlier found to be the same in sulfur- and peroxide-cross-linked
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Website: www. physik.uni-halle.de/nmr. *E-mail:
[email protected]. ORCID
Kay Saalwächter: 0000-0002-6246-4770 Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Hernández, M.; Grande, A. M.; Dierkes, W.; Bijleveld, J.; van der Zwaag, S.; García, S. J. Turning Vulcanized Natural Rubber into a SelfHealing Polymer: Effect of the Disulfide/Polysulfide Ratio. ACS Sustainable Chem. Eng. 2016, 4, 5776−5784. (2) Posadas, P.; Malmierca, M. A.; González-Jiménez, A.; Ibarra, L.; Rodríguez, A.; Valentín, J. L.; Nagaoka, T.; Yajima, H.; Toki, S.; Che, J.; Rong, L.; Hsiao, B. S. ESR investigation of NR and IR rubber vulcanized with different cross-linking agents. eXPRESS Polym. Lett. 2016, 10, 2−14. (3) Dondi, D.; Buttafava, A.; Zeffiro, A.; Palamini, C.; Lostritto, A.; Giannini, L.; Faucitano, A. The mechanisms of the sulfur-only and catalytic vulcanization of polybutadiene: An EPR and DFT study. Eur. Polym. J. 2015, 62, 222−235. (4) Tobolsky, A. V.; MacKnight, W. J.; Takahashi, M. Relaxation of Disulfide and Tetrasulfide Polymers. J. Phys. Chem. 1964, 68, 787− 790. (5) Canadell, J.; Goossens, H.; Klumperman, B. Self-Healing Materials Based on Disulfide Links. Macromolecules 2011, 44, 2536− 2541. (6) Takahashi, Y.; Tobolsky, A. V. Chemorheological Study on Natural Rubber Vulcanizates. Polym. J. 1971, 2, 457−576.
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DOI: 10.1021/acssuschemeng.7b02965 ACS Sustainable Chem. Eng. 2017, 5, 11125−11126
