Letter Cite This: ACS Macro Lett. 2017, 6, 1280-1284
pubs.acs.org/macroletters
Thermally Healable and Reprocessable Bis(hindered amino)disulfideCross-Linked Polymethacrylate Networks Akira Takahashi,† Raita Goseki,†,‡ Kohzo Ito,§ and Hideyuki Otsuka*,†,‡ †
Department of Organic and Polymeric Materials and ‡Department of Chemical Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan § Department of Advanced Materials Science, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan S Supporting Information *
ABSTRACT: A facile approach to polymethacrylate networks that contain thermally exchangeable bis(2,2,6,6-tetramethylpiperidin-1yl)disulfide (BiTEMPS) cross-linkers is reported, and the easily inducible healability and reprocessability of the obtained networks are discussed. The free radical polymerization of BiTEMPS crosslinkers and hexyl methacrylate (HMA) monomers afforded insoluble and colorless networks of poly(hexyl methacrylate) (PHMA) films, whose structures were characterized after de-crosslinking via thermal BiTEMPS exchange reactions with added lowmolecular-weight BiTEMPS. Swelling experiments and stressrelaxation measurements at elevated temperatures revealed the contribution of BiTEMPS as a polymer chain exchanger both in the gels and in the bulk. The BiTEMPS-cross-linked PHMA networks showed damage healability and repeatable reprocessability in the bulk by simple hot pressing at 120 °C under mild pressure (∼70 kPa). These results should grant facile access to various vinyl polymer networks with on-demand malleability. ue to their covalently fixed structures, polymer networks exhibit outstanding properties such as mechanical strength, shape persistence, and chemical stability. At the same time, however, the high stability of such networks leads to difficulties associated with their reprocessing and recycling, which limits the application of their superior properties in a wider context. The introduction of dynamic covalent bonds into the networks,1−4 pioneered by Leibler and co-workers with novel “vitrimer” concept,5 is one of the most effective solutions to the aforementioned problems and has been intensively explored in recent years. Many common organic bonds, for example, ester,5 imine,6 olefin,7 carbamate,8,9 siloxane,10,11 boronic ester,12−15 and dichalcogenide16−26 bonds are known to be exchangeable under bond-specific conditions and have been used to endow network structures with malleability. But although a large variety of such bonds has been reported to date,27−36 many of these require the additional presence of external catalysts and/or substituents in order to ensure high levels of malleability,5−11,17,34,35 which could potentially complicate the molecular design of materials and impede easy access to their dynamic functionality, commensurate with diminishing the versatility of this methodology. Also, the dynamic polymer networks reported so far have often been prepared using methods based on a step-growth mechanism, which leaves room for investigations into more common systems, such as vinyl polymers.14,15,29 We have previously demonstrated the dynamic covalent chemistry of bis(2,2,6,6-tetramethylpiperidin-1-yl)disulfide (Bi-
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© XXXX American Chemical Society
TEMPS), which is based on a reversible dissociation of the central disulfide bond (Figure 1a).37 Its bond exchange reactions can be easily induced by moderate heating, whereby a specific chemical environment is not required. This is due to the low bond dissociation energy of the central disulfide bond (110−130 kJ/mol) relative to that of conventional alkyl disulfide bonds (250−290 kJ/mol), which arises from the high stability of the 2,2,6,6-tetramethylpiperidine-1-sulfanyl (TEMPS) radicals that are formed.38−40 Furthermore, the unusual stability of the mediating radicals toward air enables the use of the dynamic functionality without the need for special care under atmospheric conditions. Another characteristic property of BiTEMPS is the low absorbance of visible light based on the aliphatic skeleton, rendering it a good prospective dynamic unit for polymers such as poly(methacrylate)s, which are often used for colorless and transparent materials. Since BiTEMPS predominantly exists in its dimeric, sterically hindered disulfide form at room temperature, it should tolerate the conditions of free radical polymerization at that temperature,41 which would offer facile access to vinyl polymer networks with on-demand reprocessable functionality by merely adding BiTEMPS-based cross-linkers to the polymerization reactions. Received: September 27, 2017 Accepted: October 28, 2017
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DOI: 10.1021/acsmacrolett.7b00762 ACS Macro Lett. 2017, 6, 1280−1284
Letter
ACS Macro Letters
The structure of the obtained P(HMA-co-BTA) polymer networks was subsequently characterized by decross-linking experiments.44 For that purpose, a small piece of the material was stirred at 100 °C for 12 h in 1,4-dioxane that contained a low-molecular BiTEMPS compound (∼20 equiv relative to the molar mass of BTA in the sample). Dissolution of the original gel was observed, and the GPC curve of the reaction mixture showed a broad peak for a polymeric fraction with a peak top molecular weight of 55300 g/mol (Figure S2), indicating that the BiTEMPS moieties at the cross-linking points were cleaved via thermal bond exchange reactions with the external low molecular BiTEMPS to form soluble fractions. The thus obtained polymers were purified by four reprecipitations into cold methanol, and the resulting material was analyzed by 1H NMR spectroscopy in CDCl3. The obtained spectrum was consistent with the target structure (Figure S3), and the BTA ratio (∼4.7 mol %) was estimated based on the area ratio of the peaks corresponding to the HMA and BTA moieties (cf. SI). These results strongly support the notion that the free radical polymerization furnished P(HMA-co-BTA) polymer networks. The thermal properties of the obtained PHMA networks were also examined. Although the glass transition temperature (Tg) of PHMA is about −5 °C, the P(HMA-co-BTA) of this study showed a much higher value (20 °C) in a DSC analysis. Considering the Tg values of P(HMA-co-ADSA) (3 °C) and P(HMA-co-HDA) (6 °C) (Figure S4 and Table S1), the unique Tg of P(HMA-co-BTA) should be attributed to the network structure and the rigidity of the cyclic skeleton of BiTEMPS. Furthermore, a thermogravimetric analysis (TGA) revealed comparable thermal decomposition temperatures (5% weight loss) for all PHMA networks (Figure S5), despite the differences in bond dissociation energy, which indicates that the incorporation of BiTEMPS moieties should not affect the thermal stability of the obtained materials in the temperature range examined. Subsequently, we carried out a swelling test to briefly assess the differences of the thermally induced dynamic properties of the obtained PHMA networks. For that purpose, small pieces of each PHMA sample were immersed in 1,4-dioxane at room temperature under the exclusion of light. As is common for chemically cross-linked polymers, all PHMAs reached their swelling equilibrium after several hours, confirming the presence of covalent BiTEMPS and alkyl disulfide bonds in the absence of external stimuli. The swelling ratio of P(HMAco-BTA) was somehow smaller than those of the other network PHMAs, possibly due to restricted network elongation based on rigid, less mobile BiTEMPS skeletons as was indicated with the DSC analysis. On the other hand, only P(HMA-co-BTA) showed further swelling upon raising the temperature to 80 °C, presumably due to a mesh-size expansion induced by the chainexchange reactions based on the incorporated BiTEMPS moieties (Figure 2).37,45 Neither P(HMA-co-ADSA) nor nondynamic P(HMA-co-HDA) exhibited such a thermal increase of the swelling ratio, reflecting the covalent character of the alkyl disulfide bonds under mildly increased temperatures. Thereafter, we investigated the dynamic properties of the obtained PHMA networks in the bulk. Stress-relaxation measurements were initially carried out in order to evaluate the thermal chain exchangeability of the networks. For that purpose, a square-shaped specimen of each PHMA was maintained at 3% strain at elevated temperatures, and the time-course stress transition was monitored. P(HMA-co-BTA)
Figure 1. (a) Reversible dissociation of BiTEMPS; (b) synthesis of P(HMA-co-BTA); and (c) schematic illustration of the BiTEMPScross-linked dynamic polymer networks.
Herein, we report the synthesis of BiTEMPS-cross-linked poly(methacrylate) networks and their healability and reprocessability in the bulk. We initially examined a free-radical polymerization using BiTEMPS-containing bifunctional crosslinkers. In order to reduce the number of entanglement points and thus help to observe the contribution of the BiTEMPSexchange reactions to the macroscopic malleability, n-hexyl methacrylate (HMA) with a high entanglement molar mass of ∼33000 g/mol was chosen as the main monomer42 and the cross-linker ratio was set to 5 mol %. A mixture of BiTEMPScontaining diacrylate (BTA), prepared from the diol derivative of BiTEMPS and 2-isocyanatoethyl acrylate, and 19 equiv of HMA was stirred in N,N-dimethylacetoamide (DMAc). The polymerization was carried out at
