Thermosets—polymeric materials that adopt a permanent shape upon curing—have a key role in the modern plastics and rubber industries, comprising about 20 per cent of polymeric materials manufactured today, with a worldwide annual production of about 65 million tons1,2. The high density of crosslinks that gives thermosets their useful properties (for example, chemical and thermal resistance and tensile strength) comes at the expense of degradability and recyclability. Here, using the industrial thermoset polydicyclopentadiene as a model system, we show that when a small number of cleavable bonds are selectively installed within the strands of thermosets using a comonomer additive in otherwise traditional curing workflows, the resulting materials can display the same mechanical properties as the native material, but they can undergo triggered, mild degradation to yield soluble, recyclable products of controlled size and functionality. By contrast, installation of cleavable crosslinks, even at much higher loadings, does not produce degradable materials. These findings reveal that optimization of the cleavable bond location can be used as a design principle to achieve controlled thermoset degradation. Moreover, we introduce a class of recyclable thermosets poised for rapid deployment.
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We thank the National Science Foundation (DMREF CHE-1629358) and the National Institutes of Health (1R01CA220468-01) for support of this work. P.S. was supported by a fellowship from the American Cancer Society. S.L.K. was supported by a fellowship from the Misrock Fund. We thank B. Adams and W. Massefski for assistance with NMR analysis, A. Schwartzman for assistance with AFM and nanoindentation measurements, T. McClure for assistance with ICP-OES measurements, M. Tarkanian for assistance with mould fabrication, and S.-X. Luo for assistance with Raman measurements. J.L., D.V., Y.S. and K.A.N. acknowledge support for the microparticle impact experiments from the US Army Research Office through the Institute for Soldier Nanotechnologies, under Cooperative Agreement number W911NF-18-2-0048.
P.S., W.Z., K.E.L.H. and J.A.J. are named inventors on patent applications (US Patent Application 16/542,824 and US Provisional Application 62/935,799) filed by the Massachusetts Institute of Technology on the copolymers described in this work.
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Extended data figures and tables
Extended Data Fig. 1 The reverse gel-point concept used to derive the model of degradable thermosets shown in Fig. 1b.
a, A thermoset network containing f potential crosslinks per strand, c actual crosslinks per strand and x cleavable bonds within each strand may or may not be degraded into soluble fragments after bond cleavage. A model that determines whether the material will dissolve can be described as a function of f, c and x (Fig. 1b). b, The reverse gel-point concept enables this model by assuming that the minimum x required to enable thermoset degradation for given c and f values corresponds to the value that will inhibit the gelation of degradation fragments derived from strands with f potential crosslinking sites and x cleavable bonds. We use existing gelation theories (Miller–Macosko and Flory–Stockmayer) to solve for x, given f and c. Key to the reverse gel-point concept is the assumption that the network structure formed by the crosslinking of linear copolymer strands followed by cleavage of degradable bonds in those strands is identical to the network formed by first cleaving the linear copolymer strands and then crosslinking the resulting degradation products.
a, Images of pDCPD with various amounts of iPrSi and without iPrSi. b, Images of pDCPD with and without 20 vol% SiXL. c, pDCPD doped with up to 80 vol% SiXL remains intact after 12 h of TBAF treatment.
Extended Data Fig. 3 Further quantification of the impact of silyl ether incorporation into pDCPD strands.
a, Samples containing different amounts of iPrSi (0, 2.5, 5, 7.5 and 10 vol%) were incubated in 0.5 M TBAF in THF overnight, showing iPrSi-dependent degradation. b, Loss moduli for native pDCPD and 2.5% and 5% iPrSi-doped samples before and after TBAF treatment, as measured by oscillatory rheology. The storage moduli are presented in Fig. 2c. c, THF swelling ratios (THF swollen mass divided by dry mass) for native pDCPD and 2.5% and 5% iPrSi-doped samples following TBAF treatment. Centre values denote average. Error bars denote s.e.m. n = 3 for all samples.
Extended Data Fig. 4 Characterization of mechanical and thermal properties of iPrSi-doped pDCPD by DMA.
a, Loss factor (tan(delta)) plots of pDCPD samples as a function of iPrSi incorporation. b, Storage moduli collected at Tg – 60 °C for all samples. Centre values denote average. Error bars denote s.e.m. n = 3, except for the 33% sample, for which n = 5.
a, Structure of EtSi, which differs from iPrSi in terms of the alkyl substituents on the silyl ether group. The less sterically hindered ethyl groups render this monomer more susceptible to cleavage. b, Images of 10% EtSi- or iPrSi-doped pDCPD. c, 10% EtSi dissolves in 0.5 M TBAF in THF after 12 h. d, Images of 10% EtSi-doped (left) and iPrSi-doped (right) pDCPD exposed to THF containing 15% concentrated aqueous HCl (12.1 N). The EtSi sample shows noticeably more rapid degradation under these conditions as compared to the iPrSi sample. Both samples are largely degraded within 12 h. In this case, acidic hydrolysis is facilitated by the presence of organic solvent to swell the network.
a, The weathering setup. Samples were kept inside glass vials over the course of the weathering experiments. b, Measured irradiance for samples during the weathering experiments and comparison to solar reference spectra (ASTM G177). c, Ultraviolet–visible spectra for the 0%, 10% and 20% iPrSi- and 10% EtSi-doped pDCPD samples. The sample thickness was 1 mm. d, Images of samples before and after the weathering studies.
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Shieh, P., Zhang, W., Husted, K.E.L. et al. Cleavable comonomers enable degradable, recyclable thermoset plastics. Nature 583, 542–547 (2020). https://doi.org/10.1038/s41586-020-2495-2
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