Toward Recyclable Thermosets Timothy E. Long Science 344, 706 (2014); DOI: 10.1126/science.1254401
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Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of May 15, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/344/6185/706.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/344/6185/706.full.html#related This article cites 13 articles, 4 of which can be accessed free: http://www.sciencemag.org/content/344/6185/706.full.html#ref-list-1 This article appears in the following subject collections: Materials Science http://www.sciencemag.org/cgi/collection/mat_sci
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
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PERSPECTIVES dipole moment in their unit cell (e.g., wurtzite structure) that results in asymmetric crystal shapes. Polar crystal faces find application in semiconductors (e.g., zinc oxide) and catalysis. It has been impossible to observe the growth of such crystals in real time under typical synthesis conditions (T ~ 100°C), but now there is reason for optimism that such important crystallizations can be tempted to reveal their secrets.
References 1. G. Binnig, C. F. Quate, Ch. Gerber, Phys. Rev. Lett. 56, 930 (1986). 2. G. Meyer, N. M. Amer, Appl. Phys. Lett. 56, 2100 (1990). 3. S. Manne et al., Science 251, 183 (1991). 4. J. D. Rimer et al., Science 330, 337 (2010). 5. A. I. Lupulescu, J. D. Rimer, Science 344, 729 (2014). 6. W. K. Burton, N. Cabrera, F. C. Frank, Phil. Trans. Roy. Soc. A 243, 299 (1951). 7. M. Ohara, R. Reid, Modeling Crystal Growth Rates from Solution (Prentice-Hall, Upper Saddle River, NJ, 1973). 8. R. L. Penn, J. F. Banfield, Science 281, 969 (1998). 9. D. Li et al., Science 336, 1014 (2012).
10. N. Cabrera, D. Vermilyea, Growth and Perfection of Crystals, R. Doremus, B. Roberts, D. Turnbull, Eds. (Wiley, New York, 1958). 11. J. J. De Yoreo, P. M. Dove, Science 306, 1301 (2004). 12. J. Goniakowski et al., Rep. Prog. Phys. 71, 016501 (2008).
Acknowledgments: Financial support was provided by the Dow Chemical Company through a doctoral fellowship (P.D.) and by the National Science Foundation through CBET1159746 (M.F.D.). 10.1126/science.1254259
MATERIALS SCIENCE
Toward Recyclable Thermosets
Advances in synthesis are leading to thermoset plastics that can be converted to the starting monomers.
Timothy E. Long
706
that combine the desirable thermal and chemical stability of conventional thermosets with recyclability and reprocessability. For several decades, the microelectronics, aerospace, and automotive industries have used a relatively small number of thermally stable and mechanically ductile thermosets. A nitrogen-containing heterocycle exemplifies a desirable chemical linkage in polymeric thermosets due to thermal stability and restricted rotation that provides high glass transition temperatures (4, 5). However, many polymerization processes for thermosets require high temperatures and long reaction times. Researchers are therefore trying to develop chemical processes that require less energy. The microelectronics and automotive industries also demand higher performance, including increased modulus, improved
toughness, and flame resistance. Use of existing monomers in new synthetic methods can also lead to novel compositions with desirable properties. García et al.’s novel thermoset-forming reaction is based on the well-established reactivity of monofunctional aromatic and aliphatic amines with paraformaldehyde. A monofunctional amine in combination with paraformaldehyde readily forms a low–molar mass triazine. Use of a difunctional amine enabled the in situ formation of a triazine cross-link point that is the basis of the polymeric network of García et al.’s thermosets (see the figure). The use of a multifunctional amine for making high-performance triazine polymeric networks is unprecedented. The authors first created a family of lowtemperature, hemiaminal-based thermosets,
Department of Chemistry, Macromolecules and Interfaces Institute, Virginia Tech, Blacksburg, VA 24061, USA. E-mail: [email protected]
Reversible thermoset plastics. García et al. show that the formation of a trifunctional cross-link point through reaction of a diamine and paraformaldehyde leads to a thermally stable and mechanically ductile thermoset. Low pH triggers the reversal of the thermoset architecture to the corresponding monomers.
pH < 2
+
Heat
=
N N
N O
=
NH2
H2N = O H
16 MAY 2014 VOL 344 SCIENCE www.sciencemag.org Published by AAAS
H
CREDIT: K. ZHANG/VIRGINIA TECH
R
ecycling codes on plastic food and beverage packaging serve to guide consumers’ daily decisions about the disposal of used packaging. However, technological obstacles remain for the recycling of more sophisticated polymeric packaging, ranging from multilayered food packaging to composite polymeric materials for electronic packaging. Moreover, many electronic devices contain heat-resistant, chemically stable polymers called thermosets that are not amenable to conventional collecting and recycling. On page 732 of this issue, García et al. (1) report a crucial step toward recyclable thermosets with the synthesis of ductile, insulating, temperature-resistant, and chemically inert thermosets that can be returned to their monomeric state through a pH trigger. Thermoplastics are polymers that become pliable or moldable at elevated temperatures, but return to a solid state when cooled. These polymers can thus be readily processed or reprocessed upon heating, and are therefore widely used in food and beverage packaging. In contrast, thermosets are chemically crosslinked polymers, networks, or gels with chemical bonds between chains that do not thermally dissociate, even at high temperature. They are ideal for high-temperature electronic or automotive applications, but cannot be reprocessed or recycled either by melting or by solution processing. Many researchers are now challenging this classical definition with concepts of recyclable networks, reworkable encapsulants, reversible gels, self-healing polymeric coatings, and stimuli-responsive polymeric structures (2, 3). The goal is to create reversible thermosets
PERSPECTIVES which are chemically and thermally stable to modest temperatures (
This copy is for your personal, non-commercial use only.
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of May 15, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/344/6185/706.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/344/6185/706.full.html#related This article cites 13 articles, 4 of which can be accessed free: http://www.sciencemag.org/content/344/6185/706.full.html#ref-list-1 This article appears in the following subject collections: Materials Science http://www.sciencemag.org/cgi/collection/mat_sci
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on May 16, 2014
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.
PERSPECTIVES dipole moment in their unit cell (e.g., wurtzite structure) that results in asymmetric crystal shapes. Polar crystal faces find application in semiconductors (e.g., zinc oxide) and catalysis. It has been impossible to observe the growth of such crystals in real time under typical synthesis conditions (T ~ 100°C), but now there is reason for optimism that such important crystallizations can be tempted to reveal their secrets.
References 1. G. Binnig, C. F. Quate, Ch. Gerber, Phys. Rev. Lett. 56, 930 (1986). 2. G. Meyer, N. M. Amer, Appl. Phys. Lett. 56, 2100 (1990). 3. S. Manne et al., Science 251, 183 (1991). 4. J. D. Rimer et al., Science 330, 337 (2010). 5. A. I. Lupulescu, J. D. Rimer, Science 344, 729 (2014). 6. W. K. Burton, N. Cabrera, F. C. Frank, Phil. Trans. Roy. Soc. A 243, 299 (1951). 7. M. Ohara, R. Reid, Modeling Crystal Growth Rates from Solution (Prentice-Hall, Upper Saddle River, NJ, 1973). 8. R. L. Penn, J. F. Banfield, Science 281, 969 (1998). 9. D. Li et al., Science 336, 1014 (2012).
10. N. Cabrera, D. Vermilyea, Growth and Perfection of Crystals, R. Doremus, B. Roberts, D. Turnbull, Eds. (Wiley, New York, 1958). 11. J. J. De Yoreo, P. M. Dove, Science 306, 1301 (2004). 12. J. Goniakowski et al., Rep. Prog. Phys. 71, 016501 (2008).
Acknowledgments: Financial support was provided by the Dow Chemical Company through a doctoral fellowship (P.D.) and by the National Science Foundation through CBET1159746 (M.F.D.). 10.1126/science.1254259
MATERIALS SCIENCE
Toward Recyclable Thermosets
Advances in synthesis are leading to thermoset plastics that can be converted to the starting monomers.
Timothy E. Long
706
that combine the desirable thermal and chemical stability of conventional thermosets with recyclability and reprocessability. For several decades, the microelectronics, aerospace, and automotive industries have used a relatively small number of thermally stable and mechanically ductile thermosets. A nitrogen-containing heterocycle exemplifies a desirable chemical linkage in polymeric thermosets due to thermal stability and restricted rotation that provides high glass transition temperatures (4, 5). However, many polymerization processes for thermosets require high temperatures and long reaction times. Researchers are therefore trying to develop chemical processes that require less energy. The microelectronics and automotive industries also demand higher performance, including increased modulus, improved
toughness, and flame resistance. Use of existing monomers in new synthetic methods can also lead to novel compositions with desirable properties. García et al.’s novel thermoset-forming reaction is based on the well-established reactivity of monofunctional aromatic and aliphatic amines with paraformaldehyde. A monofunctional amine in combination with paraformaldehyde readily forms a low–molar mass triazine. Use of a difunctional amine enabled the in situ formation of a triazine cross-link point that is the basis of the polymeric network of García et al.’s thermosets (see the figure). The use of a multifunctional amine for making high-performance triazine polymeric networks is unprecedented. The authors first created a family of lowtemperature, hemiaminal-based thermosets,
Department of Chemistry, Macromolecules and Interfaces Institute, Virginia Tech, Blacksburg, VA 24061, USA. E-mail: [email protected]
Reversible thermoset plastics. García et al. show that the formation of a trifunctional cross-link point through reaction of a diamine and paraformaldehyde leads to a thermally stable and mechanically ductile thermoset. Low pH triggers the reversal of the thermoset architecture to the corresponding monomers.
pH < 2
+
Heat
=
N N
N O
=
NH2
H2N = O H
16 MAY 2014 VOL 344 SCIENCE www.sciencemag.org Published by AAAS
H
CREDIT: K. ZHANG/VIRGINIA TECH
R
ecycling codes on plastic food and beverage packaging serve to guide consumers’ daily decisions about the disposal of used packaging. However, technological obstacles remain for the recycling of more sophisticated polymeric packaging, ranging from multilayered food packaging to composite polymeric materials for electronic packaging. Moreover, many electronic devices contain heat-resistant, chemically stable polymers called thermosets that are not amenable to conventional collecting and recycling. On page 732 of this issue, García et al. (1) report a crucial step toward recyclable thermosets with the synthesis of ductile, insulating, temperature-resistant, and chemically inert thermosets that can be returned to their monomeric state through a pH trigger. Thermoplastics are polymers that become pliable or moldable at elevated temperatures, but return to a solid state when cooled. These polymers can thus be readily processed or reprocessed upon heating, and are therefore widely used in food and beverage packaging. In contrast, thermosets are chemically crosslinked polymers, networks, or gels with chemical bonds between chains that do not thermally dissociate, even at high temperature. They are ideal for high-temperature electronic or automotive applications, but cannot be reprocessed or recycled either by melting or by solution processing. Many researchers are now challenging this classical definition with concepts of recyclable networks, reworkable encapsulants, reversible gels, self-healing polymeric coatings, and stimuli-responsive polymeric structures (2, 3). The goal is to create reversible thermosets
PERSPECTIVES which are chemically and thermally stable to modest temperatures (
