High-level hierarchical morphology reinforcing covalent adaptable networks

Abstract

Creep deformation presents as an inevitable drawback for lots of covalent adaptable networks (CANs). Recently in Cell Reports Physical Science, Chen and co-workers employed well-defined tri-block copolymers to construct CANs where the self-assembled microphase separation enables excellent creep resistance by restricting dynamic bond exchanges within isolated domains. Creep deformation presents as an inevitable drawback for lots of covalent adaptable networks (CANs). Recently in Cell Reports Physical Science, Chen and co-workers employed well-defined tri-block copolymers to construct CANs where the self-assembled microphase separation enables excellent creep resistance by restricting dynamic bond exchanges within isolated domains. Main textA fundamental truth in the realm of macromolecular chemistry is that the connection framework of molecules dictates the macroscopic behavior of materials. For example, let’s compare thermoplastics with thermosets: the latter, which have inter-crosslinked networks, significantly outperform their unlinked counterparts in terms of dimensional stability, mechanical strengths, and creep and chemical resistance. Conversely, although the intractable network provides edges for thermosets to serve in broad aspects of modern life, the permanently defined structure also imposes great challenges in such sustainability issues as recycling and reprocessing. Thanks to the dynamic covalent-bond chemistry, the contradiction between network integrity and reconfigurability can be reconciled by covalent adaptable networks (CANs), which have sprung up as a novel class of polymeric material in the past years.1Denissen W. Winne J.M. Du Prez F.E. Vitrimers: permanent organic networks with glass-like fluidity.Chem. Sci. 2016; 7: 30-38Crossref PubMed Google Scholar,2Kloxin C.J. Bowman C.N. Covalent adaptable networks: smart, reconfigurable and responsive network systems.Chem. Soc. Rev. 2013; 42: 7161-7173Crossref PubMed Google Scholar The dynamic covalent crosslinks enable the “on-off” switch for the topology rearrangement in CANs: under working temperatures where the exchange reactions are inhibited, the materials behave like thermosets with analogous mechanical robustness. Meanwhile, activating the dynamic bond reshuffling caused by specific stimuli (e.g., heat or light) can unleash the restrained segmental movement, allowing the effective reprocessing specific to thermoplastics to take place.Oriented to various requirements, diversified architecture designs of CANs have been continually put forth. In most state-of-the-art examples, the central philosophy is to select an appropriate kind of dynamic bond compatible with the polymer backbone structure and amenable to the desired thermal, mechanical, and rheological properties. Chemistries such as the Diels-Alder reaction, transamination, transesterification, olefin metathesis, silyl ether exchange, and dioxaborolane metathesis exhibit different characteristics of dissociative and associative mechanisms, reaction rates, and activation energies, providing a versatile toolbox for tradeoffs. With adequate methodological guidance, emphases on the catalyst addition, procedural efficiency of preparation, or other interesting functionalities can be further tailored.Despite the strengths of CANs, this kind of material also has its Achilles’ heel: because of the incomplete prohibition of exchange reactions, the dynamic networks tend to suffer from creep behavior, causing unexpected permanent deformation over time. Adopting dynamic bonds with increased activation energy can alleviate this problem at the sacrifice of reprocessability, presenting a dilemma for accessing robust materials. To address this challenge, Torkelson and co-workers introduced orthogonal crosslinks of dynamic and static bonds.3Li L. Chen X. Jin K. Torkelson J.M. Vitrimers designed both to strongly suppress creep and to recover original cross-link density after reprocessing: quantitative theory and experiments.Macromolecules. 2018; 51: 5537-5546Crossref Scopus (119) Google Scholar Delicately controlling the fraction of the permanent crosslinks to avoid percolation of the permanent network can effectively suppress the creep behavior while preserving the reprocessability. However, this strategy still imposes limitations on complete recycling of this permanent network.Apart from chemical modifications on the network architecture, morphology regulation holds extra potential from an interdisciplinary perspective between chemistry and physics: it can also be leveraged to gain bonuses on the functionalities. Although nanostructure engineering based on polymer self-assembly has motivated intensive research into its practicability in templates, membranes, photonic materials, and drug carriers,4Whitesides G.M. Grzybowski B. Self-assembly at all scales.Science. 2002; 295: 2418-2421Crossref PubMed Scopus (5795) Google Scholar,5Schacher F.H. Rupar P.A. Manners I. Functional block copolymers: nanostructured materials with emerging applications.Angew. Chem. Int. Ed. 2012; 51: 7898-7921Crossref PubMed Scopus (540) Google Scholar its implementation in CANs has rarely been addressed. Leibler and co-workers investigated the phase-separation effect in the boronic-ester-functionalized polyethylene vitrimer and accounted the increased flow inside the crosslinking matrix to it.6Ricarte R.G. Tournilhac F. Leibler L. Phase separation and self-assembly in vitrimers: hierarchical morphology of molten and semicrystalline polyethylene/dioxaborolane maleimide systems.Macromolecules. 2019; 52: 432-443Crossref Scopus (64) Google Scholar Sumerlin and co-workers employed a di-block copolymer (di-BCP) that can self-assemble into the lamellar phase in a CAN material.7Lessard J.J. Scheutz G.M. Sung S.H. Lantz K.A. Epps 3rd, T.H. Sumerlin B.S. Block copolymer vitrimers.J. Am. Chem. Soc. 2020; 142: 283-289Crossref PubMed Scopus (96) Google Scholar Because the dynamic crosslinks localized in separated domains, this di-BCP network exhibited excellent resistance to creep deformation, in contrast to its counterparts composed of random copolymers.Recently in Cell Reports Physical Science, Chen and co-workers introduced a dynamically crosslinked tri-block copolymer (tri-BCP) network system into a novel CAN structure with high-level hierarchical architecture (Figure 1).8Clarke R.W. McGraw M.L. Newell B.S. Chen E.Y.-X. Thermomechanical activation achieving orthogonal working/healing conditions of nanostructured tri-block copolymer thermosets.Cell Rep. Phys. Sci. 2021; 2: 100483https://doi.org/10.1016/j.xcrp.2021.100483Abstract Full Text Full Text PDF Scopus (3) Google Scholar They proposed that employing an ABA tri-BCP with crosslinking points in the A blocks could generate a continuous macro-phase separation network in which the crosslinked A domains are connected with each other, providing more stabilized compartmentalization of the dynamic linkages than the AB di-block system. By using a sequence-controlled Lewis pair polymerization (LPP) technique, the authors could easily access high-molecular-weight acrylic ABA tri-BCPs (G1) with well-defined block structures. Selective acid hydrolysis of the A blocks produced the hard-soft-hard PAA-b-PAE-b-PAA tri-BCP precursor (G2) as a thermoplastic elastomer, wherein poly(acrylic acid) (PAA) and poly(acrylic ester) served as hard and soft segments, respectively. Analogous to many BCP systems, self-assembly of G2 into hexagonally packed cylinder nanostructures was observed as a result of the hydrogen bonding between the PAA blocks. Subsequent condensation of the carboxylic acid units of PAA above 150°C gave the final polymer product G3, bearing both unreacted carboxylic acid moieties and newly formed anhydride crosslinks, where the phase-separation behavior could be maintained. The correspondingly derived network could undergo either associative or dissociative acid-anhydride exchange at elevated temperatures without the need for any extra catalyst or crosslinker, representing a highly efficient and straightforward preparation scheme for CAN materials.Tensile test experiments indicated dramatic enhancement of the G3 materials compared with the G2 counterparts in terms of various mechanical properties, such as tensile strength and toughness, demonstrating the superiority of the covalently crosslinked network. By altering the chemical composition of the soft middle block, Chen and co-workers could readily achieve further modulation of the specific mechanical performances; they attained the highest Young’s modulus (up to 2.9 ± 0.37 GPa) for PAAn100-b-PMA80-b-PAAn100. Along with the outstanding mechanical modulus, the G3 materials also offer impressive flexibility (up to 269% ± 19%) on par with that of traditional thermoplastic elastomers.Because the dynamic bonds are confined within isolated domains, the G3 materials are able to inhibit permanent deformation under tensile stress, even with the occurrence of exchange reactions at high temperatures. However, when compressed, the more compact bulk structure allows for productive bond exchange between the crosslinked domains, making it capable of morphology reconstruction, although a considerably long operation time of 24 h is needed. Through adjustment of the segment lengths in the block copolymers, more facile reprocessing is expected to be realized. As the authors envisioned, the “sea-island” dynamic network effect also enabled excellent creep resistance for the G3 materials; with negligible creep (<0.3%) at 150°C, they largely outperformed the G1 and G2 BCPs. To further demonstrate the necessity of tri-BCP, the authors performed comprehensive comparisons between tri-BCP, di-BCP, and random copolymer (RCP). Under identical chemical compositions, increased creep resistance was detected in the order of RCP (11.8%), di-BCP (5.5%), and tri-BCP (1.1%) over 4 h, consistent with the authors’ initial hypothesis that more stabilized phase-separation structure can provide higher-dimensional firmness under elevated temperatures. Insightful investigation of the acid-anhydride dynamic exchange was probed with the RCP materials, which can conduct effective stress-relaxation experiments. The rheological behavior followed the Maxwell model, and an activation energy (Ea) of 201 kJ/mol was derived, which is a rather high value among various dynamic bonds utilized in CANs.As an innovation to leverage self-assembly behavior into the construction of CANs, this study introduces a new dimension where the regulation of polymer topologies, coupled with modifications of backbone compositions, can be used to furnish certain functionalities. With a multitude of well-developed polymerization platforms—such as reversible deactivation radical polymerization,9Parkatzidis K. Wang H.S. Truong N.P. Anastasaki A. Recent developments and future challenges in controlled radical polymerization: a 2020 update.Chem. 2020; 6: 1575-1588Abstract Full Text Full Text PDF Scopus (179) Google Scholar ring-opening metathesis polymerization,10Bielawski C.W. Grubbs R.H. Living ring-opening metathesis polymerization.Prog. Polym. Sci. 2007; 32: 1-29Crossref Scopus (1130) Google Scholar and LPP11McGraw M.L. Chen E.Y.X. Lewis pair polymerization: perspective on a ten-year journey.Macromolecules. 2020; 53: 6102-6122Crossref Scopus (46) Google Scholar—a diversity of complex architectures ranging from block, alternating, gradient sequences to hyperbranched, dendritic, star-like structures could be readily synthesized and studied along this direction. New trials and explorations to address the existing challenges and acquire broader comprehension of the structure-property relationships for this promising next-generation material should be enticing. Standing at the intersection between thermoplastics and thermosets, CANs hold promise for creating new materials with well-versed properties in the foreseeable future. Main textA fundamental truth in the realm of macromolecular chemistry is that the connection framework of molecules dictates the macroscopic behavior of materials. For example, let’s compare thermoplastics with thermosets: the latter, which have inter-crosslinked networks, significantly outperform their unlinked counterparts in terms of dimensional stability, mechanical strengths, and creep and chemical resistance. Conversely, although the intractable network provides edges for thermosets to serve in broad aspects of modern life, the permanently defined structure also imposes great challenges in such sustainability issues as recycling and reprocessing. Thanks to the dynamic covalent-bond chemistry, the contradiction between network integrity and reconfigurability can be reconciled by covalent adaptable networks (CANs), which have sprung up as a novel class of polymeric material in the past years.1Denissen W. Winne J.M. Du Prez F.E. Vitrimers: permanent organic networks with glass-like fluidity.Chem. Sci. 2016; 7: 30-38Crossref PubMed Google Scholar,2Kloxin C.J. Bowman C.N. Covalent adaptable networks: smart, reconfigurable and responsive network systems.Chem. Soc. Rev. 2013; 42: 7161-7173Crossref PubMed Google Scholar The dynamic covalent crosslinks enable the “on-off” switch for the topology rearrangement in CANs: under working temperatures where the exchange reactions are inhibited, the materials behave like thermosets with analogous mechanical robustness. Meanwhile, activating the dynamic bond reshuffling caused by specific stimuli (e.g., heat or light) can unleash the restrained segmental movement, allowing the effective reprocessing specific to thermoplastics to take place.Oriented to various requirements, diversified architecture designs of CANs have been continually put forth. In most state-of-the-art examples, the central philosophy is to select an appropriate kind of dynamic bond compatible with the polymer backbone structure and amenable to the desired thermal, mechanical, and rheological properties. Chemistries such as the Diels-Alder reaction, transamination, transesterification, olefin metathesis, silyl ether exchange, and dioxaborolane metathesis exhibit different characteristics of dissociative and associative mechanisms, reaction rates, and activation energies, providing a versatile toolbox for tradeoffs. With adequate methodological guidance, emphases on the catalyst addition, procedural efficiency of preparation, or other interesting functionalities can be further tailored.Despite the strengths of CANs, this kind of material also has its Achilles’ heel: because of the incomplete prohibition of exchange reactions, the dynamic networks tend to suffer from creep behavior, causing unexpected permanent deformation over time. Adopting dynamic bonds with increased activation energy can alleviate this problem at the sacrifice of reprocessability, presenting a dilemma for accessing robust materials. To address this challenge, Torkelson and co-workers introduced orthogonal crosslinks of dynamic and static bonds.3Li L. Chen X. Jin K. Torkelson J.M. Vitrimers designed both to strongly suppress creep and to recover original cross-link density after reprocessing: quantitative theory and experiments.Macromolecules. 2018; 51: 5537-5546Crossref Scopus (119) Google Scholar Delicately controlling the fraction of the permanent crosslinks to avoid percolation of the permanent network can effectively suppress the creep behavior while preserving the reprocessability. However, this strategy still imposes limitations on complete recycling of this permanent network.Apart from chemical modifications on the network architecture, morphology regulation holds extra potential from an interdisciplinary perspective between chemistry and physics: it can also be leveraged to gain bonuses on the functionalities. Although nanostructure engineering based on polymer self-assembly has motivated intensive research into its practicability in templates, membranes, photonic materials, and drug carriers,4Whitesides G.M. Grzybowski B. Self-assembly at all scales.Science. 2002; 295: 2418-2421Crossref PubMed Scopus (5795) Google Scholar,5Schacher F.H. Rupar P.A. Manners I. Functional block copolymers: nanostructured materials with emerging applications.Angew. Chem. Int. Ed. 2012; 51: 7898-7921Crossref PubMed Scopus (540) Google Scholar its implementation in CANs has rarely been addressed. Leibler and co-workers investigated the phase-separation effect in the boronic-ester-functionalized polyethylene vitrimer and accounted the increased flow inside the crosslinking matrix to it.6Ricarte R.G. Tournilhac F. Leibler L. Phase separation and self-assembly in vitrimers: hierarchical morphology of molten and semicrystalline polyethylene/dioxaborolane maleimide systems.Macromolecules. 2019; 52: 432-443Crossref Scopus (64) Google Scholar Sumerlin and co-workers employed a di-block copolymer (di-BCP) that can self-assemble into the lamellar phase in a CAN material.7Lessard J.J. Scheutz G.M. Sung S.H. Lantz K.A. Epps 3rd, T.H. Sumerlin B.S. Block copolymer vitrimers.J. Am. Chem. Soc. 2020; 142: 283-289Crossref PubMed Scopus (96) Google Scholar Because the dynamic crosslinks localized in separated domains, this di-BCP network exhibited excellent resistance to creep deformation, in contrast to its counterparts composed of random copolymers.Recently in Cell Reports Physical Science, Chen and co-workers introduced a dynamically crosslinked tri-block copolymer (tri-BCP) network system into a novel CAN structure with high-level hierarchical architecture (Figure 1).8Clarke R.W. McGraw M.L. Newell B.S. Chen E.Y.-X. Thermomechanical activation achieving orthogonal working/healing conditions of nanostructured tri-block copolymer thermosets.Cell Rep. Phys. Sci. 2021; 2: 100483https://doi.org/10.1016/j.xcrp.2021.100483Abstract Full Text Full Text PDF Scopus (3) Google Scholar They proposed that employing an ABA tri-BCP with crosslinking points in the A blocks could generate a continuous macro-phase separation network in which the crosslinked A domains are connected with each other, providing more stabilized compartmentalization of the dynamic linkages than the AB di-block system. By using a sequence-controlled Lewis pair polymerization (LPP) technique, the authors could easily access high-molecular-weight acrylic ABA tri-BCPs (G1) with well-defined block structures. Selective acid hydrolysis of the A blocks produced the hard-soft-hard PAA-b-PAE-b-PAA tri-BCP precursor (G2) as a thermoplastic elastomer, wherein poly(acrylic acid) (PAA) and poly(acrylic ester) served as hard and soft segments, respectively. Analogous to many BCP systems, self-assembly of G2 into hexagonally packed cylinder nanostructures was observed as a result of the hydrogen bonding between the PAA blocks. Subsequent condensation of the carboxylic acid units of PAA above 150°C gave the final polymer product G3, bearing both unreacted carboxylic acid moieties and newly formed anhydride crosslinks, where the phase-separation behavior could be maintained. The correspondingly derived network could undergo either associative or dissociative acid-anhydride exchange at elevated temperatures without the need for any extra catalyst or crosslinker, representing a highly efficient and straightforward preparation scheme for CAN materials.Tensile test experiments indicated dramatic enhancement of the G3 materials compared with the G2 counterparts in terms of various mechanical properties, such as tensile strength and toughness, demonstrating the superiority of the covalently crosslinked network. By altering the chemical composition of the soft middle block, Chen and co-workers could readily achieve further modulation of the specific mechanical performances; they attained the highest Young’s modulus (up to 2.9 ± 0.37 GPa) for PAAn100-b-PMA80-b-PAAn100. Along with the outstanding mechanical modulus, the G3 materials also offer impressive flexibility (up to 269% ± 19%) on par with that of traditional thermoplastic elastomers.Because the dynamic bonds are confined within isolated domains, the G3 materials are able to inhibit permanent deformation under tensile stress, even with the occurrence of exchange reactions at high temperatures. However, when compressed, the more compact bulk structure allows for productive bond exchange between the crosslinked domains, making it capable of morphology reconstruction, although a considerably long operation time of 24 h is needed. Through adjustment of the segment lengths in the block copolymers, more facile reprocessing is expected to be realized. As the authors envisioned, the “sea-island” dynamic network effect also enabled excellent creep resistance for the G3 materials; with negligible creep (<0.3%) at 150°C, they largely outperformed the G1 and G2 BCPs. To further demonstrate the necessity of tri-BCP, the authors performed comprehensive comparisons between tri-BCP, di-BCP, and random copolymer (RCP). Under identical chemical compositions, increased creep resistance was detected in the order of RCP (11.8%), di-BCP (5.5%), and tri-BCP (1.1%) over 4 h, consistent with the authors’ initial hypothesis that more stabilized phase-separation structure can provide higher-dimensional firmness under elevated temperatures. Insightful investigation of the acid-anhydride dynamic exchange was probed with the RCP materials, which can conduct effective stress-relaxation experiments. The rheological behavior followed the Maxwell model, and an activation energy (Ea) of 201 kJ/mol was derived, which is a rather high value among various dynamic bonds utilized in CANs.As an innovation to leverage self-assembly behavior into the construction of CANs, this study introduces a new dimension where the regulation of polymer topologies, coupled with modifications of backbone compositions, can be used to furnish certain functionalities. With a multitude of well-developed polymerization platforms—such as reversible deactivation radical polymerization,9Parkatzidis K. Wang H.S. Truong N.P. Anastasaki A. Recent developments and future challenges in controlled radical polymerization: a 2020 update.Chem. 2020; 6: 1575-1588Abstract Full Text Full Text PDF Scopus (179) Google Scholar ring-opening metathesis polymerization,10Bielawski C.W. Grubbs R.H. Living ring-opening metathesis polymerization.Prog. Polym. Sci. 2007; 32: 1-29Crossref Scopus (1130) Google Scholar and LPP11McGraw M.L. Chen E.Y.X. Lewis pair polymerization: perspective on a ten-year journey.Macromolecules. 2020; 53: 6102-6122Crossref Scopus (46) Google Scholar—a diversity of complex architectures ranging from block, alternating, gradient sequences to hyperbranched, dendritic, star-like structures could be readily synthesized and studied along this direction. New trials and explorations to address the existing challenges and acquire broader comprehension of the structure-property relationships for this promising next-generation material should be enticing. Standing at the intersection between thermoplastics and thermosets, CANs hold promise for creating new materials with well-versed properties in the foreseeable future. A fundamental truth in the realm of macromolecular chemistry is that the connection framework of molecules dictates the macroscopic behavior of materials. For example, let’s compare thermoplastics with thermosets: the latter, which have inter-crosslinked networks, significantly outperform their unlinked counterparts in terms of dimensional stability, mechanical strengths, and creep and chemical resistance. Conversely, although the intractable network provides edges for thermosets to serve in broad aspects of modern life, the permanently defined structure also imposes great challenges in such sustainability issues as recycling and reprocessing. Thanks to the dynamic covalent-bond chemistry, the contradiction between network integrity and reconfigurability can be reconciled by covalent adaptable networks (CANs), which have sprung up as a novel class of polymeric material in the past years.1Denissen W. Winne J.M. Du Prez F.E. Vitrimers: permanent organic networks with glass-like fluidity.Chem. Sci. 2016; 7: 30-38Crossref PubMed Google Scholar,2Kloxin C.J. Bowman C.N. Covalent adaptable networks: smart, reconfigurable and responsive network systems.Chem. Soc. Rev. 2013; 42: 7161-7173Crossref PubMed Google Scholar The dynamic covalent crosslinks enable the “on-off” switch for the topology rearrangement in CANs: under working temperatures where the exchange reactions are inhibited, the materials behave like thermosets with analogous mechanical robustness. Meanwhile, activating the dynamic bond reshuffling caused by specific stimuli (e.g., heat or light) can unleash the restrained segmental movement, allowing the effective reprocessing specific to thermoplastics to take place. Oriented to various requirements, diversified architecture designs of CANs have been continually put forth. In most state-of-the-art examples, the central philosophy is to select an appropriate kind of dynamic bond compatible with the polymer backbone structure and amenable to the desired thermal, mechanical, and rheological properties. Chemistries such as the Diels-Alder reaction, transamination, transesterification, olefin metathesis, silyl ether exchange, and dioxaborolane metathesis exhibit different characteristics of dissociative and associative mechanisms, reaction rates, and activation energies, providing a versatile toolbox for tradeoffs. With adequate methodological guidance, emphases on the catalyst addition, procedural efficiency of preparation, or other interesting functionalities can be further tailored. Despite the strengths of CANs, this kind of material also has its Achilles’ heel: because of the incomplete prohibition of exchange reactions, the dynamic networks tend to suffer from creep behavior, causing unexpected permanent deformation over time. Adopting dynamic bonds with increased activation energy can alleviate this problem at the sacrifice of reprocessability, presenting a dilemma for accessing robust materials. To address this challenge, Torkelson and co-workers introduced orthogonal crosslinks of dynamic and static bonds.3Li L. Chen X. Jin K. Torkelson J.M. Vitrimers designed both to strongly suppress creep and to recover original cross-link density after reprocessing: quantitative theory and experiments.Macromolecules. 2018; 51: 5537-5546Crossref Scopus (119) Google Scholar Delicately controlling the fraction of the permanent crosslinks to avoid percolation of the permanent network can effectively suppress the creep behavior while preserving the reprocessability. However, this strategy still imposes limitations on complete recycling of this permanent network. Apart from chemical modifications on the network architecture, morphology regulation holds extra potential from an interdisciplinary perspective between chemistry and physics: it can also be leveraged to gain bonuses on the functionalities. Although nanostructure engineering based on polymer self-assembly has motivated intensive research into its practicability in templates, membranes, photonic materials, and drug carriers,4Whitesides G.M. Grzybowski B. Self-assembly at all scales.Science. 2002; 295: 2418-2421Crossref PubMed Scopus (5795) Google Scholar,5Schacher F.H. Rupar P.A. Manners I. Functional block copolymers: nanostructured materials with emerging applications.Angew. Chem. Int. Ed. 2012; 51: 7898-7921Crossref PubMed Scopus (540) Google Scholar its implementation in CANs has rarely been addressed. Leibler and co-workers investigated the phase-separation effect in the boronic-ester-functionalized polyethylene vitrimer and accounted the increased flow inside the crosslinking matrix to it.6Ricarte R.G. Tournilhac F. Leibler L. Phase separation and self-assembly in vitrimers: hierarchical morphology of molten and semicrystalline polyethylene/dioxaborolane maleimide systems.Macromolecules. 2019; 52: 432-443Crossref Scopus (64) Google Scholar Sumerlin and co-workers employed a di-block copolymer (di-BCP) that can self-assemble into the lamellar phase in a CAN material.7Lessard J.J. Scheutz G.M. Sung S.H. Lantz K.A. Epps 3rd, T.H. Sumerlin B.S. Block copolymer vitrimers.J. Am. Chem. Soc. 2020; 142: 283-289Crossref PubMed Scopus (96) Google Scholar Because the dynamic crosslinks localized in separated domains, this di-BCP network exhibited excellent resistance to creep deformation, in contrast to its counterparts composed of random copolymers. Recently in Cell Reports Physical Science, Chen and co-workers introduced a dynamically crosslinked tri-block copolymer (tri-BCP) network system into a novel CAN structure with high-level hierarchical architecture (Figure 1).8Clarke R.W. McGraw M.L. Newell B.S. Chen E.Y.-X. Thermomechanical activation achieving orthogonal working/healing conditions of nanostructured tri-block copolymer thermosets.Cell Rep. Phys. Sci. 2021; 2: 100483https://doi.org/10.1016/j.xcrp.2021.100483Abstract Full Text Full Text PDF Scopus (3) Google Scholar They proposed that employing an ABA tri-BCP with crosslinking points in the A blocks could generate a continuous macro-phase separation network in which the crosslinked A domains are connected with each other, providing more stabilized compartmentalization of the dynamic linkages than the AB di-block system. By using a sequence-controlled Lewis pair polymerization (LPP) technique, the authors could easily access high-molecular-weight acrylic ABA tri-BCPs (G1) with well-defined block structures. Selective acid hydrolysis of the A blocks produced the hard-soft-hard PAA-b-PAE-b-PAA tri-BCP precursor (G2) as a thermoplastic elastomer, wherein poly(acrylic acid) (PAA) and poly(acrylic ester) served as hard and soft segments, respectively. Analogous to many BCP systems, self-assembly of G2 into hexagonally packed cylinder nanostructures was observed as a result of the hydrogen bonding between the PAA blocks. Subsequent condensation of the carboxylic acid units of PAA above 150°C gave the final polymer product G3, bearing both unreacted carboxylic acid moieties and newly formed anhydride crosslinks, where the phase-separation behavior could be maintained. The correspondingly derived network could undergo either associative or dissociative acid-anhydride exchange at elevated temperatures without the need for any extra catalyst or crosslinker, representing a highly efficient and straightforward preparation scheme for CAN materials. Tensile test experiments indicated dramatic enhancement of the G3 materials compared with the G2 counterparts in terms of various mechanical properties, such as tensile strength and toughness, demonstrating the superiority of the covalently crosslinked network. By altering the chemical composition of the soft middle block, Chen and co-workers could readily achieve further modulation of the specific mechanical performances; they attained the highest Young’s modulus (up to 2.9 ± 0.37 GPa) for PAAn100-b-PMA80-b-PAAn100. Along with the outstanding mechanical modulus, the G3 materials also offer impressive flexibility (up to 269% ± 19%) on par with that of traditional thermoplastic elastomers. Because the dynamic bonds are confined within isolated domains, the G3 materials are able to inhibit permanent deformation under tensile stress, even with the occurrence of exchange reactions at high temperatures. However, when compressed, the more compact bulk structure allows for productive bond exchange between the crosslinked domains, making it capable of morphology reconstruction, although a considerably long operation time of 24 h is needed. Through adjustment of the segment lengths in the block copolymers, more facile reprocessing is expected to be realized. As the authors envisioned, the “sea-island” dynamic network effect also enabled excellent creep resistance for the G3 materials; with negligible creep (<0.3%) at 150°C, they largely outperformed the G1 and G2 BCPs. To further demonstrate the necessity of tri-BCP, the authors performed comprehensive comparisons between tri-BCP, di-BCP, and random copolymer (RCP). Under identical chemical compositions, increased creep resistance was detected in the order of RCP (11.8%), di-BCP (5.5%), and tri-BCP (1.1%) over 4 h, consistent with the authors’ initial hypothesis that more stabilized phase-separation structure can provide higher-dimensional firmness under elevated temperatures. Insightful investigation of the acid-anhydride dynamic exchange was probed with the RCP materials, which can conduct effective stress-relaxation experiments. The rheological behavior followed the Maxwell model, and an activation energy (Ea) of 201 kJ/mol was derived, which is a rather high value among various dynamic bonds utilized in CANs. As an innovation to leverage self-assembly behavior into the construction of CANs, this study introduces a new dimension where the regulation of polymer topologies, coupled with modifications of backbone compositions, can be used to furnish certain functionalities. With a multitude of well-developed polymerization platforms—such as reversible deactivation radical polymerization,9Parkatzidis K. Wang H.S. Truong N.P. Anastasaki A. Recent developments and future challenges in controlled radical polymerization: a 2020 update.Chem. 2020; 6: 1575-1588Abstract Full Text Full Text PDF Scopus (179) Google Scholar ring-opening metathesis polymerization,10Bielawski C.W. Grubbs R.H. Living ring-opening metathesis polymerization.Prog. Polym. Sci. 2007; 32: 1-29Crossref Scopus (1130) Google Scholar and LPP11McGraw M.L. Chen E.Y.X. Lewis pair polymerization: perspective on a ten-year journey.Macromolecules. 2020; 53: 6102-6122Crossref Scopus (46) Google Scholar—a diversity of complex architectures ranging from block, alternating, gradient sequences to hyperbranched, dendritic, star-like structures could be readily synthesized and studied along this direction. New trials and explorations to address the existing challenges and acquire broader comprehension of the structure-property relationships for this promising next-generation material should be enticing. Standing at the intersection between thermoplastics and thermosets, CANs hold promise for creating new materials with well-versed properties in the foreseeable future. Thermomechanical activation achieving orthogonal working/healing conditions of nanostructured tri-block copolymer thermosetsClarke et al.Cell Reports Physical ScienceJuly 1, 2021In BriefClarke et al. show crosslinked tri-block copolymers with nanostructured networks, constructed by a catalyst/additive-free self-crosslinking and self-assembly process. Compared with analogous random and di-block copolymers, these crosslinked tri-block copolymers show not only superior mechanical performance but also much greater creep resistance when operated on a morphology-regulated and thermomechanical activation mechanism. 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