Self-Healing Elastomer Wietor, Jean-Luc; Sijbesma, Rint P
Angewandte Chemie (International ed.),
October 13, 2008, Letnik:
47, Številka:
43
Journal Article
Recenzirano
“Heal thyself!”: A thermoplastic rubber material has recently been developed that can completely mend itself when the fracture interfaces are rejoined and left to heal for a moderate time. This ...“smart” rubber is easy to synthesize and displays excellent mechanical properties.
Biology often uses hierarchical self-assembly to produce complex functional structures from smaller components. At each level of this stepwise process, non-covalent interactions bring together the ...subunits of a lower level of complexity, using the information encoded in their structures. Applying this approach to synthetic systems represents a formidable challenge, because it requires a high degree of command of non-covalent interactions. In this tutorial review, recent developments in the hierarchical self-assembly of discrete columnar aggregates are discussed.
Ferroelectric switching is demonstrated in a hydrogen bonded columnar liquid crystalline (LC) material. Polar order induced in the LC phase can be frozen by crystallization of the alkyl chains in the ...periphery of the columns yielding thin films with remnant polarization and an unprecedented high surface potential as shown by scanning Kelvin probe microscopy.
The rheology of supramolecular polycaprolactone polymers with two ureidopyrimidinone (UPy) end groups and unimer molecular weights between 600 and 4000 was compared with that of branched tri- and ...tetrafunctional analogs. Above the melting point of polycaprolactone, the previously observed low-frequency plateau in storage and loss moduli during oscillatory frequency sweep of lower molecular weight bifunctional unimers was shown to persist up to 130−170 °C, where it gradually disappeared but reappeared upon slow cooling. Even though they are supramolecularly cross-linked, the tri- and tetrafunctional materials showed no plateau. This counterintuitive behavior was further investigated with optical microscopy, WAXS, and DSC experiments, which indicated that the plateau is closely connected to the presence of crystalline domains in the lower molecular weight fraction of bifunctionalized unimers. Because the formation of crystallites is prevented by branching, and because the network formed by the tri- and tetrafunctional unimers has a short lifetime, the branched materials do not show a low-frequency plateau.
Microphase separation of bio-based soft blocks in a hard isosorbide polycarbonate enabled the preparation of a transparent bio-based engineering plastic with improved mechanical properties and ...processability at milder conditions. The ability to process these isosorbide-containing polycarbonates at lower temperatures in combination with a lower polymerization temperature due to the use of the activated bis(methyl salicyl) carbonate as the carbonate source avoided the undesired elimination of β-hydrogens, which is commonly observed in isosorbide-containing polymers. Preparation of a wide range of custom samples with varying combinations of soft blocks, followed by characterization and statistical analysis, enabled the identification of the correlations between composition and mechanical and thermal properties, resulting in an optimized engineering plastic with facile processing, transparency, and ductility combined with >84% renewable content.
Dynamic covalent networks are a class of polymeric materials that combine the merits of classical thermosets and thermoplastics, in terms of mechanical properties and reprocessability, in one ...material. Various dynamic covalent chemistries have thus been implemented in polymeric materials with recent interests shifting toward chemistries that would allow rearrangements in network topology without the aid of external catalysts. Here we introduce transesterification in phosphate triesters as a new dynamic covalent chemistry in polymeric networks. A simple one-step synthetic strategy has been utilized to synthesize polytetrahydrofuran networks with phosphate triester cross-links. The materials showed finite viscous flow at elevated temperatures via transesterification at the cross-links without externally added catalyst. This approach provides an easy method for cross-linking OH-end-functionalized polyethers and has the potential for general use with other OH-functionalized polymers.
Bond exchange via neighboring group-assisted reactions in dynamic covalent networks results in efficient mechanical relaxation. In Nature, the high reactivity of RNA toward nucleophilic substitution ...is largely attributed to the formation of a cyclic phosphate ester intermediate via neighboring group participation. We took inspiration from RNA to develop a dynamic covalent network based on β-hydroxyl-mediated transesterifications of hydroxyethyl phosphate triesters. A simple one-step synthetic strategy provided a network containing phosphate triesters with a pendant hydroxyethyl group. 31P solid-state NMR demonstrated that a cyclic phosphate triester is an intermediate in transesterification, leading to dissociative network rearrangement. Significant viscous flow at 60–100 °C makes the material suitable for fast processing via extrusion and compression molding.
High molar mass polycarbonate is synthesized via a solution transcarbonation of bis(methyl salicyl) carbonate and bisphenol‐A at temperatures between 60 and 160 °C without the removal of the ...condensate, allowing the incorporation of thermosensitive monomers into polycarbonate. Kinetic and equilibrium studies show that the polymerization is 20–30 times faster at 120 °C compared to 60 °C, whereas the equilibrium Mw increases from 11 × 103 g mol−1 at 120 °C to 16 × 103 g mol−1 at 60 °C. This polycondensation is characterized by very high equilibrium constants ranging from 0.8 × 103 at 160 °C to 4.1 × 103 at 60 °C, corresponding to standard enthalpies and entropies of polymerization: −19 kJ mol−1 < ΔH0 < −11 kJ mol−1 and 13 J mol−1 K−1 < ΔS0 < 28 J mol−1 K−1. Without removal of the condensate, the system is shown to be dynamic and completely reversible when changing the temperature. Good predictability of this polycondensation is reported, where only at very low starting monomer concentrations, the formation of cyclics leads to deviations from the predicted behavior.
High molar mass polycarbonate is synthesized at low temperatures via a dynamic solution carbonation, which allows incorporation of thermosensitive monomers into polycarbonate. Low temperatures result in slower polymerization rates, but higher molar masses at equilibrium. The polymerization is characterized by very high equilibrium constants and shows to be fully reversible when changing the temperature.
The effect of stacking of end groups on the rheological behavior of supramolecular polymer melts is reported. Oscillatory shear experiments in the transition zone from the pseudo rubber plateau to ...the flow region of telechelic polycaprolactones (PCLs) with ureidopyrimidinone (UPy) end groups directly attached to PCL can be fitted with a single Maxwell element. This demonstrates that dimerization of the UPy groups is unidirectional and that reversible chain scission is faster than reptation. If the UPy groups are connected to the polymer via a urethane linker, a low-frequency plateau in G‘ is observed. This is ascribed to the formation of a network of stacked UPy dimers, aided by urethane hydrogen bonding. Below their melting point, these stacks form long fibers in the urethane linked supramolecular poly(methyl caprolactone), which were observed with atomic force microscopy (AFM). Steric hindrance interferes with stacking, since the plateau in G‘ is lower in a urethane linked polymer with bulky adamantyl−UPy end groups.
Three different π-conjugated oligomers (a blue-emitting oligofluorene, a green-emitting oligo(phenylene vinylene), and a red-emitting perylene bisimide) have been functionalized with ...self-complementary quadruple hydrogen bonding ureidopyrimidinone (UPy) units at both ends. The molecules self-assemble in solution and in the bulk, forming supramolecular polymers. When mixed together in solution, random noncovalent copolymers are formed that contain all three types of chromophores, resulting in energy transfer upon excitation of the oligofluorene energy donor. At a certain mixing ratio, a white emissive supramolecular polymer can be created in solution. In contrast to their unfunctionalized counterparts, bis-UPy-chromophores can easily be deposited as smooth thin films on surfaces by spin coating. No phase separation is observed in these films, and energy transfer is much more efficient than in solution, giving rise to white fluorescence at much lower ratios of energy acceptor to donor. Light emitting diodes based on these supramolecular polymers have been prepared from all three types of pure materials, yielding blue, green, and red devices, respectively. At appropriate mixing ratios of these three compounds, white electroluminescence is observed. This approach yields a toolbox of molecules that can be easily used to construct π-conjugated supramolecular polymers with a variety of compositions, high solution viscosities, and tuneable emission colors.
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