Rubbers exhibit enormous extensibility up to several hundred per cent, compared with a few per cent for ordinary solids, and have the ability to recover their original shape and dimensions on release of stress. Rubber elasticity is a property of macromolecules that are either covalently cross-linked or connected in a network by physical associations such as small glassy or crystalline domains, ionic aggregates or multiple hydrogen bonds. Covalent cross-links or strong physical associations prevent flow and creep. Here we design and synthesize molecules that associate together to form both chains and cross-links via hydrogen bonds. The system shows recoverable extensibility up to several hundred per cent and little creep under load. In striking contrast to conventional cross-linked or thermoreversible rubbers made of macromolecules, these systems, when broken or cut, can be simply repaired by bringing together fractured surfaces to self-heal at room temperature. Repaired samples recuperate their enormous extensibility. The process of breaking and healing can be repeated many times. These materials can be easily processed, re-used and recycled. Their unique self-repairing properties, the simplicity of their synthesis, their availability from renewable resources and the low cost of raw ingredients (fatty acids and urea) bode well for future applications.

Self-healing and thermoreversible rubber from supramolecular assembly

Units of 2-ureido-4-pyrimidone that dimerize strongly in a self-complementary array of four cooperative hydrogen bonds were used as the associating end group in reversible self-assembling polymer systems. The unidirectional design of the binding sites prevents uncontrolled multidirectional association or gelation. Linear polymers and reversible networks were formed from monomers with two and three binding sites, respectively. The thermal and environmental control over lifetime and bond strength makes many properties, such as viscosity, chain length, and composition, tunable in a way not accessible to traditional polymers. Hence, polymer networks with thermodynamically controlled architectures can be formed, for use in, for example, coatings and hot melts, where a reversible, strongly temperature-dependent rheology is highly advantageous.

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Reversible Polymers Formed from Self-Complementary Monomers Using Quadruple Hydrogen Bonding

Materials are said to show a shape-memory effect if they can be deformed and fixed into a temporary shape, and recover their original, permanent shape only on exposure to an external stimulus. Shape-memory polymers have received increasing attention because of their scientific and technological significance. In principle, a thermally induced shape-memory effect can be activated by an increase in temperature (also obtained by heating on exposure to an electrical current or light illumination). Several papers have described light-induced changes in the shape of polymers and gels, such as contraction, bending or volume changes. Here we report that polymers containing cinnamic groups can be deformed and fixed into pre-determined shapes--such as (but not exclusively) elongated films and tubes, arches or spirals--by ultraviolet light illumination. These new shapes are stable for long time periods, even when heated to 50 degrees C, and they can recover their original shape at ambient temperatures when exposed to ultraviolet light of a different wavelength. The ability of polymers to form different pre-determined temporary shapes and subsequently recover their original shape at ambient temperatures by remote light activation could lead to a variety of potential medical and other applications.

Light-induced shape-memory polymers

In the 21st century, soft materials will become more important as functional materials because of their dynamic nature. Although soft materials are not as highly durable as hard materials, such as metals, ceramics, and engineering plastics, they can respond well to stimuli and the environment. The introduction of order into soft materials induces new dynamic functions. Liquid crystals are ordered soft materials consisting of self-organized molecules and can potentially be used as new functional materials for electron, ion, or molecular transporting, sensory, catalytic, optical, and bio-active materials. For this functionalization, unconventional materials design is required. Herein, we describe new approaches to the functionalization of liquid crystals and show how the design of liquid crystals formed by supramolecular assembly and nano-segregation leads to the formation of a variety of new self-organized functional materials.

Functional liquid-crystalline assemblies: self-organized soft materials.

Additional functionality can be incorporated into liquid crystalline materials by using phase segregation and self-assembly. Intermolecular interactions such as hydrogen bonding and ionic interactions play key roles in the formation of these complex structures. One-, two-, and three-dimensional phase-segregated structures on various scales of length are formed by self-assembly of a variety of partially incompatible molecules. Such structures can enhance anisotropic properties such as ionic conductivity.

https://science.sciencemag.org/content/sci/295/5564/2414.full.pdf

Self-Assembly of Phase-Segregated Liquid Crystal Structures

Details of the thermal behavior, including transition temperatures, transition heats and transition entropies, are reported for three series of nematic liquid-crystalline “backbone” polyesters having the general structure 






All polysters described above were examined by differential scanning calorimetry and were found to have reproducible thermal behavior once the polymer had been annealed by heating to the isotropic phase and then subsequently cooled. Enantiotropic nematic phases were found for all 30 polysters studied. Many polymers showed multiple endotherms on melting to the nematic phase. Plots of solid–nematic and nematic–isotropic transition temperatures versus number of carbon atoms in the diacid segment (x) for each (y) reveal an even–odd alternation reminiscent of trends in homologous series of small-molecule liquid crystals. Enthalpies for the solid nematic and nematic isotropic transitions do not show such a precise alternation.

Mesogenic polymers. III. Thermal properties and synthesis of three homologous series of thermotropic liquid crystalline ``backbone'' polyesters

Effect of molecular structure on mesomorphism. 12. Flexible-center Siamese-twin liquid crystalline diesters - a "prepolymer" model

Hydrogen bonding between carboxylic acids and pyridines can be used as a means of chain extension. However, the reversibility of hydrogen bond formation does not allow the chain-extended structure to be fixed, and a temperature-dependent chain length results. Complexation between a tetrapyridyl species and a diacid is shown to counteract the reversibility of the hydrogen bonding. These complexes form thermoreversible three-dimensional supramolecular networks which have properties typical of low molar mass materials at high temperatures but polymeric properties at low temperatures. The complexes exhibited a glass transition and could form fibers from the melt yet exhibited a low-viscosity isotropic liquid at elevated temperatures. Atypically, a complex incorporating a 2,2',6,6'-tetrasubstituted biphenyl moiety was found to be liquid crystalline.

Thermoreversible Supramolecular Networks with Polymeric Properties

Toward Negative Poisson Ratio Polymers Through Molecular Design

The rheological properties of a thermotropic polyester were determined in the nematic and isotropic states. In the isotropic state, the viscosity is almost constant and the polymer is only slightly elastic. The nematic phase has a lower viscosity than the isotropic, except at low frequencies or shear rates, where the viscosity increases as though the polymer had a yield stress. There is a marked dependence of the rheology on shear history. The effects of shearing can be erased by returning the material first to the isotropic state and then back to the nematic state. The results are discussed with reference to analogous observations in small-molecule liquid crystals and in thermotropic aromatic co-polyesters.

Anselm C. Griffin

Papers

Mesogenic polymers. III. Thermal properties and synthesis of three homologous series of thermotropic liquid crystalline ``backbone'' polyesters

Effect of molecular structure on mesomorphism. 12. Flexible-center Siamese-twin liquid crystalline diesters - a "prepolymer" model

Thermoreversible Supramolecular Networks with Polymeric Properties

Toward Negative Poisson Ratio Polymers Through Molecular Design

Rheology of a thermotropic polyester in the nematic and isotropic states