I propose a very simple model accounting for the very high toughness of double network gels based on the assumption that the first, stiff network will break up forming multiple cracks when the stress ...is above a defined value. These cracks are held together by the second network. A multiply cracked damage zone will form around any macroscopic crack in the material, causing energy dissipation and shielding the second network. The toughness enhancement by this process is estimated to be about ×40. The effect of cross-linking of the second network is discussed and explained.
Mechanics of elastomeric molecular composites Millereau, Pierre; Ducrot, Etienne; Clough, Jess M. ...
Proceedings of the National Academy of Sciences - PNAS,
09/2018, Letnik:
115, Številka:
37
Journal Article
Recenzirano
Odprti dostop
A classic paradigm of soft and extensible polymer materials is the difficulty of combining reversible elasticity with high fracture toughness, in particular for moduli above 1 MPa. Our recent ...discovery of multiple network acrylic elastomers opened a pathway to obtain precisely such a combination. We show here that they can be seen as true molecular composites with a well–cross-linked network acting as a percolating filler embedded in an extensible matrix, so that the stress–strain curves of a family of molecular composite materials made with different volume fractions of the same cross-linked network can be renormalized into a master curve. For low volume fractions (<3%) of cross-linked network, we demonstrate with mechanoluminescence experiments that the elastomer undergoes a strong localized softening due to scission of covalent bonds followed by a stable necking process, a phenomenon never observed before in elastomers. The quantification of the emitted luminescence shows that the damage in the material occurs in two steps, with a first step where random bond breakage occurs in the material accompanied by a moderate level of dissipated energy and a second step where a moderate level of more localized bond scission leads to a much larger level of dissipated energy. This combined use of mechanical macroscopic testing and molecular bond scission data provides unprecedented insight on how tough soft materials can damage and fail.
•Forest plantations accumulate higher AGC compared to secondary forests.•Plantations reached AGC and basal area similar to primary forests after 42 years.•Woody recruits within plantations had ...similar AGC and basal area as secondary forests.•Timber plantations facilitated successful colonization by native woody recruits.
High deforestation rates, especially in the tropics, currently result in the annual emission of large amounts of carbon, contributing to global climate change. There is therefore an urgent need to take actions to mitigate climate change both by slowing down deforestation and by initiating new sinks. Tropical forest plantations are generally thought to sequester carbon rapidly during the initial years but there is limited knowledge on their long-term potential. In this study, we assessed the carbon sequestration in old (42–47 years) timber plantations of Aucoumea klaineana, Cedrela odorata, Tarrietia utilis, and Terminalia ivorensis, and secondary forests of similar ages, by comparing their basal areas and above-ground carbon stocks (AGC) to that of nearby primary forests. Additionally, we estimated and compared timber volume and stumpage value in the three forest types.
Systematic random sampling of ninety-three 20 m × 20 m plots in eleven forest sites (2 secondary forests, 2 primary forests, and 7 timber plantations) was undertaken to determine the effect of forest type on AGC, basal area, timber volume, and stumpage value.
After 42 years of growth, mean AGC of the timber plantations (159.7 ± 14.3 Mg ha−1) was similar to that of primary forests (173.0 ± 25.1 Mg ha−1) and both were significantly higher than the mean AGC of the secondary forests (103.6 ± 12.3 Mg ha−1). Mean basal area and timber volume of the timber plantations and secondary forests were similar to that of the primary forests, though in each case the timber plantations had significantly higher values compared to the secondary forests. Mean timber value of the plantations ($8577 ha−1) was significantly higher than both secondary ($1870 ha−1) and primary forests ($3112 ha−1). Contrary to our expectations, naturally regenerated trees (woody recruits) within the timber plantations had similar AGC levels, basal area, timber volume, and value compared to the secondary forests.
Long-rotation tropical forest plantations under low-intensity management could achieve higher AGC levels and thus have higher climate change mitigation potential and timber values compared to naturally regenerated secondary forests, and are able to reach values similar to primary forests. Monoculture timber plantations could facilitate the successful colonization of their understoreys by native woody recruits that contribute considerably to stand AGC and timber values. Long-rotation forest plantations in the tropics therefore have a critical role to play in forest rehabilitation and climate change mitigation while having the potential to provide modest financial returns to landowners through selective harvesting of timber and/or payments for carbon sequestration.
Systematic loading and unloading experiments, in uniaxial tension and uniaxial compression, have been performed on a double-network hydrogel exhibiting a very high toughness. We observed a ...significant hysteresis during the first loading cycle that increased strongly with the applied maximum deformation. A large hysteresis was not observed during a second loading cycle, implying that the initial hysteresis can be attributed to the fracture of covalent bonds in the primary network. We report this type of dissipative mechanism for polymer gels for the first time. Assuming that the entire energy dissipated during the hysteresis cycle can be attributed to the fracture of network strands by a Lake−Thomas mechanism, our results suggest that the fracture and unloading of only 1% of the bonds within the network leads to a decrease of up to 80% of the number of strands. These results also demonstrate the very large degree of heterogeneity within the hydrogel network. If such a dissipative mechanism is active at the crack tip, it will most likely greatly increase the energy necessary to propagate a macroscopic crack, elucidating the origin of the toughness in these interesting materials.
The fabrication of hydrogels with well-defined structure and high mechanical strength has become a challenging and fascinating topic. The aim of this study is to develop a new method for fabricating ...hydrogels with high mechanical strength by utilizing the well-developed structure of biological gels. We firstly studied the mechanical properties and microstructure of a biological gel-the mesogloea of edible jellyfish Rhopilema esculenta Kishinouye (JF gel). JF gel has much higher mechanical strength than normal synthetic hydrogels due to its layered porous structure with pore walls consisting of nano-structured layers and fibers. We have also synthesized hydrogels by radiation-induced polymerization and crosslinking and found that they are distinctly stronger than those produced by the classical thermal polymerization using a crosslinking agent. When a synthetic gel is incorporated into JF gel by the radiation-induced polymerization and crosslinking of a hydrophilic monomer, a novel type of hybrid hydrogel with very high mechanical strength results. The compressive and tensile strengths of the hybrid hydrogels are generally several times to more than ten times higher than those of JF gel and the corresponding component synthetic gels. The hybrid gels combine the well-developed structure of biological jellyfish gel and the unique microstructure of the synthetic gel produced by the radiation method, and strong interactions between the two networks are formed.
Slide-ring (SR) gels have been fabricated through a convenient one-pot approach using thiol–ene click chemistry. Pseudo-polyrotaxanes of monothiolated β-cyclodextrin threaded on poly(allyl glycidyl ...ether)-block-poly(ethylene glycol)-block-poly(allyl glycidyl ether) were prepared in water by sonication and subsequently photo-cross-linked by UV irradiation. The pseudo-polyrotaxanes were characterized through dynamic light scattering analysis and nuclear Overhauser enhancement spectroscopy with the chemical and physical properties of the resulting SR gels being compared to the corresponding covalent gels. In contrast to these covalent gels, the SR gels showed comparable elasticity but exhibited high stretchability and tunable degradability under acidic conditions. We believe this simple and efficient fabrication method will expand and facilitate future applications of SR gels.
Torsional actuation of twisted polymer fibres is the basis for high performance tensile actuation when these fibres are formed into coils. The thermally-induced torsional actuation of twisted ...polyamide-6 fibres can be predicted by a single helix approximation when the measured diameter and length direction thermal expansion coefficients are known. The single helix model illustrates the sensitivity of the magnitude of torsional actuation to the volume expansion anisotropy for a given volume change. The applicability of the model has been further assessed by investigating three polymer fibres that display different thermal expansion anisotropies. Commercially available polyethylene, polypropylene and polyamide-6 fibres were twisted to the maximum extent without coiling and then heat treated to fix the twisted structure. Heating the twisted fibres between 26 and 62 °C resulted in a partial untwist which was reversed during cooling. The single-helix model of the twisted fibres was used to accurately predict the torsional stroke based on the measured fibre length and diameter change during heating. Comparative torsional stroke of twisted polyamide-6, polyethylene and polypropylene was explained in terms of materials thermo-physical properties. Generated blocked torques was also correctly predicted by the single-helix model when combined with the measured fibre torsional stiffness. Variances between torsional stiffnesses were found to be dependent of different anisotropic thermal properties of tested fibres.
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•Heat-induced torsional actuation has been measured for 3 different polymer fibres.•Torsional stroke, torque generated and torsional stiffnesses have been measured.•Fibre diameter and length changes have been measured.•Torsional stroke can be predicted using the single helix model.•Torque generated can be predicted based on torsional stroke and stiffness.
Hybrid gels featuring interpenetrating covalent and ionic crosslinked networks have recently been shown to exhibit both high toughness and recoverability of strain-induced network damage. The high ...toughness results from the energy dissipated as entropically strained network strands are released by the dissociation of ionic crosslinks. As in the so-called double network hydrogels, the toughening process is inherently linked to network damage. This damage, however, can be recovered to a large degree in hybrid gels due to the reformation of ionic associations when the gel is unloaded. The stability of the ionic network under load is here investigated and it is shown that these networks show large stress relaxation at constant strain, time dependent stress-strain behaviour and rate-dependent toughness. A double exponential model is invoked to mathematically describe the stress relaxation of the hybrid gels indicating at least two relaxation mechanisms. The rate-dependent toughness and the relaxation behaviour of the hybrid gels are attributed to the labile unzipping of the ionic crosslinks which is assumed to be load and time dependent.
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•Hybrid hydrogels suffer rapid stress relaxation when stretched and held at constant strain.•Stress relaxation occurs in the ionically crosslinked network and modelling suggests two separate relaxation processes.•The hydrogels were tested in their equilibrium swollen state and toughness was reduced compared with as-synthesized gels.
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•A new type of tensile artificial muscle is introduced for the first time.•The muscle consists of a torsionally-active fiber that is serially connected to an elastomeric fiber.•Heat ...induced untwist of one fiber causes coil formation and shortening of the second fiber.•Large contraction strains of more than 10% are demonstrated.•A comprehensive mechanics model is developed that accurately describes the new actuation mechanism.
A new concept for tensile artificial muscles is introduced in which the torsional actuation of a twisted polymer fibre drives a twist to writhe conversion in a serially attached elastomeric fibre. Thermally induced torsional rotation of the twisted fibre caused formation of coils in the elastomeric fibre which resulted in overall muscle length contraction. Theoretical predictions of the muscle strain were developed by means of a modified single-helix theory. Experimental tests were conducted to measure the isotonic contraction strains for elastomeric fibres of different diameters and lengths. A good agreement between the measured and calculated results was found. Practical applicability of this muscle is evaluated by using different mechanical loading conditions. Actuation contraction strains as high as 10% were observed with excellent reversibility. Unlike original coiled fibre tensile actuators, these twist-coil artificial muscles did not require any pre-conditioning cycles.
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