Zhigang Suo

Cellulose has long been viewed as the major load-bearing component in plant primary cell walls. Recent studies, however, suggest that pectin plays a significant role in cell wall mechanics despite a kPa-scaled modulus. Here we quantify the mechanical properties of centimeter-sized internodal cell walls of Chara corallina following treatments to remove calcium crosslinks or digest pectin. We also characterize the flow of water and gas through the cell wall as a function of pressure. We found that removal of calcium crosslinks did not affect wall strength and stress-relaxation behavior, yet it decreased the elastic modulus and increased permeability to water. In contrast, pectin removal had a dramatic impact on all measurements. Samples from which pectin had been removed exhibited lower strength and stiffness and had greater stress relaxation under constant strain. Pectin removal increased the permeability of the cell wall to water above that observed when calcium was removed and reduced the pressure at which gas “tunnels” through the cell wall ∼ 2-fold. Our findings support the idea that pectin locally restricts the movement of cellulose microfibrils, with implications for understanding functional properties of primary cell walls in vascular plants and developing high-performance double-network materials.

Xylem pit membranes are discrete regions of primary cell wall that are permeable to water but prevent the spread of air embolism. Here we investigate how pectin, a cell wall hydrogel with a kPa-scale modulus, affects the functioning of intervessel pit membranes in Acer rubrum (red maple). We show that enzymatically digesting pectin significantly lowers the resistance against embolism spread, confirming earlier reports in other species and providing evidence that pectin is present in at least some portions of A. rubrum’s intervessel pit membranes. Removing calcium with a chelating agent had a smaller effect that became non-significant after accounting for radial flows. Vulnerability curves of control and calcium removal stems measured with and without the addition of a surfactant (0.1% w/v Triton-X) exhibited large differences in P50 as expected for the change in surface tension and implying an invariant effective Laplace radius. In contrast, pectin removal stems did not exhibit a significant change in P50 when vulnerability curves were measured with and without surfactant. Instead, the difference in surface tension implied large changes in the effective Laplace radius with increasing xylem tension, suggesting that pectin removal increases the local compliance of pit membranes. We hypothesize that pectin stabilizes the spacing of cellulose microfibrils, thereby contributing to the transport of water under tension.

Many applications require rubber-like materials to fulfill two important requirements: high modulus to resist excessive deformation and low hysteresis to reduce energy loss. High modulus is commonly realized by mixing rubber with inorganic fillers, but the fillers also increase the hysteresis. This conflict between the modulus and hysteresis presents a challenge in material development. Here, we show that a high modulus and low hysteresis are achieved simultaneously in a composite of long rubber chains and structured fillers. We prepare a model system using natural rubber latex with long polymer chains and structured fillers in which primary silica particles fuse into aggregates. Each aggregate has an open structure that occludes the rubber. The composite forms two networks: a network of cross-linked natural rubber chains and a network of percolated silica aggregates. The two networks intertwine with each other and interlink through covalent bonds. We measure the modulus and hysteresis of the composites prepared with various volume fractions of aggregates and molar fractions of the initiator. We discuss the mechanisms by which long rubber chains and structured silica aggregates lower hysteresis while maintaining a high modulus. This work provides insights into the development of rubber products for energy efficiency.

Natural rubber outperforms synthetic rubbers because of its long chains and strain-induced crystallization (SIC). However, these advantages are largely lost when the natural rubber chains are masticated during processing, and silica particles are added for reinforcement. Mastication eases mixing but shortens chains and lowers performance. Silica particles require covalent interlinks with rubber chains, but these interlinks restrict chain stretch and alignment, reducing SIC. Here, we show that the performance of silica-reinforced natural rubber can be markedly enhanced by preserving long natural rubber chains. We use a solvent to dissolve natural rubber latex into individual rubber chains and use the solution to uniformly disperse silica particles. After drying, the uncured compound can be stored and molded prior to curing. The long rubber chains are then sparsely crosslinked with one another and interlinked with the silica particles. The long strands readily align under stretch and increase SIC. Preserving long chains elevates toughness by an order of magnitude, from ~2 to 44 kJ m ^–2 . High toughness arises from energy dissipation across multiple length scales, over long rubber strands, silica particles, and a zone of SIC. High modulus of ~19 MPa arises from two interpenetrating networks: the network of densely entangled rubber chains and the network of percolated silica particles. The resulting material achieves high toughness while maintaining high modulus, a combination uncommon in silica-reinforced synthetic and natural rubbers.

A material tears, peels, and breaks by growing a crack. In a zone around the crack front, atoms undergo an irreversible process of breaking─and possibly reforming─bonds. Trailing behind the crack front are two layers of scars. Outside the irreversible zone and scars, atoms undergo the reversible process of elasticity. The irreversible zone is considered localized if it is small relative to the body. The idealization of localized irreversibility leads to a thermodynamic framework centered on the energy release rate. This crack driving force is defined using an ideal body in which a crack is stationary and deformation is elastic, and is applied to a real body in which a crack grows by an irreversible process. The irreversible zone scales with a material length: the fractocohesive length. We review recent advances in the development of crack-resistant elastomers and hydrogels as well as polymer networks reinforced by hard particles, fibers, or fabrics, subject to monotonic, cyclic, and static loading. Emphasis is placed on how molecular features, such as strand length, entanglements, noncovalent bonds, and chemical reactions, govern crack resistance. Design principles are highlighted that reconcile high toughness with low hysteresis through stress deconcentration. This review traces crack resistance to molecular origins, providing a foundation for designing next-generation crack-resistant materials.

In carbon black reinforced natural rubber, the synergy between sparsely crosslinked, densely entangled long chains and the percolated network of carbon particles greatly amplifies toughness while maintaining a high modulus.

Plants responding to excessive soil salinity by discharging brine onto their leaf surface risk dehydration through the osmotic continuity between the living tissue and the surface brine, which further enriches with evaporation. Cuticle cracks have long been identified as essential for salt to reach the leaf surface but enable a potentially desiccating continuity between the brine and the gland interior. Using the secreting salt gland of Nolana mollis as a model system, we integrate mathematical modeling, imaging, and physiological measurements to examine the mechanical and biochemical processes required for efficient salt removal. We find that the subcuticular space between the concentrated surface brine and the more dilute secreting cell eases the energetic limits of active salt secretion by reducing the concentration gradient of salt across the cell membrane. We show that crack size plays a critical role in balancing the osmotic and pressure gradients required for salt removal without runaway foliar desiccation.