Microbial production of megadalton titin yields fibers with advantageous mechanical properties

Manmade high-performance polymers are typically non-biodegradable and derived from petroleum feedstock through energy intensive processes involving toxic solvents and byproducts. While engineered microbes have been used for renewable production of many small molecules, direct microbial synthesis of high-performance polymeric materials remains a major challenge. Here we engineer microbial production of megadalton muscle titin polymers yielding high-performance fibers that not only recapture highly desirable properties of natural titin (i.e., high damping capacity and mechanical recovery) but also exhibit high strength, toughness, and damping energy — outperforming many synthetic and natural polymers. Structural analyses and molecular modeling suggest these properties derive from unique inter-chain crystallization of folded immunoglobulin-like domains that resists inter-chain slippage while permitting intra-chain unfolding. These fibers have potential applications in areas from biomedicine to textiles, and the developed approach, coupled with the structure-function insights, promises to accelerate further innovation in microbial production of high-performance materials.

Modelling Damage Accumulation During Cyclic Loading in Elastomeric Gels With Interpenetrating Networks

Double network (DN) elastomers are a class of reinforced gels that benefit from a significantly high stretch-ability and toughness. However, DN gels lose their toughness due to the accumulation of damage under cyclic loading during their lifetime. While recent advances in the process and characterization of the DN gels have led to significant improvements in their properties, our understandings of the accumulated damage mechanisms within the material remain sparse and inconclusive. Here, a physically motivated constitutive model is presented for DN gels subjected to a high number of cyclic deformations, which will eventually approach a steady-state after thousands of cycles. The model can be particularly used to elucidate the inelastic features, such as permanent damage during deformation of each cycle. The observed damage may be induced from the chain scission, chain slippage, or polymer relaxation. Therefore, irreversible chain detachment and decomposition of the first network due to its highly cross-linked structure are explored as the underlying reasons for the nonlinear stress softening phenomenon. The model is validated against the experimental tests. The model contains a few numbers of material constants and shows good agreement with cyclic uni-axial tensile test data.

Effect of cross-link density and the healing efficiency of self-healing poly(2-hydroxyethyl methacrylate) hydrogel

In this study, hydrogels of poly(2-hydroxyethyl methacrylate) with different cross-link density were prepared by the free-radical polymerization method. l-Cystine, which acts as a cross-linker, was prepared at different concentrations, ranging from 0.02 to 0.08 mol/l, to identify the concentration that provided the highest mechanical strength and healing efficacy. Healing of the hydrogels was achieved by heating above their glass transition temperature. Intermolecular diffusion of the dangling chain or chain slippage led to the healing of the gels. Results showed that 0.04 mol/l of l-cystine in poly(2-hydroxyethyl methacrylate) hydrogels provided the highest ultimate tensile strength (0.780 N/mm2) and healing recovery (92%). This healing capability was also observed using optical microscopy.

Molecular deformation mechanisms and mechanical properties of polymers simulated by molecular dynamics

Virtual polymeric materials were created and used in computer simulations to study their behavior under uniaxial loads. Both single-phase materials of amorphous chains and two-phase polymer liquid crystals (PLCs) have been simulated using the molecular dynamics method. This analysis enables a better understanding of the molecular deformation mechanisms in these materials. It was confirmed that chain uncoiling and chain slippage occur concurrently in the materials studied following predominantly a mechanism dependent on the spatial arrangement of the chains (such as their orientation). The presence of entanglements between chains constrains the mechanical response of the material. The presence of a rigid second phase dispersed in the flexible amorphous matrix influences the mechanical behavior and properties. The role of this phase in reinforcement is dependent on its concentration and spatial distribution. However, this is achieved with the cost of increased material brittleness, as crack formation and propagation is favored. Results of our simulations are visualized in five animations.

Thermoelastic Behaviour of Synthetic Rubber/Organoclay Nanocomposites at Low Elongations

Melt-compounded nanocomposites of synthetic styrene- co-butadiene rubber (BUNA SL18) and clay particles pretreated with three different modifiers were characterised by stretching calorimetry in the range of relative elongations λ < 1.3. In contrast to the pristine rubber, all nanocomposites exhibited irreversibility of both mechanical work and heat effects in stretching/contraction cycles at fairly low elongations. The observed irreversibility was considered as evidence for chain slippage effects.

The Chain End Distributions and Crosslink Characteristics in Black-Filled Rubbers

A black-filled rubber compound consists of two phases: the free polymer phase, where no particle exists, and the carbon black agglomerate phase, where highly concentrated particles are bound by a small amount of the polymer—so called bound rubber. The Charlesby—Pinner virtual linear number-average molecular weight Mn1 of the polymer in each phase is determined for black-filled compounds to obtain information about the chain end distribution in the compounds. The nominal crosslink density of the bound rubber is also measured by means of the swelling measurement of the “carbon gel” to characterize the crosslink variation in the vulcanizates. The results indicate that the chain end density is much higher in the agglomerate phase than in the free polymer phase due to enhanced chain scission during mixing. The enhancement of scission is considered due to the free radical crosslinking which imposes restriction to chain slippage in the flow field. This together with the previous findings suggests the features of the phase construction in the filled vulcanizates: tightly crosslinked free polymer phase with fewer chain ends, and loosely crosslinked agglomerate phase with more chain ends but, with rather suppressed chain mobility due to the dense nano-scale particles. The features match well with the energy dissipating and the toughened nature of the filled vulcanizates.