Search for: All records

Abstract An aqueous emulsion of conducting polymer is commonly applied on a substrate to form a coating after drying. The coating, however, disintegrates in water. This paper reports a coating prepared using a mixture of two emulsions: an aqueous emulsion of conducting polymer, and an aqueous emulsion of hydrophobic and rubbery chains copolymerized with silane coupling agents. When applied on a substrate and dried, particles of the mixed emulsion merge into a continuous film. While the conducting polymer forms percolated nanocrystals, the silane groups crosslink the rubbery chains and interlink the rubbery chains to the substrate. The percolated nanocrystals make the coating highly conductive. The covalent network of hydrophobic polymer chains stabilizes the coating in water. The high conductivity and stability in water may enable broad applications. 

Rubbers reinforced with rigid particles are used in high-volume applications, including tyres, dampers, belts and hoses1. Many applications require high modulus to resist excessive deformation and high fatigue threshold to resist crack growth under cyclic load. The particles are known to greatly increase modulus but not fatigue threshold. For example, adding carbon particles to natural rubber increases its modulus by one to two orders of magnitude1,2,3, but its fatigue threshold, reinforced or not, has remained approximately 100 J m−2 for decades4,5,6,7. Here we amplify the fatigue threshold of particle-reinforced rubbers by multiscale stress deconcentration. We synthesize a rubber in which highly entangled long polymers strongly adhere with rigid particles. At a crack tip, stress deconcentrates across two length scales: first through polymers and then through particles. This rubber achieves a fatigue threshold of approximately 1,000 J m−2. Mounts and grippers made of this rubber bear high loads and resist crack growth over repeated operation. Multiscale stress deconcentration expands the space of materials properties, opening doors to curtailing polymer pollution and building high-performance soft machines. 

We mix the amphiphilic monomer acrylic acid, the hydrophobic polymer poly(methyl methacrylate), and water. We report various morphologies, which we interpret by invoking that the amphiphilic monomer can bridge the hydrophobic polymer and water. 

ABSTRACT Corners concentrate elastic fields and often initiate fracture. For small deformations, it is well established that the elastic field around a corner is power-law singular. For large deformations, we show here that the elastic field around a corner is concentrated but bounded. We conduct computation and an experiment on the lap shear of a highly stretchable material. A rectangular sample was sandwiched between two rigid substrates, and the edges of the stretchable material met the substrates at 90° corners. The substrates were pulled to shear the sample. We computed the large-deformation elastic field by assuming several models of elasticity. The theory of elasticity has no length scale, and lap shear is characterized by a single length, the thickness of the sample. Consequently, the field in the sample was independent of any length once the spatial coordinates were normalized by the thickness. We then lap sheared samples of a polyacrylamide hydrogel of various thicknesses. For all samples, fracture initiated from corners, at a load independent of thickness. These experimental findings agree with the computational prediction that large-deformation elastic fields at corners are concentrated but bounded. 

Prior to fracture, a polyacrylamide hydrogel has a stress-stretch curve of nearly perfect elasticity, but it has been suggested that an inelastic zone exists around a crack tip. This inelastic zone, however, has never been observed directly in a polyacrylamide hydrogel. Here we identify the inelastic zone using digital image correlation (DIC). We prepare a polyacrylamide hydrogel with a precut crack. While a sample of the hydrogel is stretched, the speckle patterns are recorded using a microscope or a camera, with pixel size 2.3 μm and 22.7 μm, respectively. The speckle patterns recorded by the microscope and camera are processed using the DIC software, and merged to provide the deformation field over the entire sample. The measured field of deformation is used to calculate the field of energy density according to the neo-Hookean model. When the body is perfectly elastic, the field of energy density around the crack tip is inversely proportional to the distance from the crack tip. The difference between the measured field and the predicted elastic field identifies the inelastic zone. The measured size of the inelastic zone is ∼ 0.6 mm. We further confirm that, when a sample is much larger than the inelastic zone, an annulus exists, in which the elastic crack tip field prevails. 

Poly(ethyleneterephthalate)(PET)is ather moplastic of high-volu me applications, andisiden- tified as Nu mber 1inthe ResinIdentification Code onsingle-use packages. The ester bondsinthe poly mer chains are prone to hydrolysis, but the rate of hydrolysis is extre mely lo w at roo m temperature. Here weshowthathydrolysiscausesPETtogrowcracksevenatroomtemperature and under lo w loads. The hydrolytic cracks greatly outrun erosion. When PET is sub merged in water andsubjectedto a fixedload,the crack velocityincreases with p H. At highloads,the crack gro ws rapidly, and hydrolysis is negligible, so that the crack gro ws with substantial plastic defor mation andthefracturesurfaceisrough. Atlo wloads,the crack gro wsslo wly and hydrolysis isfastenough,sothatthecrackgrows withnegligibleplasticdeformationandthefracturesurface is s mooth. These observations sho w that hydrolysis e mbrittles PET. Under develop ment for sus- tainability and healthcare are biodegradable and bio mass-derived poly mers, many of which have hydrolysablegroupsinthe mainchainsorcrosslinks.Theyareallpotentiallysusceptibletohy- drolyticcrackgrowthandembrittlement.Itishopedthatthisstudy willaidthedevelopmentand applications ofthese poly mers. 

Traditional polymer processing breaks polymer chains. The resulting networks of short chains have a low fatigue threshold. This paper shows that a low-intensity process preserves long chains, leading to a network of an increased fatigue threshold. 

Hydrogels are being developed to bear loads. Applications include artificial tendons and muscles, which require high strength to bear loads and low hysteresis to reduce energy loss. However, simultaneously achieving high strength and low hysteresis has been challenging. This challenge is met here by synthesizing hydrogels of arrested phase separation. Such a hydrogel has interpenetrating hydrophilic and hydrophobic networks, which separate into a water-rich phase and a water-poor phase. The two phases arrest at the microscale. The soft hydrophilic phase deconcentrates stress in the strong hydrophobic phase, leading to high strength. The two phases are elastic and adhere through topological entanglements, leading to low hysteresis. For example, a hydrogel of 76 weight % water, made of poly(ethyl acrylate) and poly(acrylic acid), achieves a tensile strength of 6.9 megapascals and a hysteresis of 16.6%. This combination of properties has not been realized among previously existing hydrogels. 

For a thin layer of elastomer sandwiched between two rigid blocks, when the blocks are pulled, numerous cavities grow in the elastomer like cracks. Why does the elastomer grow numerous small cracks instead of a single large crack? Here we answer this question by analyzing an idealized model, in which the elastomer is an incompressible neoHookean material and contains a penny-shaped crack. To simulate one representative crack among many, the model is axisymmetric with zero radial displacement at the edge. When the rigid blocks are pulled by a pair of forces, a hydrostatic tension develops in the elastomer. At a critical hydrostatic tension, a small crack deforms substantially, as predicted by an elastic instability, resulting in an unbounded energy release rate. Consequently, the small crack initiates its growth, regardless of the toughness of the elastomer. As the crack grows, the energy release rate decreases, so that the crack arrests. Meanwhile, the rigid blocks constrain deformation of the elastomer far away from the crack, where hydrostatic tension remains high, allowing other cracks to grow. For an elastomer of thickness H, shear modulus , and toughness , the crack radius and spacing decrease as the normalized toughness increases. Therefore, a tough elastomer of small modulus and thickness will grow numerous small cracks when confined by two rigid blocks and pulled beyond a critical force. 

Living tissues and some engineering materials contain water. When a wet material loses water, high triaxial tensile stress may build up and cause instability. The mechanism of instability under triaxial tension has attracted great attention, but quantitative study remains an ongoing chal- lenge. Here we develop an experimental method to apply well-controlled triaxial tensile stress and observe osmotic instability in situ. We synthesize a hydrogel in an elastomer tube with strong adhesion between them. The elastomer dissolves minute amount of water, but allows water to diffuse out and places the hydrogel under homogeneous, equal-triaxial, tensile stress. We develop a method to determine the stress as a function of time. The transparent setup enables observation of various types of osmotic instabilities, including cavity nucleation, crack propagation, and surface undulation. Notably, our method enables the measurement of crack speed from ~10−5 m/ s to a limit comparable to the Rayleigh wave speed ~1 m/s. We observe a large jump in crack speed at a critical energy release rate. This work opens opportunities to study the physics of soft materials under high triaxial tension.