Hydrogels (original) (raw)
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Macromolecules, 2014
Networks combining physical and covalent chemical cross-links can exhibit a large amount of dissipated inelastic energy along with high stretchability during deformation. We present our analysis of the influence of the extent of covalent cross-linking on the inelasticity of hydrogels. Four model networks, which are similar in structure but strongly differ in elasticity, have been studied. The aim was the identification of a key structural factor responsible for observing a hysteresis or an elastic deformation. In the employed molecular dynamics study this factor is derived from the underlying structure of each particular hydrogel network. Several structural characteristics have been investigated like the extent of damage to the network, chains sliding, and the specific properties of load-bearing chains. By means of such a key factor, one can predict the deformation behavior (hysteresis or elasticity) of some material, provided a precise description of its structure exists and it resembles any of the four types of a network. The results can be applied in the design of bio-inspired materials with tailored properties.
Universality in Nonlinear Elasticity of Biological and Polymeric Networks and Gels
Networks and gels are part of our everyday experience starting from automotive tires and rubber bands to biological tissues and cells. Biological and polymeric networks show remarkably high deformability at relatively small stresses and can sustain reversible deformations up to 10 times their initial size. A distinctive feature of these materials is highly nonlinear stress−strain curves leading to material hardening with increasing deformation. This differentiates networks and gels from conventional materials, such as metals and glasses, showing linear stress−strain relationship in the reversible deformation regime. Using theoretical analysis and molecular dynamics simulations, we propose and test a theory that describes nonlinear mechanical properties of a broad variety of biological and polymeric networks and gels by relating their macroscopic strain-hardening behavior with molecular parameters of the network strands. This theory provides a universal relationship between the strain-dependent network modulus and the network deformation and explains strain-hardening of natural rubber, synthetic polymeric networks, and biopolymer networks of actin, collagen, fibrin, vimentin, and neurofilaments.
Polymer, 2014
Tough double network (DN) hydrogels are a kind of interpenetrating network (IPN) gels with a contrasting structure; it consists of a rigid and brittle 1 st network with dilute, densely cross-linked short chains and a soft and ductile 2 nd network with concentrated, loosely cross-linked long chains. In this work, we focus on how the brittle gel changes into a tough one by increasing the amount of ductile component. By comparing the molecular structures of the individual first network and second network gels, we found that the true key factor that controls the brittleductile transition is the density of elastically effective polymer strands of the two networks, and. When / < 1, the second network fractures right after the fracture of the first network, and the gels are brittle. When / > 1, only the first network fractures. As a result, the brittle first network serves as sacrificial bonds, imparting toughness of DN gels. This result provides essential information to design tough materials based on the double network concept. Scheme 1: Illustration of super-tough double network (DN) gels with a contrasting structure. The blue and pink lines represent the 1 st and 2 nd networks respectively and the filled circles indicate chemical cross-linking points. The two networks do not have internetwork chemical bond, creating a truly interpenetrating network (IPN) structure. The chemical structures of PAMPS and PAAm as first and second networks, respectively, is shown on the right.
Dynamic Mechanical Response of Hybrid Physical Covalent Networks − Molecular Dynamics Simulation
Macromolecular Symposia, 2017
In this article, we present two principal stimuli that affect the deformation response: additional covalent crosslinking and external deformation. Additional crosslinking leads to hybrid gels which show improved toughness in comparison to the physical gels. The article considers also the change of response of the model as a consequence of external mechanical loading. In this case, the structural changes driven by deformation induce permanent structural changes owing to extensive stretching analogous to filament drawing in spider silk or collagen. Almost identical atomistic configurations are transformed into various materials, depending on the stimulus and the conditions. The intensity of stimuli can be regulated also in real materials. The degree of covalent crosslinking can be regulated for example by ultraviolet irradiation. The permanent changes are observed only when the stretching ratio is sufficiently large and stretching is quick.
Polymer, 2018
To establish relationships between the molecular structure of polyolefines and their physical characteristics which determine possible commercial applications, structural changes and tensile deformation response up to deformations beyond the natural draw ratio were investigated using a variety of experimental approaches. True stress-strain curves were measured at different temperatures so as to estimate the available effective network density, which will eventually define the failure mode of the material under investigation. Analysis of the deformation by means of tensile strain hardening, assuming the Haward-Thackray spring dashpot decoupling assumption by means of Edward-Vilgis' non-Gaussian rubber-elastic slip-link model, reveals the role of transient and fixed network nodes. It was established by differential scanning calorimetry and X-ray diffraction analysis that the transformation from lamellar to fibrillar morphology passes through the several pronounced stages: deformation of initial lamellae ( < 1.5); destruction of lamellar structure through the tilt; slippage of molecules in the crystallites; simultaneous formation of fibrils with structural characteristics depending on the molecular structure and on deformation conditions; deformation of the formed fibrillar structure; tiltingformation of chevrons for high molecular weight low density polyethylene or slippage of fibrils and void formation. Distinction between fixed and transient slip link network contributions reveals neatly that although there is a slight drop in the fixed link network density with increasing temperature, this contribution remains Manuscript Click here to view linked References
Models for stiffening in cross-linked biopolymer networks: A comparative study
Journal of the Mechanics and Physics of Solids, 2008
In a recent publication, we studied the mechanical stiffening behavior in two-dimensional (2D) cross-linked networks of semiflexible biopolymer filaments under simple shear [Onck, P.R., Koeman, T., Van Dillen, T., . Alternative explanation of stiffening in cross-linked semiflexible networks. Phys. Rev. Lett. 95, 178102]. These simulations make use of a geometrically nonlinear finite-element technique, taking into account the discreteness of the biopolymer network. As an alternative to the prevalent view, these computations relate the stiffening to nonaffine network reorientations. However, this discrete-network model neglects any interaction of the filaments with the surrounding fluid, which is the origin of entropic stiffening in single filaments and in biopolymer networks, according to MacKintosh et al. For this reason, this article is devoted to a thorough study of the difference between both approaches on the 2D singlefilament level. In addition, we investigate the deviation from affine deformation behavior, by comparing the discrete calculations with an affine-network model. r
Assessment of Local Heterogeneity in Mechanical Properties of Nanostructured Hydrogel Networks
ACS Nano, 2017
Our current understanding of the mechanical properties of nanostructured biomaterials is rather limited to invasive/destructive and low-throughput techniques such as atomic force microscopy, optical tweezers, and shear rheology. In this report, we demonstrate the capabilities of recently developed dual Brillouin/Raman spectroscopy to interrogate the mechanical and chemical properties of nanostructured hydrogel networks. The results obtained from Brillouin spectroscopy show an excellent correlation with the conventional uniaxial and shear mechanical testing. Moreover, it is confirmed that, unlike the macroscopic conventional mechanical measurement techniques, Brillouin spectroscopy can provide the elasticity characteristic of biomaterials at a mesoscale length, which is remarkably important for understanding complex cell−biomaterial interactions. The proposed technique experimentally demonstrated the capability of studying biomaterials in their natural environment and may facilitate future fabrication and inspection of biomaterials for various biomedical and biotechnological applications.
Journal of Polymer Science Part B: Polymer Physics, 1987
Polyurethane elastomers were prepared from a series of poly(ethylene oxide) samples by end-linking t h e chains into "model" trifunctional networks. The molecular weight M , between crosslinks in such networks is simply the number-average molecular weight M , of the precursor polymer. End-linking samples separately gave networks with unimodal distributions of network chain lengths, whereas end-linking mixtures of two sanlples having very different values of M, gave bimodal distributions with average values of M , equal to the average value of M,, for the two samples. Stress-strain isotherms in elongation were obtained for thesc networks, both unswollen and swollen to various extents. Strain-induced crystallization was manifestd in elastic properties t h a t changed significantly with changes in temperature. Swelling has more complicated effects, since it causes deformation of the network chains as well as melting of some of the crystallites. Comparisons among stress-strain isotherms a t constant M , indicate that bimodality facilitates strain-induced crystalli,mtion.
Characterization of internal fracture process of double network hydrogels under uniaxial elongation
Soft Matter, 2013
Previously we revealed that the high toughness of double network hydrogels (DN gels) derives from the internal fracture of the brittle network during deformation, which dissipates energy as sacrificial bonds. In this study, we intend to elucidate the detailed internal fracture process of DN gels. We quantitatively analysed the tensile hysteresis and re-swelling behaviour of a DN gel that shows a well-defined necking and strain hardening, and obtained the following new findings: 1) Fracture of the 1 st network PAMPS starts far below the yielding strain, and 90% of the initially load-bearing PAMPS chains already breaks at the necking point. 2) The dominant internal fracture process occurs in the necking and hardening region although the softening mainly occurs before necking. 3) The internal fracture efficiency is very high, 85% of the work is used for the internal fracture and 9% of all PAMPS chains break at sample failure. 4) The internal fracture is anisotropic, fracture occurs preferentially perpendicular to the tensile direction than other two directions, but the fracture anisotropy decreases in the hardening region. Result 1) and 2) is in agreement with a hierarchical structural model of PAMPS network. Based on these findings, we present a revised description of the fracture process of DN gels.
Yielding Behavior of Tough Semicrystalline Hydrogels
Macromolecules, 2017
Supramolecular semicrystalline hydrogels are soft functional materials consisting of water-swollen hydrophilic polymer chains interconnected by hydrophobic segments forming lamellar crystals. Although such hydrogels with high crystallinity are mechanically strong, with elastic moduli and tensile strength of 80−300 MPa and 4−7 MPa, respectively, they are brittle and rupture at a stretch of less than 20% without yielding. Here, we report that the incorporation of a small amount of a weak hydrophobe into semicrystalline hydrogels significantly increases their toughness and stretchability without losing their high modulus and high strength. We design a highly entangled physical network based on poly(N,N-dimethylacrylamide) (PDMA) chains containing n-octadecyl acrylate (C18A) and lauryl methacrylate (C12M) segments with side chain lengths of 18 and 12 carbons, respectively. By including 0.1−0.4 mol % C12M into the PDMA backbone containing 30 mol % C18A segments, we were able to create more ordered and thinner lamellar crystals with a layered structure. Simultaneously, a brittle-to-ductile transition was observed due to the appearance of necking behavior leading to 10-fold increase of toughness. The significant toughness improvement upon incorporation of C12M into the semicrystalline hydrogels could be explained with the appearance of active tie molecules under external force interconnecting the lamellar clusters. The hydrogels also exhibit reversible tensile deformation induced by heating above the melting temperature of crystalline domains.