Abstract
Meso-scale models for hydrogels are crucial to bridge the conformation change of polymer chains in micro-scale to the bulk deformation of hydrogel in macro-scale. In this study, we construct coarse-grain bead-spring models for polyacrylamide (PAAm) hydrogel and investigate the large deformation and fracture behavior by using Dissipative Particle Dynamics (DPD) to simulate the crosslinking process. The crosslinking simulations show that sufficiently large diffusion length of polymer beads is necessary for the formation of effective polymer. The constructed models show the reproducible realistic structure of PAAm hydrogel network, predict the reasonable crosslinking limit of water content and prove to be sufficiently large for statistical averaging. Incompressible uniaxial tension tests are performed in three different loading rates. From the nominal stress-stretch curves, it demonstrated that both the hyperelasticity and the viscoelasticity in our PAAm hydrogel models are reflected. The scattered large deformation behaviors of three PAAm hydrogel models with the same water content indicate that the mesoscale conformation of polymer network dominates the mechanical behavior in large stretch. This is because the effective chains with different initial length ratio stretch to straight at different time. We further propose a stretch criterion to measure the fracture stretch of PAAm hydrogel using the fracture stretch of C-C bonds. Using the stretch criterion, specific upper and lower limits of the fracture stretch are given for each PAAm hydrogel model. These ranges of fracture stretch agree quite well with experimental results. The study shows that our coarse-grain PAAm hydrogel models can be applied to numerous single network hydrogel systems.
Highlights
A hydrogel is a network of polymer chains swollen in water
We propose a method to construct the mesoscale PAAm hydrogel models and investigate the large deformation and fracture mechanism of PAAm hydrogel using Dissipative Particle Dynamics (DPD) simulations
It shows that the formation of polymer network in crosslinking process only occurs when the diffusion length of polymer beads is much larger than the mean distance of polymer beads
Summary
A hydrogel is a network of polymer chains swollen in water. Synthetic hydrogels have developed rapidly since the landmark research by Wichterle and Lím (1960). Examples include double-network hydrogels (Gong et al, 2003; Henderson et al, 2010), poly(vinyl alcohol) hydrogels with crystalline domains (Peppas and Merrill, 1976; Stauffer and Peppast, 1992), hydrogels with hybrid physical and chemical crosslinkers (Kong et al, 2003; Shull, 2012; Sun et al, 2012), and hydrogels with transformable domains (Brown et al, 2009) It is well-known that the hyper-elastic nature of hydrogel originates from its crosslinking polymer network (Liu Z. et al, 2015). In order to reveal the true nature of the crosslinking polymer network during the deformation and fracture of hydrogels, we have to bring ourselves down to the mesoscale or even molecular scale, where molecular dynamics (MD) simulations and Monte-Carlo simulations could be an effective approach
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