Study on Large Deformation Behavior of Polyacrylamide Hydrogel Using Dissipative Particle Dynamics

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 CC 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.

A statistical mechanics framework for polymer chain scission, based on the concepts of distorted bond potential and asymptotic matching

“…Lower bound of chain scission: Considering the rapid time scale of thermal fluctuations, it is assumed that the chain becomes ruptured when the energy needed for a single segment to dissociate is imparted to the chain as a whole. εdiss c /β is taken to be approximately equal to E char ν (Lei et al, 2021(Lei et al, , 2020. This criterion neglects both the probabilistic nature of thermal fluctuations and the reduction in scission activation energy under an increasing chain force.…”

Interplay of Spatial and Topological Defects in Polymer Networks

“…Including both spatial and topological heterogeneities in model network structure is important for accurate descriptions of micro and nanogels. , Additionally, the elasticity and mechanical behavior of networks is inherently related to their molecular network structure. − While there are examples of computational studies , which explicitly study heterogeneity effects; however, they did not determine the effect on the gel point. While many computational studies ,− incorporate topological defects and potentially spatial variations by employing reaction mechanisms to form gels, commonly, the heterogeneous structure of the network is not explicitly controlled or systematically investigated.…”

“… 30 − 33 While there are examples of computational studies 34 , 35 which explicitly study heterogeneity effects; however, they did not determine the effect on the gel point. While many computational studies 16 , 36 − 39 incorporate topological defects and potentially spatial variations by employing reaction mechanisms to form gels, commonly, the heterogeneous structure of the network is not explicitly controlled or systematically investigated.…”

Mechanism Study on Mechanical Properties of Physical–Chemical Hybrid Hydrogels by Coarse-Grained Molecular Dynamics Simulations

“…Different from traditional AAMD, CGMD performs better for studying polymer systems in many cases due to its computability to allow longer length and time scales, and it is also more suitable for numerous hydrogel systems. 30 Therefore, CGMD has been used to explore interesting materials including the slide-ring polymer network 31 and ioncontaining polymers. 32 Although numerous studies have been performed to investigate the synthesis, mechanical tests, and the relationship between mechanical properties and polymer networks of triblock copolymer hydrogels, the molecular mechanisms of hybrid triblock copolymer gels are seldom of concern.…”

“…MD simulation is a useful tool for illustrating the deformation mechanisms by analyzing the elastic, yielding, and strain-hardening regimes as well as the effects of temperature, nanoparticles, conductivity, and cross-linkers on mechanical behaviors. − The good agreement between these results and experimental data demonstrates the great feasibility of MD on hydrogel systems. Different from traditional AAMD, CGMD performs better for studying polymer systems in many cases due to its computability to allow longer length and time scales, and it is also more suitable for numerous hydrogel systems . Therefore, CGMD has been used to explore interesting materials including the slide-ring polymer network and ion-containing polymers …”