An official website of the United States government
Abstract The development and understanding of antifreezing hydrogels are crucial both in principle and practice for the design and delivery of new materials. The current antifreezing mechanisms in hydrogels are almost exclusively derived from their incorporation of antifreezing additives, rather than from the inherent properties of the polymers themselves. Moreover, developing a computational model for the independent yet interconnected double-network (DN) structures in hydrogels has proven to be an exceptionally difficult task. Here, we develop a multiscale simulation platform, integrating ‘random walk reactive polymerization’ (RWRP) with molecular dynamics (MD) simulations, to computationally construct a physically-chemically linked PVA/PHEAA DN hydrogels from monomers that mimic a radical polymerization and to investigate water structures, dynamics, and interactions confined in PVA/PHEAA hydrogels with various water contents and temperatures, aiming to uncover antifreezing mechanism at atomic levels. Collective simulation results indicate that the antifreezing property of PVA/PHEAA hydrogels arises from a combination of intrinsic, strong water-binding networks and crosslinkers and tightly crosslinked and interpenetrating double-network structures, both of which enhance polymer-water interactions for competitively inhibiting ice nucleation and growth. These computational findings provide atomic-level insights into the interplay between polymers and water molecules in hydrogels, which may determine their resistance to freezing.
more »
« less
A multiscale polymerization framework towards network structure and fracture of double-network hydrogels
Abstract Double-network (DN) hydrogels, consisting of two contrasting and interpenetrating polymer networks, are considered as perhaps the toughest soft-wet materials. Current knowledge of DN gels from synthesis methods to toughening mechanisms almost exclusively comes from chemically-linked DN hydrogels by experiments. Molecular modeling and simulations of inhomogeneous DN structure in hydrogels have proved to be extremely challenging. Herein, we developed a new multiscale simulation platform to computationally investigate the early fracture of physically-chemically linked agar/polyacrylamide (agar/PAM) DN hydrogels at a long timescale. A “random walk reactive polymerization” (RWRP) was developed to mimic a radical polymerization process, which enables to construct a physically-chemically linked agar/PAM DN hydrogel from monomers, while conventional and steered MD simulations were conducted to examine the structural-dependent energy dissipation and fracture behaviors at the relax and deformation states. Collective simulation results revealed that energy dissipation of agar/PAM hydrogels was attributed to a combination of the pulling out of agar chains from the DNs, the disruption of massive hydrogen bonds between and within DN structures, and the strong association of water molecules with both networks, thus explaining a different mechanical enhancement of agar/PAM hydrogels. This computational work provided atomic details of network structure, dynamics, solvation, and interactions of a hybrid DN hydrogel, and a different structural-dependent energy dissipation mode and fracture behavior of a hybrid DN hydrogel, which help to design tough hydrogels with new network structures and efficient energy dissipation modes. Additionally, the RWRP algorithm can be generally applied to construct the radical polymerization-produced hydrogels, elastomers, and polymers.
more »
« less
- Award ID(s):
- 1825122
- PAR ID:
- 10218028
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- npj Computational Materials
- Volume:
- 7
- Issue:
- 1
- ISSN:
- 2057-3960
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The double‐network (DN) concept, initially applied to hydrogels, has been adapted to elastomers, resulting in materials that combine exceptional toughness with tunable elasticity. This article delves into the constitutive and fracture behaviors of DN elastomers, elucidating the pivotal role of prestretch and composition in tailoring their properties. An incompressible hyperelastic model is employed to predict the stress–strain behavior and energy release rate of a DN elastomer, focusing on how the interactions between the two networks influence its overall material properties. The influence of prestretch and composition on increasing the stiffness and energy release rate of a DN elastomer is analytically determined. The analytical predictions are validated experimentally through comprehensive mechanical and fracture testing using a DN elastomer fabricated by a two‐step crosslinking process to decouple the prestretch and composition. The results show that manipulating these processing parameters can finely tune the mechanical responses of DN elastomers, optimizing them for specific applications. The findings provide new insights into the mechanics of DN elastomers.more » « less
-
Abstract Charged double network (DN) hydrogels are widely studied for their desirable mechanical strength and tunable properties. In this work, the influence of polymer concentration on microstructure and properties of agarose/polyacrylic acid DN hydrogels is studied. Agarose, the first network, is a brittle biopolymer, while polyacrylic acid (PAAc) is a weak polyelectrolyte. The microstructure, visualized in liquid environment, displays an agarose scaffold coated and interconnected by PAAc, deviating from the common assumption of an entangled double network. Importantly, the charging of PAAc in the hydrogel is regulated not only by the pH and weak polyelectrolyte effects, but also by the restricted swelling of the double network, and hence, it is an inherent regulation mechanism of charged hydrogels. The interactions between the hydrogel and the ionic environment induce microstructural changes and charging of the double network, impacting surface properties such as topography, stiffness, and adhesion, which are spatially resolved by liquid‐environment atomic force microscopy. The responsiveness of the DN hydrogels significantly depends on both polymer concentrations and ion concentrations. These findings provide insights into the responsive behavior of double network hydrogels and reveal universal mechanisms for charged hydrogels, which can guide the future development of functional soft materials.more » « less
-
null (Ed.)The fabrication of hydrogel materials has gained increased attention for a broad range of biomedical and biotechnological applications. However, one longstanding challenge in the field is to develop hydrogels that can be easily processed into the desired form factor, while achieving the necessary final physical and biochemical properties. Herein, we report a shear-thinning hydrogel ink that can be photo-cured to create a stretchable, suturable hydrogel whose polymer network is formed via the combination of thiol-Michael addition and radical polymerization. A shear-thinning hydrogel based on bis-methacrylated Pluronic® F-127 was modified with varying equivalents of 2,2′-(ethylenedioxy)diethanethiol (EDT) as an additive. We observed that aging the hydrogel over time prior to extrusion allowed the relatively slow thiol-Michael addition to occur (between thiol and methacrylate) prior to UV initiated photopolymerization of the methacrylates. The viscoelastic properties of these hydrogels could be tuned based on the amount of EDT added, and the aging time of the hydrogel formulation. The changes to the physical properties of the hydrogels were attributed to the increased chain length between network junctions that resulted from the thiol-Michael addition reactions. The optimized hydrogel composition was then extruded from a coaxial nozzle to produce hydrogel tubes that, after curing, were resistant to tearing and were suturable. These extrudable synthetic hydrogels with tunable viscoelastic properties are promising for tissue engineering applications and as surgical training models for human vasculature.more » « less
-
ynthesis-property relation is fundamental to materials science, but many aspects of the relation are not well understood for many materials. Impetus for this paper comes from our recent appreciation for the distinct roles of entanglements and crosslinks in a polymer network. Here we study the synthesis-property relation of polyacrylamide hydrogels prepared by free radical polymerization. Some of the as-prepared hydrogels are further submerged in water to swell either to equilibrium or to a certain polymer content. The synthesis parameters include the composition of a precursor, as well as the polymer content of a hydrogel. Series of hydrogels are prepared along several paths in the space of synthesis parameters. For each hydrogel, the stress-stretch curve is measured, giving four properties: modulus, strength, stretchability, and work of fracture. We interpret the experimentally measured synthesis-property relation in terms of entropic polymer networks of covalent bonds. When the precursor has a low crosslinker-to-monomer molar ratio, the resulting polymer network has on average long polymer segments. When the precursor has a low water-to-monomer molar ratio, the resulting polymer network has on average many entanglements per polymer segment. We show that crosslinks lower strength, but entanglements do not. By contrast, both crosslinks and entanglements increase modulus. A network of highly entangled long polymer segments exhibits high swell resistance, modulus, and strength.more » « less
