Session F12: Polymer Physics Prize

Polymer Physics Prize Winner: Toughening hydrogels with sacrificial bonds

Hydrogels are a class of soft materials composed of cross-linked polymer networks and abundant water. Due to their rubbery nature, conventional hydrogels lack an energy dissipation mechanism during deformation and thus have weak mechanical properties. The invention of double-network hydrogels with two interpenetrating, contrasting network structure demonstrates a strategy to significantly toughen such rubbery materials (Gong et al., Adv. Mater. 2003). In double-network hydrogels, the hard and brittle short-strand network breaks upon deformation to dissipate large amounts of energy, while the soft and stretchable long-strand network disperses the stress and maintains the integrity of the materials. Therefore, the covalent bonds of the brittle network act as "sacrificial bonds" for toughening the materials [Gong, Soft Matter, 2010]. This sacrificial bond principle has been found to be general and also applicable to elastomers [Ducrot, Creton et al., Science, 2014]. This principle has been extended to non-covalent bonds to further endow materials with self-healing capabilities [Sun, Suo et al., Nature, 2012; Sun, Gong et al., Nature Mater. 2013]. The sacrificial bond principle further leads to a more general strategy for designing tough soft materials: deliberately combining a mechanically fragile structure or weak bonds to make the entire material tough. This strategy gives greater freedom in molecular design, not only limited to double or multiple network systems but also applicable to dually cross-linked single-network systems. Recently, it has been demonstrated that "self-renovative" materials that behave like muscles through repetitive mechanical training can be developed by using sacrificial bonds in double network system [Matsuda, Nakajima, Gong et al., Science 2019].   

A molecular picture of elastomer fracture

In threshold conditions the molecular model of Lake and Thomas predicts that when a network breaks, the minimum amount of energy dissipation is due to the breakage of chemical bonds crossing the fracture plane over a thickness of the order of the mesh size, and each broken strand frees an energy which evolves linearly with the strand’s length. This model predicts the right scaling and order of magnitude for fracture energy as a function of strand areal density and length even when viscoelastic dissipation isn’t negligible. Up to this day, why this model works so well remains an open question. The stochastic nature of the polymer network inevitably leads to early breakage of shorter strands which encourages us to believe that bonds have to break over a thickness much larger than the mesh size when a crack propagates. The existence of significant bond scission in the bulk was confirmed experimentally by Slootman et al. for a simple acrylate networks.

In this work, we synthesized soft networks of poly ethyl acrylate (Tg = -20°C) at various crosslink densities, labeled with mechanosensitive crosslinkers that become fluorescent when they break. Crack propagation tests were then carried out on notched samples and we extracted the energy release rate during crack propagation and mapped the concentration of broken crosslinkers from the activation of mechanophores.  For a given energy release rate, the areal density of broken crosslinkers was less dependent than expected on crosslink density but each broken crosslink appeared to dissipate energy proportional to its average length to the power 2.4.  In our experimental system the two errors compensate each other. This experimental observation brings light to why the molecular model of Lake and Thomas predicts so well the scaling of the fracture energy with network structure even though the mechanism is physically more complex.

[1] Lake, G.J. and A.G. Thomas, Proc. Roy. Soc. A, 1967. A300: p. 108-119.

[2] Slootman, J., et al., Physical Review X, 2020. 10(4): p. 041045.

    

Three Faces of Polymer Entanglements

There are four classes of flexible polymer networks: unentangled networks well-described by the classical phantom model and three classes of networks with qualitatively different types of entanglements: collective, pairwise, or transient. The classical model of polymer entanglements is the confining tube model that represents the topological effects of neighboring polymers on a given one by a confining potential. Since the potential cannot store any energy upon network deformation, the diameter of the confining tube and the conformations of network strands change non-affinely with macroscopic deformation. In contrast, the pairwise entanglements model and the primitive path analysis based on it identify topological restriction between a pair of chains (similar to slide-rings locally holding these two chains together). Such pairwise topological entanglements deform affinely with macroscopic deformation of polymer network and therefore are qualitatively different from confining tube entanglements. We develop a theory connecting these two models of polymer entanglements and show that the molecular weight between entanglements in the confining tube model is smaller than the one obtained from the primitive path analysis. The number of monomers in the affine strand increases upon network swelling due to the dis-interpenetration of network chains and saturates at a value corresponding to the pairwise entanglements determined by the primitive path analysis. This saturation corresponds to the collective to pairwise entanglement transition. Both collective and pairwise entanglements improve mechanical properties, such as the toughness of polymer networks by re-equilibrating tension in network strands. Similar features of entanglements of strands of the second network in the fine mesh of the first network lead to exceptional properties of double-networks. The third type of topological interactions between polymers is observed upon polymer deswelling and is similar to entanglements forcing non-concatenated rings into compact loopy globular conformations. Such transient entanglements formed upon deswelling of polymer gels store the extra length of polymer strands in fractal loopy conformations and allow super-elasticity of de-swollen gels – their very large reversible deformability.   

Tough and Rapidly Recoverable Polymer Gels: Self-Reinforcement by Strain-Induced Crystallization

Hydrogels, cross-linked polymer networks containing water, are expected to be applied for biomaterials such as artificial cartilage, ligaments, and prosthetic joints due to their high water content and biocompatibility. For the applications as the biomaterials, the mechanical toughness and instant recoverability are needed because artificial ligaments and prosthetic joints should survive under repeated high stress at a high frequency (ex. 1 Hz). Most tough hydrogels are reinforced by introducing sacrificial structures that can dissipate input energy [1]. However, since the sacrificial damages cannot recover instantly, the toughness of these gels drops substantially during consecutive cyclic loadings. In this presentation, we propose a damageless reinforcement strategy to realize tough and instantly recoverable hydrogels utilizing strain-induced crystallization (SIC) [2].

We used slide-ring (SR) gel containing movable figure-of-eight cross-links that work as pulleys to eliminate stress heterogeneities during deformation. SR hydrogel was prepared from polyrotaxane in which only 2 % of its polyethylene glycol (PEG) axis was threaded with cyclodextrin rings. The SR gel can survive at high stress (>5 MPa) and high strain (> 1000%). Also, the mechanical hysteresis under repeated cycle is quite small, and the instant recovery of extension energy between two consecutive cycles is 95 %. From in-situ wide-angle X-ray scattering (WAXS) experiments on the SR gel under repeated tensile deformation, we found that crystalline of PEG in the gels forms at a large strain and destructs quickly when applied stress is reduced. The reversible strain-induced crystallization yields the high toughness and instant recoverability of the SR gel.

[1] J. P. Gong, Soft Matter, 6, 2583 (2010).

[2] C. Liu, N. Morimoto, L. Jiang, S. Kawahara, T. Noritomi, H. Yokoyama, K. Mayumi, K. Ito, Science 372, 1078 (2021).

    

Tanglemer: a polymer network in which entanglements greatly outnumber crosslinks

It has long been appreciated that both entanglements and crosslinks stiffens a polymer network. Recently we have demonstrated that entanglements and crosslinks behave differently when the network fractures. Crosslinks embrittle the network, but entanglements do not. When a polymer of dense entanglements and sparse crosslinks is stretched, before a polymer chain breaks, tension transmits in the chain along the entire chain, and to many other chains through entanglements. The deconcentration of tension leads to high toughness, strength, and fatigue resistance. This talk describes methods to prepare highly entangled networks, as well as the mechanical behavior of such networks.