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1. Enabling Cut-Resistant Superhydrophobic Surfaces Using A Highly Entangled Soft Polymeric Substrate

   Cheng, Junce; Liu, Tingyi (June 2024, Hilton Head Workshop 2024: A Solid-State Sensors, Actuators and Microsystems Workshop)

   This paper presents a novel idea to enable superhydrophobic (SHPo) surfaces with exceptional resistance to cutting while remaining soft and stretchable. For the first time, we achieve these unprecedented mechanical properties through strategic heterogeneous integration of a highly entangled polymeric substrate with a top layer of SU-8 micro-pillars. To realize resistance to cutting in soft materials, our innovation utilizes a polymer within which the entanglement outnumbered the crosslinks so that the cutting stress can be redistributed along the long polymer chains and to many other chains. We demonstrate the unique cut-resistant property of the highly entangled hydrogel, the integration of the hydrogel to fabricate SHPo surfaces, and water-repellency tests against cutting. 

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2. Making Highly Elastic and Tough Hydrogels from Doughs

   https://doi.org/10.1002/adma.202206577  

   Nian, Guodong; Kim, Junsoo; Bao, Xianyang; Suo, Zhigang (December 2022, Advanced Materials)

   Abstract A hydrogel is often fabricated from preexisting polymer chains by covalently crosslinking them into a polymer network. The crosslinks make the hydrogel swell‐resistant but brittle. This conflict is resolved here by making a hydrogel from a dough. The dough is formed by mixing long polymer chains with a small amount of water and photoinitiator. The dough is then homogenized by kneading and annealing at elevated temperatures, during which the crowded polymer chains densely entangle. The polymer chains are then sparsely crosslinked into a polymer network under an ultraviolet lamp, and submerged in water to swell to equilibrium. The resulting hydrogel is both swell‐resistant and tough. The hydrogel also has near‐perfect elasticity, high strength, high fatigue resistance, and low friction. The method is demonstrated with two widely used polymers, poly(ethylene glycol) and cellulose. These hydrogels have never been made swell‐resistant, elastic, and tough before. The method is generally applicable to synthetic and natural polymers, and is compatible with industrial processing technologies, opening doors to the development of sustainable, high‐performance hydrogels. 

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3. Highly entangled hydrogels with degradable crosslinks

   https://doi.org/10.1016/j.eml.2022.101953  

   Shi, Meixuanzi; Kim, Junsoo; Nian, Guodong; Suo, Zhigang (March 2023, Extreme Mechanics Letters)

   This paper studies a polymer network in which crosslinks are degradable but polymer chains are not. We show that entanglements markedly enhance the mechanical properties of the polymer network before degradation and slow down degradation. We synthesize polyacrylamide hydrogels with disulfide crosslinks. In a precursor of a low water-to-monomer molar ratio and low crosslinker-to-monomer molar ratio, the monomers are crowded and the resulting polymer chains are long, so that the entanglements greatly outnumber crosslinks. The as-synthesized hydrogels are submerged in pure water to swell to equilibrium. We show that entanglements enhance the swell resistance of the hydrogel, as well as stiffen and toughen the hydrogel. We further show that entanglements slow down degradation when the hydrogel is submerged in an aqueous solution of cysteine. This work demonstrates that entanglements substantially expand the properties space of degradable polymers. 

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4. Forming Covalent Crosslinks between Polymer‐Grafted Nanoparticles as a Route to Highly Filled and Mechanically Robust Nanocomposites

   https://doi.org/10.1002/adfm.201905168  

   Kubiak, Joshua_M; Macfarlane, Robert_J (August 2019, Advanced Functional Materials)

   Abstract Filler aggregation in polymer matrix nanocomposites leads to inhomogeneity in particle distribution and deterioration of mechanical properties. The use of polymer‐grafted nanoparticles (PGNPs) with polymers directly attached to the particle surfaces precludes aggregation of the filler. However, solids composed of PGNPs are mechanically weak unless the grafted chains are long enough to form entanglements between particles, and requiring long grafts limits the achievable filler density of the nanocomposite. In this work, long, entangled grafts are replaced with short reactive polymers that form covalent crosslinks between particles. Crosslinkable PGNPs, referred to as XNPs, can be easily processed from solution and subsequently cured to yield a highly filled yet mechanically robust composite. In this specific instance, silica nanoparticles are grafted with poly(glycidyl methacrylate), cast into films, and crosslinked with multifunctional amines at elevated temperatures. Indentation and scratch experiments show significant enhancement of hardness, modulus, and scratch resistance compared to non‐crosslinked PGNPs and to crosslinked polymer films without nanoparticle reinforcement. Loadings of up to 57 wt% are achieved while yielding uniform films that deform locally in a predominantly elastic manner. XNPs therefore potentially allow for the formulation of robust nanocomposites with a high level of functionality imparted by the selected filler particles. 

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5. Polyacrylamide hydrogels. VI. Synthesis-property relation

   https://doi.org/10.1016/j.jmps.2022.105099  

   Wang, Yecheng; Nian, Guodong; Kim, Junsoo; Suo, Zhigang (January 2023, Journal of the Mechanics and Physics of Solids)

   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. 

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