Fracture of Highly Entangled Polymer Networks

Abstract

Elastomers and gels are polymer networks, in which polymers entangle and crosslink. The topology of the polymer network significantly affects the material properties of elastomers and gels. The role of entanglements on deformation has been studied, but their effects on other properties, such as fracture, fatigue, and friction, are less well understood. This thesis studies how entanglements affect various material properties and how to fabricate highly entangled polymer networks using either monomers or preexisting polymers. Also, this thesis introduces two theories on polymer networks, regarding rate-dependency of toughness and thermodynamic model. Lastly, a method to make hydrogels as stiff as plastics using phase separation will be discussed. To study the role of entanglements, we synthesize polymers in which entanglements greatly outnumber crosslinks, by polymerizing the monomers with small amounts of crosslinker, initiator, and solvent. The dense entanglements enable the transmission of tension in a polymer chain along its length and to many other chains. The sparse crosslinks prevent the polymer chains from disentangling. These polymers have high toughness, strength, and fatigue resistance. After the polymers are submerged in water and swell to equilibrium, the resulting hydrogels have low hysteresis, low friction, and high wear resistance. Many hydrogels are also fabricated from preexisting polymers, covalently crosslinked into a network. However, densifying entanglements with preexisting polymers is challenging compared to monomers, due to their huge viscosity. We make a hydrogel from a dough, formed by mixing long polymers with a small amount of water and initiator. The dough is homogenized by kneading and annealing at elevated temperatures, during which the crowded polymers densely entangle. We demonstrate the method with widely used polymers, poly(ethylene glycol) and cellulose. Highly entangled hydrogels can be made degradable by using disulfide crosslinks. We study how entanglements affect degradation. A lightly entangled polymer network dissolves in an aqueous solution of cysteine within a few hours, whereas a highly entangled polymer network degrades over one month. It is likely because the entanglements retard the dissolving of polymer chains and provide more time to re-bond. We also show that elastic hydrogels can have rate-dependent toughness when the polymer chains are long. When the hydrogel is stretched, at the crack tip, some polymer chains slide relative to others. When the hydrogel is stretched at a high rate, the polymer chain at the crack tip does not have time to slip fully. The high tension only transmits over a short segment of the chain, which will lead to a lower toughness. When a polymer network is stretched, some polymer strands do not bear load but all strands contribute to swelling. We distinguish such a difference by modifying the Flory-Rehner model with three parameters: the density of load-bearing strands, N, the density of all strands, M, and the interaction parameter, χ. We synthesize polyacrylamide hydrogels and determine the parameters by three tests: free swelling, fast tensile test, and stress relaxation. In all samples tested, M is several times larger than N, whereas χ is nearly a constant. To increase the stiffness of hydrogels further, we self-assemble a nanocomposite using a hydrogel-forming polymer and a glass-forming polymer. The process separates the polymers into a hydrogel phase and a glass phase. The two phases arrest at the nanoscale and are bicontinuous. The nanocomposite maintains the structure and resists further swelling. We demonstrate the process using commercial polymers, achieving high water content, as well as load-bearing capacity comparable to that of polyethylene. During the process, a rubbery stage exists, enabling us to fabricate objects of complex shapes and fine features.