Viscoelastic Response of Double Dynamics Polymer Networks under Transient Elongation

Double dynamics networks (DDNs) are a class of polymer networks combining two or more distinct dynamic modes within the same material. Traditional permanent networks can resist flow, but show limited processability and recyclability; physical networks are more processable and recyclable, but they creep at long time. In DDNs, distinct features of permanent and physical networks are combined. As a result, DDNs can display multi-scale viscoelastic responses, making them ideal for applications in our daily life.

Knowledge on rheology, which is the science of flow and deformation of material, is necessary for application of DDNs. On one hand, it is closely connected to the network structure obtained from synthesis. On the other hand, it can have an impact on mechanical properties of DDNs towards application. It is thus desired to obtain a deep understanding of DDNs dynamics in both linear and nonlinear deformations, to uncover their multi-scale responses. While linear rheology provides information close to equilibrium conditions, nonlinear rheology is more relevant for processing and practical uses.

This thesis presents the experimental study of linear and nonlinear rheology of two classes of DDNs. Nonlinear rheology is mainly characterized by extensional rheology. Reliable extensional measurements have been performed on a Filament Stretching Rheometer with an online control scheme. The two classes of DDNs studied are i) entangled linear polymers with stickers along the chain, and ii) blends of entangled linear and ring polymers. In class 1, we first study linear and nonlinear rheology of polystyrene based copolymer with heatlabile hydrogen bonds (poly(styrene-co-4-vinylbenzoic acid)). The mechanical properties of the stretched and quenched anisotropic solids are also studied, and the results are compared to the nonmodified polystyrene. Secondly, the rheological and mechanical properties of diamine neutralized entangled poly(styrene-co-4-vinylbenzoic acid) ionomers are investigated. In class 2, the topological conformation of a symmetric linear-ring blends in extensional flow is probed by a combination of extensional rheology, Molecular Dynamics simulation and ex-situ small angle neutron scattering. Subsequently, the influences of molar mass and ring fraction on dynamics of linear-ring polymer blends are elucidated.