Thermomechanical Behavior of Intramolecular Cross-linked Thermoplastics

Abstract

Polymers are materials composed of a large number of macromolecular chains, exhibiting a broad range of thermomechanical properties. Polymer properties are determined by interactions between chains and interactions between monomers within individual chains. Since these two different types of interactions, intermolecular and intramolecular, can be tuned, polymers exhibit a broad range of properties. For example, poly(methylmethacrylate) and poly(urethane) have different monomers and exhibit different properties. These interactions are dependent on the conformation of chains as well as monomer chemistry, molecular weight, etc. Creating covalent cross-links between different macromolecular chains changes the configuration of chains and inter- and intramolecular interactions. However, intermolecular cross-linking changes material processability as well as its underlying structure. Intermolecular cross-linking results in thermosets, polymers that are not thermally processable or reprocessable. In contrast to intermolecular cross-linking, intramolecular cross-linking literally creates a cross-link within a single macromolecular chain. While this intramolecular cross-linking has been widely studied on single macromolecular chains because of its precise control of chain morphology and ability to generate small polymer nanoparticles, the effects of intramolecular cross-linking in bulk polymers in the solid state have attracted little attention. In this dissertation we characterize the structure-dependent properties of polymers assembled solely from intramolecular cross-linked macromolecular chains. We adopt intramolecular cross-linking as a means of changing the underlying structure of polymers while keeping the polymers as thermoplastics. This class of polymeric materials are investigated by molecular modeling and simulations and synthesis and experiments. Firstly, we study the effects of intramolecular cross-linking on individual, independent macromolecular chains by using a molecular dynamics (MD) model. MD models of different cross-linking degrees are generated and used to study the effects of intramolecular cross-linking on size, shape, chain mobility/dynamics, and mechanics of individual, independent intramolecular cross-linked chains in vacuum. Our simulations demonstrate that the higher the CL degree, the less mobile the monomers of single cross-linked chains, and the stiffer and more rigid the chains. Individual, independent cross-linked chains are studied only by MD modeling and simulations. Next, we extend our research to studying the effects of intramolecular cross-linking on the thermomechanical properties of bulk polymers. We build MD models representative of bulk polymers made from the assembly of intramolecular cross-linked chains by using MD models of single intramolecular cross-linked chains as building blocks. These MD models, representative of bulk polymers, are studied in terms of chain topology and thermomechanical properties. In addition to modeling and simulation, we mechanically characterize by experiments the same class of materials, synthesized by collaborating chemists. These materials are analyzed by uniaxial tensile tests and cyclic tests. We find from simulations and experiments that the higher the CL degree, the less unfolded and entangled the macromolecular chains are in bulk polymers. These differences in the underlying structure due to intramolecular cross-linking result in tailored thermomechanical properties. We find that the material becomes stiffer, stronger, and more brittle at the glassy state, and stiffer, stronger, and more stretchable at the rubbery state with increasing CL degree. Our simulation results are vital in explaining the mechanisms underlying these enhanced mechanical properties.