Ph.D Thesis


Ph.D StudentGalant Or
SubjectIntramolecular Cross-Linking: From Single Chain to
Bulk Materials
DepartmentDepartment of Polymer Engineering
Supervisors ASSOCIATE PROF. Charles E. Diesendruck
ASSOCIATE PROFESSOR Maya Davidovich-Pinh
Full Thesis textFull thesis text - English Version


Abstract

New advances in polymer chemistry lead to new polymer architectures and consequently, new ways to tailor polymer properties. Historically, polymers are classified to two main groups; either they are thermoplastics or thermosets, according to their thermal response which are a direct consequence of their polymer architecture. Thermoplastics are materials in which the chains are not covalently connected to each other, and, given enough energy, can move independently. Consequently, thermoplastics are thermoformable and this formability is repeatable if no additional chemical transformations occur. Unlike thermoplastics, thermosets cannot be reformed. The presence of interchain cross-links in thermosets leads to stronger materials, albeit this comes with a ‘price’. An alternative way to change properties is by addition of additives during processing. These additives affect the chain-chain interaction; however, they can ‘leach’ out with time or under extreme conditions. Another way to change the mechanical properties, which can be less destructive to its processability and recyclability, is by tuning the polymer architecture. Three main aspects of polymer architecture are composition, functionality and topology. In this thesis, I focus on comparing polymer topologies, leading to changes in properties. Natural polymers, such as proteins, are in a type of topology where the chains are in a chemically folded architecture, and when assembled to bio-based materials, present unparalleled properties. Therefore, inspired by Nature, numerous studies investigated chain folding of synthetic polymers by using different cross-linking chemistries to form a polymer architecture known as single chain polymer nanoparticles (SCPNs). This thesis explains the effects of chain folding on the molecular level and bulk polymer properties. A bottom-up approach is taken, in which the polymer’s properties can be tuned by a rational design of the SCPNs chemistry and nanostructure. In this research, I prepared polymers with different levels of intramolecular cross-link, and studied their thermomechanical properties at the single molecule level, as well as in bulk materials at both the glassy and rubbery states. Initial studies in solution and single-molecule level indicate that chain folding affects the polymer chain conformation, reducing its available sites for interaction, leading to reduced hydrodynamic volume compared to its linear precursor. With these results in hands, properties in bulk could be studied and explained. Bio-based materials assembled from folded proteins present mechanical properties unparalleled in synthetic polymers, such as materials which present both high stiffness and toughness. In synthetic plastics, these properties tend to be mutually exclusive, i.e., material enhancements that increase stiffness tend to make the materials more brittle. In this study I show that the bulk polymer becomes more brittle as a consequence of increase in intramolecular cross-linking, which cause a reduction in entanglements between chains and in intermolecular interactions. However, in mildly glassy materials which yield at low strains, an ideal balance between intra- and intermolecular interactions can be found at intermediate intramolecular cross-linking degree. This is further seen in rubbery materials, in which remarkable improvements in mechanical properties are seen. The combination of entanglements and intramolecular cross-links lead to materials with improved moduli, strength, toughness, and amazingly high elasticity.