Ph.D Thesis


Ph.D StudentLevy Avishai
SubjectMechanochemistry of Intramolecular Cross-Linked Polymers
DepartmentDepartment of Chemistry
Supervisor ASSOCIATE PROF. Charles E. Diesendruck
Full Thesis textFull thesis text - English Version


Abstract

Mechanochemistry is the transduction of mechanical energy into chemical energy. The most common mechanochemical reaction is homolytic bond scission in polymers that occur after polymers are stretched and unfold. These reactions result in chain fragmentation and reduction of molecular weight, and, as a consequence, worsening of the polymer physical properties. This phenomenon is known to be faster as synthetic polymer chains are longer. Yet, natural molecular springs, which are typically very long proteins, do not suffer from fragmentation despite absorbing mechanical energy, as a consequence of their folded architecture. These proteins are folded by reversible intramolecular hydrogen bonds that selectively break when the proteins unfold as mechanical energy is absorbed, but reform and allow refolding when the latter is removed. This thesis demonstrates that changing the architecture of synthetic polymers from linear to individually folded chains (which resemble molecular springs) by intramolecular cross-links using both covalent and non-covalent interactions, significantly inhibit the rate of mechanochemical fragmentation. By studying the self-folding of chains by covalent interactions it was possible to demonstrate that the architectural change made to the polymers, lead to improved resistance to mechanochemical fragmentation since the unfolding of polymers requires breaking higher number of bonds before fragmentation occurs. Covalent interactions were used to show that in folded architecture (unlike linear), the degree of polymerisation is actually an advantage. In a constant cross-link density, the difference in rate of fragmentation (due to ultrasound irradiation) between folded polymers and their linear precursors becomes larger as chains are longer, because of larger number of intramolecular cross-links per chain that absorb mechanical energy before fragmentation. In addition, comparison between four parameters of the cross-linker (length, strength, position and valency) showed that incorporation of chemical bonds of different strengths in the cross linker, is the most effective way to inhibiting or accelerate fragmentation. The length of the cross-linker is also effective, but only up to a limit which above it, no significant change is seen. Positioning the cross-links between distant positions along the polymer's backbone and using multiple valency cross-linker helped inhibiting fragmentation, but not to the same extent as the other parameters. Finally, by folding linear chains with reversible weak non-covalent interactions, and incorporating mechanical force sensitive molecules (mechanophores) that are able to measure the selectivity of mechanical energy absorbance, behaviour similar to natural molecular springs was obtained. Selectivity towards scission of cross-links was observed as the cross-linking density was higher. Refolding of the polymers due to the reversibility of the cross-linkers was observed if the polymers were allowed to relax between ultrasound pulses.